HEC-HMS Model

[EngineerUtah]

I have been modeling small industrial and commercial sites (larger than an acre) using the HEC-HMS software because it is required by the city in which these projects are being done. It is difficult at times to provide the required storage in the detention pond due to site constraints. I have modeled these site in two different ways based on how difficult it is going to be to provide the required storage and satisfy the city.

Scenario 1: Orifice control at detention pond. Calculate weighted CN for entire site (post development) and input that into model. Input "0" as percent impervious. This seems to be a less conservative method. I can reduce my required storage and meet the required release rates more easily.

Scenario 2: Orifice control at detention pond. Calculate weighted CN for pervious areas only (post development) and input that into model. Input area of impervious surfaces as percent impervious. This seems to be a more conservative method. It increases the required storage.

It seems obvious to me that reducing the storage is the way to go as long as it meets the requirements of the city.

Can anyone tell me if either of these scenarios is more accurate and acceptable than the other and why?

Thanks.

 

[SMIAH]

If you're using Loss Method : SCS Curve Number you have to define a initial abstraction. By default, if you don't put a "0", it will be (25400/CN-254 x 20%) in S.I. This is overly conservative in here and I often tweak it to 10% or input a number.

You should be getting the same results if you do a 1st run with a CN = 98 and initial abstraction = 0 and another run with a CN = 1 and 100% impervious,

 

[Drew08]

What is more accurate depends on the layout of the site. How to include a impervious(%) area depends on if the impervious area is directly connected to the outlet. Meaning runoff from the impervious surface has no opportunity to infiltrate, such as parking lot runoff into a culvert system. Use a composite CN that includes the impervious area if runoff from an impervious surface has an opportunity to infiltrate such as roof top runoff with gutter discharge into grass.

HMS treats the impervious(%) separately when calculating the runoff depth (Q) assuming zero infiltration. This is equivalent to CN = 100. Where Impervious% = 100% Runoff depth will equal the rainfall depth. Normal impervious surface has some puddling loss so a CN = 98 is used (not truly impervious). This accounts for some of the difference.

Here is an example assume 50% impervious watershed, and 50% grass CN = 61. And 5 inches of rainfall.

For using Composite CN:
CN = (98 x 0.5) + (61 x 0.5) = 79.5 and runoff depth = 2.85 inches

For using Impervious(%):
Pervious runoff depth = 1.37 inches (CN = 61), and impervious runoff depth = 10 inches (CN = 100: zero infiltration)
Total runoff depth = (1.37 x 0.5) + (10 x 0.5) = 5.69 inches, a pretty significant difference.

This difference becomes less though for higher rainfall depths. Approaching PMP levels, the Composite CN option actually results in more runoff depth, though just slightly.