Saad Merayyan^1,, Jeremy Hill²

This study presents a case study for estimating the 100-year peak flow for Middle Creek Watershed in Northern California. The watershed contains several stream flow gages; however, the precipitation data is only available as daily data, which was not usable form for this study. Thus considering that the watershed to be ungagged. In order to overcome this shortcoming in the hydrologic analysis, other approaches were considered. Therefore, the precipitation point frequency estimates were obtained from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14. The Hydrologic Engineering Center’s Hydrologic Modeling System (HMS) was used to create the hydrologic model to estimate the peak flows at key points in the watershed. The purpose of using the HMS model was to predict eh rainfall-runoff analysis for this watershed, which only has steam gage data. Other parameters needed for the HMS model were obtained from various sources as suggested in the United States Army Corps of Engineers (USACE) Central Valley Hydrology Study (CVHS): Technical procedures document. The 100-year flows from the HMS model results were then calibrated/validated by comparing to the 100-year flow frequency curves computed using the Hydrologic Engineering Center’s Flood Frequency Analysis (FFA) program, FEMA USACE, and USGS Regression methods. Sensitivity analysis of several of the model parameters was analyzed to determine the results confidence level. The HMS modeled results were in good agreement with the results obtained from the Flood Frequency method and the USGS regression studies. The procedure described herein for developing and validating hydrologic models for ungagged watersheds can be used for other similar ungagged watersheds.

HEC-HMS, partially gagged watersheds, computational methods, flooding