developing runoff hydrograph

[calculor]

trying to determine the amount of storage required for detention pond. using CN=95 and calculated tc=5mins, design storm is based on 5 min intervals for 2 hour and 24 hour storm (based on idf curves). Should I use tc=5mins to develope my unit hydrograph or should I use a 5 min interval, which means tc would be approximately .6hrs. Since my storm interval is 5 mins, I would think this would be the same for the unit hydrograph when I convolute the effective rainfall to get the runoff hydrograph. Any suggestions?

 

[bltseattle]

Detention design methods/assumptions/criteria vary substantially from region to region. Where is this, what method are you trying to use, and is the procedure you discuss the method accepted by the local agency?

 

[calculor]

The location is eastern canada. The method I am using is the SCS unit hydrograph and yes this method is accepted by local jurisdiction. I am confused since my time of concentration is 5mins which means my time to peak for the unit hydrograph is around 3.3mins. This also means delta t would be less than 5mins. If my interval for the hyetograph is 5 mins. How would I calculate the runoff hydrograph since my hyetograph and unit hydrograph would have different intervals?

 

[sam74]

Sounds like you are trying to develop a spreadsheet or are trying calcs by hand. The software I have only allows you to input time intervals that are < 50% of the time to peak of the Unit Hydrograph. The listed values below 5 min. Tc on my software (Hydraflow Hydrographs) are 2 and 1 minute intervals. The program will not accept a 5 minute interval and a 5 minute Tc. Why don't you try downloading the HydroCAD free sampler listed at the top of the page and let your computer do the dirty work. If you have troulbe using the software I believe there is a forum devoted to it exclusively under engineering computer programs. I've never used it but most of the hydrology programs I have used are pretty similar.

 

[LeoSC]

I suggest the following document for a very useful explanation of the SCS unit hydrograph method:

http://www.info.usda.gov/CED/ftp/CED/neh630-ch10.pdf

Once you understand the method, then you can apply a computer model such as HydroCAD.

What you will find in hydrograph methods, is that the convoluted hydrograph time to peak will be very similar, if not equal, to the time to peak of the storm.

 

[ALBUD1962]

You need to use an interval similar to your TC, .1 hr or 6 minutes is common. I would download one of the widely used models,(they are free). Hec-HMS will let you model a complex system, but doing such its complicated to use. I have been using win tr55 and win tr20 to model simple watersheds. Both can be downloaded at the NCRS website.

Many designers choose the peak to be at the time of concentration.

Al