N.H. Code Admin. R. Env-Wq 1508.04

Current through Register No. 25, June 20, 2024

Section Env-Wq 1508.04 - Stormwater Treatment Practices: Stormwater Wetlands

Stormwater wetlands, including but not limited to shallow wetlands, extended detention wetlands, and pond/wetland systems but excluding gravel wetlands, shall comply with the following:

(a) The stormwater wetland design shall include a sediment forebay for pretreatment that meets the criteria specified in Env-Wq 1508.11;

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

(c) If extended detention is used, then:

(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 wetland shall be curvilinear;

(e) The side slopes of each wetland shall be no greater 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 elevation of the wetland shall be based upon:

(1) An outlet control structure at an elevation 2 feet below the SHWT; or

(2) Preparation of 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 wetland and that the wetland will not be inundated with an excess of water;

(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 wetland shall be able to discharge the 50-year, 24-hour storm without overtopping the embankment crest; and

(n) The maximum water surface elevation shall not extend more than 4 feet above the permanent pool during the 50-year, 24-hour storm;

(o) 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.

N.H. Code Admin. R. Env-Wq 1508.04

(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

Amended by Volume XXXVII Number 32, Filed August 10, 2017, Proposed by #12342, Effective 8/15/2017, Expires 8/15/2027.