Dry and Wet Swales Design Criteria
2:1 maximum, 3:1 preferred
Notes:
The design approach for sizing dry and wet swales is based on temporarily storing the WQv within a shallow ponding area. This methodology incorporates a volume based sizing criteria for the WQv, and a rate based criteria for checking the erosive potential for the 2 year frequency storm and capacity for the 10 year frequency storm.
The swales should generally be trapezoidal in shape, although a parabolic shape is also acceptable (provided the underlying soil bed design width, for dry swales, is equal to or greater than, the design bottom width for a trapezoidal cross section). Size the bottom width between two and eight feet. The two feet minimum allows for construction considerations and ensures a minimum filtering surface for water quality treatment. The eight feet maximum reduces the likelihood of flow channelization within a portion of the bottom of the swale. The side slopes of the channel should be no steeper than 2:1 for maintenance considerations (mowing). The slope of the swale should be moderately flat to permit the temporary ponding of the WQv within the channel without having excessively deep water at the downstream end. A slope between 1.0% and 2.0% is recommended.