Anthropogenic effects and climate change threats on the flood diversion of Erchung Floodway in Tanshui River, northern Taiwan

The Erchung Floodway, one of the successful measures of the Taipei Flood Prevention System, was constructed to mitigate the Tanshui River floods in the Taipei metropolitan area. The Taipei metropolitan area is the most crowded region in Taiwan. More than one-third of the total population of the country resides in this area. However, its function has changed due to urban development and natural alterations over time. The main concerns of this study are to investigate the current diversion capacity and the current vulnerability of the Taipei Flood Prevention System in accordance with influential environmental factors, including anthropogenic effects and climate change threats. Thirty-two scenarios were established for sensitivity analysis using HEC-RAS model. The results indicate that the capacity of the Erchung Floodway diversion has noticeably decreased from 9,200 to 6,300 m3/s under a 200-year recurrence flood. Three vulnerable locations have been identified: Shihtzutou, Shezi, and Wugu. It was also found that the Taipei Flood Prevention System will encounter challenges if the roughness of the riverbed within the Erchung Floodway increases by over 50 %, the roughness of the riverbed within the Tanshui River increases by over 25 %, the water stage at the river mouth rises to 5.03 m, or the 200-year recurrence flood increases to 28,300 m3/s. Two proposed cost-effective mitigation strategies in the present study are: (1) to remain or decrease the riverbed elevation and roughness within the Tanshui River (Taipei Bridge section); (2) to decrease the riverbed roughness by at least 25 % within the upstream and midstream of the Erchung Floodway.