Vegetated Swale Slope

[Sethd]

Hi all,

I'm working on a site that needs stormwater detention and treatment. What I need help with is the swale. What is the minimum slope of the swale? I have to follow certain criteria, so it would be nice if I could make the swale nearly flat. The city has hydrology design requirements to meet: 9 min hydraulic residence, water quality velocity of 0.9 fps and water quality depth of 4 inches, and conveyance storm depth of 12" inches.

Thanks!

 

[1503-44]

Usually not very steep if covered only with veggies. 1 rise to 2 run is probably max maybe 1:1.5 Anything more usually needs stone cover, or a gabion mattress.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[TerryScan]

Sethd,

I would first be sure to double check the City requirements for a published minimum slope. I would then ask the City reviewer directly. The requirement can vary from one regulatory entity and one geographical area to another, so you could get answers here that won't satisfy your reviewer.

I design primarily in a coastal plain where we often use very flat slopes in channels. We use a minimum of 0.5% slope and using that minimum can result in issues with the ability of contractors to construct with positive drainage. Also, it is easy to cause ponding in the swale post construction with minor disturbances (such as tire ruts from maintenance/mowing equipment with damp soil). For these reasons, when possible,we try to go with 1.0% min.