Slide 26 of 43
Notes:
The following criteria should be considered when sizing grass channels for stormwater quality treatment, while also accommodating larger storms.
The channel should be trapezoidal or parabolic in shape. The trapezoidal cross section is the easiest to construct and a more efficient hydraulic configuration. For a trapezoidal cross section, size the bottom width between two and six feet. The two feet minimum allows for construction considerations and ensures a minimum filtering surface for water quality treatment. The six feet maximum prevents shallow flows from concentrating and potentially gullying, thereby maximizing the filtering by grass blades. The sides slopes should be flat as possible to aid in providing pretreatment for lateral incoming flows and to maximize the channel filtering surface. The slope of the channel should be steep enough to ensure uniform flow and which can be constructed using conventional construction equipment without ponding, but not steeper than 4.0%. A minimum slope of 1.0% is recommended. The maximum flow depth for water quality treatment should be approximately the same as the height of the grass. Since most channels will be mowed relatively infrequently the vegetation may reach heights of 6" or more. However, since higher grass is prone to fallover during higher flows, a maximum flow depth of 4" is required for water quality design. The flow depth for the 2 year and 10 year storms will depend on the flow rate and channel geometry.