2 CCR 408-1
DEPARTMENT OF NATURAL RESOURCES Colorado Water Conservation Board RULES AND REGULATIONS FOR REGULATORY FLOODPLAINS IN COLORADO 2 CCR 408-1 [Editor’s Notes follow the text of the rules at the end of this CCR Document.] Rule 1. Title:
The formal title of the previous rules and regulations was “Rules and Regulations for the Designation and Approval of Floodplains and of Storm or Floodwater Runoff Channels in Colorado” as approved in 1988. The new title for these 2005 rules and regulations is “Rules and Regulations for Regulatory Floodplains in Colorado” . They may be referred to herein collectively as the “Rules” or individually as “Rule” . These 2005 rules supersede the 1988 rules.
Rule 2. Authority:
These rules are promulgated pursuant to the authority granted the Colorado Water Conservation Board (Board or CWCB), in sections 37-60-106(1)(c), 37-60-106(1)(k), 37-60-108, 30-28-111(1) and (2), 31-23- 301(1) and (3), and 24-65.1-403 (3) and 24-4-103, C.R.S. (2004). Rule 3. Purpose and Scope:
A. Purpose. The purpose of these rules is to provide uniform standards for regulatory floodplains (or floodplains) in Colorado, to provide standards for activities that may impact regulatory floodplains in Colorado, and to stipulate the process by which floodplains will be designated and approved by the Colorado Water Conservation Board. Rules for 100-year floodplains are of statewide concern to the State of Colorado and the Colorado Water Conservation Board. These rules will also assist the CWCB and communities in Colorado to develop sound floodplain management practices and to assist with the implementation of the National Flood Insurance Program (NFIP).
B. Scope a. Zoning. These rules apply to all floodplain information developed for zoning and for floodplain permitting purposes for streams in the State of Colorado by, but not limited to, individuals, corporations, local government agencies, regional government agencies, state government agencies, Indian tribes, and federal government agencies.
b. Subdivisions. These rules generally apply to the approval of subdivision drainage reports that provide 100-year floodplain information, which is a responsibility of local government and is covered in Rule 6. However, local governments are encouraged to ensure that site- specific floodplain delineations prepared during development activities are consistent with floodplain information designated and approved by the Board.
c. Design Criteria. These rules do not apply to the selection of optimal economic criteria for the construction of roads, bridges, irrigation structures, or any other facility in the floodplain.
d. Dam Failure floodplain. These rules do not apply to the identification of the area potentially inundated by the catastrophic or sudden failure of any man-made structure such as a dam, canal, irrigation ditch, pipeline, or other artificial channel. Rule 4. Definitions:
Floodplain Rule Terms Defined. The following definitions are applicable to these Rules and Regulations for Regulatory Floodplain in Colorado.
Term Definition Alluvial Fans A fan-shaped sediment deposit formed by a stream that flows from a steep mountain valley or gorge onto a plain or the junction of a tributary stream with the main stream. Alluvial fans contain active stream channels and boulder bars, and recently abandoned channels.
Approximate floodplain Floodplain information information that significantly reduces the level of detail for topographic mapping or hydraulic calculations to arrive at floodplain delineations without a comparison of water surface profiles with a topographic map of compatible accuracy. The level of detail for hydrology is consistent with that of detailed floodplain information.
Base Flood Synonymous with 100- year flood and it means a flood having a one percent chance of being equaled or exceeded in any given year.
Base Flood Elevation The elevation shown on a (BFE) FEMA Flood Insurance Rate Map for Zones AE, AH, A1-A30, AR, AR/A, AR/AE, AR/A1-A30, AR/AH, AR/AO, V1- V30, and VE that indicates the water surface elevation resulting from a flood that has a one percent chance of equaling or exceeding that level in any given year.
Basin The total land surface area from which precipitation is conveyed or carried by a stream or system of streams under the force of gravity and discharged through one or more outlets.
Channel Low lying area where water flows regularly or intermittently with a perceptible current between observable banks, although the location of banks may vary under different conditions.
Channelization The artificial creation, enlargement or realignment of a stream channel.
Code of Federal (CFR) is the codification Regulations of the general and permanent rules published in the Federal Register by the executive departments and agencies of the Federal Government. It is divided into 50 titles that represent broad areas subject to Federal regulation. FEMA regulations fall under 44 CFR.
Community Any political subdivision in the state of Colorado that has authority to adopt and enforce floodplain management regulations through zoning, including, but not limited to, cities, towns, unincorporated areas in the counties, Indian tribes and Drainage and Flood Control Districts.
Debris Flows Movement of mud and water downward over sloping terrain. The flow typically consists of a mixture of soil, rock, woody debris and water that flows down steep terrain.
Designation and Certification by formal Approval action of the Board that technical information developed through scientific study using accepted engineering methods suitable for making land use decisions under statutorily authorized zoning powers.
Detailed Floodplain Floodplain information Information prepared utilizing topographic base mapping, hydrologic analysis, and hydraulic calculations to arrive at precise water surface profiles and floodplain delineations suitable for making land use decisions under statutorily authorized zoning powers.
Development Any man-made changes to improved or unimproved real estate, including, but not limited to, buildings or other structures, mining, dredging, filling, grading, paving, excavation or drilling operations.
DFIRM Database Database (usually spreadsheets of data and analyses that accompany DFIRMs). The FEMA Mapping Specifications and Guidelines outline requirements for the development and maintenance of DFIRM databases.
Digital Flood Insurance FEMA digital floodplain Rate Map (DFIRM) map. These digital maps serve as “regulatory floodplain maps” for insurance and floodplain management purposes.
Digital Terrain Model Digitally encoded (DTM) information about the elevation (or variation of relief) of a given area.
Federal Register The official daily publication for rules, proposed rules, and notices of Federal agencies and organizations, as well as executive orders and other presidential documents FEMA Federal Emergency Management Agency FEMA - Guidelines & Floodplain mapping Specifications for Flood specifications published Hazard Mapping by FEMA. The current Partners guiding documents are posted on FEMA’s website and provide the minimum national standards for base map data and floodplain information.
500-year floodplain An area that has a 0.2 percent chance of flooding in any given year.
"Flood" or "Flooding" A general and temporary condition of partial or complete inundation of normally dry land areas from:1. The overflow of water from channels and reservoir spillways;2. The unusual and rapid accumulation or runoff of surface waters from any source; or3. Mudslides (e.g., mudflows) excess surface water that is combined with mud and other debris that are sufficiently fluid so as to flow on and over the surface of normally dry land areas, as when earth is carried by a current of water and deposited along the path of the current.
Flood Contour A line shown on a map joining points of equal elevation on the surface of floodwater that is perpendicular to the direction of flow.
Flood Insurance Rate A FIRM is the official Map (FIRM) map of a community on which FEMA has delineated both the special hazard areas and the risk premium zones applicable to the community.
Flood Mitigation A project within or Project adjacent to a flooding source that is specifically intended to reduce or eliminate the negative impacts caused by excessive floodwaters through improvement of drainage, flood control, flood conveyance or flood protection.
Floodplain The area of land that could be inundated as a result of a flood including the area of land over which floodwater would flow from the spillway of a reservoir.
Floodplain Management The operation of an overall program of corrective and preventive measures for reducing flood damage, including, but not limited to, zoning or land-use regulations, flood control works, and emergency preparedness plans.
Floodplain Management Zoning ordinances, Regulations subdivision regulations, building codes, health regulations, land-use permits, special purpose ordinances (floodplain ordinance, grading ordinance, or erosion control ordinance) and other applications of police powers. The term describes state/local regulations that provide standards for flood damage preservation and reduction.
Floodplain Maps Maps that show in a plan view the horizontal boundary of floods of various magnitudes or frequencies. Such maps include, but are not limited to, Flood Hazard Boundary Maps (FHBM), Flood Insurance Rate Maps (FIRM), and Digital Flood Insurance Rate Maps (DFIRM) published by FEMA, Flood Prone Area Maps published by the U.S. Geological Survey (USGS), Flooded Area Maps published by the U. S. Army Corps of Engineers (COE), Flood Hazard Area Delineations
Floodplain Studies A formal presentation of the study process, results, and technical support information developed for floodplain maps.
Floodway Highest hazard portion of the floodplain where floodwater is likely to be deepest and fastest. It is the area of the floodplain that must be kept free of obstructions to allow floodwaters to move downstream.
Foreseeable The potential future Development development of, or changes in, the land uses that are likely to take place during the period of time covered by a community's adopted master land use plan, or comprehensive community plan, over a 20-year period. If there is no adopted community plan, then potential development patterns based on zoning, annexations, and other relevant factors should be evaluated.
Freeboard The vertical distance in feet above a predicted water surface elevation intended to provide a margin of safety to compensate for unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood such as bridge openings and the hydrological effect of urbanization of the watershed.
Geographic Information Computer software that Systems (G.I.S.) utilizes databases and terrain mapping to store and display special and tabular data, such as floodplains, as layers (e.g.
Hydraulic analysis The determination of flood elevations and velocities for various probabilities based on a scientific analysis of the movement and behavior of floodwaters in channels or basins.
Hydrogeomorphology Study of the physical appearance and operational character of the river as it adjusts its boundaries to the magnitude of stream flow and erosional debris produced within the attendant watershed.
Hydrologic Analysis The determination of the peak rate of flow, or discharge in cubic feet per second, for various selected probabilities for streams, channels, or basins based on a scientific analysis of the physical process.
Letter of Map An amendment to the Amendment (LOMA) currently effective FEMA map that establishes that a property is not located in a Special Flood Hazard Area. LOMAs are issued by FEMA.
Letter of Map Revision An official amendment to (LOMR) the currently effective FEMA map. It is issued by FEMA and changes flood zones, delineations, and elevation.
LIDAR (Light Detection LIDAR uses the same and Ranging) principle as RADAR. The LIDAR instrument transmits light out to a target. The transmitted light interacts with and is changed by the target.
Metadata Data about the data used in a floodplain study.
Mitigation The process of preventing disasters or reducing related hazards.
Mudflow A river of liquid and flowing mud on the surfaces of normally dry land areas, as when earth is carried by a current of water. Other earth movements, such as landslide, slope failure, or a saturated soil mass moving by liquidity down a slope, are not mudflows.
National Flood FEMA’s program of flood Insurance Program insurance coverage and (NFIP) floodplain management administered in conjunction with the Robert T. Stafford Disaster Relief and Emergency Assistance
North American Datum Refers to the North 1927 (NAD 1927) American Datum of 1927.
North American Datum Refers to North American 1983 (NAD 1983) Datum of 1983.
National Geodetic Based on the sea level Vertical Datum 1929 vertical datum of 1929.
(NGVD 1929)
North American The vertical adjustment Vertical Datum 1988 system using the new sea (NAVD 1988) level datum from the 1980's. It redefines the heights of several hundred thousand benchmarks across North America.
Notification Written notice to FEMA, CWCB, and all local governments affected by a proposed stream alteration activity.
100-year Flood A flood having a recurrence interval that has a one-percent chance of being equaled or exceeded during any year (1% chance exceedance probability). The terms "one-hundred-year flood"
100-year Floodplain The area of land susceptible to being inundated as a result of the occurrence of a one- hundred-year flood. This term is synonymous with the term “state regulatory floodplain” .
Post Wildfire Hydrology Methodologies and calculations developed to account for the increased stormwater runoff following forest fires.
Raster Images A collection of electronic dots called pixels. Each pixel is a tiny colored square. When an image is scanned, the image is converted to a collection of pixels called a raster image. Scanned graphics and web graphics (JPEG and GIF files) are the most common forms of raster images.
Regulatory Floodplain Synonymous with the 100-year, or 1% chance, floodplain.
Stream Alteration Any manmade activity Activity within a stream or floodplain that alters the natural channel, geometry, or flow characteristics of the stream for purposes other than Flood Mitigation Projects that are intended for the improvement of drainage, flood control, flood conveyance or flood protection.
Topography Configuration (relief) of the land surface; the graphic delineation or portrayal of that configuration in map form, as by contour lines.
Triangulated Irregular A significant alternative Network Model (TIN) to the regular raster of a DEM that has been adopted in numerous GIS programs and automated mapping and contouring packages. The TIN model was developed in the early 1970's as a simple way to create a surface from a set of irregularly spaced points.
Universal Transverse The Universal Transverse Mercator (UTM) Mercator (UTM)
Vector Images A collection of connected lines and curves that produce objects. When creating a vector image in a vector illustration program, node or drawing points are inserted and lines and curves connect notes together. This is the same principle as "connect the dots". Each node, line and curve is defined in the drawing by the graphics software by a mathematical description.
Water Surface Profile A graph that shows the relationship between the vertical elevation of the top of flowing water and of the streambed with the horizontal distance along the stream channel.
Rule 5. State Regulatory Floodplain:
The regulatory floodplain is the 100-year floodplain. “Storm or Floodwater Runoff Channels” are within the 100-year floodplain. Sections 24-65.1-101 & 24-65.1-202(2)(a)(I) and 24-65.1-302(1)(b)&(2)(a) and 24-65.1-403(3) and 24-65.1-404(3), C.R.S. (2004) deems the designation of floodplains a matter of statewide importance and interest and gives the CWCB the responsibility for the designation of the 100- year floodplain.
Rule 6. Standards for Delineation of Regulatory Floodplain Information:
A. Intent of this Rule. This rule contains standards for approximate and detailed floodplains.
B. Level of Detail.
a. Approximate floodplain information will be based on detailed hydrology computed for the 100- year flood. Hydraulic information shall be produced using approximate, field, or limited techniques and best available topographic/survey data.
b. Detailed floodplain information will be based on detailed hydrologic and hydraulic determinations for the 100-year flood. Flood profiles and floodplain delineations for the 100-year flood and other frequencies, if any, shall be plotted, preferably using a digital technique. Floodplain delineations for the 500-year flood are encouraged, but not required, by the CWCB. The CWCB shall only designate and approve 100-year floodplain information.
C. Base Mapping. Base mapping for floodplain studies shall meet the minimum standards as set forth in Appendix A and any other method approved by the Board.
D. Topography and Surveys. Topographic and field survey information for floodplain studies shall meet the minimum standards as set forth in Appendix A.
E. Vertical Datum and Horizontal Control.
a. New topographic information obtained for floodplain studies in Colorado shall be produced using a vertical datum standard based on NAVD 1988. Existing flood profiles and Base Flood Elevations based on NGVD 1929 Datum shall be converted to NAVD 1988 for the purposes of new and revised floodplain mapping studies. New studies, utilizing previous topographic information that was originally developed using NGVD 1929 vertical datum, shall be produced on NAVD 1988 datum by appropriately converting the vertical information to the new datum for modeling and mapping purposes.
b. The accepted Horizontal Projection standards are UTM (Zone 12 or 13 depending on Geographic location within the state) and State Plane coordinates.
c. Additional requirements are set forth in Appendix A.
F. Geographic Information Systems (GIS). GIS information for floodplain studies in Colorado shall meet the minimum standards as set forth in Appendix A. New floodplain studies submitted for CWCB designation and approval shall be in conformance with CWCB approved GIS standards for digital floodplain information.
G. Hydrology. Hydrologic analyses for floodplain studies in Colorado shall be completed using the information contained in Appendix B and other methods approved by the CWCB. In addition, the following rules apply to hydrology studies:
a. All floodplain studies, regardless of the level of detail (e.g. approximate or detailed) shall utilize detailed hydrologic information. The CWCB recognizes existing and future watershed conditions for the purposes of computing flood hydrology. Future watershed conditions, in addition to existing conditions, shall be evaluated when Foreseeable Development is expected.
b. Any new study performed by a sponsor to evaluate precipitation information and/or design storm criteria shall be completed in such a way that it is scientifically defensible and technically reproducible.
c. All jurisdictions and communities affected by revised precipitation data, due to their geographic proximity and/or their location within a particular watershed, are encouraged to participate in the update process, and shall be given the opportunity by the study sponsor to review and comment on the revised information. Opponents to the revised information shall present technically accurate and sound scientific data at a CWCB hearing that clearly demonstrates that the information in question is inaccurate. The CWCB shall make the final determination regarding disputes.
d. Within any given watershed, or hydrologic subregion, consistency in precipitation data and runoff methodology shall be pursued to the extent possible through cooperation of all affected jurisdictions and entities.
H. Detailed Hydraulic Method . Hydraulic analyses for floodplain studies in Colorado shall be completed using protocols as set forth in Appendix C.
