For proposed projects that substantially improve residential structures in a designated floodway for the primary purpose of reducing risk of flood damage, or for all substantially damaged residential structures, other than farmhouses, located in a designated floodway, the department, at the request of the town, city, or county with land use authority over the structures, is authorized to assess the risk of harm to life and property posed by the specific conditions of the floodway. Based upon scientific analysis of depth, velocity, flood-related erosion and debris load potential, the department may exercise best professional judgment in recommending to the local permitting authority substantial improvement, repair, replacement or relocation of a residential structure. The property owner shall be responsible for submitting to local government any information necessary to complete the assessment required by this section when such information is not otherwise available.

1. (1) Recommendation to substantially improve a residential structure, or to repair or replace a substantially damaged residential structure located in the regulatory floodway shall be based on the flood characteristics at the site. In areas of the floodway that are subject to shallow and low velocity flooding, low flood-related erosion potential, and adequate flood warning time to ensure evacuation, the department may recommend the substantial improvement of the residential structure, or the replacement or repair of the damaged structure. Any existing residential structure located in the regulatory floodway in a high risk zone based on the flood characteristics will not be recommended to be substantially improved, or repaired or replaced. Flood warning times must be 12 hours or greater, except if the local government demonstrates that it has a flood warning system and/or emergency plan in operation. For purposes of this subsection flood characteristics must include:

   1. (a) Flood depths cannot exceed more than three feet; flood velocities cannot exceed more than three feet per second.

   2. (b) No evidence of flood-related erosion. Flood erosion will be determined by location of the project site in relationship to channel migration boundaries adopted by the local government. Absent channel migration boundaries, flood erosion will be determined by evidence of existing overflow channels and bank erosion.

      At the request of local government, the department will prepare a report of findings and recommendations for local government concurrence on substantial improvements, or repair or replacement of substantially damaged residential structures located in the regulatory floodway.

      Without a recommendation from the department for the substantial improvement, or repair or replacement of a substantially damaged residential structure located in the regulatory floodway, no substantial improvement, or repair or replacement is allowed per WAC 173-158-070(1).

2. (2) Before the substantial improvement, repair, replacement, or reconstruction is started, all requirements of the National Flood Insurance Program, the state requirements adopted pursuant to RCW 86.16.031(8), and all applicable local regulations must be satisfied. In addition the following conditions must be met:

   1. (a) There is no potential safe building location for the replacement residential structure on the same property outside the regulatory floodway.
   2. (b) A replacement residential structure is a residential structure built as a substitute for a previously existing residential structure of equivalent use and size.
   3. (c) Substantial improvements, repairs or reconstruction or replacement of a residential structure shall not increase the total square footage of floodway encroachment.
   4. (d) The elevation of the lowest floor of the substantially improved, or substantially damaged or replacement residential structure is a minimum of one foot higher than the base flood elevation.
   5. (e) New, improved, and replacement water supply systems are designed to eliminate or minimize infiltration of flood water into the system.
   6. (f) New, improved, and replacement sanitary sewerage systems are designed and located to eliminate or minimize infiltration of flood water into the system and discharge from the system into the flood waters.
   7. (g) All other utilities and connections to public utilities are designed, constructed, and located to eliminate or minimize flood damage.

[Statutory Authority: RCW 86.16.041. WSR 25-16-033 (Order 24-13), s 173-158-076, filed 7/29/25, effective 8/29/25. Statutory Authority: Chapter 86.16 RCW. WSR 02-15-093 (Order 00-26), § 173-158-076, filed 7/16/02, effective 8/16/02.]