| Summary: | Flash flood is becoming more prevalent nowadays in big cities in Malaysia. Rapid and uncontrolled development projects aggravate the problem. . Lack of space for the construction of flood mitigation facilities has prompted authorities to look for other solutions for flood control. One of the approaches is to regulate flow at the upstream area. That is why this study area (Taman Wangsa Melawati), which is located at the upstream of Klang river basin was selected for the study. Taman Wangsa Melawati catchment, which drains to Sg. Gisir is used in this study to evaluate the impacts of RROSD design for storm water management at a small scale. The catchment is fully developed where 83 percent of the area is covered by impervious surfaces. The RROSD with a total storage capacity of 5.0 cubic meter located at one of the houses. 3.3 cubic meter is for rainwater storage, while 1.7 cubic meter is for flood storage. The ROSD was evaluated in its efficiency in reducing peak flow from roof runoff. The cumulative impact of peak flow reduction at the outlet was also evaluated, if each of the houses was equipped with this facility. XP-SWMM model was used to quantify the cumulative reduction of peak flow by the usage of multiple ROSD in the study area. The model was calibrated prior to its application of various hypothetical scenarios. Outflow from the ROSD is released automatically through pipe outlets to the receiving drain. The result shows that the existing ROSD design performs satisfactorily at individual house level, though it did not comply with the PSD limit as set by MASMA. However, the performance was not extendable to the catchment level. The fact that the percentage of overall peak flow reduction at the outlet is below the percentage of reduction of individual ROSD shows that the cumulative effect has reduced the performance of the ROSD. The result shows that, it is best to place the ROSD at the end of each street rather than at a single house. Storm water reuse for this ROSD provides another potential in not only in reducing peak flow but also in reducing storm water volume. Result from data collection at this house shows that about 3.4 m3 of storm water were reduced from entering the receiving drain in a month. If every house within the area is equipped with ROSD, significant volume of runoff (823 m3/month) is prevented from flowing to the outlet. This will help alleviate the problem of cumulative effect at the outlet
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