Sizing Rip Rap Down stream of large dam spillway

[mbherlihy]

Working in Georgia. I've been asked to size a rip rap channel downstream of a large 16' tall concrete principle spillway. Spillway is the primary outlet control to a large golf course lake. Common practice for our company, during civil site work, for outflowing storm pipes is to use a Rip Rap (St) nomograph (see attached). My problem is that I have been informed by the dam design engineer that their Q100 is approx 30000 cfs with a Velocity (100 yr) of approx 9 fps. I cannot read from my attached chart with that high of a Q, let alone,... I'm not confident this is the best resource to pull from. This is traditionally for sizing rip rap pads for storm pipes with lower Q's. Can anyone help? Spillway's concrete chute is 25' wide. I'm not sure why the dam engineers did not spec this during their design.... but that is my dillema

 

[msquared48]

Is there a plunge pool, and, if so, how deep is it? If it is big and deep enough, much of the energy will be dissipated here instead of on the riprap.

Do you have the book "Design of Small Dams"?

Mike McCann
MMC Engineering

 

[cvg]

9 fps is on the high side for a riprap lined channel, but can be done. concrete might be better unless you can slow down the velocity. For the reach near the stilling basin with 9 fps, you might try USBR EM 25 (see attached file) for sizing the rock. For rock on channel side slopes, you might want to increase the size. And make sure it is angular rock, not rounded.

 

[gbam]

Are you trying to size rock for the stilling basin? You may want to review FHWA's HEC-14. For riprap revetments FHWA's HEC-11.

 

[civilperson]

The 30000 cfs is a flow 133 feet high at 9 fps and 25 feet wide. I suspect a arithmetic error or the emergency spillway will also be flooded. Three man stones will be carried away in this torrent unless grouted solid.

 

[mbherlihy]

It is 3,000 cfs, typo on my part.. but I still think the water level is too high.... I will consult with the client and my collegues on a stilling basin...my initial thoughts are that due to the topography, the gentle and flat nature of exiting water (and aesthetics of a golf course) that the client is not going to want to construct this. Any other leads or thoughts?

 

[gbam]

Have you considered gabions/wire tied riprap? It can be vegetated at least above the water line.

You can design riprap based on incipient motion and shear?

 

[cvg]

9 fps sounds low for a spillway chute flow and I assume that this is the velocity after leaving the stilling basin. If this is the calculated exit velocity from the chute, than it should be re-checked.

 

[Takazviona]

Your explanation of the question is not clear. If 30000cfs flows at approx 9 fps through a rip-rap 25', the water level will be about 20' high, using mannings formula??? Something is not right with you question.

You did not mention something about the slope of the rip-rap. The chart you attached is not the right one.

First sort out the allowable velocity using slope and flow capacity before you start to select stone sizes.

 

[mbherlihy]

I have since rechecked some of my initial assumptions. First the 1/3 pmp velocity is 9.5 fps. The height calculates to be approx 9 ft (100 year).

Another consulting engineer recommends slurrying the rip rap and sizing it to larger than 24" dia stone.

Where do I find a reference to specifying stone that large?

Where can I get familiar, enough to specify...slurry operations. I know what it is, but apart from that...?

 

[Prepakt1]

Basically you are just grouting the rip-rap to form a concrete. Should not be a problem to do ,provided its done in the dry. Probably best to use something like 8000 psi adhesive type grout and cover the rock 2/3 of the way to ensure a good anchoage of the rock. Leave the rest exposed to slow the flow. If wet its a different problem entirely as its difficult to grout in flowing water.
You would have to divert and do in sections.

Intrusion Prepakt /marineconcrete.com

 

[RWF7437]

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14appd.cfm