I. Floodplain Delineations. Floodplain delineations shall be completed using protocols that are approved by the CWCB and shall comply with the technical quality assurance standards as follows:
a. The flood elevations and the floodplain delineations on the maps must correlate reasonably to the best available topographic information for the stream and adjacent corridor and must meet an acceptable level of technical accuracy.
b. The planimetric features on the floodplain maps (including but not limited to streets and highways, stream centerlines, bridges and other critical hydraulic features, corporate limits, section lines and corners, survey benchmarks) must be consistent with the best available aerial photographs or other suitable information for the stream and the adjacent corridor as determined through prevailing industry practices and must meet an acceptable level of technical accuracy.
J. Special Floodplain Conditions. There are a number of special floodplain conditions, or natural flood hazards, in Colorado that fall outside of the standard riverine environment. Studies for the 100- year flood involving special conditions shall be completed using protocols that are approved by the CWCB. The special conditions are:
a. Alluvial fan and debris flow floodplains located within foothill and mountainous regions of Colorado shall be considered on a case-by-case basis.
b. Post-wildfire hydrology shall be considered in forested areas immediately following moderate to intense wildfires resulting in approximately 15% or greater burn area of the affected watershed (actual wildfire impacts shall be evaluated on a case-by-case basis). Interim flood advisory maps, based on burned watershed conditions, shall be produced at the request of the local governing authority or by Board initiative. The interim floodplain maps shall be produced using CWCB Flood Response Program funding using post- wildfire hydrology that shows increased runoff from hydrophobic soils and lack of vegetation. The post-wildfire maps shall be evaluated every 3 to 5 years to assess the need for further revision based on watershed recovery, forest regrowth, and other factors.
c. Ice jam flooding shall be considered within stream reaches where this phenomenon is known to occur. Ice jam flooding can be analyzed utilizing methodologies available through the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL), located in Hanover, New Hampshire.
d. Erosion zones and stream migration problems shall be considered on a case-by-case basis, either at the request of the sponsoring local government or by Board initiative, within stream reaches where these problems are anticipated or known to occur.
K. Written reports and maps. The results of the hydrologic analyses, hydraulic analyses, and floodplain delineations shall be summarized in a written report and submitted to the CWCB. All approximate and detailed floodplain information that is presented to the CWCB for designation and approval shall be properly titled, dated, organized, and bound as a stand-alone document. In addition to the hard copy final report, the CWCB requires that a digital copy of the final report be submitted in MS Word and PDF formats. All pertinent technical backup data such as GIS files, hydrologic and hydraulic models, and all pertinent technical backup data shall also be provided to the CWCB in acceptable digital formats. The CWCB shall electronically distribute to interested parties, to the extent possible, pertinent study information. Access to original GIS information will be provided to local governments and other authorized users through a secure and protected website or other secure means.
a. The flooded area maps shall show, at a minimum, the flood boundaries, the location of all cross sections used in the hydraulic analysis, the reference line drawn down the center of the floodplain or low flow channel, and a sufficient number of flood contours in order to reconstruct the flood water surface profiles.
b. Flood contours, such as Base Flood Elevations, shall be drawn as wavy lines drawn normal to the direction of flow of floodwater and shall extend completely across the area of the 100- year floodplain. Each flood contour shall indicate its elevation to the nearest whole foot.
c. The flooded area map scale shall be 1-inch equals 500 feet or such map scale showing greater detail. FEMA map panels may also be published at 1 inch equals 1,000 feet or 1 inch equals 2000 feet.
d. Where discrepancies appear between flooded area maps and water surface profiles, any 100- year water surface profile designated and approved by the Board shall take precedence over any corresponding flooded area map for the same stream reach or site location.
L. Contractor Qualifications a. Qualified engineers licensed in Colorado shall direct or supervise the floodplain mapping studies and projects within the regulatory floodplain. All floodplain maps, reports and project designs within the regulatory floodplain shall be certified and sealed by the Colorado registered professional engineer of record.
b. Federal agencies or other recognized and qualified government authorities may produce floodplain mapping work as a study proponent or on behalf of a study proponent. Rule 7. Standards for Regulatory Floodways:
A. Designation of floodways. Designation and approval of floodway information shall be considered by reference as being within the designation and approval of corresponding 100-year regulatory information. For waterways with base flood elevations for which floodways are not computed, the community shall have the discretion to apply floodway regulations according to its own determination, as outlined in FEMA Regulation 44 CFR 60.3(c)(10) (2004) and incorporated by reference into this rule.
B. Establishment of Floodway Criteria. The CWCB recognizes that designated floodways are administrative limits and tools used by communities to regulate existing and future floodplain developments within their jurisdictions. Communities may choose to delineate floodways based on FEMA’s 1-foot rise criteria or based on more strict criteria (e.g. depth and velocity criteria, 0.5- foot rise, etc.). The CWCB floodway rule is synonymous with communities’ adopted floodway criteria. Where no local floodway criteria exist, the CWCB recommends the use of the minimum FEMA standard.
C. Incorporation of FEMA’s Floodway Regulations. All regulations defined in FEMA Regulation 44 CFR 60.3(c)(10) and 44 CFR 60.3(d) (2004) are hereby incorporated by reference into this rule. All communities participating in the National Flood Insurance Program that have Base Flood Elevations defined for one or more of the waterways within their jurisdictions can adopt and enforce these floodway regulations. Failure to enforce floodway regulations may impact the community’s standing in the National Flood Insurance Program and may eliminate or reduce eligibility for federal or state financial assistance for flood mitigation and disaster purposes.
D. Communities in Which This Rule Applies. Communities with designated Regulatory Floodplains that have Base Flood Elevations defined for one or more of the waterways within their jurisdictions shall be required to establish a technical (quantified) criteria for floodway determination and regulation.
Rule 8. Criteria for Determining the Effects of Dams on Regulatory Floodplains:
A. Flood Control Dams. If a publicly owned, operated and maintained dam is specifically designed and operated either in whole or in part for flood control purposes then its effects shall be taken into consideration when delineating the floodplain below such a dam. The effects of the dam shall be based upon the 100-year flood under Foreseeable Development, with full credit to be given to the diminution of peak flood discharges, which would result from normal dam operating procedures.
B. Non-Flood Control Dams. If a dam is not specifically designed and operated, either in whole or in part, for flood control purposes, then its effects, even if it provides inadvertent flood routing capabilities which reduce the 100-year flood downstream, shall not be taken into account and the delineation of the floodplain below such a dam shall be based upon the 100-year flood that could occur absent the dam’s influence. However, if adequate assurances have been obtained to preserve the flood routing capabilities of such a dam, then the delineation of the floodplain below the dam may, but need not, be based on the assumption that the reservoir formed by the dam will be filled to the elevation of the dam's emergency spillway and the 100-year hydrology can be routed through the reservoir to account for any flood attenuation effects.
C. Adequate Assurances . For the purposes of Rule 8.B. "adequate assurances" shall, at a minimum, include appropriate recognition in the community's adopted master plan of: (1) the flood routing capability of the reservoir, as shown by comparison of the 100-year floodplain in plan and profile with and without the dam in place in order that the public may be made aware of the potential change in level of flood protection in the event that the reservoir flood routing capability is lost, (2) the need to preserve that flood routing capability by whatever means available in the event that the reservoir owners attempt to make changes that would decrease the flood routing capability, and (3) a complete Operations and Maintenance Plan.
In addition, an agreement shall be executed between the Board and the affected local governments (or between the Urban Drainage and Flood Control District (District) and the affected local governments if the subject floodplain is within the District) that expresses the intent of the parties to assure that the flood routing capabilities of the reservoir will be maintained by whatever means necessary if the reservoir owners attempt to make changes to the reservoir. Rule 9. Criteria for Determining Effects of Levees on Regulatory Floodplains:
A. Ownership. Privately owned, operated, or maintained levee systems will not be considered in the hydraulic analysis to be performed pursuant to rule 5 or rule 6 unless a local ordinance mandates operation and maintenance of the levee system and the criteria set forth below are met. Levees for which the community, State, of Federal government has responsibility for operations and maintenance will be considered provided that the criteria set forth below are met.
B. Freeboard. A minimum levee freeboard of 3 feet shall be necessary, with an additional 1-foot of freeboard within 100 feet of either side of structures within the levee or wherever the flow is constricted such as at bridges. An additional 0.5-foot above this minimum is also required at the upstream end, tapering to the minimum and the downstream end of the levee.
C. Field Inspection and Maintenance. The levee shall be structurally sound and adequately maintained. Sedimentation effects shall be considered for all levee projects. Certification from a federal agency, state agency, or a Colorado registered professional engineer that the levee meets the minimum freeboard criteria as stated above and that it appears, on visual inspection, to be structurally sound and adequately maintained shall be required on an annual basis. Levees that have obvious structural defects, or that are obviously lacking in proper maintenance, shall not be considered in the hydraulic analysis.
D. Internal Drainage. Where credit will be given to levees providing 100-year protection, the adequacy of interior drainage systems shall be evaluated. Areas subject to flooding from inadequate interior drainage behind levees will be mapped using standard procedures.
E. Human Intervention and Operation . In general, evaluation of levees shall not consider human intervention (e.g. capping of levees by sandbagging, earth fill, or flashboards) for the purpose of increasing a levee's design level of protection during an imminent flood. Human intervention shall only be considered for the operation of closure structures (e.g. gates or stop logs) in a levee system designed to provide at least 100-year flood protection, including adequate freeboard as described above, provided that such human operation is specifically included in an emergency response plan adopted by the community.
F. Analysis . For areas protected by a levee providing less than 100-year protection, flood elevations shall be computed as if the levee did not exist. For the unprotected area between the levee and the source of flooding, the elevations to be shown shall be obtained from either the flood profile that would exist at the time levee overtopping begins or the profile computed as if the levee did not exist, whichever is higher. This procedure recognizes the increase in flood elevation in the unprotected area that is caused by the levee itself. This procedure may result in flood elevations being shown as several feet higher on one side of the levee than on the other. Both profiles shall be shown in the final report and labeled as "before levee overtopping" and "after levee overtopping" respectively.
Rule 10. Stormwater Detention.
Hydrologic determinations and increased runoff from development and urbanization shall be considered through detention measures to mitigate the higher runoff characteristics.
A. Stormwater/Floodwater Runoff Detention. The hydrologic analysis shall consider the effects of on-site detention for rooftops, parking lots, highways, road fills, railroad embankments, diversion dams, refuse embankments (including but not limited to solid waster disposal facilities), mill tailings, impoundments, siltation ponds, livestock water tanks, erosion control dams, or other structures only if they have been designed and constructed with the purpose of impounding water for flood detention and are owned, operated and maintained by a government body. Detention structures that are randomly located, privately owned, or privately maintained shall not be included in the hydrologic analysis unless it can be shown that they exacerbated downstream peak discharges.
B. Irrigation Facilities. The CWCB recommends that irrigation facilities (including but not limited to ditches and canals) not be used as stormwater or flood conveyance facilities, unless specifically approved and designated by local governing jurisdictions and approved by the irrigation facility owners. The flood conveyance capacity of irrigation facilities shall be acknowledged only by agreement between the facility owners and local governing jurisdictions. The CWCB will designate and approve 100-year floodplain information for irrigation facilities if the above recommendations are met.
Rule 11. Effects of Flood Mitigation Measures and Stream Alteration Activities on Regulatory Floodplains In order to assist the CWCB in carrying out its mission to protect the health, safety, and welfare of the public, through the prevention of floods in Colorado, the CWCB requires the following:
A. Detention/flood control storage shall be designed and constructed as part of a basinwide program for the watershed.
B. Detention facilities shall adequately consider flow rates and flow volumes.
C. Flood control channels shall include a low-flow channel with a capacity to convey the average annual flow rate, or other appropriate flow rate as determined through a hydrogeomorphological analysis, without excessive erosion or channel migration, with an adjacent overbank floodplain to convey the remainder of the 100-year flow. The channel improvement shall not cause increased velocities or erosive forces upstream or downstream of the improvement.
D. Channelization and flow diversion projects shall appropriately consider issues of sediment transport, erosion, deposition, and channel migration and properly mitigate potential problems through the project as well as upstream and downstream of any improvement activity. A detailed geomorphological analysis shall be performed to assist in determining the most appropriate design.
E. Low-lying areas (below BFE) “protected” by levees, flood fringe areas raised by fill, or perched flow diversion or channelization projects shall not be considered to be removed from the 100-year floodplain for the purpose of building basements.
F. Project proponents for a mitigation activity must evaluate the residual 100-year floodplain. Proponents are also encouraged to map the 500-year residual floodplain for the evaluation of critical facilities.
G. All flood protection and mitigation projects shall be maintained to ensure that they retain their structural and hydraulic integrity. Annual inspections including, as appropriate, field surveys of stream cross-sections, shall demonstrate to the appropriate regulatory jurisdictions that the project features are in satisfactory structural condition, that adequate flow capacity remains available for conveying flood flows, and that no encroachment by vegetation, animals, geological processes such as erosion, deposition, or migration, or by human activity, endanger the proper function of the project. If any significant problems are noted in such annual inspections, the local regulatory jurisdiction shall notify the CWCB within 30 days of the inspection.
H. Any stream alteration activity proposed by a project proponent must be evaluated for its impact on the regulatory floodplain and be in compliance with federal, state and local floodplain rules, regulations and ordinances.
I. Any stream alteration activity shall be designed and sealed by a Colorado registered professional engineer.
J. Stream alteration activities shall be properly permitted by local, state and federal agencies and shall be in conformance with FEMA Regulations 44 C.F.R. parts 59, 60, 65, and 70 (2004).
K. Stream alteration activities shall not be constructed unless the project proponent demonstrates through a floodway analysis and report, sealed by a Colorado registered professional engineer, that there are no adverse floodway and floodplain impacts resulting from the project.
L. No adverse floodway impact means that there is a 0.0-foot rise in the proposed conditions compared to existing conditions floodway.
M. The Stream Alteration proponent shall provide Notification to the CWCB whenever the proposed Stream Alteration activity would result in proposed water surface profile increases to the regulatory 100-year flood profile in excess of 0.3 vertical feet (unless the local governing authority has adopted more stringent standards). Such Notification by the proponent shall be in writing, and meet the intent of notice procedures as described in 44 CFR parts 59, 60, 65, and 70. In addition, whenever a proposed Stream Alteration activity in combination with all other previous floodplain alteration activities results in a cumulative increase of 1.0 vertical feet or greater, Notification shall also be provided by the Stream Alteration proponent. Rule 12. Process for Designation and Approval of Regulatory Floodplains:
A. Designation and Approval Requirements. The Board will designate and approve regulatory floodplains and storm or floodwater runoff channels by the adoption of written resolutions based only upon such floodplain information as the Board determines meets the standards set forth in Rule 6, as applicable, with consideration of the effects of dams and levees being subject to the criteria or Rules 8 and 9, respectively and any mitigation activity in Rule 11.
B. Base Flood. The 100-year flood shall be the basis for all designation and approvals by the Board, for zoning purposes, of regulatory floodplains in Colorado.
C. Conditions. All designations and approvals of approximate floodplain information by the Board shall be based on the Board's designation action. The community shall be notified by a CWCB resolution that a case-by-case review of the approximate floodplain information will be required, and that a detailed hydrologic and hydraulic analysis will be necessary prior to development activities taking place in the identified approximate 100-year floodplain.
D. Provisional Designation. The CWCB may designate and approve, on a provisional basis and for a maximum period of time not to exceed three years, floodplain information that does not meet the minimum requirements as set forth in Rule 6.
