Development of Synthetic Rating Curves: Case Study in Iowa

This case study describes an economically feasible approach to generate synthetic rating curves that enhance utility of stage-only river gauges. The study is based on a network of 250 bridge-mounted river-stage sensors (BMRSS) that the Iowa Flood Center has deployed in Iowa. The authors investigated using the step-backwater model with the Hydrologic Engineer Center's River Analysis System (HEC-RAS) to develop a stage-discharge relationship. The authors installed BMRSS at eight sites collocated with USGS gauging stations with well-established stage-discharge ratings to serve as reference. They also surveyed the channel cross sections in the upstream and downstream vicinity of the sensor locations. To account for uncertainty of channel roughness and free surface slope, they ran the model using a Monte Carlo simulation. The resulting rating curve realizations were compared with the USGS reference. The study reports the distribution of relative errors in the synthetic curve estimates, conditioned on the value of discharge. The results show average discharge errors of less than 5% for flows within the main channel banks and less than 3% in the floodplain. The average stage error is 0.43 m (range of 0.2-0.9 m). The authors also explored the error induced by using airborne light detection and ranging (LiDAR)-based topographic surveys for the cross-sectional geometry. They assessed the model-based methodology at an additional 19 locations where USGS maintains rating curves and concluded that the results were inferior compared with those obtained using conventional geodetic surveys. The study provides a basis for expanded monitoring of streams and rivers with stage-only sensors.