I will start with the fact that I would like to do a floodplain analysis with HEC-HMS and HEC-Ras.
I am a new user to both softwares and I write to you because I can’t figure out the following problem related to HEC-HMS:
I have one rainfall gauge station on my study area (of about 30 km2). The rain data is daily (i.e. one record per day).My flash flood spans for 12 days. I would want to generate the hydrograph on the basis of this rain data, using the Loss SCS Curve Number method and the SCS Unit Hydrograph Transform method. I determined the CN following the Purdue university tutorial, along with all the hydrologic parameters needed. I also determined the Basin Lag and the Time of Concentration according to the HEC HMS users’ manual.
My problem is that the hydrograph generated has values 10 to 15 times smaller than the streamflow measured data.
I have no idea at all what do I am doing wrong, considering that a few months ago, on the same river basin, with the same type of data, I managed to generate a hydrograph that fitted the measured streamflow hydrograph.
Any suggestions would help a lot.