E. Process for Taking Designation and Approval Actions . The Board shall consider the designation and approval of floodplain information either by request of a community or by acting on its own initiative.
a. Consideration at a Community's Request. The Board shall consider designation and approval of floodplain information upon written request from the governing body of any community having jurisdiction in the area where the floodplain information is applicable. The letter of request shall identify the report title, date, author or agency which prepared the report, stream name (s), upstream and downstream limits of the stream reach (es) to be designated, stream length (s) in miles, type of designation requested (detailed or approximate), and any other relevant information. The Board shall receive such a request at least 30 days prior to the Board meeting at which consideration of designation and approval is requested.
b. Consideration at the Board's initiative . If designation and approval of a floodplain would be in the best interest of the health, safety, and welfare of the citizens of the State of Colorado, then the Board may take action at its own initiative to consider the designation and approval of floodplain information. In such cases, the Board shall notify the affected communities in writing at least 45 days prior to the Board meeting at which it will consider the designation and approval of floodplain information within their jurisdiction.
c. Notification of Adopted Resolutions . The CWCB shall send signed copies of each adopted resolution of designation and approval to the legislative bodies of each community having jurisdiction in the study area and to FEMA. Rule 13. Designation and Approval of Changes to Regulatory Floodplains: When changes are made to the characteristics of a floodplain that do not result in a revision of a community’s Flood Insurance Rate Maps or Flood Hazard Boundary Maps (and a subsequent designation of the new map), the Board will designate and approve changes to the regulatory floodplain caused by development, new or better technical information, or other sources. This designation of changed floodplains will be by the adoption of written resolutions based upon such floodplain information as the Board determines meets the standards set forth in Rules 6-12.
A. Conditions. All changes to designated floodplains shall meet the same conditions as those required for original approval and designation.
B. Process for Designation and Approval of Changes to a Regulatory Floodplain. The Board may consider the designation and approval of floodplain information either by request of a community or by acting on its own initiative.
a. Consideration at a Community’s Request. The Board shall consider designation and approval of changes to a regulatory floodplain upon written request from the governing body of any community having jurisdiction in the area where the floodplain information is applicable. The Board shall receive such requests at least 30 calendar days prior to the Board meeting at which consideration of designation and approval is requested.
b. Consideration at the Board’s Initiative. If designation and approval of a floodplain would be in the best interest of the health, safety, and welfare of the citizens of the State of Colorado, then the Board may take action at its own initiative to consider the designation and approval of floodplain information. In such cases, the Board shall notify the affected communities in writing at least 45 days prior to the Board meeting at which it will consider the designation and approval of floodplain information within their jurisdiction.
c. Notification of Adopted Resolution . The CWCB shall send signed copies of each adopted resolution of designation and approval of changes to a regulatory floodplain to the legislative bodies of each community having jurisdiction within the limits of the changed floodplain within 30 calendar days of designation and approval.
C. Identification of Designations of Changes to a Regulatory Floodplain. The designation of the changes to the regulatory floodplain will be given a reference identification number that will differentiate the changed designation from the original. It is implied that designations to changes to a regulatory floodplain will only rescind the affected portions of the previously designated floodplain information. All other unaffected reaches will remain as originally designated.
D. Map Revisions to Flood Insurance Rate Maps or Flood Hazard Boundary Maps. Floodplain map revisions (e.g. FEMA Letters of Map Revision) will be designated twice annually by the CWCB during a regularly scheduled Board meeting and will not be subject to a full technical review by the CWCB staff.
Rule 14. Variances A. Consideration by the Board. Request for a variance to any of these rules may be considered by the Board provided the entity requesting the variance has submitted a written request to the CWCB Director and notice of the request is given to the community, if different from the entity requesting the variance, that would be affected by the variance, if granted.
B. Contents of a Request for Variance. The request for a variance shall identify:
a. The rule from which the variance is requested, b. The communities that would be affected by the variance, c. The reasons why the rule cannot be complied with, d. The estimated difference in water surface elevations, flood velocities and flood boundaries that will result if the requested variance is granted than if the calculations are made through strict compliance with the rule, e. The estimated number of people and structures that will be impacted by granting of the variance, and f. Any other evidence submitted by the community, the Colorado Water Conservation Board staff, or other party of interest.
C. Factors to be considered. Variances may be issued by the Board if it can be determined that:
a. There is a good and sufficient cause, and b. The variance is the minimum necessary, considering the flood hazard, to afford relief, and c. Failure to grant the variance would result in exceptional hardship to the community and that the hardship is not the community's own making, and d. The granting of a variance will not result in increased vulnerability to flood losses, additional threats to public safety, extraordinary public expense, create nuisances, cause fraud or victimization of the public, hide information of significant interest to the public or conflict with existing local laws or regulations.
Rule 15. Incorporation by Reference:
FEMA Regulations 44 CFR Parts 59, 60, 65, and 70 (2004) are incorporated herein by reference. Materials incorporated by reference are those in existence as of the effective date of this regulation and do not include later amendments. These Rules may be updated to reflect changes to the FEMA regulations that are incorporated by reference. The material incorporated by reference is available for public inspection during regular business hours at the Colorado Water Conservation Board, 1313 Sherman Street, Room 721, Denver, CO 80203 or may be examined at any state or federal publications depository library, or on the FEMA or CWCB website. These regulations are hereby incorporated by reference by the Colorado Water Conservation Board and made a part of these Rules and Regulations for Regulatory Floodplains in Colorado. Parties wishing to inspect these materials should contact the State of Colorado NFIP Coordinator, located at the Colorado Water Conservation Board. Rule 16. Severability:
If any portion of these Rules is found to be invalid, the remaining portion of the Rules shall remain in force and in effect.
Rule 17. Recommended Activities for Regulatory Floodplains: The following list contains floodplain management activities and actions suggested by the CWCB to increase a community’s overall level of flood protection. Communities and other authorized government entities may:
A. Adopt local standards above and beyond the FEMA and CWCB minimum requirements.
B. Develop a Flood Response Plan that identifies responsibilities/actions before, during and after a flood event.
C. Enroll in FEMA’s National Flood Insurance Program (NFIP) and possibly FEMA’s Community Rating System (CRS) Program.
D. Develop an early warning flood detection system (flood warning system) using available technologies such as automated precipitation and stream flow gages linked to an appropriate notification system.
E. Coordinate with lenders, insurance agents, real estate agents, and developers to prepare and discuss educational tools based on state and federal requirements.
F. Promote wise floodplain development and support effective structural and non-structural flood mitigation projects.
G. Conduct floodplain studies in areas of Foreseeable Development that do not currently have detailed floodplain studies.
H. Maintain an electronic or paper library of local flood related data.
I. Develop a flood risk outreach program and notify flood prone residents annually of flood hazards and the need for flood insurance.
J. Encourage elevation of floodprone structures and floodproofing of structures in the floodplains.Utilize available state/federal mitigation and preparedness funds.
L. Require certified floodplain managers to review proposed land developments.
M. Advise the public at large that flooding does occur above and beyond the 100-year flood. Communities would be wise to consider using the 500-year floodplain for regulating critical facilities such as fire stations, hospitals, police stations, nursing homes, electrical power stations, water supply treatment plants, and other important facilities. Floods greater than 500-year floods do occur, and loss of life and property is possible in areas mapped outside of both the 100-year and 500-year floodplains. Communities are encouraged to identify areas prone to flooding outside of the 500-year floodplain where loss of life or substantial property damage may occur.
N. Utilize the concept of “No Adverse Impact” floodplain management where the action of one property owner does not adversely impact the rights of other property owners, as measured by increased flood peaks, flood stage, flood velocity, and erosion and sedimentation. No Adverse Impact could be extended to entire watersheds as a means to promote the use of retention/detention or other techniques to mitigate increased runoff from urban areas.
O. Prohibit the construction of new levees that are intended to remove land from a regulatory floodplain for the purpose of allowing new development activity to take place in areas that are otherwise flood prone.
P. Require an appropriate level of freeboard at bridges between the 100-year water surface elevation and the lowest elevation of the lowest structural member to allow for passage of waterborne debris.
Q. Maintain a flood hazard page on the community website with links to the CWCB, FEMA Flood Map Store, National Flood Insurance Program, National Weather Service, local building codes, and local permitting information.
Rule 18. Effective Date:
These Rules shall apply to the designation and approval of all floodplain information made by the Board on or after December 1, 2005 and are, therefore, not retroactive to any floodplain information designated and approved by the Board prior to the effective date.
APPENDIX A GUIDANCE FOR AERIAL MAPPING AND SURVEYING Map Modernization - Federal Emergency Management Agency FEMA’s Flood Hazard Mapping Program Guidelines and Specifications for Flood Hazard Mapping Partners APPENDIX B HYDROLOGIC ANALYSIS SECTION 1 – METHODS AND APPLICATIONS SECTION 2 – STATISTICAL ANALYSIS SECTION 3 – REGIONAL REGRESSION ANALYSIS SECTION 4 – RAINFALL SECTION 5 – RUNOFF SECTION 1 – METHODS AND APPLICATIONS INTRODUCTION Due to the complexity of the natural terrain, orographic effects of the Rocky Mountains, and semi-arid climate of the region, the type and duration of storm events vary substantially within the State. However, rainstorm events can be generally defined as either short duration convective storms or long duration general rainstorms.
The short duration convective storms (cloudbursts/thunderstorms) can produce high rainfall intensities for a short period and generally cover smaller watershed areas. Convective storms are commonly known to be responsible for high peak flows and flooding problems for many small drainage basins. The long duration general rainstorms can produce rain coverage over a large watershed area for a periods ranging from several hours up to several days. General rainstorms can produce large amount of total rainfall runoffs and sometimes generate higher peak flows than the convective storms. Depending on the purpose of the hydrologic analysis, it may be necessary to analyze both types of rainstorms in order to estimate the high peak flow rate and the high runoff volume for a given drainage basin. There are many different flow estimation analysis methods available. However, not all of the methods can be effectively utilized in Colorado. Some methods are not applicable for the hydrologic conditions that exist in Colorado, and other methods cannot be utilized easily or accurately due to the lack of measured data. Also, the computed flow estimates may vary considerably depending on the methods utilized for a given watershed. Therefore, it is necessary to define minimum standards for hydrologic analysis in order to promote accuracy and consistency in the computed flow rates. Presented in this section are a list of accepted hydrologic analysis methods and approaches and their appropriate uses in the State of Colorado. The information presented is the state-of-art information available at the time of preparation of this Rules and should be updated as better techniques and new rainfall and stream gage data become available in the future. HYDROLOGIC ANALYSIS The following hydrologic analysis methods have been used and accepted widely throughout the State to estimate flow rates and hydrographs resulting from surface runoffs for various design storm events: - Statistical analysis of recorded stream gage data - Regional regression analysis - Synthetic rainfall-runoff modeling Flow rates and hydrograph estimates for the purpose of floodplain analysis and drainage design in Colorado should be computed using one or more of the listed hydrologic analysis methods. Analysis Requirements For detailed floodplain/floodway delineation projects, the hydrologic analysis should include, at a minimum, calculations for the 10-, 50-, 100-, and 500-year frequency discharges. It is optional but recommended that the peak discharge for 2- and 5-year flood events be calculated in addition to the other discharges. If using rainfall-runoff modeling, 500-year flow rates may be estimated by multiplying the 100-year flow rates by a factor of 1.7 For approximate floodplain delineation projects, the 100-year frequency discharge should be estimated at a minimum.
Flow hydrographs (total flow volume, timing of peak flows, etc.) should also be computed if it is necessary to determine effects of flow routings, detentions/dam storages, diversion flows, etc. Previous Studies Where appropriate, previously approved hydrologic studies should be used so that previous work by federal, state, or local agencies is not duplicated. When such data is not available, conditions have changed significantly, or the methodologies or data used in previous studies are not appropriate, a new hydrologic analysis for each stream should be prepared. If a new hydrologic analysis is prepared, a comparison of new discharges with all available published or not published discharge data that exist for the study area should be provided. If the new hydrologic analysis results are significantly different than the previously adopted flows, the following criteria should be used in deciding which flow estimate should be used. However, the site-specific limitations/conditions may warrant a deviation from the evaluation criteria below. The project engineer should coordinate with the appropriate agencies in deciding which flow estimate should be used.
- For streams with at least 50 years of stream-flow gage records, the following general FEMA evaluation criteria should be used.
- For all other cases, the new hydrologic analysis results should be used if the new analysis is proven to be technically superior and if the resulting peak flow rate change is greater than ten percent.
APPLICATIONS The following guidelines should be used in determining the appropriate hydrologic analysis method for a given waterway.
- When at least 50 years of stream-flow gage records are available, a flow frequency statistical analysis should be performed to determine the flood peaks of the selected recurrence intervals. - When 25 to 50 years of stream-flow gage records are available, the hydrologic analysis should include a statistical analysis and a comparison with established flow rates for similar watersheds. Similar watersheds are defined as watersheds that have similar hydrologic characteristic (precipitation depth and distribution, slope, size, elevation, vegetation cover, etc.) as the watershed being studied. If the estimated flow rates using the statistical method are determined inaccurate after comparison with similar watersheds, additional hydrologic analysis should be performed using a regional regression analysis and/or synthetic rainfall-runoff modeling methods to validate the flow rates.
- When 10 to 24 years of stream-flow gage records are available, the hydrologic analysis should include a statistical analysis, comparisons with similar watersheds, and flood hydrograph estimates using synthetic hydrologic models and precipitation records.
The estimated flow rates for 2-, 5-, and 10-year design storm events using the statistical method should be reasonably accurate. However, the estimated flow rates for 50-, 100-, and 500-year storm events using the statistical method may not be reliable, since only 10 to 24 years of stream flow gage records are used in the analysis.
A rainfall-runoff model should be prepared and calibrated to the estimated 10-year flow rates using the statistical method. Then, the calibrated rainfall –runoff model may be used to estimate flow rates for other design storm events.
All drainage basin characteristics that affect the rainfall-runoff relationship should be documented, including, but not limited to, delineation of basin and subbasin boundaries, size, shape, length, slope, general aspect, elevation extremes, time of concentration, land use, and soil types and compositions. When actual precipitation records of major recorded storm events are available from area rain gage stations, such data should be used in conjunction with rainfall data. - When less than 10 years of stream-flow gage records are available, the hydrologic analysis should include a regional regression analysis and flood hydrograph estimates using synthetic hydrologic models and precipitation records.
The computed flows using a rainfall-runoff model should compared to the confidence limits of the existing flows or estimated flow rates using a regional regression analysis. Additional Requirements Depending on the floodplain analysis and drainage design requirements, it may be necessary to develop a synthetic rainfall-runoff model, even when sufficient amount of stream-flow gage records are available. The synthetic rainfall-runoff models should be calibrated to match the statistical analysis results. The calibrated synthetic model can then be used to generate hydrographs for various design storm events. The following is a list of some of these cases: - Various flood frequency hydrographs are required, but the statistical analysis alone cannot generate the necessary hydrographs.
- The subject watershed is undergoing or projected to undergo a substantial amount of new development.
- Comparison of before and after development hydrographs to quantify potential increase in flows due to the proposed developments.
Watershed Development Conditions Hydrologic analysis should be performed, at a minimum, to reflect the existing watershed development conditions. Public works projects in progress that are planned to be completed within 12 months following the hydrology study completion should be included in the analysis. Where construction of a publicly owned, operated and maintained flood control facility will not be completed within 12 months following completion of the study, but adequate progress has been made, the impact/benefit of the project may be included in the hydrologic analysis. The project engineer should coordinate with the public agency in charge of the facility design and construction, effected local agencies and Colorado Water Conservation Board (CWCB) to determine whether to include the subject facility in the existing conditions hydrologic analysis or not.
As new developments occur, the estimated existing conditions peak flow rates may change substantially, depending on the nature and amount of new developments within a watershed. Therefore, local communities are encouraged to develop future (built-out) conditions flow rates and floodplain information in addition to the existing conditions, especially when the area plan indicates substantial amount of future developments.
DETENTION FACILITIES The hydrologic analysis should include detention facilities designed and constructed with the purpose of impounding water for flood detention that are owned, operated, and maintained by a government body. Detention structures that are randomly located, privately owned, or privately maintained should not be included in the hydrologic analyses unless it can be shown that they exacerbate downstream peak discharges.
If existing detention basins are not included in the hydrologic analysis, discussions should be provided in the report describing the detention basins and reasons why they were not considered in the analysis. Storage Routing Method The flow attenuation effect of a detention basin can be determined using the Modified Puls routing method. The Modified Puls routing method can be used in HEC-HMS, UDSWM, or HEC-1 computer programs to route hydrographs through dams and reservoirs. Only the storage specifically reserved for the flood attenuation purposes should be included in the analysis. SPECIAL HYDROLOGIC CONDITIONS The following hydrologic conditions may exist within the study watersheds: - Rain on snow - Vegetation cover loss due to fire - Flow diversion structures - Frozen soils - Affects of dams and reservoirs These hydrologic conditions should not be ignored in the watershed analysis. The practical ways to deal with the listed conditions vary depending on the selected analysis method, and they are described later in this section. Discussions on other uncommon drainage conditions including alluvial fan, mud and debris flow, irrigation-stormwater interaction, and Ice Jam are provided in the Rules. DYNAMIC FLOW ROUTING MODEL For certain flow routing conditions, it may be desired or necessary to route flows using more comprehensive hydraulic flow routing models (i.e. HEC-RAS Unsteady, FLO-2D, etc.) in place of simplistic flow routing methods utilized by the rainfall-runoff programs. The engineer should coordinate with appropriate local, state, and federal agencies in determining the appropriate dynamic flow routing models for a given waterway.
