It should only slow the water leaving until the water depth gets above the peaks of the rip-rap. Then water will flow at full rate with no restriction.
Whatever a person does, either concrete, rip-rap or even grass, it is important to have it go up the bank spillway area high enough to account for any eventual potential water flow level.
If a person only has rock lining across the bottom of the overflow area, it will protect fine at normal flow rates. But if excessive flow rates happen where water is above the area where the rocks (or concrete) are covering the soil there could be erosion at that point (on each side of the rip-rap or concrete) and still have dam failure. The lining material has to extend up beyond any point the water depth could potentially get to for full protection.
Also the rock size has to be such that the velocity of water can not wash the rocks down stream. Wider overflow areas with higher flow capacity at lower velocities would be better than narrow overflows with high velocity. Something like concrete that can't move might be a better choice if the only option is a relative narrow overflow area that could see high velocity water flow.
No expert. Just what I see in my minds eye and experience with farm machinery crossings in creeks.
Last edited by snrub; 05/29/15 11:08 AM.
