N.H. Code Admin. R. Env-Wq 1508.08

Current through Register No. 25, June 20, 2024

Section Env-Wq 1508.08 - Stormwater Treatment Practices: Treatment Swales

Treatment swales shall be used only as follows:

(a) Swales shall not be used in any of the areas identified in Env-Wq 1507.02(c)(1)-(4) unless the practice has an impermeable liner;

(b) The swale length shall be at least 100 feet long;

(c) Any portion of the swale that is in a roadside ditch which collects runoff from the adjacent roadway surface shall not count towards the minimum length specified in (c), above;

(d) The bottom of the swale shall be no more than 8 feet wide, provided that widths up to 16 feet shall be allowed if a dividing berm or structure is used such that neither channel is more than 8 feet wide;

(e) The bottom of the swale shall not be within the SHWT;

(f) The swale side slopes shall be no steeper than 3:1 and no flatter than 20:1;

(g) The swale shall have a longitudinal slope between 0.5% and 2% without check dams or 2% to 5% with check dams;

(h) The maximum flow depth in the swale shall be:

(1) Four inches at the WQF; or

(2) If a detention structure is used immediately upstream of the treatment swale, the maximum flow depth in the swale shall be 4 inches at the 2-year, 24-hour post-development peak flow rate;

(i) The swale's hydraulic residence time shall be greater than 10 minutes for:

(1) The WQF; or

(2) The 2-year, 24-hour post-development peak flow rate routed through a detention structure;

(j) The swale shall be sized to discharge the 10-year, 24-hour storm; and

(k) The swale shall have at least 85% vegetated growth prior to directing runoff to it.

N.H. Code Admin. R. Env-Wq 1508.08

(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.