SECTION 2 – STATISTICAL ANALYSIS INTRODUCTION For basins with reliable stream gage records, the preferred method of estimating various frequency flow rates is Statistical Analysis Method using recorded stream flow gage data. The reliability of the statistical approach is generally better than the rainfall-runoff modeling, provided that the period of gage record is sufficiently long. A minimum of 10 years of reliable stream gage data should be used in the flow frequency analysis.
The statistical analysis method acceptable for use in Colorado is the one that utilizes Log Pearson Type III Distribution as described in “Guidelines for Determining Flood Flow Frequencies,” Bulletin 17B, Water Resources Council (March 1982). The following two computer programs may be used to assist in the flow-frequency analysis using Log Pearson Type III Distribution: - U.S. Army Corps of Engineers, “Flood Flow Frequency Analysis," Computer Program HEC-FFA, Hydrologic Engineering Center - U.S. Geological Survey, “Annual Flood Frequency Analysis,” Computer Program PEAKFQ Methodology Detailed analysis procedures and guidelines for determining peak flow frequency curves using Log Pearson Type III Distribution are provided in the following publications: - Water Resources Council, ” Guidelines for Determining Flood Flow Frequency,” Bulletin 17B, Hydrology Committee, Washington, D.C., March 1982 All flow frequency statistical analysis should be performed in accordance with the procedures and guidelines outlined in the Bulletin 17B. The main purpose of statistical analysis is to use the recorded runoff events for a given period of record as means of extrapolating to a longer period of time. In the statistical approach to determining the size of flood peaks, the assumption involved is that nature over a period of years has defined a flood magnitude-frequency relationship that can be derived by studying actual occurrences. A period of record of a particular basin where the floods have been measured and recorded is considered to be a representative period. Floods that occurred during the period can be assumed to occur in a similar future period, that is, the period may be expected to repeat itself. For example, using a 25-year record period, the largest recorded flood is generally considered to have a recurrence interval of about 25 years. At the end of this 25-year period, because the period can be assumed to repeat itself, one could expect the largest flood of record to be equaled or exceeded once more during the next 25 years. For any given year, the probability of a flood of any given frequency happening in that year is the same as the probability of it happening in any other year. Thus, the 100-year flood has a 1 percent chance of being equaled or exceeded in any given year.
The statistical analysis has the greatest applicability to natural streams where the basins will remain in a natural state. Such streams include those with large basins where the urbanization effects on runoff will be negligible, and on small streams where the basin primarily consists of undevelopable land or land comprising greenbelt areas. In urban areas, the use of statistical analysis approach can be limited 1) by almost total lack of adequate runoff records, 2) by the effects of rapid urbanization, and 3) by man-induced changes in the watershed which may include reservoirs, flow diversion structures, canalization of natural streams, etc. Weighted Skew Coefficient Skew coefficient value computed based on a small sample of gage records is not reliable. Therefore, the skew coefficient should be estimated by weighting the computed station skew coefficient with a generalized skew coefficient. The following skew weighting equation is presented in the Bulletin 17B:
The previously referenced computer programs HEC-FFA and PEAKFQ can be used to compute weighted skew coefficients to meet the guidelines provided in the Bulletin 17B. Evaluation of Gage Data The reliability and accuracy of the estimated peak flow frequency curves depend greatly on the duration and accuracy of the measured gage data. There are different types of gage records that may be available through various agencies (i.e. USGS, CWCB, UDFCD, etc.) including annual maximum peak flows and stages, flow volumes, mean daily flows, daily peak flows, etc. However, only the annual maximum records should be used in determining the peak flow frequency curves using Log Pearson Type III Distribution.
The collected stream gage data should be carefully evaluated by a qualified professional engineer to determine the reliability and uniformity of the data. The measured data should represent homogeneous watershed hydrologic conditions throughout the record period. The following factors and conditions may result in non-homogeneous gage records: - Significant urbanization of the watershed - Construction of reservoirs, dams, and other flood control facilities - Substantial changes in the flow storage and diversion regulations - Changes in the shape and capacity of the channel at the gaging station - Loss of vegetation due to fire over large portions of the watershed If any of the above conditions existed within the watershed during the gage record period, the data should be adjusted to make the entire record homogeneous. Adjustment Of Collected Data One of the basic assumptions incorporated into the frequency statistical analysis is that the recorded peak flows are homogeneous. However, during the past decade, many watersheds have experienced substantial changes including urbanization, manmade flood control facilities, reservoirs, etc. Therefore, the peak flows during a record period may have resulted from different hydrologic watershed conditions. The recorded data should be evaluated and adjusted to reflect uniform watershed hydrologic conditions.
If the gage data during a record period reflects both natural and altered watershed conditions, then the flow rates based on the altered conditions should be adjusted to reflect the unregulated natural conditions to make the entire population uniform. Professional engineering judgment should be exercised in determining whether the adjustment should be made or not. If the changes in the subject watershed are relatively minor, the adjustment may not be necessary. General discussions on the common conditions that may require adjustment of the recorded data are provided in this section. For detailed discussions on how to adjust the recorded gage data for various altered watershed conditions, readers are referred to the following publications: - U.S. Army Corps of Engineers (USCOE), Engineer Manual No. 1110-2-1415, Engineering and Design, Hydrologic Frequency Analysis, March 1993 - Water Resources Council, Guidelines for Determining Flood Flow Frequency, Bulletin 17B, Hydrology Committee, Washington, D.C., March 1982 Urbanzation Urbanization of a watershed can substantially alter the resulting peak flows by reducing pervious areas, natural depressions, and the flow concentration time. In most cases, urbanization results in increased flood peak flows for downstream locations. Generally, the increases in peak flows are greater for more frequent flood events compared to less frequent events. Also, urbanization often results in increased base flows. Many streams that used to be dry most of time may experience continuous base flows due to irrigation return flows.
Adjustment of the recorded peak flow data skewed by urbanization is usually made utilizing a calibrated rainfall-runoff model.
Manmade Facilities Manmade flood control facilities are usually designed to reduce and/or confine peak flood flows in order to protect human lives and private and public structures. Consequently, these facilities (channels, detention basins, flood control reservoirs, levees, flow diversion structures, etc.) can substantially alter the resulting downstream peak flow rates. The resulting changes in the peak flows for a given watershed may vary considerably depending on the location and size of the facilities and magnitude and distribution (storm-centering) of storm events. The effects of manmade facilities on flood peak flows can usually be quantified by routing several representative floods thru the facilities. Using the routing analysis results, relations between with-facilities peak flows vs. without-facilities peak flows can be determined and plotted on a graph.
Loss Of Vegetation Due To Fire Vegetation loss due to fire over a large portion of a watershed can significantly change the flooding characteristics. Without the benefits provided by vegetation cover including rainfall interception, absorption, and erosion protection, the resulting flood flows can be increased substantially with high concentration of sediment and debris. Similar to the urbanization adjustment procedures, the recorded peak flows altered by temporary vegetation loss due to fire can be adjusted by using a calibrated rainfall-runoff model. Stage-Dicharge Relations Many gaging stations are equipped to measure flood stages, and the peak flow rates are estimated using a pre-determined stage-discharge relations of the channel section. Consequently, if the channel section at a gaging station experiences substantial scour or sediment deposition (gradual or rapid), the stage-discharge relations need to be updated to reflect the “changed” channel conditions.
Snowmelt And Rainfall Flood Events The collected peak flow gage data should be examined to determine the need to segregate the data. Two distinctively different types of flood events may cause stream peak flows in any give year; spring snowmelt and rainstorm. The largest annual peak flows for each flooding conditions should be selected. Peak flow frequency curves should be determined separately using annual snowmelt flood peaks and annual rainfall event flood peaks. The final flow frequency curve should be generated by combining (see Bulletin 17B) the two curves determined using the segregated annual peak flows.
If the gage data cannot be separated into two annual peak flows due to lack of data, then, the mixed population data should be treated as if the data is homogeneous. Historic Flood Events Historic flood events that occurred prior to the systematic record period can be used to extend the gage record period. The reliability of the historic flood information should be carefully evaluated by a qualified professional engineer. The procedures outlined in the Bulletin 17B should be followed to compute a historically adjusted flow frequency curves. Confidence Limits The flow frequency curve represents “expected’ flow rates for various recurrence intervals as computed based on the sample gage peak flow data. The accuracy of the computed flow frequency estimates can be illustrated by defining the confidence limits. In general, there are 5 percent chance that the true flow value for a given frequency flood event is greater than the value estimated from the 5 percent confidence curve and 5 percent chance that the true value is smaller than the value estimated from the 95 percent confidence curve. In other words, there is 90 percent chance that the true flow value can be found between the two curves. By understanding the reliability of the computed flow frequency curves, engineers and planners can make informed decisions on the appropriate uses of the computed flow rates (i.e. additional freeboard requirements, etc.).
The 5 percent and 95 percent confidence limits should be established using the “Non-Central T Distribution” . Detailed discussions on the determination of confidence limits can be found in the previously referenced Bulletin 17B and USCOE publications. Flood Hydrographs If flood hydrographs are needed for the floodplain analysis or drainage facilities design, it may be necessary to develop a synthetic rainfall-runoff model. Synthetic rainfall-runoff models should be calibrated to match the statistical analysis results. The calibrated rainfall-runoff model can then be used to generate hydrographs for various design storm events. The following is a list of some of these cases:
Various flood frequency hydrographs are required, but the statistical analysis alone cannot generate the necessary hydrographs.
- The subject watershed is undergoing or projected to undergo a substantial amount of new development.
- Comparison of before and after development hydrographs to quantify potential increase in flows due to the proposed developments.
SECTION 3 – REGIONAL REGRESSION ANALYSIS INTRODUCTION Regional regression analysis method is a simplified procedure of estimating peak flow rates for various frequency storm events for unregulated streams with short or no streamflow records. In Colorado, the regional regression equations presented in this section may be used for the following purposes: - Computation of peak flow rates for use in delineation of 100-year floodplain boundaries - As a check to validate the computed flow rates using rainfall-runoff models or other methods The State of Colorado has been divided into seven major hydrologic regions, and the regression equations are assigned for each region. The regression equations are based upon unregulated streamflows and regulated streamflows adjusted to unregulated conditions. The subjected watershed should be carefully evaluated by a qualified professional engineer to determine the applicability of the regression method. If natural or manmade features exist within the watershed (i.e., reservoirs, dams, etc.) that could have substantial impacts on the resulting peak runoff, the regression equations should be used only for validation of an unregulated rainfall/runoff model of the watershed. The use of regression equations should be limited to watersheds with minimal flow regulations and no significant urban developments.
REGIONAL REGRESSION ANALYSIS Regional regression flood hydrology in Colorado is based upon delineation of seven (7) major hydrologic regions.
The western portion of the state was divided into four major regions using the Mountain, Rio Grande, Southwest, and Northwest regions from the USGS publication. The eastern portion of the state was divided into three major regions using Arkansas River, South Platte River, and Republican River basin boundaries.
Limitations Regression equations can be used to estimate peak flow rates for unregulated streams and for validation of rainfall/runoff models based upon unregulated conditions. The following general limitations apply to the use of these regression equations: - The computed peak flow rates, without validation with a rainfall/runoff model, may only be used for delineation of approximate 100-year floodplain boundaries. Flow rates should be determined using either statistical analyses or rainfall-runoff models with validation with regional regression equation results for detailed floodplain studies and design and analyses of drainage facilities. - The regression equations may be used as a check to validate the computed flow rates using rainfall- runoff models when recorded gage data for the stream is not available. - The regression equations should only be used for unregulated rural natural streams with minimal flow regulations and no significant urban developments. If natural or manmade features exist within the watershed (i.e., reservoirs, dams, etc.) that could have substantial impacts on the resulting peak runoff, the regression equations can be used to validate rainfall/runoff modeling using unregulated conditions.
- The applicable minimum and maximum drainage basin area, slope, and mean annual precipitation limitations should be adhered to.
Basis Of Regional Regression Equations For Colorado The information provided in the following two regression analysis studies for Colorado should be used to determine the hydrologic regional boundaries and regression equations presented in this section:
For Western Colorado:
- U.S. Geological Survey (USGS), Water-Resources Investigations Report 99-4190, Analysis of the Magnitude and Frequency of Floods in Colorado, 2000 For Eastern Colorado:
- Colorado Water Conservation Board (CWCB), Guidelines for Determining 100-Year Flood Flows for Approximate Floodplains in Colorado, Version 5.0, June 2004 Site Specific Regional Analyses Regional regression analyses may also be performed on a case-by-case basis using selected stream gages, in the vicinity of a hydrologic point of interest, that are deemed to be appropriate for a more detailed or site specific purpose.
SECTION 4 – RAINFALL INTRODUCTION Presented in this section are the rainfall depths and distributions for various design storm events to be utilized with the four selected deterministic runoff modeling methods. The four deterministic methods include the Rational Method, the NRCS TR-55 Method, the NRCS Unit Hydrograph Method, and the CUHP/UDSWM Method. The criteria to be used in selection of the appropriate rainfall-runoff model for a given stream are provided in this section.
The information presented in this section is the state-of-art information available at the time of preparation of the Rules and should be updated as better techniques and new rainfall data become available in the future.
RAINFALL DATA The rainfall data published by National Oceanic and Atmospheric Administration (NOAA) in their “Precipitation-Frequency Atlas of the Western United States, Volume III – Colorado, 1973” should be used to perform necessary rainfall-runoff calculations within the State of Colorado, unless site-specific rainfall studies have been performed and adopted by the local government agency having jurisdiction over the study area.
The NOAA Atlas 6-hour and 24-hour precipitation frequency maps for various storm events for the State of Colorado are typically used. If needed, these point rainfall values can then be used to develop 5- minute, 10-minute, 15-minute, 30minute, 1-hour, 2-hour, 3-hour, and 12-hour rainfall depths. METHOD SELECTION There are many different rainfall-runoff deterministic models available. However, not all of the methods can be effectively utilized in Colorado. Some methods are not applicable for the hydrologic conditions that exist in Colorado, and other methods cannot be utilized easily or accurately due to the lack of data. Also, the computed flow estimates may vary considerably depending on the methods utilized for a given watershed. Therefore, it is necessary to define minimum standards for the analysis in order to promote accuracy and consistency in the computed flow rates.
The following four deterministic rainfall-runoff modeling methods have been selected for use in Colorado: - Rational Method - NRCS TR-55 Method - NRCS Unit Hydrograph Method - CUHP and UDSWM Method - Other models or methods approved by local, state, and federal study partners on a case-by-case basis. The recommended rainfall depths and distributions to be used with the listed runoff methods are presented in this section. Due to the incorporated assumptions and limitations of the listed modeling methods, not all of the selected deterministic methods can be used for all hydrologic conditions. The methodology used to generate the rainfall-runoff data should be selected based on the size and location of the drainage basin to be studied and the intended use of the computed flow rates and hydrographs. The Rational Method for determining runoff is widely accepted as providing a sufficient level of detail for generating runoff from relatively small basins and can be used for drainage basins with a total contributing area of less than 160 acres. The Rational Method utilizes rainfall data in the form of time-intensity- frequency curves. Since the assumptions used in the Rational Method become less valid for larger areas, larger basins require a more rigorous analysis to generate runoff data. For drainage basins with an area greater than 90 acres, NRCS TR55 Method, NRCS Unit Hydrograph Method, or CUHP/UDSWM Method should be used depending on the location and hydrologic complexity of the drainage basin to estimate the runoff data. For drainage basins with an area between 90 acres and 160 acres, any of the four selected methods maybe utilized. The NRCS TR55 Method was first developed and documented in 1975 to provide a simplified procedure for estimating runoff and peak discharges from small urban and urbanizing watersheds. Two peak runoff determination techniques are available: Graphical Method and Tabular Method. The Graphical Peak Discharge method estimates only the peak runoff. The Tabular Hydrograph method can produce a runoff hydrograph. A synthetic 24-hour regional design rainfall distribution is used in the TR-55 runoff computations.
