Flow rate if Oroville Dam aux spillway fails?

[Steve R]

Greetings,

I work on river issues in California. I am trying to figure out how to estimate the potential flow rate if the Oroville Dam auxiliary spillway fails. Can you help?

worst case scenario - the whole spillway fails, creating a 25' x 1450' opening, with 25' of head at first, 25 square mile reservoir. what is the max flow rate out of the dam if failed completely.

more likely scenario - the spillway fails partially, creating ~ 10' x 250' opening, with 10' head at first, 25 sq mi reservoir.

Can someone help with an equation to estimate flow?

 

[GeoPaveTraffic]

You need to run a dam break analysis.

Should be one in the EAP for the dam.

Mike Lambert

 

[Steve R]

Thank you Mike. I will look for it in the records. Steve

 

[psmart]

The limiting case would be an instantaneous failure, which could be estimated by the basic weir equation. In reality, it would take some time for the breach to develop, so the actual flow would be somewhat less. A dam breach/break analysis would be necessary to evaluate a progressive failure.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[GeoPaveTraffic]

Also, in looking at some of the photos and video from today; it appears that the failure would most likely occur through the right abutment. I doubt there is a dam break analysis for that case that has been ran. However, a dam break analysis of the embankment would be more conservative than one through the abutment.

All in all, looks like it is going to be hard to save the existing spillways and if the erosion starts moving toward the embankment, the whole thing could go. Quite a mess, will be interesting to see why they think the spillway slab failed.

Mike Lambert

 

[cvg]

the dam will not fail. The headcut will proceed back to the control section and according to data I have seen that would be 30 feet or so below the maximum elevation of the reservoir. The width of that headcut would gradually widen, depending on the erodibility of the ground.

If you want a rough estimate, just assume a broadcrested weir.

weir coefficient ~ 2.8

typical case
width 250 feet
head 10 feet
Discharge is approx 22,000 cfs

worse case
width 1400 feet
head 30 feet
Discharge is approx 640,000 cfs

 

[Steve R]

Peter,

That sounds right. I did you the weir equation to get a ballpark of absolute worst case scenario - over 400,000 cfs. I think it is more reasonable to think worst likely case is more like 300,000 cfs - which is still double the capacity of the downstream levees.

Thanks, Steve

 

[Steve R]

CVG,

Thank you for those estimates. 640,000 cfs is a frightening number. Fortunately virtually impossible for the conditions to set up that way. Half of that is still a devastating flow for the region - likely inundate well over 150 sq miles, tens of thousands of homes.

Thanks again.

 

[cvg]

I did not do any failure analysis since I have no data. But, obviously the water level would drop rapidly before a full spillway head-cut breach would develop, so the maximum rate would never happen. And assuming there is some bedrock in there, the head-cut might not ever develop fully as assumed. Also, the weir equation might be overly conservative, there could be some tail-water effects. Either way, it illustrates the magnitude which could be enormous. The inundation would need to be mapped with 2-dimensional analysis. Something like FLO-2D might be used to evaluate the actual inundation limits.

 

[Steve R]

That has been my assumption about the failure too. I haven't seen any info on the underlying geology, but at some point there has to be some bedrock below - it is a ridge after all.