Application of Low Impact Development for Flood Mitigation in Highly Urbanized Area─ A Case Study of Chung Ho and Yung Ho District of New Taipei City

Taiwan is located on the Tropic of Cancer which means the island is extended across the tropics and subtropics, and is often threatened by typhoons or heavy rains because of abundant precipitation causing from surrounding by the sea. In addition to our special topography such as high mountains, steep slopes and torrent water, the increasing impervious areas ratio in urban area is more important role for not only the increasing direct runoff volume but also the hydraulic efficiency resulting in the concentrated peak flow. Therefore, we are facing the inundation endangering the people''s lives and property.
All of the hydraulic engineering facilities and the flood control constructions have certain degree of protection ability. And the urban drainage systems to the storms are general designed certain criteria. If the flood volume exceeds the designed criteria, especially the heavy rain form typhoons and the concentrated short duration storms, the flood over the capacity of drainage systems will make surcharge and inundation phenomena. Thus, it is important issue and essential to understand the phenomena of urban flooding, and adopt effective measures to improve the situation caused by highly urbanized. The flood could be managed either by the temporary solution like the traditional concentrated design of hydraulic engineering facilities or the root of the problem like Low Impact Development (LID) techniques to reduce the threaten of flood.
The idea of LID in the United State has been promoted over ten years. The application of LID devices include vegetative swales, rain barrels, permeable pavement, bioretention cells, rain gardens and so on. LID emphasizes on maintaining or restoring the site hydrological conditions back to predevelopment. And its strategy is combining distributed, parallel connection and small-scale devices functioning as infiltration, interception, rainwater storage or detention facilities to increase infiltration chances for runoff and lengthen the time of concentration. By this source control methods, the environmental impact could be reduced and maintained hydrological conditions of predevelopment such as surface runoff, infiltration, surface and groundwater exchange effects.
The purpose of this study is combing Storm Water Management Model (referred SWMM) and 2D flooding model to simulate direct runoff, surcharge volume, peak discharge, peak time and inundation areas for 6 hydrological events. And, evaluating the effectiveness of LID for urban flood control as indicators of direct runoff, surcharge volume, peak discharge, peak time and inundation areas by several simulation scenarios.
The results showed that the more application of LID to replace impervious pavement the more benefits of flood control. And the maximum benefits of LID reducing direct runoff is 60%, because of the soil has its limitation on infiltration capacity.