The NRCS TR-55 Method can be effectively used for simple watershed runoff modeling. However, NRCS Unit Hydrograph Method utilizing the US Army Corps of Engineer’s HEC-HMS or HEC-1 computer program should be utilized for rainfall-runoff modeling of watersheds that involve multiple sub-basins and routing elements and reaches.
The NRCS Unit Hydrograph Method has been widely used and accepted in the western United States for hydrologic modeling of small to large watersheds with multiple subbasins. The NRCS Unit Hydrograph Method can be used with a wide range rainfall distribution types and durations (6-hour, 24-hour, etc.). However, to promote consistency in the computed results and to simplify the analysis, a 24hour balanced storm distribution should be used with this method.
The CUHP (Colorado Urban Hydrograph Procedure) method has been used exclusively for the Urban Drainage and Flood Control District (UDFCD) jurisdictional area. The CUHP method was developed and calibrated to effectively model short duration convective storms within the Denver Metro area. Therefore, the CUHP method should only be used within the Urban Drainage and Flood Control District boundaries. The CUHP model can be used to generate sub-basin hydrographs and the UDSWM (Urban Drainage Storm Water Management) computer program can be used to route and combine hydrographs. Rainfall Depths The 6-hr and 24-hr point precipitation values for 2-, 5-, 10-, 25-, 50-, and 100 - year storm events can be estimated from available data for the study area. The point precipitation values for each storm duration (6- and 24-hr) obtained from the isopluvial maps should be plotted, and a straight line of best fit should be drawn. If any rainfall value deviates substantially from the best-fit line, the value read from the line should replace the original point precipitation value from the map. Once the 6- and 24-hr rainfall values have been obtained and adjusted (if necessary), the rainfall depths for other durations can be estimated using the following procedures from the NOAA Atlas II, Volume III, 1973. The State of Colorado has been divided into four (4) geographic regions by NOAA. Before applying the empirical methods outlined below, it is necessary to determine the region and apply appropriate equations for the drainage basin. If the drainage basin is located within few miles of a regional boundary, computations should be made using equations for both regions and the average rainfall values should be used for the rainfall-runoff analysis.
The 1-hour frequency values for 2- and 100-year storm events can be estimated utilizing the appropriate regional equations. Once computed, the 2-year and 100 year, 1-hour values can be plotted, and a straight line between the two values can be drawn. Then, the 1-hour values for return periods between 2- and 100 year events can be obtained from the line. Rainfall depths for the 2-hour and 3-hour events can be estimated using the following formulas (NOAA Atlas 2, 1973).
Region 1 D = 0.342*D + 0.658*D X,2 X,6 X,1 Region 2 D = 0.341*D + 0.659*D X,2 X,6 X,1 Region 3&4 D = 0.250*D + 0.750*D X,2 X,6 X,1 Where D = "X"-year, 2-hour rainfall depth (Inches)
X,3 D = "X"-year, 1-hour rainfall depth (Inches)
X,6 Based on Figure 17 in the NOAA Atlas 2, the 12-hour duration rainfall depth for the desired recurrence frequency is essentially the average of the 6-hour and 24 - hour storm events (NOAA, 1973).
D = (D + D )/2 X,12 X,6 X,24 Where D = "X"-year, 12-hour rainfall depth (Inches)
Rainfall depths for durations less than 1-hour can be estimated using adjustment ratios and the estimated "X"-year, 1-hour rainfall depth (NOAA, 1973). These adjustment ratios were originally published in the US Weather Bureau Technical Paper No. 40 in 1961, and later evaluated and adopted by NOAA.
D = D * RATIO X,Y X,1 X,Y Where D = "X"-year, Y -minute rainfall depth (Inches)
Utilizing the estimated rainfall depths of the 5-, 10-, 15-, 30-, and 60-minute durations for a given recurrence frequency, rainfall intensities can be estimated by dividing the rainfall depth by the duration of the storm.
I = D /Duration X,Y X,Y Y Where I = "X"-year, Y-minute rainfall intensity (Inches/Hour) X,Y D = "X"-year, Y-minute rainfall depth (Inches)
The NOAA Atlas 2 precipitation depths are related to rainfall frequency at an isolated point. Storms, however, can cause rainfall to occur over extensive areas simultaneously, with more intense rainfall typically occurring near the center of the storm. Standard precipitation analysis methods require adjusting point precipitation depths downward in order to estimate the average depth of rainfall over the entire storm area. This is normally performed using depth-area reduction factors (DARF) relating to a point precipitation reduction factor to storm area and duration. The application of DARF for large watersheds is complicated by the necessity to determine the “storm centering” which produces the greatest peak flow and/or volume at the selected design point. In order to obtain consistent results and to simplify the application of DARF, the flow value at a given concentration point should be determined using the depth-area reduction value for the total watershed area tributary to the subject point of interest. As runoff flows through a subject watershed, the contributing drainage area increases and the associated depth-area reduction factor will vary. To account for this, a range of depth-area reduction factors may need to be estimated for large watersheds that have several sub-basin design points. For example, if a total watershed area is 15 square miles, three depth-area reduction values may be used to estimate runoff for a design point at 5 square miles, one at 10 square miles, and one at 15 square miles. The respective depth-area reduction values would be 0.992, 0.985, and 0.978 Rainfall Distribution For CUHP Method The CUHP (Colorado Urban Hydrograph Procedure) computer model has been used within the Urban Drainage and Flood Control District (UDFCD) jurisdictional area to estimate urban sub- basin hydrographs. The CUHP method was developed and calibrated to simulate short duration convective storms in the Denver Metro area and other similar urban drainage environments. Convective storms are commonly known to be responsible for high peak flows and flooding problems for many small drainage basins.
CUHP Storm Distribution The rainfall intensity and distribution analysis performed by UDFCD using 73-years of rainfall record data at the Denver rain gage revealed that the majority of the past intense rainstorms produced their largest rainfall within the first hour of the storm. The analysis further discovered that out of the 73 storm events analyzed, 68 events produced the most intense rainfall beginning and ending within the first hour of the storm and 52 events produced the most intense rainfall beginning and ending within the first half hour of the storm. The UDFCD analysis concluded that these “leading intensity” convective storms were the main cause of most of the flooding problems in the Denver Metro Region.
The rainfall distributions recommended to be used with CUHP were developed to reflect the “leading intensity” characteristics of the previously recorded convection storms in the Denver Region, and they vary from 2- to 6- hours depending on the size of the drainage basin. The rainfall distributions for 2-, 3- and 6-hour storm durations can be developed using the following procedures from the UDFCD.
For drainage basins less than 10 square miles but greater than 90 acres, two-hour storm distribution rainfall values without area adjustments of the values should be used with CUHP. For drainage basins between ten and twenty square miles, three-hour storm distribution rainfall values with the area-adjustment should be used. For basins equal to and larger than 20 square miles, six-hour storm distribution values with the area-adjustment should be used. Area adjustments of the rainfall values for drainage basins equal to or greater than 10 square miles are necessary to determine the average depth of precipitation over the entire drainage basin being analyzed.
The 1-, 3-, and 6-hour point rainfall depths estimated using the NOAA Atlas 2 procedure can be used to develop storm distributions for a given recurrence frequency. The estimated NOAA point precipitation values can be distributed to develop 2-, 3- or 6-hour temporal distribution values using a 5-minute time increment following the distribution procedures from the UDFCD. The 2-hour temporal distribution for a given recurrence frequency can be developed by multiplying the NOAA 1-hour rainfall depth by the incremental distribution percentages (0 to 120 minutes). The 2-hour design storm distribution can be used without further modifications with CUHP for drainage basins less than 10 square miles.
The 3-hour storm distribution can be developed by adding incremental precipitation values for the period between 125 minutes and 180 minutes to the 2-hour distribution discussed above. The incremental precipitation values for the period between 125 minutes and 180 minutes can be determined by evenly distributing the difference between the NOAA 3-hour rainfall depth and the 2-hour total precipitation. In a similar approach, the 6-hour distribution can be developed by evenly distributing the difference between the NOAA 3-hour and 6-hour rainfall depths over the period of 185 minutes to 360 minutes. The first three hours of the 6-hour distribution is same as the three-hour distribution discussed above.
Depth-Area Adjustment The NOAA precipitation depths are related to rainfall frequency at an isolated point. Storms, however, can cause rainfall to occur over extensive areas simultaneously, with more intense rainfall typically occurring near the center of the storm. Rainfall depth-area adjustment is necessary to determine the average depth of precipitation over the entire drainage basin being analyzed. This is normally performed using depth-area reduction curves relating point precipitation reduction factor to drainage basin area and storm duration. In order to assist engineers with the depth-area adjustment application procedures, UDFCD developed an adjustment factor table for drainage basins between 10 and 75 square miles. The 3- and 6- hour storm distribution values can be adjusted by multiplying each incremental rainfall depth by the appropriate adjustment factor for a given time increment and the size of the drainage basin.
SECTION 5 – RUNOFF INTRODUCTION This section presents the criteria and methodologies for determining the storm runoff peaks and volumes to be used in the preparation of drainage studies and facilities design in the State of Colorado. Practical analysis guidelines for four deterministic hydrologic methods (Rational Method, TR-55 Method, Unit Hydrograph Method, and CUHP/UDSWM Method) are provided in this section. RATIONAL METHOD The Rational Method has been used commonly for the sizing of storm sewers and for determining rainfall- runoff design values for small drainage basins with an area less than 160 acres. Even though this method has frequently come under academic criticism for its simplicity, no other practical drainage design method has evolved to such a level of general acceptance by practicing engineers. The Rational Formula method, when properly understood and applied, can produce satisfactory results for determining peak discharge estimates. The limit of application of the Rational Method is approximately 160 acres. The assumptions used in the Rational Method become less valid for larger areas. Therefore, the Rational Method should not be for drainage basins equal to or larger than 160 acres. Methodology The Rational Formula method is based on the following equation: Q = CIA Q is defined as the maximum rate of runoff in cubic feet per second (actually, Q has units of acre inches per hour, which is approximately equal to the units of cubic feet per second). C is a runoff coefficient and represents the runoff-producing conditions of the subject land area. I is the average intensity of rainfall in inches per hour for a duration equal to the time of concentration. A is the contributing basin area in acres. The time of concentration is defined as the time required for water to flow from the hydraulically most distant part of the drainage area to the point under consideration.
Assumptions The basic assumptions made when applying the Rational Formula method are as follows:
1. The computed maximum rate of runoff to the design point is a function of the average rainfall rate during the time of concentration to that point.
2. The maximum rate of rainfall occurs during the time of concentration, and the design rainfall depth during the time of concentration is converted to the average rainfall intensity for the time of concentration.
3. The maximum runoff rate occurs when the entire area is contributing flow. However, this assumption has been modified from time to time when local rainfall/runoff data was used to improve calculated results.
Limitations on methodology The Rational Formula Method can adequately approximate the peak rate of runoff from a rainstorm in a given small basin. The critics of the method usually are unsatisfied with the fact that the answers are only approximations. A shortcoming of the Rational Formula Method is that only one point on the runoff hydrograph is computed (the peak runoff rate). Therefore, the estimated total runoff volume using the triangular hydrograph is not very accurate. Time Of Concentration As previously mentioned, the time of concentration is defined as the time required for runoff to flow from the hydraulically most distant part of the drainage basin to the desired point in the basin.
The time of concentration consists of two components, the initial or overland flow time (usually as sheet flow) and the time of travel in a concentrated form (i.e., in a storm sewer, gutter, swale, channel, etc.). The initial flow time, ti, is a function of the slope, surface cover, travel distance, soil, depression storage, and antecedent rainfall. The concentrated travel time, tt, is a function of hydraulic properties (i.e., surface roughness, slope, area, etc.) of the conveyance feature and the length of travel path. The time of concentration, for both urban and non-urban areas, is represented by the following equation:
The initial travel time equation was originally developed by the Federal Aviation Administration (FAA, 1970) for use with the Rational Formula Method. However, the equation is also valid for computation of the initial or overland flow time for the SCS Unit Hydrograph method using the appropriate flow runoff coefficient. Where ti = initial or overland flow time (minutes)
The overland flow distance is the flow length where the runoff flows as sheet flow. The maximum overland flow distance is 500 feet. Usually after a 500-foot flow length (although it may be less), the sheet flow will concentrate into swales, gutters, etc. and should be considered using the travel time equation.
After the runoff concentrates and flows in swales, gutters, or channels, the flow time should be determined using the travel time equation:
V = flow velocity (feet per second)
If necessary, the concentrated flow path in a given basin can be divided into multiple reaches, and the travel time for each reach should be computed separately and then combined to estimate the total travel time for the basin. The total time of concentration (t ) is then the sum of the initial flow time (t ) and the c i travel time (t ). The minimum time of concentration value, in non-urban watersheds, t should be 10 minutes, and if the calculated value of tc is less than 10 minutes, then 10 minutes should be used.
Time Of Concentration In Urbanized Basins The time of concentration at the first design point in urbanized areas should be estimated using two different methods. The first method should use a maximum overland flow length of 300 feet. In an urban setting, overland flow occurs from the back of the lot to the street, in parking lots, in greenbelt areas, etc. and the length until the runoff concentrates is usually less than in a non-urban environment.
The minimum time of concentration value using these two methods should be used to determine the rainfall intensity, which will be described in a subsequent section. The minimum time of concentration for an urbanized area should not be less than 5 minutes, and if the calculations provide a value less than 5 minutes, then 5 minutes should be used.
The time of concentration calculated at the second design point and all subsequent design points should be calculated by adding the travel time to the downstream design points to the time of concentration calculated for the first design point. This relationship is represented by the following equation:
Rainfall Intensity The rainfall intensity, I, is the average rainfall rate in inches per hour for the period of maximum rainfall of a given frequency and duration. A time-intensityfrequency curve of a given drainage basin for various frequency events can be developed using standard procedures. The rainfall intensity for a given design storm event can be determined from the time-intensity-frequency curve using the calculated time of concentration.
Runoff Coefficient The runoff coefficient, C, represents the integrated effects of infiltration, evaporation, retention, flow routing, and interception, all of which affect the time distribution and peak rate of runoff. Determination of the coefficient requires judgment and understanding on the part of the engineer. A composite runoff coefficient can be computed on the basis of the percentage of different types of surfaces in the drainage basin. A composite C analysis will result in more accurate peak flow estimation.
Application Of The Rational Formula Method The first step in applying the Rational Formula method is to obtain a topographic map and define the boundaries of all the relevant drainage basins. Basins to be defined include all basins tributary to the area of study and sub-basins within the study area. A field check and possibly field surveys should be made for each basin. At this stage of planning, the possibility for the diversion of trans-basin waters should be identified.
Major Storm Analysis The major storm drainage basin configuration does not always coincide with the minor storm drainage basin. This is often the case in urban areas where a low flow will stay next to a curb and follow the lowest grade, but when a large storm occurs, the water will be deep enough so that part of the water will overflow street crowns and flow into a new sub-basin. When analyzing the major runoff occurring on an area that has a storm sewer system sized for the minor storm, care should be used when applying the Rational Formula method. Common application of the Rational Method assumes that all the runoff is collected by the storm sewer. For the minor storm design, the time of concentration is dependent upon the flow travel time in the pipe system. However, during the major storm runoff, the pipes will probably be at capacity and would not carry the additional water flowing to the inlets. This additional water then flows overland past the inlets, generally at a lower velocity than the flow in the storm sewers. If a separate time of concentration analysis is made for the pipe flow and surface flow, a time lag between the surface flow peak and the pipe flow peak will occur. This lag, in effect, will allow the pipe to carry a larger portion of the major storm runoff than would be predicted using the minor storm time of concentration. The basis for this increased benefit is that the excess water from one inlet will flow to the next inlet downhill, using the overland route. If that inlet is also at capacity, the water will often continue on until capacity is available in the storm sewer. The analysis of this aspect of the interaction between the storm sewer system and major storm runoff is complex. The simplified approach of using the minor storm time of concentration for all frequency analysis is acceptable for use in the State of Colorado.
