Treatment swales shall be used only as follows:

1. (a) Swales shall not be used in any of the areas identified in Env-Wq 1507.02 unless the practice has an impermeable liner;
2. (b) The swale length shall be at least 100 feet long;
3. (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 (b), above;
4. (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;
5. (e) The bottom of the swale shall not be within the SHWT;
6. (f) The swale side slopes shall be no steeper than 3:1 and no flatter than 20:1;
7. (g) The swale shall have a longitudinal slope between 0.5% and 2% without check dams or 2% to 5% with check dams;

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

   1. (1) Four inches at the WQF; or
   2. (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;

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

   1. (1) The WQF; or
   2. (2) The 2-year, 24-hour post-development peak flow rate routed through a detention structure;
10. (j) The swale shall be sized to discharge the 10-year, 24-hour storm; and
11. (k) The swale shall have at least 85% vegetated growth prior to directing runoff to it.

Source. #9343, eff 1-1-09; amd by #9817-A, eff 12-1-10; ss by #12342 (formerly Env-Wq 1508.07); ss by #13758, eff 9-28-23 (formerly Env-Wq 1508.08); ss by #14472, eff 1-19-26, EXPIRES: 1-19-36