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1
- #1
delibo
Civil/Environmental
- Jun 13, 2012
- 1
In developing a PMF, previously in Hec-1 you would have used the Standard Project Storm (SPS) method and HMR-33. However, this method is no longer recommended for various reasons of rish based analysis, etc. The Hec-HMS manuals clearly state the the Standard Project Storm method was included in the software only to aide in the recreation of old Hec-1 models with HMR-33 data.
With that in mind, how should you model the PMP in Hec-HMS? Using the Frequency Storm?
Since HMR-32/33 has been replaced by HMR-51/52 one now follows the procedures outlined in HMR-51 for calculating a depth vs area and a depth vs. duration curve for the specific project drainage area. (page 42, section 5: Use of PMP Charts). Using the depth duration curve you can determine depths for the 5-min, 15-min, 1-hr, 2-hr, etc durations. Essential a Frequency Storm in Hec-HMS. The Hec-HMS Application Guide, Chapter 6 uses the Freq Storm to perform a PMF.
If you compare a Freq. Storm HMS to a SPS you will get a significantly different runoff hydrograph. My observation is that this is caused by the significantly higher rainfall depths detrived using the methods in HMR 51 as opposed to the SPS method of using the 24-hr rainfall depth for the closest drainage area curve (i.e if your drainage area is 8 sq miles you would use the 10 sq. mile curve.) For example if you were to use the 10 sq mile / 24 hr curve for a location on Long Island, NY you would get 34 inches for the SPS. However, if you were to follow the procedures in HMR-51 and transfer the precip for the 8 sq miles you might get 41 inches for 24 hours as one point on your depth vs. duration curve. Using this and the other values in the Freq Storm your runoff hydrograph will be larger than the SPS.
The issue is if you are comparing your new HMS analysis using the Freq. Storm and HMR-51 to a previous analysis using Hec-1 and the SPS you are possibly going to show that the Spillway is signifcantly under sized. Leaving out the applicability of sizing for the PMF and accepting that the dam safety agency requires it regardless of its error.
Your thoughts, comments or experience would be appreciated.
With that in mind, how should you model the PMP in Hec-HMS? Using the Frequency Storm?
Since HMR-32/33 has been replaced by HMR-51/52 one now follows the procedures outlined in HMR-51 for calculating a depth vs area and a depth vs. duration curve for the specific project drainage area. (page 42, section 5: Use of PMP Charts). Using the depth duration curve you can determine depths for the 5-min, 15-min, 1-hr, 2-hr, etc durations. Essential a Frequency Storm in Hec-HMS. The Hec-HMS Application Guide, Chapter 6 uses the Freq Storm to perform a PMF.
If you compare a Freq. Storm HMS to a SPS you will get a significantly different runoff hydrograph. My observation is that this is caused by the significantly higher rainfall depths detrived using the methods in HMR 51 as opposed to the SPS method of using the 24-hr rainfall depth for the closest drainage area curve (i.e if your drainage area is 8 sq miles you would use the 10 sq. mile curve.) For example if you were to use the 10 sq mile / 24 hr curve for a location on Long Island, NY you would get 34 inches for the SPS. However, if you were to follow the procedures in HMR-51 and transfer the precip for the 8 sq miles you might get 41 inches for 24 hours as one point on your depth vs. duration curve. Using this and the other values in the Freq Storm your runoff hydrograph will be larger than the SPS.
The issue is if you are comparing your new HMS analysis using the Freq. Storm and HMR-51 to a previous analysis using Hec-1 and the SPS you are possibly going to show that the Spillway is signifcantly under sized. Leaving out the applicability of sizing for the PMF and accepting that the dam safety agency requires it regardless of its error.
Your thoughts, comments or experience would be appreciated.