Evaluating the Impact of Climate Change on Future Flooding Extent Using HEC-HMS and HEC-RAS Models: A Case Study of the Grand and Chagrin Rivers in Northeast Ohio

This study assessed future flooding extent using Climate Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6) climate models and provided a general overview of future flood patterns in Northeast Ohio, specifically within the Grand and Chagrin Rivers of the Lake Erie Basin. The hydrologic model was developed for both rivers using the hydrologic engineering center-hydrological modeling system (HEC-HMS), and the hydraulic model was developed using the hydrologic engineering center-river analysis system (HEC-RAS). These models were calibrated and validated using historical climate and streamflow data. Both models performed consistently well, both in calibration and validation, while evaluating through statistical indices. For example, Nash Sutcliffe Efficiency (NSE) for the HEC-HMS model was in the range of 0.70 to 0.71 for calibration and 0.67 to 0.68 for validation of both rivers. Similarly, the NSE for HEC-RAS was in the range of 0.65 to 0.94 for both calibration and validation of both rivers. The coupled HEC-HMS and HEC-RAS models were used to generate flood inundation maps for baseline (1980–2019), near-future (2020–2059), and far-future (2060–2099) periods for 10-year, 50-year, and 100-year return periods and across multiple climate projections from CMIP5 and CMIP6. Results indicated a consistent increase in peak discharge, ranging from 6% to 70% in the Grand River and 12% to 57% in the Chagrin River, across all return periods and climate scenarios. The high-emission scenarios from both climate models resulted in the most significant increases in flood inundation area, ranging from 15% to 25% in the near future and 22% to 36% in the far future. Additionally, the study examined how watershed size influences flood inundation. Results indicated that larger subwatersheds generally showed a greater percentage increase in flood inundation area compared to subwatersheds of smaller area for the 100-year return period flood. Additionally, the one-dimensioanal (1D) HEC-RAS model showed a relatively lower percentage increase in flood inundation area compared to the two-dimensional (2D) HEC-RAS model. Our analyses indicated that for each 1% increase in precipitation, runoff was projected to increase by 1.36 times the increase in precipitation in the Chagrin River and 1.23 times the increase in precipitation in the Grand River. Corresponding increases in flood extent were 1.07 and 0.73 times of increase in precipitation for the Chagrin and Grand Rivers, respectively.