A quantitative analysis of the increased flood hazard in the Wenchuan Earthquake-hit region was carried out. Flood routing was performed for the reach of Tongkou River from Tangjiashan Quake Dam to Beichuan City and the change of flood defense capacity of Beichuan City was examined. It was shown that since the river bed was seriously raised by the Tangjiashan Quake Lake drainage and subsequent Weijiagou Debris Flow, the present flood defense capacity near Beichuan City had become much lower than that before the earthquake (i.e., the capacity against a 20-year flood) and far away from the requirement of protecting a national commemoration earthquake site. Meanwhile, the predicted water level and erosion depth were sensitive to the manning roughness coefficient for flood routing. It would be desired in future to explore the manning roughness coefficient changing with river geomorphology and bed material composition.
