Thickness of the Water Sheet over the Spillway

During hurricane Sandy, in late December 2011, the question arose:How high does the water flow over the dam in an extreme rainfall case?"

That was an intersting question

When the lake fills up to maximum capacity, the water begins to flow over the concrete spillway. As more water comes into the lake the thickness of the sheet of water flowing over the spillway becomes thicker. The top of the dam is much higher than the spillway. So one way to look at the question is: How thick does this sheet get during an unusual rain event?

This begs the question: What is an unusual rain event?

Two of us did a theoretical analysis of the thickness of the sheet. We both assumed that the worst rain event is one that drops all of the rain, that we normally get in one year, in one day over the whole watershed.

The results of the analyses can be found here; by Morgan France and by Pete Versteegen , who used two very different approaches, to come to essentialy the same conclusion. The water sheet would be about 5 to 6 ft thick which is still about 8 to 7 ft lower than the crest of the dam.

The conclusion: The water would probably never reach the top of the dam!

Ideal representation of the spillway