Bio-swale

Technical Standards for Bio-swales

1.. Select sites that are nearly level along the edge of parking lots, roadways, walkway, open spaces  or athletic fields. 

2. Slopes shall not exceed two (2) percent slope. The minimum length in the direction of flow  must be 100 feet. LD 

3. Minimum width shall be thirty-six (36) inches maximum width shall be sixty (60) inches.

4. Bio-swales must have dense grass or vegetation coverage to reduce erosion potential. Periodically, thin out vegetation to a minimum of 35% density as a means of keeping vegetation healthy.

5. Three different plant types must be included in the design. They shall be of different heights and composed of  grasses, sedges,  rushes or  evergreen broad leaved herbaceous material. Suggested species include iris, elephant ear, black rush, pickerel weed, alligator weed, delta duck potato, cut grass or switch grass. Colonies of plants are preferred over spot planted vegetation so  so no more than five (5) species of plants shall be used per  one hundred (100) linear feet of swale.                                         

6. Maximum depth of flow for 1-inch, 6-hour storm event is 4inches. Alignment may be straight but curvilinear is preferred to control slope and flow.

7. Bio-swales shall have a trapezoidal cross-sectional shape with slide slopes flatter than 3:1 (h:v) and a minimum bottom width of two (2) feet  to pass the calculated design flow from an articulated drainage basin not to exceed 5000 sf.    

8. Bio-swales must be designed to pass the 10-yr, 25-yr and 100-yr storm event without causing erosion or flood damage problems.

9. A permeable filter media and under drain  system may be provided in locations of heavy clay soils.

10.. Maximum ponding duration is 48hours and minimum ponding duration is 30 minutes. Weirs may be provided to allow for increased settling and infiltration time.  

11. The slope of the swale must be 2% or less with an under drain. Check dams may be used to produce an effective slope of 2% or less.

12. Drainage area must be a maximum of one (1) acre with no dry weather flow. Minimum area shall be five thousand (5000) sf.