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Culvert with side channel and spillway

Culvert with side channel and spillway

Culvert with side channel and spillway

The design consists of a culvert and a spillway that function as one structure.  The 300 lf, 72” diameter RCP culvert is designed to convey the 2-10yr storms with the overflow spillway channel designed to provide additional capacity for larger storms, 25yr-100yr.  The overflow channel uses the top of the culvert headwall as the spillway elevation and stretches downstream for approximately 290 ft at 1% slope adjacent the culvert alignment.  The overflow channel then transitions to match existing grade just downstream of the culvert outlet via a 25% slope spillway (approx. 8’ drop in 32’).

Looking at the flow over the spillway aside from the culvert (430 cfs for the 100 yr storm), the flow will transition down the spillway from sub- to super- and back to sub-critical flow while creating a hydraulic jump at the bottom of the spillway.   

When designing the spillway or the stilling basin, should we take into consideration the tailwater condition from the culvert?  Or do we conservatively design it as if there were no tailwater condition at the bottom of the spillway?   

Are there any typical details or guidelines for a spillway or stilling basin regarding concrete thickness, structural reinforcement, etc., beside the USBR?

Are there other appropriate spillway alternatives other than concrete that we could consider?  (Gabions, matting, etc

RE: Culvert with side channel and spillway

unless the pipe is totally plugged, you will have tailwater.  In addition, once the spillway begins to flow, the 430 cfs will create it's own tailwater.  Is there a trashrack on the pipe?  Is it sized to allow full flow even when 50% plugged?  If not, then maybe you could consider reduced flow through the pipe.

RE: Culvert with side channel and spillway

I like assuming tailwater too.  Unless there is a history of plugging sometimes you can just keep adding conservative assumptions up.

The NRCS has an SAF basin and a lot of smaller designs which would fit your situation.  I like the USBR manual though for 30 fps and higher.

With only any 8' drop a sometimes cheap method in humid areas I have seen work well is a grassed block structure. Install either filter fabric or graded filter with concrete blocks over it.  Fill in with sand and seed. The NRCS has some references.  Plus it can double as an access point.
Illinois NRCS has some design software as well:

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