Estimation of design discharge for an ungauged overflow-receiving watershed using one-dimensional hydrodynamic model

Design discharge at a location of interest is conventionally determined using flood frequency analysis, precipitation frequency analysis, or national flood frequency equations. However, none of these three methods can be used to estimate the design discharge for ungauged watershed where high flows are from overflows of the watershed's bordering stream(s), such as the Hartsville Coulee watershed located in northwestern Minnesota. The historical high flows in the Hartsville Coulee were from the overflows of the Red River of the North (RRN). The objective of this study was to set up and use a HEC-RAS hydrodynamic model to determine the maximum overflow rate from RRN into the Hartsville Coulee in 1997, when the record flooding occurred. The model was calibrated using the observed daily streamflows at four US Geological Survey (USGS) gauging stations along RRN. The breach, over which the floodwater was spilled into the Hartsville Coulee in 1997, was defined using the topographic data surveyed by US Army Corps of Engineers (USACE). The results indicated that the maximum overflow rate was likely between 250 and 310 m(3) s(-1). The computed overflow rate in this study was well compatible with the value estimated by USGS based on a field reconnaissance as well as the value estimated by USACE using Manning equation. In generalization, the HEC-RAS hydrodynamic module can be used to estimate design discharges for overflow-receiving drainage areas such as the Hartsville Coulee watershed.