Changes in the global climate have induced densified rainfall and caused natural hazards across the world in recent years. Formed by a central mountain range and a corridor of alluvial plains to the west, Taiwan is at risk of flood hazards owing to its low-lying lands as well as the distinct seasonality of rainfall patterns. The rapid discharge of surface runoff and a growing number of impervious surfaces have also increased flood hazards during recent typhoon landfalls. A century ago, ancestors in Taoyuan City constructed a system of water channels composed of thousands of ponds to fulfill the needs of agriculture and aquaculture. During the expansion of urban areas, land reformation replaced a majority of earlier ponds with residential and industrial zones. However, the remaining ponds could potentially serve as on-site water detention facilities under the increasing risk of floods. In this research, we first renewed an outdated pond database by deploying a novel unmanned aerial vehicle (UAV) system with a micro-sonar to map the bathymetry of 80 ponds. Next, a simplified inundation model (SPM) was used to simulate the flood extent caused by different scenarios of rainfall in Bade District of Taoyuan City. Assuming that extremely that heavy rainfalls at 25, 50, 75, and 100 mm occurred in a very short period, the flood area would decrease by 96%, 75%, 52%, and 37%, respectively, when the ponds were preparatorily emptied.