A Tale of Two Dams Under Extreme Weathers in Upper Langat River Basin

This paper provides an updated review of the performance of morning glory spillways during extreme weather conditions under hydrological dam safety assessment exercises. The revised performance evaluation was undertaken on two existing water supply reservoirs in the upper Langat River basin, one of the major river systems in the state of Selangor. For uniformity, both dams were assessed and compared deterministically in this study of their respective spillway discharge capacities using a new set of much higher Probable Maximum Precipitation (PMP) by NAHRIM (2008). This set of PMPs were derived some 20 years post dam commission. The PMPs adopted in the design of the dam were based on an earlier derivation reported in SMHB (1992, 1994). The findings of this study cause a grave concern on the safety aspects of the dam in light of this recent set of PMPs, therefore, a general consensus by various stakeholder is required to evaluate deterministically that probability and likelihood of the extreme floods of PMP magnitude overtopping the dam crests. A positive co-relationship was also presented for selected existing water supply dams by relating the draining catchment area to the product of equivalent spillway length and headroom.