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A Water Quality Study of the Selangor River, Malaysia

Malaysia’s rapid economic and demographic development has put pressures on its water supplies and consequently on the quality of its river water. The Selangor River, close to the nation’s capital, is now a major source of water and there are fears that its water quality will deteriorate. The Malaysian Government in its Vision for Water 2025 states that rivers should achieve Class II as measured by Malaysia’s Water Quality Index (WQI) (Class I is cleanest). The objectives of this thesis are to investigate the effects of flow through the 10 major tidal control gates (TCGs) which regulate run-off from the oil-palm plantations into the river, and to predict the water quality for the river in 2015, 2020 and 2030. In order to achieve these objectives it was necessary to set-up, calibrate and validate a commercial one-dimensional numerical model, InfoWorks, which includes both the hydrodynamics and water quality of the river-estuary network. It was concluded that there was insufficient hydrodynamic (stage and current) and water quality data to fully calibrate and validate the InfoWorks model but it performed well when compared with measured salinity transects. The model was found to be relatively insensitive to the choice of diffusion parameters but needed a high value for the oxygen transfer velocity, 0.3 m h-1, to get reasonable values for the dissolved oxygen (DO) along the river. The effect of run-off through the TCGs was less than expected and attributed to the high oxygen transfer velocity and needs to be addressed before the model can properly represent run-off through the TCGs. The model shows the WQI of the lower reaches of the river to be Class III in both wet and dry seasons except close to the estuary where it is Class II due to tidal flushing. The dissertation identifies several deficiencies in the model; the lack of an operational ramp function at the estuary boundary, the use of a single value of the oxygen transfer velocity throughout, and the exclusion of water extraction. Land-use changes above Rantau Panjang, the upper boundary of the InfoWorks model, and water quality data were used to estimate the water quality and its uncertainties at Rantau Panjang in 2015, 2020 and 2030 due to predicted development in the upper catchment for both wet and dry seasons. InfoWorks models of water quality along the river in 2015, 2020 and 2030, which included extraction at the Batang Berjuntai barrage, predict little change in the WQ (Class II/III boundary) below the barrage during the dry season but a rapid deterioration in the wet season (down to Class III/IV by 2030) showing the importance of water extraction to the water quality of the river. Overall, because of its relative simplicity and ease of operation, InfoWorks is considered to be a useful tool for river management in Malaysia.