Hydrological Risk Assessment: A Comarison of Morning Glory Spillway Capacity

An investigation on dam overtopping risk was carried out for ten single purpose water supply dams with a common outlet feature of drop shaft (or morning glory) spillway. These dams had been designed as early as the 1930’s in the West Coast region of peninsula Malaysia. Task were carried out using uniformed and common methodology for each selected dam, firstly by selection of appropriate Probable Maximum Precipitation PMPs which served as influx into the reservoir. Subsequently an empirically derived synthetic unit hydrograph (SUH) approach was then used to translate the PMPs into probable maximum flood (PMFs). Finally, the outflows and corresponding flood rises over the sills of spillways were estimated accordingly. The outcomes of this study have indicated that, in general, the flood rise level for the most of the dams were lower than the ECL level by at least 0.50 m and this in essence, has provided some factors of safety in light of heavy and gusty wind surges at the top of the reservoir water surface. Thus, this has provided some flexibilities during emergency operations at the onset of a probable PMP/PMF event. In conclusion, the dams had been safe from the risk of overtopping based on an extreme meteorological event of PMP/PMF magnitude.