Risk Assessment Scenario of Machap Dam Overtopping Using New PMP Malaysian Series

A hydrological dam safety analysis was carried out for existing Machap dam (CA= 77 km2) due to change in the extreme rainfall condition in the watershed. It is important to ensure that extreme meteorologically induced flood rises do not exceed its crest level. Step-by-step procedures were carried out in tandem to evaluate the hydrological performance of the gated spillway capacity in light of an extreme storm event of PMP/PMF magnitude. This study adopts a newly developed “inland” type of PMPs compared to the original design PMPs. A catchment routing and reservoir procedures were then used to translate the PMPs to PMFs and estimate the outflows and corresponding flood rises over the crest level for all durations. The results of the PMPs/PMFs were comparable to the “Creager” type of catchment area-PMP relationship of various dams in Malaysia. A conventional reservoir routing procedure by modified puls technique is then carried out for all PMP/PMF durations; i.e., 1 to 120 hours. Three (3) scenarios are performed; namely, (1) when the gates are fully closed, (2) when the gates are partially open in light of impeding PMP/PMF event and (3) when the gates are fully open. The last scenario (case 3) represents the most efficient and effective management of the reservoir gate operation as the original designer intended. The flood rises for all durations are considered lower than the embankment crest level. The results also show expectedly the failures of both cases (1 and 2) for their inadequate capacity to allow safe passage of flood water of PMP/PMF magnitude.