HydroCAD vs. HEC-HMS
HydroCAD vs. HEC-HMS
(OP)
Hello All,
I'm working on a 6 sq. mi. drainage study. I first ran HEC-HMS, then HydroCAD to get a quick sense of the storage volume required. The numbers were significantly different (2800 cfs vs. 2400 cfs) and I've set them up to use the same calculation methods. I'm using SCS unit hydrograph method, SCS Type 1A temporal distribution, NOAA 100-yr 24hr rainfall depth of 6.5", TR-55 for Tc (Hey HEC, how about incorporating the TR-55 equations?!)
The most curious is when I isolate one subshed and HMS and HydroCAD yield the same runoff depth (4.3") but have such different peaks (2200 cfs vs. 1900 cfs). 5% error I would still question, but 15% is very high, wouldn't you agree?
Thanks in advance for your thoughts!
Nate
I'm working on a 6 sq. mi. drainage study. I first ran HEC-HMS, then HydroCAD to get a quick sense of the storage volume required. The numbers were significantly different (2800 cfs vs. 2400 cfs) and I've set them up to use the same calculation methods. I'm using SCS unit hydrograph method, SCS Type 1A temporal distribution, NOAA 100-yr 24hr rainfall depth of 6.5", TR-55 for Tc (Hey HEC, how about incorporating the TR-55 equations?!)
The most curious is when I isolate one subshed and HMS and HydroCAD yield the same runoff depth (4.3") but have such different peaks (2200 cfs vs. 1900 cfs). 5% error I would still question, but 15% is very high, wouldn't you agree?
Thanks in advance for your thoughts!
Nate





RE: HydroCAD vs. HEC-HMS
Now for the real dilemma...you need to design and which do you choose? Conservative design or over-design?
I would run it on with the actual TR55 program, and choose the model which yields the closest. Submit the actual TR55 results for your hydrology, then submit the HEC or HydroCAD output for hydraulic design basis.
Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve
RE: HydroCAD vs. HEC-HMS
You should also check the unit hydrograph and the calculation time step. A comparison to TR-20 would also be informative. But keep in mind that none of these tools is claimed to accurately predict exact runoff rates. They are best at predicting changes in runoff from an existing to a proposed (developed) condition. As long as you use the same techniques througout a given project you should be OK.
RE: HydroCAD vs. HEC-HMS
I suppose I will run the TR- routines at some point. Right now I'm pretty tired of the uncertainty of hydrology. I'd rather study String Theory!
Nate
RE: HydroCAD vs. HEC-HMS
However, in your original post you said the runoff depth (and therefore the volume) were the same. This would indicate that the same runoff equation and CN value are being used.
Different Tc values or rainfall distributions will affect the runoff timing and peak flow, but not the volume.
I suggest you go back and re-check all your input data. I suspect this is a simple discrepancy in the input.
RE: HydroCAD vs. HEC-HMS
using a variety of methods. I have yet to see a rainfall runoff event mathced using several methods for the same event.
Try using actual storm data (USGS Water Resources Division is a good source) to calibrate yes calibrate you model. Then the method you use and variations using other methods can be scientifically explained.
RE: HydroCAD vs. HEC-HMS