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Riprap design at toe of Concrete Spillway

Riprap design at toe of Concrete Spillway

Riprap design at toe of Concrete Spillway

(OP)
Hello everyone,

I'm looking for some guidance on the design of spillways, with riprap at the toe of an embankment. This is specifically pertaining to the riprap.

Background
The project has 2 ponds (pond 1 for irrigation, pond 2 for mitigation), which are connected by a spillway. Since the only means of conveyance to the 2nd basin is this spillway, this spillway both acts as the primary and auxiliary spillway. The first basin is used to collect storm water, which is redistributed through out the project for the purpose of irrigation. This means that my controlling flow rate is the unmitigated 100-year storm, since I'm assuming no storage is provided by this basin. Basin 1 is existing and Basin 2 is proposed. The embankment slope is 3:1, with a vertical difference between the upstream spillway crest basin 2 is 14 ft. The incoming flow is 920 cfs.


What I've been looking for is guidance/equations on designing the riprap at the toe of the spillway. I understand that riprap/rock sizing is very specific to the application, but I can't find literature that specifically deals with this situation. The closest equation/literature relevant to this situation was the "Rock Chutes" by published by ASAE "Design of Rock chutes", however, that appears to be the design procedure for a situation where the spillway/chute is not concrete lined, rather, rock lined all the way to the toe. I also figured I could use shields diagram by determining the shear stress at the toe of the spillway, perform iteration on different rock sizes until the critical shear stress of the rock is greater than the shear stress at the toe from the incoming storm water. I considered using Hec-14 Riprap apron for culverts (equation 10.4), but I feel that also is not correct.


There is so much literature on the subject of rock sizing for erosion and I feel like I may be overthinking this.


RE: Riprap design at toe of Concrete Spillway

US Corps of Engineers have loads of stuff in their references. Likely US Forest Conservation (?) Service have that also. Try US Navy Design Manuals, available via internet. Search for NAVFAC DM# MY old DM7 has some. Newer numbes may be different.

RE: Riprap design at toe of Concrete Spillway

(OP)
Oldestguy & cvg, thanks for the input. These are good resources.

Regarding stilling basins, my understanding is that these are for larger dams/basins. I've seen in many design plans that a simple use of riprap at the end of the spillway is used. When is it appropriate to transition from using riprap, where i'd determine the D50 and apron dimensions, to a stilling basin? I would imagine a stilling basin to be more expensive right?

and regarding sizing the riprap apron and d50 for this scenario (Spillway with riprap energy dissipator) I'm still sieving through the resources that shows how to do this.

RE: Riprap design at toe of Concrete Spillway

EM25, section 11 has a good chart
I think your key is to provide sufficient tailwater to cause a jump and get the flow subcritical

RE: Riprap design at toe of Concrete Spillway

NRCS has great information on riprap chutes.

RE: Riprap design at toe of Concrete Spillway

Recommend looking into

hydraulic design of stilling basins and energy dissipators

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