Underground and in-ground detention basins shall comply with the following:

1. (a) Untreated stormwater shall not be directed to unlined detention basins;
2. (b) Basins shall not be located in an area of RSA 482-A jurisdiction, unless a permit that specifically allows the detention basin has been issued pursuant to RSA 482-A;
3. (c) Underground detention basins shall have access manholes located upstream, downstream, and at intermediate locations to provide access for maintenance; and

4. (d) If the practice includes one or more in-ground detention basins, the following requirements also shall be met:

   1. (1) The side slopes of the basin shall be 2:1 or flatter;
   2. (2) The crest shall be at least 4 feet wide;
   3. (3) Any outlet structure having a 6-inch or smaller diameter orifice or a 6-inch wide or narrower weir shall have a trash rack to minimize clogging;
   4. (4) The lowest orifice shall be at least 6 inches above the basin floor;
   5. (5) Energy dissipation shall be provided at the inlet and outlet to prevent scour;

   6. (6) The detention basin shall be:

      1. a. Constructed with an emergency spillway, in accordance with Env-Wq 1507.02(d); and
      2. b. Able to discharge the 100-year, 24-hour storm without overtopping the embankment crest;
   7. (7) All areas of the detention basin, including the basin floors, side slopes, berms, impoundment structures, or other earth structures shall have vegetation suitable for the soil type, the moisture content, the amount of sun exposure, and the level of inundation to which it is exposed; and

   8. (8) If RSA 482 requires a dam permit to be obtained for the basin, then:

      1. a. The permit shall be obtained prior to construction; and
      2. b. Any conditions imposed under such permit that are more stringent than those listed above shall apply.

Source. #9343, eff 1-1-09; ss by #12342 (formerly Env-Wq 1508.16); ss by #13758, eff 9-28-23 (formerly Env-Wq 1508.17); ss by #14472, eff 1-19-26, EXPIRES: 1-19-36