Section Env-Wq 704.04 - Details of Design and Construction of Gravity Sewers(a) A gravity sewer may be smaller than 8 inches in nominal diameter only if the sewer: (1) Is constructed on a dead-end or cul-de-sac street;(2) Serves, or is planned to serve, no more than 10 residences;(3) Has a total estimated flow no greater than 2,000 gpd; and(4) Has a nominal diameter of 6 inches and a minimum pipe slope of 0.01 feet per foot.(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.(c) 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.(d) 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 |
(e) The slope requirements set forth in (d), above, shall not be met by using sewers that are larger than required based on design flow calculations.(f) Sewers smaller than 48 inches in nominal diameter shall be laid with straight alignment between manholes.(g) 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.(h) 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.(i) Where velocities greater than 10 feet per second are anticipated, the design shall incorporate pipe anchors on steep sections and eliminate the potential for hydraulic jumps. Velocities shall be calculated based on the peak hourly flow and hydraulic elements related to the depth of flow.(j) Sewers on 15 percent slopes or greater shall be securely anchored.(k) Sewers crossing streams or located within 10 feet of a stream embankment shall be protected against erosion.N.H. Code Admin. R. Env-Wq 704.04
(See Revision Note at chapter heading for Env-Wq 700) #8590, eff 3-25-06
Amended byVolume XXXIV Number 46, Filed November 13, 2014, Proposed by #10693, Effective 10/15/2014, Expires10/15/2024.