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 1508.09 - Stormwater Treatment Practices: Treatment Swales

(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:

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

(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.