72-Hour Rainfall Distribution
72-Hour Rainfall Distribution
(OP)
I will likely soon be asked to analyze a basin and spillway for the 72-hour 1/2 PMF. I can easily find the rainfall amount for that storm, but when it comes to modeling the runoff, where can I find (or create) the 72-hour rainfall distribution for my region (central Virginia)?
The drainage basin is about 800 acres and I am using PondPack (grrrr...) My Tc is less than 2 hours, and I think the 72-hour storm is excessive, but this 72-hour analysis may be the rule in the future. For the 24-hour storm, I am using the NRCS Type II storm event. I was thinking perhaps to use that as a basis somehow to interpolate by adding time intervals or something like that.
Any ideas?
The drainage basin is about 800 acres and I am using PondPack (grrrr...) My Tc is less than 2 hours, and I think the 72-hour storm is excessive, but this 72-hour analysis may be the rule in the future. For the 24-hour storm, I am using the NRCS Type II storm event. I was thinking perhaps to use that as a basis somehow to interpolate by adding time intervals or something like that.
Any ideas?





RE: 72-Hour Rainfall Distribution
RE: 72-Hour Rainfall Distribution
What is HMR 51?
Where would one find a copy of it ?
I'll Google it but a direct link would help.
rwf7437
RE: 72-Hour Rainfall Distribution
HMR 51
Probable Maximum Precipitation Estimates, United States East of 105th Meridian
HMR 52
Application of Probable Maximum Precipitation Estimates - United States East of 105th Meridian
RE: 72-Hour Rainfall Distribution
I would double check your Tc also, it seems high.
I believe HMR-51 nests PMP events for different durations(though cvg seems familiar with it)
Another good source for distributing the rainfall is the Bureau of Reclamation's Flood Hydrology Manual by Cudworth. A lot of of it is in Design of Small Dams by the Bureau, which is probably the best book I have ever read and relied on. The Bureau typically uses a late weighted scheme that for a single basin should give you the greatest discharge flow rate. The COE uses a center weight. (Like cvg says your dam safety regulator should let you know what they allow).
One thing to note is that for stability analysis it is sometimes unconservative to peak your event too much. It is sometimes good to flatten the hyetograph. Most PMP distributions are concerned with determing maximum capacity and may not be appropriate for stability analysis.
RE: 72-Hour Rainfall Distribution
My Tc is just a shade over an hour. I mentioned the Tc because a 72-hour storm analysis seems excessive if the Tc is so short. The dam in question impounds a reasonably small amount of water and is not connected to an extensive river system which could potentially take >24 hours to see peak flows.
The dam safety regulations are intentionally vague and it even says right in the text that "...rigid design flood criteria or standards is not intended." Oh great. Thankfully they are under technical review right now and I hope they get better. Perhaps this is a topic for another day.
I will look into the Design of Small Dams book. I agree it is a wonderful reference.
Thanks to all for the advice.
RE: 72-Hour Rainfall Distribution
For those interested, for my particular application, the storms in ascending order of runoff were: 12-hr, 6-hr, then 24-hr being largest. Storm was 1/2 PMP.
RE: 72-Hour Rainfall Distribution
If someone knows where the 1/2 comes from I would love to know.
RE: 72-Hour Rainfall Distribution
My thought was that for such a small basin, I could use the PMP from HMR 51 (figure 20) for 24hr for a 10 sq mi basin and use a 24-yr hyetograph for distribution and use the SCS TR-55 to get generate a hydrogaph.
I know I'm doing something wrong/stupid so any insight would be appreciated.
RE: 72-Hour Rainfall Distribution
RE: 72-Hour Rainfall Distribution
I would use the hyetograph procedure as shown in the HMR. If you use HEC-HMS at least for the hydrograph generation if not for the pond. You can use the SCS methods as desired in HMS but still run the meteorology like the HMR. The SCS stuff sometimes have their hyetograph distributions built into their charts and software.
I cited it earlier, but for large flood events on isolated sheds the Bureau of Reclamation's Flood Hydrology Manual by Cudworth is really a great resource.
RE: 72-Hour Rainfall Distribution
if you are designing a dam - and I assume you are since you are considering using the PMP, it might be beneficial to meet with your state dam safety engineer to determine what the requirements for hydrology will be. In virginia, you can check on the web at:
http://www.dcr.virginia.gov
http:
RE: 72-Hour Rainfall Distribution
cvg makes a good point. It might be even better to find out the criteria for a "dam" and stay below it. I remember using an internal 24.9' high criteria for designing dams that the state was happy with since they looked at 25' and higher. Of course camber almost got me in trouble.