Calculating Piped Spillway Capacity

[KatieTX]

I am doing a spillway analysis for a small pond/dam. The spillway is a 24-inch pipe. I understand how I would calculate the rating curve if it was a short culvert with a known tailwater (or range of tailwater levels). What if the spillway pipe extends thousands of feet before it gets to an open water body (ditch or something)? What if the spillway is infinitely long (stays in a pipe forever)? Can a program like HY-8 handle this kind of situation, or is it only for "short" culverts, and if so, how would you define "short"?

 

[elev8848]

KatieTX,

I haven't used HY-8.

Theoretically,
an infinitely long 24 inch diameter pipe, if it is steep and the discharge is below the full flow capacity, will flow partially full. It will have supercritical flow. Any tailwater, in such a case, will have no effect on the flow.

 

[cvg]

HY8 can't handle complex pipe drain configurations. It is designed to handle highway culverts. If your spillway is constructed like a typical culvert with inlet outlet and straight grade in between then HY8 can handle it. Otherwise, you will need to do it another way. For high headwater depths such as found in a dam or pond, flow through the spillway is generally under inlet control so that tailwater or pipe length, slope would have no effect on the capacity. For lower flow rates, tailwater may control your capacity. Analyze your storm drain system separately from the outfall up to the dam (using stormcad or other method) to see if the predicted hydraulic grade line is higher than what HY8 predicts.

In my opinion, connecting the spillway for a dam directly into a long storm drain is not advisable. I would discharge the spillway into a stilling basin and then let it flow into your storm drain system.