TR-55 METHOD The NRCS TR-55 Method (formerly known as SCS TR-55 Method) was first developed and documented in January 1975. Its purpose was to provide a simplified procedure for estimating runoff and peak discharges from small urban and urbanizing watersheds. The method was derived from typical hydrographs prepared by procedures outlined in Chapter 16 of the National Engineering Handbook 4 (SCS, 1985). The computations were made using the computerized hydrologic model TR-20 (SCS, 1983). The method is similar to the NRCS Unit Hydrograph Method discussed herein in that a Curve Number (CN) is used to determine rainfall excess and the unit hydrograph theory is used to develop a runoff hydrograph. The method differs, however, from the Unit Hydrograph Method as follows: 1 Synthetic, 24-hour regional design rainfall distributions are used. 2 Two peak runoff determination methods are available. The Graphical Peak Discharge method estimates only the runoff peak. The Tabular Hydrograph method produces a runoff hydrograph. 3 The Tabular Hydrograph method uses pre-routed hydrographs from the specified sub-basins to produce the estimated runoff hydrograph.
Both Graphical and Tabular methods can be used to estimate peak flow rates. However, the Tabular Hydrograph Method should be used if runoff hydrograph estimation is required. The reader is referred to the NRCS TR-55 document for a more detailed explanation of the two methods. Additional computation forms and worksheets are also provided in the TR-55 document. The TR-55 Tabular Hydrograph Method may be used to combine hydrographs from multiple subbasins. However, hydrologic analysis of watersheds that require routing of flows (channel, overland, and storage) and combining of hydrographs, the NRCS Unit Hydrograph Method using HEC-HMS or HEC-1 should be used.
Graphical Peak Discharge Method The Graphical Peak Discharge method determines the peak discharge, qp, from a basin based on the following equation:
Q = Runoff (in.)
F = Pond and Swamp Adjustment Factor p The unit peak discharge, q is determined based on the composite CN, for the basin and the u total rainfall, P. With CN and P values, an I /P ratio may be determined using the following a equation:
I /P = ((200/CN) – 2) / P a Where: I = Initial Abstraction (in)
a c u The basin runoff, Q, is then determined.
The pond and swamp adjustment factor, F , accounts for ponds and swamps not in the t flow p c path.
Tabular Hydrograph Method The Tabular Hydrograph method determines the runoff hydrograph at a given design point by combining pre-routed subbasin hydrograph coordinates which are computed from the following equation:
Limitations on Methodology The following limitations are applicable to the Graphical Peak Discharge method:
1. The watershed should be hydrologically homogeneous (describable by one CN). Land use, soils, and cover are distributed uniformly throughout the watershed.
2. The watershed may have only one mainstream or, if more than one, the branches should have nearly equal t s.
c 3. Accuracy of the peak discharge estimated by this method will be reduced if Ia/P values are used that are outside the recommended range.
4. The Graphical Method should be used only if the weighted CN is greater than 40.
5. When the Graphical Method is used to develop estimates of peak discharge for both present and developed conditions of a watershed, use the same method for estimating t for c both conditions.
The limitations of the Tabular Hydrograph Method are stated in TR-55 (NRCS, 1986). The user is encouraged to research this document before applying the Tabular Hydrograph Method. NRCS Unit Hydrograph Method The NRCS Unit Hydrograph Method (formerly known as SCS Unit Hydrograph Method) was originally developed for the Soil Conservation Services (SCS) by Mr. Victor Mockus. The SCS Unit Hydrograph was derived from a large number of natural unit hydrographs from watersheds varying widely in size and geographic location. The SCS Unit Hydrograph has been in use for many years and has produced satisfactory results for many applications. Methodology The NRCS Unit Hydrograph Method uses the unit hydrograph theory as a basis for runoff computations. The unit hydrograph theory computes rainfall excess hydrographs for a unit amount of rainfall excess applied uniformly over a sub-basin for a given unit of time (or unit duration). The rainfall excess hydrographs are then transformed to a sub-basin hydrograph by superimposing each excess hydrograph lagged by the unit duration. The shape of the NRCS Unit Hydrograph is based on studies of various natural unit hydrographs. The basic governing parameters of this curvilinear hydrograph are as follows: 1 The time-to-peak, T , of the unit hydrograph approximately equals 0.2 times the time-of-base, p T .
1 The effects of all physical characteristics of a given drainage basin are reflected in the shape of the storm runoff hydrograph for that basin.
2 At a given point on a stream, discharge ordinates of different unit graphs of the same unit time of rainfall excess are mutually proportional to respective volumes. 3 A hydrograph of storm discharge that would result from a series of bursts of excess rain or from continuous excess rain of variable intensity may be constructed from a series of overlapping unit graphs each resulting from a single increment of excess rain of unit duration.
Lag Time Input data for the NRCS dimensionless unit hydrograph method consists of a single parameter, TLAG, which is equal to the lag (in hours) between the center of mass of rainfall excess and the peak of the unit hydrograph.
For small drainage basins (less than one square mile) with basin slopes less than ten percent, the lag time may be related to the time of concentration, t , by the following empirical relationship: c TLAG = 0.6 t c The t can be computed assuming that the flow runoff coefficient, R, should be calculated using c the following equation. The equation was developed by converting curve numbers (CN) to typical C runoff coefficients.
S = Representative (average) slope of the longest watercourse (feet per mile) This lag equation is based on the United States Bureau of Reclamation's analysis of the above parameters for several drainage basins in the southwest desert, Great Basin, and Colorado Plateau area (USBR, 1989). Since the NRCS (formerly SCS) and the USBR define lag differently, this equation was developed by modifying the USBR's S-graph lag equation to correspond to the NRCS's definition of the dimensionless unit hydrograph lag equation. In order to obtain comparable results between the t calculation and the TLAG calculation, it is c recommended that either method be used as a check of the other method for drainage areas around one square mile in size.
Roughness Factor The selection of a proper roughness factor for use in the lag time calculation is highly subjective. Therefore, in order to obtain more consistent lag time and runoff analysis results, the roughness factor, K , should be determined using accepted procedures. For partially developed basins, the n roughness factor should be interpolated in relationship to the percent of each land use in the basin.
Unit Storm Duration The minimum unit duration, delta t, is dependent on the time of concentration of a given basin. If the basin is large (i.e., > one square mile), a larger unit duration may be used. If the basin is small (i.e., <one square mile) a smaller unit duration should be used. The unit duration, delta t, should be ≤ .25 T , where T is the time-to-peak of the unit hydrograph. For the State of p p Colorado, the unit storm duration of 5 minutes should be used unless conditions warrant otherwise.
Precipitation Losses Land surface interception, depression storage, and infiltration are referred to as precipitation losses. Interception and depression storage are intended to represent the surface storage of water by trees or grass, in local depressions, in the ground surface, in cracks and crevices in parking lots or roofs, or in a surface area where water is not free to move as overland flow. Infiltration represents the movement of water to areas beneath the land surface. Two important factors should be noted about the precipitation loss computations to be used for the NRCS Unit Hydrograph Methods. First, precipitation which does not contribute to the runoff is considered to be lost from the system. Second, the equations used to compute the losses do not provide for soil moisture or surface storage recovery. The precipitation loss component of the NRCS Unit Hydrograph method is considered to be sub- basin average (uniformly distributed over an entire sub-basin). In some instances, there are negligible precipitation losses for a portion of a sub-basin. This would be true for an area containing a lake, reservoir or impervious area. In this case, precipitation losses should not be computed for a specified percentage of the area labeled as impervious. There are several methods that can be used to calculate the precipitation loss. These methods include the Initial and Uniform Loss Rate, Exponential Loss Rate, Holtan Loss Rate, Horton Loss Rate, Green-Ampt and NRCS Curve Number Method to name a few. The NRCS Curve Number Method (formerly known as SCS Curve Number) is recommended because there is a lack of data for use of other methods. The Curve Number method has been used widely by the practicing engineers because the necessary data can be relatively easily obtained and the method can be easily applied to practical applications. In the Curve Number method, an average precipitation loss is determined for a computation interval and subtracted from the rainfall hyetograph. The resulting precipitation excess is used to compute an outflow hydrograph for a sub-basin. Curve Number (CN)
The NRCS (formerly SCS) has instituted a soil classification system for use in Soil Survey maps across the country. Based on experimentation and experience, the agency has been able to relate the drainage characteristics of soil groups to curve numbers. The Soil Survey provides information on relating soil group type to the curve number as a function of soil cover, land use type and antecedent moisture conditions.
Precipitation loss is calculated based on supplied values of CN and the initial surface moisture storage capacity, IA. CN and IA are related to a total runoff depth for a storm by the following relationships:
P = Accumulated rainfall depth (inches)
The CN Method uses a soil cover complex number for computing excess precipitation. The curve number is related to hydrologic soil group (A, B, C, or D), land use, treatment class (cover), and antecedent moisture condition. The soil group is determined from published soil maps for the area. These maps are usually published by NRCS. Land use and treatment class are usually determined during investigations in conjunction with aerial photographs. The procedures for determining land use and treatment class are found in Chapter 8 of the National Engineering Handbook (NEH), Section 4. The antecedent moisture condition (AMC) of the watershed is explained as follows:
The amount of rainfall in a period of 5 to 30 days preceding a particular storm is referred to as antecedent rainfall, and the resulting condition of the watershed in regard to potential runoff is referred to as an antecedent moisture condition. In general, the heavier the antecedent rainfall, the greater the direct runoff that occurs from a given storm. The effects of infiltration and evapo- transpiration during the antecedent period are also important, as they may increase or lessen the effect of antecedent rainfall. Because of the difficulties of determining antecedent storm conditions from data normally available, the conditions are reduced to three cases, AMC-I, AMC-II and AMC-III.
For the State of Colorado, the AMC-II condition should generally be used for determining storm runoff.
There are a number of methods available for computing the percentage of impervious area in a watershed. Some methods include using U.S. Geological Survey topographic maps, land use maps, aerial photographs, and field reconnaissance. Care should be exercised when using methods based on such parameters as population density, street density, and age of the development as a means of determining the percentage of impervious area. The available data on runoff from urban areas is not yet sufficient to validate widespread use of these methods. Sub-Basin Sizing The determination of the peak rate of runoff at a given design point is affected by the discretization of sub-basins in the subject watershed. Typically, the more discrete the analysis of a given watershed (more sub-basins), the larger the peak flow rate as compared to analysis of the basin with no sub-basins. Therefore, in order to obtain more consistent results between different designers as well as between different runoff models (i.e. Rational Formula Method, TR-55 method), the following general guidelines are recommended for basin discretization: - Subbasins should be delineated so that the land use and surface characteristics within each sub-basin are homogeneous.
- The sub-basin sizing should be consistent with the level of detail needed to determine peak flow rates at various design points within a given watershed. However, the maximum size of a subbasin should not exceed one square mile.
Routing Of Hydrographs Whenever a large or a non-homogeneous basin is being investigated, the basin should be divided into smaller and more homogeneous subbasins and the storm hydrograph for each subbasin should be calculated. The user then should route and combine the individual subbasin hydrographs to develop a network of storm hydrographs for the entire watershed. There are several methods available for use in flow routing which include:
a. Muskingum b. Convex c. Direct Translation d. Storage-Discharge (Modified Puls)
e. Kinematic Wave f. Diffusion Wave g. Dynamic Wave h. Muskingum-Cunge The most commonly used channel and overland flow routing techniques are Muskingum, Muskingum-Cunge (an approximate diffusion router), and Kinematic Wave (a finite-difference technique) methods.
The Muskingum-Cunge method provides reasonable results over a wide range of channel flow conditions and is relatively easy to use. The Muskingum-Cunge technique should be used to route flows in channels with standard prismatic shapes or channels with irregular cross section shapes. In some instances, an error message will occur with Muskingum-Cunge method that terminates the program computations. In this instance the Muskingum method should be used to route flows.
The Muskingum method should be used to route flows over a wide shallow floodplain. The Muskingum weighting factor, X coefficient (0 ≤ X ≤ 0.5), should be selected carefully to represent the routing reach conditions. The “X” coefficient of 0.2 to 0.3 is recommended for an average well-established natural channel, and 0.5 for a concrete lined channel. The Kinematic Wave method should only be used in relatively short reaches such as those encountered in an urban environment. Numerical errors introduced when solving the Kinematic Wave technique may cause a greater attenuation of the peak flow than actually occurs. The Kinematic Wave technique can only be used for specific types of channel shapes (i.e., trapezoidal, rectangular, etc.).
The reader is referred to the HEC-1 and HEC-HMS User's Manual for details on the development of Muskingum, Muskingum-Cunge and Kinematic Wave techniques and details on the input parameters needed for their use in the computer programs. RESERVOIR ROUTING OF HYDROGRAPHS The procedures for manual computation of reservoir routing are presented in this section to enhance understanding of the method. However, the storage-stagedischarge routing calculations can be automated using HEC-HMS or HEC-1 computer programs.
Modified Puls Routing Method The procedure for the original Puls Method was developed in 1928 by L.G. Puls of the U.S. Army Corps of Engineers. The method was modified in 1949 by the Bureau of Reclamation simplifying the computational and graphic requirements. The Modified Puls method is also referred to as the Storage-Indication or Goodrich Reservoir Routing Method. The differences, if any, are mainly in the form of the equation and means of initializing the routing. The procedures presented herein were obtained from Hydrology for Engineers (LINSLEY, 1975). The principle of mass continuity for a channel reach can be expressed by the equation: (I-D)t = delta S Where I is the inflow rate, D is the discharge rate, t is the time interval, and delta S is the change in storage. If the average rate of flow during a given time period is equal to the average of the flows at the beginning and end of the period, the equation can be expressed as follows: (I + I ) t/2 - (D + D ) t/2 = S - S 1 2 1 2 2 1 Where the subscripts 1 and 2 refer to the beginning and end of time period t. Rearranging the equation gives the following form used for the Modified Puls method: I + I + (2S /t - D ) = (2S /t + D )
The Colorado Urban Hydrograph Procedure (CUHP) is a synthetic unit hydrograph methodology developed and calibrated for the Denver Metro area.
Therefore, the CUHP method should only be used for urban areas with similar hydrologic characteristics as Denver Metro area. The computer version of CUHP can be used to compute hydrographs from drainage basins larger than 90 acres.
The Urban Drainage and Flood Control District’s UDSWM program can be used to route the sub-basin hydrographs generated by CUHP through conveyance elements and storage facilities located within a drainage basin. UDSWM is the runoff block of EPA’s SWMM (Storm Water Management Model, Version 2), as modified by the U.S. Army Corps of Engineers. UDSWM provides channel, pipe, and reservoir routing, and has been calibrated to work with CUHP. UDSWM can add and combine the hydrographs from sub-basins and conveyance elements as the flow proceeds downstream. The Colorado Urban Hydrograph Procedure (CUHP) is a method of hydrologic analysis based upon the unit hydrograph principle. It has been developed and calibrated using rainfall-runoff data collected in Colorado (mostly in the Denver/Boulder metropolitan area). This section provides a general background in the use of the computer version of CUHP to carry out stormwater runoff calculations. Effective Rainfall For CUHP Effective rainfall is that portion of precipitation during a storm event that runs off the land to streams. Those portions of precipitation that do not reach streams are called abstractions and include interception by vegetation, evaporation, infiltration, storage in all surface depressions, and long-time surface retention.
Pervious-Impervious Areas The urban landscape is comprised of pervious and impervious surfaces. The degree of imperviousness is the primary variable that affects the volumes and rates of runoff calculated using CUHP. When analyzing a watershed for design purposes, the probable future percent of impervious area must first be estimated. The pervious-impervious area relationship can be further refined for use in CUHP as follows:
1. DCIA—Impervious area portion directly connected to the drainage system.
2. UIA—Impervious area portion that drains onto or across impervious surfaces.
3. RPA—The portion of pervious area receiving runoff from impervious portions.
4. SPA—The separate pervious area portion not receiving runoff from impervious surfaces. Depression Losses Rainwater that is collected and held in small depressions and does not become part of the general surface runoff is called depression loss. Most of this water eventually infiltrates or is evaporated. Depression losses also include water intercepted by trees, bushes, other vegetation, and all other surfaces. The CUHP method requires numerical values of depression loss as inputs to calculate the effective rainfall.
When an area is analyzed for depression losses, the pervious and impervious loss values for all parts of the watershed must be considered and accumulated in proportion to the percent of aerial coverage for each type of surface.
Infiltration The flow of water into the soil surface is called infiltration. In urban hydrology much of the infiltration occurs on areas covered with grass. Urbanization can increase or decrease the total amount of infiltration.
