In this study, Tangozeki Park, located in Chiba City, Chiba Prefecture, is used as the target site to propose a new urban park that contributes to both flexible community development against flood damage caused by heavy rainfall and preservation of biodiversity. It will also enhance the water friendliness of the entire community by accepting rainfall as a more familiar presence. When the water level of the Miyako River adjacent to the target site, Tango Weir Park, rises, flood risk will be reduced by allowing river water to temporarily flow into the park. It will also increase biodiversity in the park by partially purifying rainwater within the park. Furthermore, the temporary acceptance of river water within the park will change the appearance of the park, and park users will be able to enjoy the differences even on rainy days.