HEC-HMS Gage weights method and SCS Storm Type II 1

[guanxt]

Hi I'm a newbie in Hec-HMS, sorry for my entry-level question.

I have a basin with 3 rainfall stations around it. I would like to use the gage weights method in meteorologic models to calculate rainfall depths for each sub-basin. But my client ask me to use SCS storm type II to distribute the rainfall.

Can I apply scs storm patten while using the gage weights method?

Thank you so much!
Tony

 

[SMIAH]

You have to define a 24-hour total rainfall depth.

Are the rainfall stations recording the rainfall depth?
Are they recording it on a hourly basis or twice a day, etc.?

If you're interested into a flow rate associated to a particular return period, you can perform a statistical analysis to relate the daily recorded rainfall depth to a particular probability. Then use a program (or Excel sheet) to generate the SCS-II (distribute it).

Or, you may want to find a station that is already giving you this total rainfall depth associated to a probability (e.g. 6 inches for 24 hours for a 25-year return period). Then you could still use your own storms (i.e. generated from the 3 stations) based on the recorded data.

Some thoughts...

 

[guanxt]

This is what I don't understand.

All the rainfall data for the 3 stations are daily data only. I do have the rainfall depth for 24-hour design from the county technical manual (isohyets).

Could you explain what you said in detail?

Thanks!!!
Tony

 

[SMIAH]

Then you could perform a statistical analysis using the recorded 24 hour rainfall data. But this is probably an overkill for your project.

Using the rainfall depth for 24-hour design from your county: write it in the Excel computation spreadsheet attached. Then you will have the 24-hour SCS-II storm to use in HEC HMS.

The format would then be "cumulative inches" in HMS.

This Excel sheet is not from me. If someone can provide the original link...

 

[SMIAH]

To answer your initial question of Can I apply scs storm pattern while using the gage weights method?[/]:

You have to define a 24-hour rainfall height.
You may want to use the maximum recorded value, the mean, the lowest, etc. Really depends on what you're looking for.

One could extract all the maximum annual recorded rainfall height. Then perform a statistical analysis using those values. Then it would depend on how many recorded years.

 

[guanxt]

Thanks for your help!!!

So now I have the 24 hours SCS II storms but how can i use it in hec-hms? Under meteorologic models, should I still use weighted station method or i have to pick some different method?

I'm asking this because I want to give a weight for each rainfall station and then using recorded date to calculate a weighted rainfall depth for each sub-basin. then enter them to hec-hms. Hopefully, this is the right approach.

Thanks!

 

[SMIAH]

You will have to use "specified hydrograph" for the SCS storm. And be sure to enable "include Subbasins".

By the Menu/Component/Time Series Data Manager/
Under "Data Type" choose "Precipitation Gage" then "new" and name it.

Then go to the left menu of HMS, browse until you find your precipitation gage. Then fill the information up (time windows, data, Table and Graph).

Hope this helps.