HEC-HMS: Outflow and effects of Intensity Duration and Time Interval

[froude]

Fellow forum users:

A little background to my question -

I just developed "existing condition" flows for a portion of a watershed that was previously studied during a FEMA FIS study. I calculated peak outflows for the 10-,50-,100-, and 500-year return events. When comparing my flows to the FIS flows, I noticed that my flows were higher. This is expected as there has been development in the watershed since the FIS study. However, the percent difference in flows increases for each return event (i.e. my calculated 10-year flows are only 5% more than the FIS, but the increase is 13%, 18%, and 28% for the other 3 events). I should mention that I am using Frequency Storms based on data developed by a local jurisdiction.

Questions -

A. Why is the percent difference in increase not constant (or, is there a flaw in my assumption that the percent difference should not change?)

B. If the percent difference should not change, could it be due to the Intensity Duration and Time Interval specified in HEC-HMS? Should the Intensity Duration and Time Interval be changed for the higher return-events? I understand that the time interval is dependent on lag-times...but I'm curious as to the sensitivity of hydrologic models on these paramters.

Any insight would be appreciated!

 

[RWF7437]

"I should mention that I am using Frequency Storms based on data developed by a local jurisdiction."

Be careful. First, find our what method was used by FEMAs contractor to arrive at the flows used in the Flood Study. It is probable they used regional regression equations, BASED ON STREAMFLOW DATA. I don't know what 'Frequency Storms developed by local jurisdiction' are.

"Why is the percent difference in increase not constant (or, is there a flaw in my assumption that the percent difference should not change?)"

First, the differences you are finding are not large by hydrology standards ( Voodoo Hydrology). Plus/minus 50% could be considered "good" agreement.
Second, I don't know of any reason to expect these differences to be exactly proportional.

Third, flows developed in HEC-HMS are not likely to be the same as the flows used by FEMA. Differences found can be the result of a multitude of factors. It is very unlikely that changes in land use or development in the watershed.

It would help to know Why you are doing this dangerous comparison.

 

[RWF7437]

Please see FAQ 194-1462

 

[RWF7437]

http://www.stormh2o.com/july-august-2006/urban-hydrology-methods.aspx

 

[froude]

I was not aware that inquisitively comparing numbers was dangerous. I will cease at once!

 

[RWF7437]

Correction:

"It is very unlikely that changes in land use or development in the watershed"......are the reasons you are finding differences.

"It would help to know Why you are doing this dangerous comparison"

Interpret the word "dangerous" in the same way as you would "truthiness".