HEC-HMS and the SCS Type II Storm

[raptorboy]

Hi,
I'm a new user of HEC-HMS and I have been trying to model a basin using an SCS Type II distribution curve. The problem is, the agency requires that I use a 6-hour distribution curve for teh SCS hydrograph instead of the 24-hour distribution curve that is built into the program. I can't seem to find a way to set up my model for a 6-hour SCS Type distribution curve. Is this possible using HEC-HMS or am I out of luck?

 

[psmart]

The standard SCS rainfalls are defined primarily for 24-hour duration. There are also some less-used 48-hour versions, but as far as I know, there aren't any standard SCS storms of 6-hour duration. Your "agency" may be using a local version of the Type II storm, in which case the details (ie mass curve data) should be in their stormwater regulations.

 

[queque]

I FACEDTHE SAME PROBLEM HERE IN MARICOPA COUNTY, AZ.
psmart showed me how to load the County's 6 hour distribution data into HydroCAD. I used both distribtuions and the results indicated a higher paek was obtained with the 24 hour distribtion. I presented both results and used the 24 hour storm results for culvert design.

 

[psmart]

Compresing a 24-hour storm into 6 hours will increase the peak intensity four-fold, so this generally isn't the right solution. A better way to do the conversion would be to take the middle 6 hours out of the 24-hour storm. But the best solution is to use the rainfall table specified by your agency.

 

[cvg]

Maricopa county doesn't compress the storm as you indicate, but has a separate distribution for both the 6- and 24-hour storms. For example, a 100-year, 6-hour depth might be 3.00 inches of rain and a 100-year, 24-hour would be 3.60 inches and each has a different distribution. The 24-hour storm is an SCS type 2 and the 6-hour is a custom distribution specific to Phoenix. The distributions are specified in the county drainage manual. Typically, for smaller watersheds (under 20 sq mi) the 6-hour storm will govern, however, for larger watersheds, both are typically used and the one with the highest peak is used. For retention facilities, a 2-hour storm pattern is used.

 

[blueoak]

The type II is a 24 hour distribution. NRCS has a 6 hour that is typically for spillway design. This is called the Type B. It is pretty easy to add as a time series though most NRCS pubs has in 7.2 minute increments so you have to convert. Just a heads up for any New Mexicans, the NRCS uses a Type IIA after a bunch of dam failures. There are 4 different Type IIA distributions.

The best thing in HMS for these little storms (less than a 500 or 100 yr event) is using the frequency method and then entering in the data from NOAA Atlas 14 if you are lucky to have it done for your state.

I have to say that at least you ask, I see too much where people abuse NRCS methods than complain about the results.