Frequency Storm

I usually use HEC-1, so I am new to HEC-HMS.

I have three questions about the meteorologic model in HEC-HMS v2.1.3. If you use the "Frequency Storm" method, you configure the storm by selecting the exceedance probability, max intensity duration, storm area, etc.

1) Storm Area (sq. mi) - The user manual says, "Enter a storm area equal to the total darinage area at the point where exceedance probability will be inferred for computed flow." Is this the total watershed area? What does this actually mean?

2) Peak Center - Can you add a user specified center of peak? If so, how?

3) I want to see a hyetograph plot of the synthetic storm HMS creates from the frequency parameters. When I ran a test using a 100-yr user specified hyetograph and a 100-yr frequency storm, I got similar flow and timing results. The test was used on a very simple basin model so I'm not quite convinced of the method of interpretation for a frequency storm. Should I be?

Much thanks, Eric Stewart

Hello Eric;

It helps that you've worked with HEC-1. Let me try:

1- Storm Area :An excerpt taken from the HMS help on 'Hypothetical Frequency Storms' follows: " The precipitation depth obtained from the NWS criteria are point values that are commonly assumed to apply to areas up to 10 square miles. For larger areas, the average precipitation over the area is less than the value for a point, and adjustments are required. When evaluating the effects of a given storm at a particular location in the watershed, the storm area used in HEC-HMS should be set equal to the total drainage area at the point of interest". So it seems that you may enter the total basin area if you're using a local DDF. Similarly, HEC-1 uses an Areal Reduction Factor,derived from the TP#40, to convert a point rainfall to the areal (=average basinwide depth).

2- Peak Center : HEC-1 uses a Storm Advancement Coef (R) of 50% (i.e., the Ratio of the time before the peak intensity to the total duration of the storm is .5 or, the peak intensity occurs in the middle of the storm) whereas in HMS you have 5 choices for the R ,viz. .25, .33, .50 ,.67 and .75. You may simply select a 'Peak Center' when you input your 'Frequency Storm'.

3- No you should not! It's very unlikely that your User Specified Hyetograph has a temporal pattern exactly like the one produced by the 'Frequency Storm' option of the HMS. HMS, just like HEC-1 uses a triangular pattern to distribute the storm depth-duration data; quite similar to what is called the Alternative Block Method (see Chow,et al,1988:Applied Hydrology,p.466) -- but for R=50% e.g.,the COE procedure places the largest increment or block @ the center ,the 2nd largest block BEFORE the largest ,the 3rd largest after the largest and so on and so forth. This is the way HMS uses the Peak Center and the depth-duration data to construct the hypothetical hyetograph. You of course can build it yourself and input your own (User Specified) synthetic hyetograph.

To summarize,the 'Frequency Storm' option in HMS is very much like to what is called the 'Hypothetical Storm' option in HEC-1 where you can use a PH card/record to define your storm event. The only apparent difference here is that HEC-1 uses a fixed (R= 50% ) but HMS gives you more choices. Hope this helps.

And I have a question for the list:

How can a HEC-1 Precipitation Depth - Drainage Area relation (which uses PH+JD or PI/PC+JD cards) can be implemented in HMS? Thank you.

Mostafa Bozorg-Zadeh Independent Consulting Engineer