posted 10 years ago
When my swales or check-dams have broken, it's been during thunderstorm caused flash flood events. And at that time there is already so much water, mud, boulders, and vegetation flowing that a little bit more is inperceptible. The materials flow downstream until they get hung-up on some obstruction or other. That might be a few feet, or a few miles. The flow of water tends to be concentrated on the point of failure, so ravine formation is typical just below the point of failure. Stilling pools or rip-rap just below spillways really helps with structural integrity during flash flows.
So far, I have only built swales on bedrock.