Flash flood routing modeling for levee-breaks and overbank flows due to typhoon events in a complicated river system

There has been a yearly increase in precipitation in Taiwan, consistent with trends seen across the world. In the summer and fall, typhoons or tropical cyclones with torrential rainfall frequently occur as a result of Taiwan's subtropical climate. Flash floods may cause a levee-break and/or the overtopping of banks at narrow neck locations in a river system, which may in turn produce inundation in urban areas. Therefore, a model that predicts flash floods is of vital importance for river management. The present study is based on a flash flood routing model, which incorporates levee-break and overbank functions to calculate the discharge hydrographs in the complicated Danshuei River system of northern Taiwan. The numerical model was calibrated and verified against observed water stages using three typhoon events. The results indicate reasonable agreement between the model simulations and the observed data. The model was then used to calculate the levee-break and overbank flow hydrographs due to Typhoon Talim (2005) and Typhoon Nari (2001), respectively. The simulated results indicate that several parameters significantly affect the flow hydrograph during a levee-break and should be carefully monitored when levee-break events occur in the river system. The simulated water stages at several stations are consistent with observed data from Typhoon Nari. The simulated overbank flow results quantitatively agree with reported information. The data also confirm that most of the overbank events occurred at the upper reaches of the Keelung River, consistent with the low levee height protection.