HEC-1 Input

[johnengr]

I need to locate the PC Record(Cumulative Precipitation Time Series) for the 12 hour pmp storm event. I have the 6 hour(51points) and the 24 hour(241points) PC data. When I change the IN value to 1.5 on the 24 hours file and also the PB file for PMP it does not correlate to the 6 hour (51 point) results. Type II SCS. Location is Virginia. Thanks

 

[RWF7437]

http://www.weather.gov/oh/hdsc/

 

[johnengr]

I obtained the 6, 12 and 24 hour pmp depth from this reference, could not located any cumulative precipitation time series data.

 

[RWF7437]

You haven't said how you plan to use this information when you get it. It is very hard to answer your questions without that.

In some areas of the country storm patterns are provided by reviewing agencies if they are required for a particular purpose. These are usually based on some historic storm event and sometimes they are completely arbitrary. There is NO universal "standard" way of doing these things.

Have you checked with whatever agency will be reviewing your work?
Have you read FAQ 194-1462 ?

 

[RWF7437]

Also see thread 162-233863

"The new Virginia Impounding Structure Regulations require analysis of the 6, 12 and 24-hour storm events, although there is no guidance on how. Wide open for a little "Voodoo Hydrology".

 

[johnengr]

The state of Virginia's new Impounding Structure Regulations now require a developed PMF Hydrograph for 6, 12 and 24 hour durations with the hydrograph that creates the largest peak outflow to be used to determine capacity for nonfailure and failure analysis. To my knowledge the reviewing agency does not provide Cumulative precipitation time series for these storm events. HEC-1 with the Dam Breach Simulation is an accepted procedure. Previous regulations only neccesitated using the 6 hour duration PMP event. The 6 hour and 24 hour distribution data I am using is provided by an adjacent states dam safty program and used throughout there numerous example simulations. The structure being evaluated has a drainage area of 680 acres. Thanks for your interest.

 

[RWF7437]

"The state of Virginia's new Impounding Structure Regulations now require a developed PMF Hydrograph for 6, 12 and 24 hour durations with the hydrograph that creates the largest peak outflow to be used to determine capacity for nonfailure and failure analysis."

If PMF means Probable Maximum Flood, is that to be based on the NOAA published Probable Maximum Precipitation?

You will almost certainly find that the 24 hour runoff Volume is greater than the 12 hour Volume which is greater than the 6 hour Volume, for just about anywhere in the U.S.

You will similarly almost certainly find the the peak Inflow Rate to the impoundment for the 24 hour storm will be smaller than the 12 hour Peak Inflow which will be smaller than the 6 hour inflow.

If the impoundment does NOT "fail" structurally or suddenly then the outflow rate can be controlled and can be made to depend on the size of the outlet and the depth of water above the outlet.

The VOLUME of storage required will usually be controlled by the design storm with the greatest INFLOW VOLUME; NOT the greatest inflow rate. Of the three storm durations cited that will always be the 24 hour storm. BUT, longer duration storms can and do occur and may result in greater runoff volumes.

If the impoundment is assumed to fail then the peak outflow rate will depend mainly on the depth of water impounded. This outflow rate is almost never used to size anything. But it is often used to estimate the risk of damage to downstream property and people and to estimate the warning time available to evacuate the floodable area.


"To my knowledge the reviewing agency does not provide Cumulative precipitation time series for these storm events."

To make sense, such data, and the methods to be used, need to be specified. Otherwise, everybody is at liberty to invent their own method ?

"HEC-1 with the Dam Breach Simulation is an accepted procedure."

Did you mean HEC-RAS.... ? I've never used HEC-I but I didn'think it had a dam breach option. I also thought HEC-I had been replaced by HEC-HMS. Is HEC-I Accepted by Virginia?

"The 6 hour and 24 hour distribution data I am using is provided by an adjacent states dam safty program and used throughout there numerous example simulations."

Why would you use an adjacent State's methods in VA ?
What State's standards are you trying to meet ? What program for dam breach simulation does the adjacent State use? Does VA accept this ?

I'm in the State of Confusion now.

 

[RWF7437]

http://directives.sc.egov.usda.gov/TR_210_60_Second_Edition.pdf

 

[johnengr]

Hec-1 with Dam Breach simulation has been accepted by VA Dam Safety in the past. Input information and output analysis utilizes other calculation procedures not in HEC-1; Breach input factors are calculated from NWS Simple Dam Break and Breach Predictor equations. It is my understanding that the PC record data noted in the first posting also has been called a hyetograph which provides a cumulative rainfall record over a given time period. Thanks

 

[johnengr]

RWF7437: Thanks for the link. It will let me in but not as far as TR 210.

 

[RWF7437]

johnengr,

I had similar problems. The link was intended to lead you to the NRCS publication, Earth Dams and Reservoirs, TR-60 ( not TR-210). When you get there you are supposed to be able to download a .pdf of the publication ( 48 pages). I tried that but had no luck so, I went to my local NRCS office this morning and they downloaded it and printed it for me at no cost.

It looks, at first glance, to be very useful. In fact, you may even get the State of Virginia to adopt it if you speak very sweetly to them.

Persevere.

Russ

 

[johnengr]

Thanks: I will try to get a copy early next week.