1. (a) A gravity sewer may be smaller than 8 inches in nominal diameter only if the sewer:

   1. (1) Is constructed on a dead-end or cul-de-sac street;
   2. (2) Serves, or is planned to serve, no more than 10 residences;
   3. (3) Has a total estimated flow no greater than 2,000 gpd; and
   4. (4) Has a nominal diameter of 6 inches and a minimum pipe slope of 0.01 feet per foot.
2. (b) Sewers shall be buried to a minimum depth of 6 feet below grade in all roadway locations and to a minimum depth of 4 feet below grade in all cross-country locations.

3. (c) Sewer pipe diameter shall be determined using the following methodology:

   1. (1) New sewers up to 15 inches in diameter shall be designed to flow half full;
   2. (2) New sewers larger than 15 inches shall be designed to flow three-fourths full; or
   3. (3) An engineering analysis that considers current and future design flow information and provides commensurate reserve capacity in the sewer.
4. (d) Sewers shall be designed and constructed at such slopes as to prevent deposition of solids, with a minimum flow velocity for design purposes of 2 feet per second when flowing full.

5. (e) The minimum allowable slope shall be as set forth in Table 704-1, below:

   Table 704-1 Minimum Pipe Slope

   Nominal Pipe Diameter

   (Inches)

   Minimum Slope

   (feet/foot)

   8

   0.0040

   10

   0.0028

   12

   0.0022

   14

   0.0017

   15

   0.0015

   16

   0.0014

   18

   0.0012

   21

   0.0010

   24

   0.0008

   27

   0.0007

   30

   0.0006

   36

   0.0005

   42

   0.0004

   48 and larger

   0.0003

6. (f) The slope requirements set forth in (e), above, shall not be met by using sewers that are larger than required based on design flow calculations.
7. (g) Sewers smaller than 48 inches in nominal diameter shall be laid with straight alignment between manholes.
8. (h) When a smaller sewer joins a larger one, the invert of the larger sewer shall be lowered sufficiently to maintain the same hydraulic gradient. An approximate method which may be used for securing these results is to place the 0.8 depth point of both sewers at the same elevation.
9. (i) A reduction in the size of the outgoing sewer from a manhole shall be allowed only on sewers larger than 24-inch diameter and only if the capacity of the outgoing sewer is not exceeded.
10. (j) Where velocities greater than 10 feet per second are anticipated, the design shall incorporate measures to protect against pipe abrasion, displacement, and potential effects of hydraulic jumps. Velocities shall be calculated based on the peak hourly flow and hydraulic elements related to the depth of flow.
11. (k) Sewers on 15 percent slopes or greater shall be securely anchored.
12. (l) Sewers crossing streams or located within 10 feet of a stream embankment shall be protected against erosion.

Source. (See Revision Note at chapter heading for Env-Wq 700) #8590, eff 3-25-06; ss by #10693, eff 10-15-14; ss by #14258, eff 7-1-25, EXPIRES: 7-1-35