Unused Raft Foundation Spaces as Detention Ponds to Mitigate Urban Flooding

Urbanization is changing the original catchment properties which are altering the urban hydrology. With increased impermeable pavements, including buildings and roads leading to less infiltration and more runoff during precipitation comes with the upsurge in urban flooding frequency. To mitigate urban flooding, onsite source control measures integrated with existing drainage systems are widely used. The onsite source control is aimed to increase the ability of infiltration and detention to decrease and delay the surface runoff. In addition to building a new structure, the existing and unused raft foundations in buildings can be utilized to detain excess runoff. This study investigated 4 buildings in Taipei City, Taiwan, and found that raft foundations show high potential as an onsite detention method. The legislated minimum water detention is 0.078 m3 per square meter of the construction area (m2); however, the average of 0.23 m3/m2 can be achieved when an unused raft foundation is employed. Under this possible scenario, the flood mitigation capacity of Taipei city can be increased from 78.8 mm/hr to 82.0 mm/hr.