Grassed Swale

Technical Standards for Grassed 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.

3. Swales must have dense grass coverage or vegetation with permanent reinforcement to reduce erosion potential. Vegetative flow diffusers should be located on all down slope edges.

4. Grassed swales are generally planted with warm season turf grass or pasture grass that can be mowed on a monthly schedule during the April to October growing season. Suggested species include Pennsacola bahia, centipede or improved Bermuda or panicum switch grass.  The swale shall be planted with only one species of grass that may or may not be the same species used for near by grass lawns.  

5. Maximum depth of flow for the 1-inch, 6-hour storm event is 4inches.

6. Grassed swales shall have a trapezoidal cross-sectional shape with slide slopes flatter than 3:1 (h:v) and a minimum bottom width of  four (4)  feet and a minimum top width of ten (10) feet.

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

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

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

10. 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. 

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

12. The minimum length in the direction of flow must be 100 feet.

13. Alignment should be curvilinear to control slope, flow rate and blend in successfully with other site features such as lawns, open spaces, preserved forests or remnant wetlands.