Nothing beats corrugated steel. Also, "bigger is better" We live at the bottom of steep hills on three sides, and the runoff after a heavy rainstorm converges on our house with no place to go on occasion. We have had to rebuild twice in two years after being flooded nearly a foot deep in part of our house during several days of heavy rain. If the minimum culvert diameter is 15", I strongly suggest using a 24" or larger. The extra cost in both labor and materials will be minimal, compared to having to rebuild after experiencing flood damage. BTW, federal flood insurance covers things a normal homeowner's policy simply refuses to address! Buy it!
Jerry
