New Hampshire Code of Administrative Rules
Env - Department of Environmental Services
Subtitle Env-Wq - Water Quality and Quantity Programs
Chapter Env-Wq 1500 - ALTERATION OF TERRAIN
Part Env-Wq 1508 - PERMANENT METHODS FOR PROTECTING WATER QUALITY
Section Env-Wq 1508.03 - Stormwater Treatment Practices: Stormwater Ponds

Universal Citation: NH Admin Rules Env-Wq 1508.03

Current through Register No. 13, March 27, 2025

Stormwater ponds, including but not limited to micropool extended detention ponds, wet ponds, wet extended detention ponds, multiple pond systems, and pocket ponds, shall comply with the following:

(a) The stormwater pond design shall include a sediment forebay for pretreatment that meets the criteria specified in Env-Wq 1508.12;

(b) Stormwater ponds shall have a permanent pool, or combination of permanent pool and extended detention, greater than or equal to the WQV;

(1) The extended detention volume shall:

a. Not comprise more than 50% of the WQV; and

b. Have a minimum 24-hour drawdown; and

(2) The outlet shall discharge at a maximum flow rate of twice the average flow rate, where the average flow rate is calculated as the extended detention volume divided by 24 hours;

(d) The perimeter of each pond shall be curvilinear;

(e) The side slopes of the pond shall be no steeper than 3:1 and no flatter than 20:1;

(f) The minimum length to width ratio shall be 3:1, where:

(1) Length is measured along the flow path between the inlet and outlet at mid-depth; and

(2) Width is computed by summing the average top width and the average bottom width and dividing by 2;

(g) The permanent pool depth shall be:

(1) Not less than 3 feet; and

(2) Demonstrated by providing:

a. A stormwater pond having a pond floor at least 5 feet below the SHWT or the lowest elevation pond outlet, whichever is lower; or

b. A hydrologic budget that accounts for the inflow to, outflow from, and storage in the stormwater pond, showing that sufficient water is available to maintain the water depth in the permanent pool;

(h) The permanent pool depth shall not be greater than 8 feet;

(i) The inlet and outlet shall be located as far apart as possible;

(j) Energy dissipation shall be provided at the inlet and outlet to prevent scour;

(k) 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;

(l) If elevations allow, a manually-controlled drain shall be provided to dewater the pond over a 24-hour period;

(m) The stormwater pond shall be able to discharge the 50-year, 24-hour storm without overtopping the embankment crest; and

(n) A planting plan that does not include any invasive species and that replicates the spatial and compositional diversity of a natural wetland shall be developed by an individual having knowledge of wetlands ecosystems and, in particular, wetlands plant species.

(See Revision Note #1 at chapter heading for Env-Wq 1500) #9343, eff 1-1-09; (See Revision Note #2 at chapter heading for Env-Wq 1500) amd by #9817-A, eff 12-1-10

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