Soil type is the most important factor in determining the infiltration rate. When the soil has a large percentage of well-graded fines, the infiltration rate is low. In some cases of extremely tight soil, there may be, from a practical standpoint, essentially no infiltration. If the soil has several layers or horizons, the least permeable layer near the surface will control the maximum infiltration rate. The soil cover also plays an important role in determining the infiltration rate. Vegetation, lawn grass in particular, tends to increase infiltration by loosening the soil near the surface. Other factors affecting infiltration rates include slope of land, temperature, quality of water, age of lawn and soil compaction.
As rainfall continues, the infiltration rate decreases. When rainfall occurs on an area that has little antecedent moisture and the ground is dry, the infiltration rate is much higher than it is with high antecedent moisture resulting from previous storms or land irrigation such as lawn watering. Although antecedent precipitation is very important when calculating runoff from smaller storms in non-urbanized areas, the runoff data from urbanized basins indicates that antecedent precipitation has a limited effect on runoff peaks and volumes in the urbanized portions of the UDFCD.
There are many infiltration models in use by hydrologists. These models vary significantly in complexity. Because of the climatic condition in the semi-arid region and because runoff from urban watersheds is not very sensitive to infiltration refinements, the infiltration model proposed by Horton was found to provide a good balance between simplicity and reasonable physical description of the infiltration process for use in CUHP. in which:
t = time (seconds)
In developing the equation, Horton observed that infiltration is high early in the storm and eventually decays to a steady state constant value as the pores in the soil become saturated. The coefficients and initial and final infiltration values are site specific and depend on the soils and vegetative cover complex. It is possible to develop these values for each site if sufficient rainfall- runoff observations are made. However, such an approach is rarely practical. Since 1977, the UDFCD has analyzed a considerable amount of rainfall-runoff data. The NRCS Hydrologic Soil Groups C and D occur most frequently within the UDFCD; however, areas of NRCS Group A and B soils are also fairly common. Consult NRCS soil surveys for appropriate soil classifications.
To calculate the maximum infiltration depths that may occur at each time increment, it is necessary calculate the values for each time increment. Very little accuracy is lost if the infiltration rate is calculated at the center of each time increment. This “central” value can then multiplied by the unit time to increment to estimate the infiltration depth. This was done for the four NRCS hydrologtic soil groups.
Rainfall The CUHP computer program requires the input of a design storm, either as a detailed hyetograph or as a 1-hour rainfall depth. A detailed hyetograph distribution is generated by the program for the latter using the standard 2-hour storm distribution. Catchment Description The following catchment parameters are required for the program to generate a unit and storm hydrograph.
1. Area—Catchment area in square miles.
2. Catchment Length—The length in miles from the downstream design point of the catchment or siub-catchment along the main stream path to the furthest point on its respective catchment or sub-catchment. When a catchment is subdivided into a series of sub- catchments, the sub-catchment length used shall include the distance required for runoff to reach the major stream from the farthest point in the sub-catchment.
3. Centroid Distance—Distance in miles from the design point of the catchment or sub- catchment along the main stream path to its respective catchment or sub-catchment centroid.
4. Percent Impervious—The portion of the catchment’s total surface area that is impervious, expressed as a percent value between 0 and 100.
5. Catchment Slope—The length-weighted, corrected average slope of the catchment in feet per foot.
There are natural processes at work that limit the time to peak of a unit hydrograph as a natural stream becomes steeper. To account for this phenomenon, it is recommended that the slope used in CUHP for natural streams and existing manmade grass-lined channels be adjusted. When a riprap channel is evaluated, use the measured (i.e. uncorrected) average channel invert slope.
In concrete-lined channels and buried conduits, the velocities can be very high. For this reason, it is recommended that the average ground slope (i.e., not flow-line slope) be used where concrete- lined channels and/or storm sewers dominate the basin streams. There is no correction factor or upper limit recommended to the slope for concrete-lined channels and buried conduits. Where the flow-line slope varies along the channel, calculate a weighted basin slope for use with CUHP. Do this by first segmenting the major stream into reaches having similar longitudinal slopes. Then calculate the weighted slope using the equation: in which:
6. Time of Concentration—As an option for small urbanized areas (e.g., less than 90 acres), the CUHP user must enter time of concentration in minutes. For catchments between 90 acres and 160 acres, the user may enter the time of concentration to determine the difference between flow calculated using CUHP parameters and the flow using input time of concentration.
7. Pervious Retention—Maximum depression storage on pervious surfaces in inches.
8. Impervious Retention—Maximum depression storage on pervious surfaces in inches.
9. Infiltration Rate—Initial infiltration rate for pervious surfaces in the catchment in inches per hour. If this entry is used by itself, it will be used as a constant infiltration rate throughout the storm.
10. Decay—Exponential decay coefficient in Horton’s equation in “per second” units.
11. Final infiltration—Final infiltration rate in Horton’s equation in inches per hour. The program computes the coefficients C and C ; however, values for these parameters can 1 p be specified by the user as an option.
The shaping of the unit hydrograph also relies on proportioning the widths at 50% and 75% of the unit hydrograph peak. The proportioning is based on 0.35 of the width at 50% of peak being ahead of the “time to peak” and 0.45 of the width at 75% of peak being ahead of the “time to peak” . These proportioning factors were selected after observing a number of unit hydrographs derived from the rainfall-runoff data collected by the USGS for the UDFCD. It is possible for the user to override the unit hydrograph widths and the proportioning of these widths built into the program. For drainage and flood studies within the UDFCD, the program values shall be used. If the user has derived unit hydrographs from reliable rainfall-runoff data for a study catchment and can develop a “calibrated” unit hydrograph for this catchment, this option permits reshaping the unit hydrograph accordingly.
The following catchment parameters are also optional inputs and are available to the user to account for the effects of directly connected/disconnected impervious areas:
1. DCIA—specifies the directly connected impervious area (DCIA) level of practice.
2. D—Defines the fraction of the total impervious area directly connected to the drainage system. Values range from 0.01 to 1.0 3. R—Defines the fraction of total pervious area receiving runoff from the “disconnected” impervious areas. Values range from 0.01 to 1.0.
4. WQCV—Defines the “water quality capture volume” to be modeled as detained in swales, berms, or other facilities (for DCIA level 3 only) in watershed-inches.
5. DTIME—Defines the duration over which the WQCV is released in hours (up to 40 hours). Catchment Delineation Criteria The maximum size of a catchment to be analyzed with a single unit hydrograph is limited to 5 square miles. Whenever a larger catchment is studied, it should be subdivided into sub- catchments of 5 square miles or less and individual sub-catchment storm hydrographs should be routed downstream using appropriate channel routing procedures such as the UDSWM model. The routed hydrographs are then added to develop a single composite storm hydrograph. The catchment shape can have a profound effect on the final results and, in some instances, can result in underestimates of peak flows. Experience with the 1982 version of CUHP has shown that, whenever catchment length is increased faster than its area, the storm hydrograph peak will tend to decrease. Although hydrologic routing is an integral part of runoff analysis, the data used to develop CUHP are insufficient to say that the observed CUHP response with disproportionately increasing basin length is valid. For this reason, it is recommended to subdivide irregularly shaped or very long catchments (i.e., catchment length to width ratio of four or more) into more regularly shaped sub-catchments. A composite catchment storm hydrograph can be developed using appropriate routing and by adding the individual sub-catchment storm hydrographs. UDSWM HYDROGRAPH ROUTING PROCEDURE UDSWM is a computer model that is used to generate surface runoff hydrographs from sub- catchments and then route and combine these hydrographs. The procedure described here is limited to the routing of hydrographs generated using CUHP software. UDSWM is a modified version of the Runoff Block of the Environmental Protection Agency’s (EPA’s) SWMM (Storm Water Management Model). It has been modified by the UDFCD so that it may be used conjunctively with CUHP.
UDSWMM Description The original Runoff Block was developed as part of the EPA’s SWMM. It simulates both the quantity and quality of runoff from an urban watershed and the routing of flow and contaminants to the major sewer lines. Later, the Missouri River Division of the U.S. Army Corps of Engineers (USACE) modified the model to expand the routing capabilities of the Runoff Block. This version of the Runoff Block was adopted by the UDFCD and converted to a personal computer version called UDSWM. It permits simulation of surface runoff rates and volumes and can also be conjunctively used with CUHP.
UDSWM represents the watershed by an aggregate of idealized channels, gutters, and pipes. The program can accept rainfall hyetographs and make a step-by-step accounting of rainfall infiltration losses in pervious areas, surface retention, overland flow, and gutter flow leading to the calculation of hydrographs. However, this portion of the model is normally not used by the UDFCD because the calculation of hydrographs for each sub-catchment is generally carried out using the CUHP software. If, however, the UDSWM is used to calculate runoff, the model must be calibrated against the CUHP calculations for the same watershed being studies. After the CUHP software is used to calculate hydrographs from a number of sub-watersheds, the resulting hydrographs from all sub-watersheds can be summer and routed through gutters (i.e., channels) or pipes to compute the resultant hydrographs at design points. Overflow channel sections may be used in conjunction with gutters and pipes. Detention basins with a specified storage-outflow relationship may also be used. Also, the program allows the user to specify a flow diversion table for a number of routing elements. All pipes, gutters, diversions, and detention basins are sometimes referred to as “conveyance elements” in this computer program. Surface Flows And Flow Routing The core of the Runoff Block is the routing of hydrographs through the stream system. This is accomplished by a combination of channel, gutter, reservoir, and pipe routing. The CUHP hydrographs are routed through the conveyance elements of the watershed. Pipes and channel sections are permitted to surcharge when full or, if desired, overflow sections may be provided to convey the flow exceeding the pipe or the initial channel capacity. The routing is based on a kinematic wave approach using Manning’s equation both for sub-watersheds and conveyance elements.
There are five standard types of conveyance elements and three special flow-routing conveyance elements. The five standard conveyance elements are as follows:
1. Channel—A trapezoidal channel to represent or approximate an open channel/gutter condition. This can be used for natural channels, constructed channels, and gutters.
2. Pipe—A circular pipe of any diameter.
3. Direct Flow (non-routed)—This element provides only instantaneous direct translation of the flows from the upstream to the downstream conveyance element and does not modify the hydrograph. It is often used to sum two or more upstream hydrographs and act as a design point. Its use upstream of detention basins that have more than one element (gutter or runoff) is strongly recommended.
4. Channel With Overflow Channel—Same as the channel element above except that a larger trapezoidal channel is also specified to accept the flows exceeding the capacity of the initial channel cross section. This is often used to model a 100-year flood channel with a low-flow channel.
5. Pipe With Overflow Channel—Same as the channel element above except that a trapezoidal channel is also specified to convey the flows exceeding the capacity of the pipe element. This is often used to model a storm sewer with excess stormwater carried in a gutter or channel.
The three special flow routing-conveyance elements are as follows:
1. Diversion—A table of flows in a conveyance element versus the flow diverted to another conveyance element may be specified using this option. This option may be used with any routing element.
2. Storage Reservoir—A table of reservoir storage in acre-feet versus outflow in cfs may be specified using this option. This option may be used in conjunction with the “pipe” routing element. The pipe capacity has to be exceeded before the storage-outflow function is used.
3. Inflow Hydrograph—This option may be used to specify an input hydrograph table of time in hours versus the flow in cfs for any routing element.
APPENDIX C HYDRAULIC ANALYSES 1 – DETAILED METHOD 2 – LIMITED DETAILED METHOD 3 – APPROXIMATE METHOD SECTION 1 – DETAILED METHOD INTRODUCTION Detailed flood hazard area information including floodplain and floodway limits, flood water surface elevations, flow velocities, etc. can be determined based on the detailed hydraulic analysis methods and guidelines outlined in this section. The detailed hydraulic analysis approach should be used for the following general cases:
- To determine new detailed floodplain and/or floodway boundaries for streams that are located adjacent to existing and/or planned developments.
- To revise existing detailed floodplain/floodway delineations to reflect changes in topography or hydrology caused by natural or manmade activities.
- To determine potential impacts or benefits of proposed improvements within the delineated floodplains. - To delineate detailed floodplain/floodway boundaries for streams that have been previously studied and delineated using approximate methods.
- To check the flow conveyance capacity of designed or newly constructed drainage facilities. Detailed analyses generally consider flooding from the 10-, 50-, 100-, and 500-year as well as defined floodways. The information presented in this section is the state-of-art information available at the time of preparation of the Rules and should be updated as better analysis and modeling techniques become available in the future.
PREVIOUS STUDIES Before proceeding with a detailed hydraulic study, the project engineer should evaluate the applicability of all available hydrologic and hydraulic studies for the subject stream. The previously approved studies should be used whenever practical, unless the watershed/stream conditions have changed substantially and/or the original analysis methodology was determined inappropriate or inadequate. Where applicable, a comparison of the calculated 100-year water surface elevations (WSEL) at the study limits with the previously approved WSELs for the stream should be provided. Except where clearly identified changes in flooding characteristics or error in the existing water surface profile can be shown, the proposed 100-year flood elevations at the study limits should agree with those of other contiguous studies on the same stream. The 100-year water surface elevations should match within +/- 0.5 foot of the existing valid elevations. Where elevations cannot be reconciled to within +/- 0.5 foot because of changed flooding conditions or an error in the previous analysis, a full explanation and justification for the difference should be provided.
HYDROLOGIC ANALYSIS Hydrologic analysis should be performed based on the criteria outlined in Appendix B of the Rules. Peak flow rates should be computed using statistical analysis, rainfall-runoff models, or regional regression methods.
TOPOGRAPHIC MAPPING For discussions and specifications on the topographic mapping standards for detailed floodplain delineation studies, please refer to Appendix A of the Rules. Cross Sections The riverine cross-section data for detailed hydraulic modeling purposes should be obtained based on the following methods:
- Photogrammetric methods at the time of map compilation - From DTM, DEM, or TIN models - From the map contours and spot elevations - Through field surveys All field-surveyed cross section points should be within ±0.5 foot of the true elevations. In general, cross sections should be aligned perpendicular to the direction of flow and spaced to adequately represent the stream. Additional cross sections should be placed at appreciable changes in flow area, roughness, or stream gradient, bridges and culverts, the head and tail of levees, confluences with tributaries, and all flow control structures. HYDRAULIC ANALYSIS APPROACHES The open channel/floodplain hydraulics can be very complex, encompassing many different flow conditions from steady-state uniform flows to unsteady, rapidly varying flows. The calculations for uniform and gradually varying flows are relatively straightforward, however, rapidly varying flow computations can be very complex and the solutions are generally empirical in nature. Flow hydraulics is three-dimensional in actuality. However, flow hydraulics for most streams can be adequately modeled by using one of the following three modeling approaches: - One-dimensional Steady Flow Analysis - One-dimensional Unsteady Flow Analysis - Two dimensional Steady/unsteady Flow Analysis There are limitations on all of the three modeling approaches. Therefore, the hydraulic properties of the study stream should be carefully evaluated and compared to the modeling limitations before selecting the appropriate modeling approach. Te modeling engineer should coordinate with the CWCB, local jurisdictions,and other study sponsors to select the most appropriate modeling approach and specific model for the stream being studied.
One-Dimensional Steady Flow The one-dimensional steady flow analysis is the most commonly used modeling approach due to its simplicity. This approach is widely accepted for modeling of streams with steady and gradually varying flow conditions. The most common occurrence of gradually varying flow is the backwater created by culverts and channel constrictions. For these conditions, the flow depth will be greater than normal depth in the channel, and the water surface profile should be computed using backwater techniques.
Following limitations generally apply to one-dimensional steady flow modeling techniques and programs:
- Flow condition is steady and gradually varied.
- Only the velocity in the direction of flow can be accounted for - Flow rate is constant for a given channel reach through out the duration of a flood event (only peak flow rates can be used, not hydrographs)
- Channel slope is relatively flat, less than 1 percent. - Cannot model effects of flow attenuation due to storage Flood water surface profiles may be calculated using the standard step backwater method employing the Bernoulli energy equation with energy losses due to friction evaluated with the Manning equation. Many computer programs are available for computation of backwater curves. The most general and widely used programs are US Army Corps of Engineers' HEC-RAS and HEC-2. Both HEC-RAS and HEC-2 programs can be used to model one-dimensional subcritical and supercritical flow conditions. In addition, HEC-RAS can be used to model mixed flow conditions.
