Grass Channels Design Criteria
Parameter Design Criteria
Manning’s n 0.15 for water quality treatment
0.15-0.03 for depths 4-12”
0.03 minimum for depths greater than 12”
Flow Velocity 1.0 fps for water quality treatment
4.0-5.0 fps for 2-year storm
7.0 fps for 10-year storm
Length length required for minimum 10
Notes:
The channel is initially designed for the water quality storm, and then checked against the larger 2 year storm to ensure a non-erosive condition. Finally, the capacity for conveyance of the 10 year frequency storm is checked and a minimum freeboard is applied. The design procedure is a rate based sizing criteria which uses Manning's equation to compute velocities and depths based on specified channel geometry and slope.