HEC-HMS

Maricopa County’s Hydrologic Modeling Procedures Added to US Army Corps of Engineers’ HEC-HMS 4.9 or Newer Versions
(FCDMC, August 29, 2023)

Three major Maricopa County’s hydrologic modeling procedures have been added to HEC-HMS software by the Hydrologic Engineering Center (HEC) of US Army Corps of Engineers (USACE) in collaboration with the Flood Control District of Maricopa County (FCDMC). The added hydrologic modeling procedures include the GIS-based Green-Ampt parameter estimation, Clark unit hydrograph, and S-graph unit hydrograph based on the 2018 Drainage Design Manual for Maricopa County, Arizona, Volume II, Hydrology (Hydrology Manual). The procedures have been implemented in HEC-HMS version 4.9 or newer versions. The software can be downloaded from the USACE website (https://www.hec.usace.army.mil/software/hec-hms/downloads.aspx).

The tutorials for the added Maricopa County hydrologic modeling procedures can be found from USACE HEC web site:(https://www.hec.usace.army.mil/confluence/hmsdocs/hmsguides/gis-tutorials-and-guides/applying-the-maricopa-county-parameter-estimation-tools#ApplyingtheMaricopaCountyParameterEstimationTools-CreatingaGeo-referencedBasinModel).

It may be noted that the original JD card approach for point rainfall area reduction in HEC-1 modeling is no longer needed in HEC-HMS because HEC-HMS internally runs the rainfall-runoff model for each of the selected concentration points or subbasins. HEC-1 performs the interpolation for the peak flows based on the JD cards (maximum 9 index storms) while HEC-HMS does not rely on index storms to perform any interpolation for the peak flows. The area-reduced rainfall is automatically computed by interpolation within HEC-HMS for each of the selected concentration points or subbasin based on the point rainfall area reduction curve. For Maricopa County 6-hour storms that require five drainage-area-based rainfall distributions, the rainfall distribution is automatically computed by interpolation within HEC-HMS for each of the selected concentration points or subbasins.