Natural riverine flood water surface profiles for the purpose of floodplain delineations should be determined using subcritical flow regime calculations. Critical depths should be used for the natural stream reach where supercritical flow depths occur. Supercritical flow modeling may be used for man-made channels designed to handle supercritical flows. One Dimensional Unsteady Flow The main difference between the one-dimensional steady and unsteady flow models is that unsteady flow models can compute the effects of flow attenuation due to channel and floodplain storages. Instead of using single point peak flow rates, users can input entire flow hydrographs and route the hydrographs through the channel/floodplain system to compute water surface profiles and routed resulting hydrographs. The US Army Corps of Engineers' HEC-RAS computer program is recommended for 1-D unsteady flow modeling of riverine hydraulics. While this modeling approach is superior to 1-D steady flow modeling techniques, the following limitations still apply:
- Flow condition is steady and gradually varied.
- Only the velocity in the direction of flow can be accounted for - Channel slope is relatively flat, less than 1 percent. Developing 1-D unsteady flow models can be complex and costly. Therefore, this approach has not been used as frequently as the 1-D steady modeling approach. However, drainage systems with significant storage components should be modeled using the 1-D unsteady modeling techniques.
Two-Dimensional Flow While most of the riverine hydraulic conditions can be adequately modeled using either 1-D steady or 1-D unsteady flow modeling techniques, some flooding conditions (i.e., alluvial fans, shallow flooding) may require the use of two-dimensional modeling techniques in order to correctly model and delineate the flood hazard areas. Two-dimensional hydraulic computer programs can be used to model flood flows in two horizontal directions. For these cases, project engineer should coordinate with the local agencies and CWCB in selecting the appropriate modeling program for the drainage system being studied. The most commonly used two-dimensional hydraulic computer programs are MIKE FLOOD and FLO-2D. Detailed discussions on the modeling of alluvial fans, readers are referred to Chapter 12 of this manual.
STARTING WATER SURFACE ELEVATION One of the model boundary conditions that need to be defined by the modeling engineer is the starting water surface elevation (WSEL). For a subcritical model run, starting WSEL for the most downstream cross section should be defined, and for a supercritical run, starting WSEL for the most upstream cross section should be defined.
For a riverine reach not affected by backwater, the starting water surface elevation may be estimated based on normal depth calculations, unless a known water surface elevation for the starting cross section can be obtained from an existing model or previous recorded flood events. If normal depth calculation is used to compute the starting water surface elevation, several cross-sections (minimum of 2) should be placed outside of the study limit (upstream or downstream depending on the model flow regime) to improve the accuracy of the computed water surface elevation at the study limit. ROUGHNESS VALUES Recommended Manning’s “n” values for various channel and floodplain conditions can be found in published documents from the U.S Geological Survey and other common documents used in the industry. Manning’s roughness coefficients should be estimated considering the following factors: - Channel bed materials - Type, density, and height of existing vegetations - Existing structures in the overbanks - Roughness variations with different flow depths - Channel maintenance operations - Past flood data Past flood data, if available, should be used to calibrate roughness coefficients, taking into consideration any alteration in the channel subsequent to the floods. The calibrated roughness coefficients should closely match the observed channel and floodplain conditions. Photographs should be taken of the study reaches of the stream channel and floodplain to support roughness coefficients used for hydraulic computations.
SPLIT FLOW ANALYSIS Spilt flows occur when streams overflow the channel banks and take different flow paths away from the main floodplains. Undersized channels and crossing structures (i.e., culverts, bridges) are the most common reasons for the flow splits.
Flows that split away from the main floodplain may return back to the stream at a downstream location or may divert away to an adjacent stream. The amount of flow splits should be calculated and the downstream flow rates/hydrographs for the stream being studied should be adjusted accordingly. Flow splits can be estimated using the built-in split flow computational options in HEC-RAS or HEC-2 programs. It is important that the modeling engineer review the computed results to determine the accuracy of the results. Also, the flood hazard areas resulting from the split flows should be studied and delineated depending on the following factors:
- Purpose of the study - Amount of flow splits - Existing and proposed land uses within and adjacent to the flow path (i.e. residential vs. agricultural) CROSSING STRUCTURE Numerous roadways have been constructed across streams, and commonly, culverts or bridges are provided at these locations to convey flows beneath roadways. These drainage-crossing structures usually cause increase in water surface elevations by creating additional energy losses. The additional hydraulic energy losses, including contraction and expansion losses, at the crossing structures should be accounted for in the hydraulic analysis to compute water surface elevations. Culvert and bridge hydraulics can be modeled using HEC-RAS or HEC-2 programs. Readers are referred to the previously referenced program users manuals for detailed discussions on the subject. The following hydraulic elements should be considered in the analysis: - Size, type, and material of the structure - Invert elevations (including length and slope) of the structure - Location of the representative cross sections on both sides of the structure - Roadway profile for the weir flow computation - Ineffective flow areas - Debris and sediment blockage Blockage All culverts and bridges should be considered for the potential to become blocked by floating debris and sediment loads. In determining the potential for blockage, and subsequent reduction in the flow conveyance capacity, the following factors should be considered: - Old photographs - History of maintenance during high flows - Ongoing maintenance operations - Watershed characteristics such as erodibility of channel banks - Amount and type of vegetation along the stream - Size and characteristics of the waterway Blockage may be accounted for in computer runs by increasing width of piers, raising streambed elevation or reducing waterway opening by a percentage. Debris removal activities during flood events (i.e. snagging) should not be considered.
INEFFECTIVE FLOW AREA Ineffective flow areas may store water during flood events but the velocity in the direction of flow is zero or negligible. Therefore, these areas should be blocked out for the flow conveyance hydraulic analysis. However, the flow storage benefit of the ineffective areas should be modeled when using either one- dimensional unsteady or two-dimensional flow analysis approach. Ineffective flow areas commonly exist at both ends of culverts and bridges. Ponds, local depression areas, and backwater pools may, not always, also act as ineffective flow areas.
MODEL CALIBRATION Hydraulic models should be calibrated to match the reliable flood data from previous flood events, if available, within 0.5-foot +/- accuracy. Model calibration should involve adjustment of the hydraulic parameters that were estimated (i.e., Manning’s “n” values, contraction and expansion coefficients, etc.). However, the adjusted values should still be reasonable and closely represent the observed stream conditions.
AREAS PROTECTED BY LEVEES In order for a levee system to be recognized as providing flood protections, the levee should be structurally sound and adequately maintained. Certification from a federal or state agency that the levee meets the minimum freeboard criteria and that it appears, on visual inspection, to be structurally sound and adequately maintained will be required. Levees that have obvious structural defects, or that are obviously lacking in proper maintenance, should not be modeled as providing flood protections in the hydraulic analysis.
Further information regarding levees is included in Rule 9 of the Rules. Floodplain Analysis The natural floodplain areas protected from a 100-year event by a levee system can be designated as 100-year Shallow Floodplain with 1-foot average depth (FEMA Zone X). However, the areas inundated by the interior drainage behind the levees should be defined, and the 100- year water surface elevations, flooding limits and depths, flood hazard zone designations should be clearly identified.
If levees protecting the subject area do not meet the necessary requirements, the 100-year flood elevations of the protected area should be computed as if the levees did not exist. For the unprotected areas between the levee and the source of flooding, the 100-year flood elevations should be obtained from either the flood profile computed with the levees in place or the profile computed as if the levees did not exist, whichever is higher. This procedure recognizes the increase in flood elevation in the unprotected area caused by the levees. This procedure may result in the 100-year flood elevations being shown as several feet higher on one side of the levee than on the other. Both profiles should be shown in the final delineation with a line drawn along the levee centerline separating the areas with different BFEs. If levees exist on both sides of a stream, several levee failure scenarios should be considered including simultaneous levee failure, left levee only failure, and right levee only failure scenarios. Where flood protection credit will be given to levees providing 100-year protection, the adequacy of interior drainage systems should be evaluated. Areas subject to flooding from inadequate interior drainage behind levees will be mapped using standard floodplain mapping procedures. AREAS PROTECTED BY DAMS Flood Control Dams If a publicly owned, operated and maintained dam or a publicly controlled privately owned dam is specifically designed and operated, either in whole or in part, for flood control purposes, then its effects should be taken into consideration when delineating the floodplains below such a dam. Full credit should be given to the diminution of peak flood discharges, which would result from normal dam operating procedures.
Flood control dams that are not owned and maintained by public agencies should not be considered in the floodplain analysis.
Non-Flood Control Dams If a dam is not specifically designed and operated, either in whole or in part, for flood control purposes, then its effects, even if it provides inadvertent flood routing capabilities which reduce the flooding downstream, should not be taken into account. The delineation of the floodplains below such a dam should be based upon the floods that would occur absent of the dam. However, if adequate assurances have been obtained to preserve the flood routing capabilities of such a dam, then the flood attenuation effects may, but need not, be taken into consideration when delineating the floodplains below such a dam. The project engineer should coordinate with appropriate government agencies and CWCB in determining whether a non-flood control dam should be included in the analysis or not.
If existing dams are not included in the hydrologic analysis, discussions should be provided in the report describing the dams and reasons why they were not considered in the analysis. ALLUVIAL FAN FLOODING Alluvial fan flooding is quite different than a riverine flooding, and consequently, the alluvial fan floodplains should be studied and delineated based on a different set of criteria. Alluvial fan flooding can be characterized by unpredictable flow paths, mud-flows, high flow velocity, and erosion and sediment deposition. Alluvial fans typically do not have a well-defined channel capable of conveying a 100-year flows, although, it is not unusual to have smaller defined channel(s). Typically, flood flows do not spread over the entire alluvial fan surface, but are conveyed down from the apex to the toe of the fan by a network of old and new flow paths/channels.
FLOODWAY ANALYSIS The floodway represents the community’s regulatory limit of encroachment into the 100-year floodplain for those watercourses with the established floodway boundaries. Communities may choose to delineate floodways based on FEMA’s 1-foot rise criteria or based on stricter criteria by allowing a lesser amount of rise above the base flood elevations (BFEs).
Equal Conveyance Reduction Method Floodway limits should be determined based on the “equal conveyance reduction” method. This method reduces an equal amount of flow conveyance from both overbanks, allowing potential development areas on both sides of the waterway. This floodway analysis method is available in both HEC-2 and HEC-RAS programs.
Special Conditions Floodways can be delineated for most streams with channel and overbank flooding conditions. However, It is not practical to designate floodways for all flooding conditions. Floodways should not be delineated for the following general cases:
- Split flow areas - Sheet flow area (divided flow areas)
- Alluvial fans If the above condition(s) exists for only a small portion of the stream, the floodplain may be set equal to floodway for that portion, and floodway limits for the remaining parts of the stream may be defined using the equal conveyance reduction method. SECTION 2 – LIMITED DETAILED METHOD INTRODUCTION Limited Detailed flood hazard area information including floodplain limits, flood water surface elevations, flow velocities, etc. can be determined based on the detailed hydraulic analysis methods and guidelines described in Section 1 of Appendix C of the Rules. The Limited Detailed hydraulic analysis approach should be used for the following general cases:
- To determine new Limited Detailed floodplain boundaries for streams that are located adjacent to existing and/or planned developments.
- To revise existing Limited Detailed floodplain delineations to reflect changes in topography or hydrology caused by natural or manmade activities.
- To determine potential impacts or benefits of proposed improvements within the delineated floodplains. - To delineate Limited Detailed floodplain boundaries for streams that have been previously studied and delineated using approximate methods.
- To check the flow conveyance capacity of designed or newly constructed drainage facilities. Limited Detailed flood hazard area information is identical in nature and level of detail as the Detailed Method described in Section 1 of Appendix C of the Rules except that Limited Detailed studies need only analyze the 100-year flood event. In addition, floodway analyses are optional for Limited Detailed studies. SECTION 3 – APPROXIMATE METHOD INTRODUCTION The Approximate Method results in the delineation of approximate 100-year floodplain boundaries without base flood elevations (BFEs). The primary advantage of using Approximate Method is that approximate 100-year floodplain limits can be determined with a reasonably low cost and effort. This analysis approach may be used to delineate approximate 100-year floodplain boundaries for the following general cases:
- To update the currently designated 100-year approximate floodplain boundaries based on changes in the watershed or more current data and methodologies.
- To delineate new approximate floodplain delineations for streams that do not already have them. New approximate floodplains are typically done in areas that do not have existing development, or in areas that may potentially be developed in the long-term future. If new developments are proposed or expected within or adjacent to a previously delineated approximate 100-year floodplain, the previously delineated approximate floodplain limits should be restudied by using the detailed method as outlined in Section 3.
HYDROLOGIC ANALYSIS Hydrologic analysis should be prepared based on the criteria outlined in Appendix B of the Rules. Hydrologic calculations for the approximate method should provide for 100-year peak flow rates at a minimum.
The peak flow rates for approximate 100-year floodplain boundaries can be computed using statistical analysis, rainfall-runoff models, or regional regression equations. TOPOGRAPHIC INFORMATION The best available topographic data should be used to develop approximate floodplain information. Such base map should, at a minimum, meet the requirements outlined in Appendix A of the Rules. Field surveyed channel and floodplain cross-sections can be obtained to supplement the available topographic data as needed.
HYDRAULIC ANALYSIS The following hydraulic analysis methods or a more detailed method should be used for delineation of approximate 100-year floodplain boundaries:
- Software packages that work together with GIS and digital base mapping to form automated routines for delineating approximate 100-year floodplain boundaries. - Simplified HEC-RAS or similar computer models that reasonably represent hydraulic conditions of the stream of interest using cross-section spacing and other factors that are less rigorous than for detailed analyses.
- Calculate 100-year water depths for the cross sections that are representative of the stream reach being studied using normal-depth calculations (Manning’s Equation). It should be noted that the normal depth approximations do not incorporate backwater effects that may occur due to roadways, dams, etc.
- Published culvert rating charts (i.e., FHWA, 1985) may be used to compute headwater depths at culvert inlets. Weir equations may be used to compute approximate flow depths over roadways. - Simple computer hydraulics programs may be used to compute normal depths for the selected representative cross sections (i.e., FEMA Quick-2, Heastad Flowmaster, etc.) A sufficient amount of cross-sections should be used to adequately represent and analyze the physical features (narrows, culverts, bridges, etc.) of the stream. Normal Depth Computation Normal depth occurs for a stream section when the flow is uniform, steady, and not effected by downstream channel features (i.e., drop structures, weirs, etc.). Open-channel flow is said to be uniform if the depth of flow is the same at every section of the channel. For a given channel geometry, roughness, discharge, and slope, the only possible depth for maintaining uniform flow is the normal depth.
The computation of normal depth at a cross section can be performed using the Manning’s formula as follows:
P = Wetted perimeter (feet)
R = A/P = Hydraulic radius (feet)
S = Slope of the energy grade line (feet/feet)
For prismatic channels, the energy gradeline (EGL) slope, hydraulic gradeline (HGL) slope, and the channel bottom slope are assumed to be the same for uniform, normal depth flow conditions. Therefore, the channel bottom slope may be used for the normal depth calculation. The channel and overbank Manning’s “n” values may be estimated from field observations and using published methods. A, is the flow conveyance area below the water surface elevation. Wetted Perimeter, P, is the length of the channel/overbank along the ground surface of the cross section, below the water surface elevation.
Weir Flow Depth Computation There are two main types of weirs: sharp-crested and broad-crested. A sharp-crested weir has a sharp upstream edge. A broad-crested weir has a horizontalor nearly horizontal crest sufficiently long in the direction of flow so that the overflowing sheet of water will be supported and hydrostatic pressures will befully developed for at least a short distance. For most of roadways, the flow overtopping depths can be estimated using thefollowing broad crested weir equation:
H = Head (feet)
Weir coefficient, C, varies from approximately 2.5 to 3.1 for most broad crestedweirs, and the weir coefficients for various weir sizes are summarized in Table CH10-T201. Weir coefficient of 3.0 is reasonable for weir flow over paved roadways.
Effective horizontal length, L, is the effective width of the weir cross section,perpendicular to the direction of flow. Weir head, H, is the difference between theupstream energy grade and road crest elevations. For approximation purposes, weir head can be assumed to be equal to the difference between the upstream water surface and road crest elevations. Surveyed High Water Marks If surveyed high water marks from a previous flood event are available for the stream of interest, the high water mark elevations may be used to supplement and calibrate the computed 100-year water surface elevations or depths.
___________________________________________________ Editor’s Notes History