Peak flow in single-building lots in a residential areas with rainwater storage facilities in Taiwan

In this study, the Storm Water Management Model of the United States Environmental Protection Agency and different rainfall return periods were employed to investigate peak flow, runoff allocation, and potential legal revisions following the installation of rainwater storage facilities in compliance with Taiwan’s Building Technical Regulations. This study focused on single-building lots covering an area of 63.1 ha within the residential areas of Danhai New Town, New Taipei City. The study area comprised 180 subcatchments, 173 of which are single-building lots that are located in the residential areas and have a size ranging from 0.09 to 2.80 ha. The capacity (m3) of the rainwater storage facilities in these lots was calculated by multiplying the building lot area by 0.045. The results revealed that despite a reduction in peak flow of 0.0018–0.0166 m3/s during 1-h and 24-h rainfall events with different return periods following the installation of rainwater storage facilities in the single-building lots, further room for improvement in the peak flow reduction existed. On the basis of the results as well as the regulations and the floor area ratio incentives implemented for building lots with various areas, the percentage reduction in peak flow ranged from 4.434 to 4.476% for lots smaller than 0.3 ha. This finding suggests that rainwater storage facilities are a suitable water retention solution for such small lots. However, for lots larger than 0.3 ha, in which a higher floor area ratio is incentivized for residential development, the percentage reduction in peak flow of approximately 4.303–4.404% was slightly lower than the aforementioned range. Consequently, due to the building lots with large area and floor area ratio incentives that result in higher runoff, additional runoff allocation responsibilities and proactive water retention strategies need to be implemented. Overall, to minimize the peak flow in building lots and flood damage in urban areas, Taiwan should promote the installation of rainwater storage facilities across various types of building lots. At the same time, the responsibility for runoff allocation from public facility zones and private development zones should be increased, and potential legal revisions should be added to the low-impact development (LID) facilities for various land uses in urban areas.