It's hard to believe what a flooding rain can do. I try to overengineer things myself. I have a wetland that the Indiana Department of Transportation had built to mitigate one that they destroyed. I am glad it is a mitigation because the water floods over the berm regularly and will wash out. There is no emergency spillway and it only has a 6" pipe drain. My upstream pond has a 15" drain, and even with that, the pond's emergency spillway gets used occasionally. The downstream wetland gets even more water from its watershed. The emergency spillway should really be constructed in virgin dirt. They will have to fix it when the berm washes out. Don't really think that they knew how to engineer it, but they sure did a great job with the plantings and it is wonderful for wildlife.