The water deficit due to a rise in water demand, and inundation due to an increase in impervious surface area are some of the challenges brought upon by urbanization. Rooftop rainwater harvesting system (RTRWHS) has been a subject of various researches because of its great potential as an additional water resource and as a flood mitigation scheme. This study investigated the reliability of rainwater tanks in meeting non-drinking usage of water and how the implementation of RTRWHS affects the reduction of flood peak and flood volume using HEC-HMS at a selected outlet along A.S. Fortuna St., Mandaue City, Cebu, Philippines. Prior to its modelling, the rainwater tanks to be used for each building within the selected catchment area were sized according to a targeted reliability of 60%-80% using the Storage Estimation and Reliability Tool (SARET). A 25-year daily rainfall data acquired from the Philippine weather agency was used in the simulation to estimate the reliabilities of various tank sizes. One of the factors that can affect the reliability of a certain tank size is the roof area. By using the building footprint shapefile acquired from the Center for Geographical Information and Environmental Solutions (CenGES), the roof areas of the buildings found in the study area were estimated using a measuring tool in ArcMap. The building classifications and roof areas were archived in a geodatabase. The reduction in flood peak and flood volume were estimated by comparing hydrographs for a basin without RTRWHS and with RTRWHS obtained from simulations for rainfall events of 2-, 5-, 10-, and 25-year return periods using HEC-HMS. A reliability of 60%-80% can be achieved for roof areas of 100-550 sq. m. for a corresponding tank size of 1-7 cu. m. when the demand is 280-500 L/day. RTRWHS can reduce flood volume by 6.03%-15.27% and 2.03%-3.35% for flood peaks.

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