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.
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.
This paper presents a review of water yields for six reservoir schemes within Selangor and Kuala Lumpur. The study was carried out using up-to-date hydrometric database up to 2009. Three approaches were used for yield calculations; namely, (1) Drought Sequence, (2) Longterm records, and (3) Storage-yield-reliability model techniques/methodologies. It was found that the gross yields of various reservoir schemes were agreeable with one another, at least within the margin of difference which is about 5%.
This study investigates the feasibility of converting four (4) existing single purpose water supply reservoir schemes into conjunctive hydropower operation that could supply raw waters to respective water treatment plants and at the same time, generate power. These dams with proper retrofitting exercises could be a potential hydropower cum direct water supply reservoir operation. The change in operation does not only berth well with current trend of dual or multipurpose functions of the reservoir scheme but also in light of emerging role of hydropower as a mature and steady source of renewable energy. These dams were assessed for retrofitting with turbine to generate hydroelectricity and at the same time, to supply raw water to both water supply schemes. Despite the advantages of this dual function of the existing reservoirs schemes, this study however concludes infeasible tasks as only less than 1 MW of firm power can be harnessed. To reconfigure these existing infrastructures for purpose other than for the single purpose of water supply requires optimum storage capacity and hydraulic head, the two most important variables, in essence, especially for viable hydropower generation. In addition, the height of the dam dictates critically the magnitude of effective head but unfortunately unavailable readily for most of the existing dams evaluated in this Study. They are less than 60 m in height in general. The design philosophy for the reservoir schemes at their preliminary conceptualization also does not support future multipurpose components of the reservoir operation. However a positive development of these potential schemes could be beneficial to supply power to the local community in the surrounding areas in additional to plant consumption.
Dams are supposedly designed and built with stringent criteria for no overtopping under the most adverse meteorological event. This may not be the case for dams that were built earlier before current methodological guidelines and practices are followed. The objective of assessing the safety criteria, i.e., overtopping of the Bengoh dam structure was achieved by carrying out the independent tasks of derivation of probable maximum precipitation (PMP) by reviewing other studies and works in Malaysia: a PMP-to-PMF (probable maximum flood) routing using a modified version of the synthetic unit hydrograph approach and a conventional reservoir routing procedure, i.e., modified Puls procedure to estimate the flood rise during a fateful PMP/PMF event. In essence, the task carried out is to evaluate and assess the adequacy of the step spillway capacity of the Bengoh dam. Bengoh dam, which is made of rolled compacted concrete (RCC) with a step spillway, is considered safe in the event of a PMP/PMF event, with a flood rise generally lower than the embankment crest level, i.e., þ86.2 m MSL (mean sea level).
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