SEDIMENT YIELD AND SEDIMENTATION IN A SHALLOW TIMAH TASOH RESERVOIR, PERLIS, MALAYSIA

摘要

This paper reports the first year observation of a study carried out to investigate the sediment yield of river inputs and lake sedimentation in Northern Malaysian State of Perlis. Water samples and gauging were carried out every two weeks with additional gauging carried out intensively during storms and storm water samples were collected automatically using automated water samplers. These samples were correlated with two continuous water level data recording stations at two main river inputs feeding the reservoir. Reservoir sedimentation rate was measured using sediment traps collected monthly. Two main river inputs to the reservoir were monitored- the Jarum and Pelarit. The minimum concentrations of sediment at Pelarit were always lower than that at Jarum. However, the sediment concentrations increase immediately once storms occur. The quarrying activity in the Pelarit catchment only contributed its sediment during rainy days. The rainy season's suspended sediment concentration range was 4.8mg·l~(-1)-1,396.8 mg·l~(-1), while the dry season concentration ranges from 1.2 mg·l~(-1) to 73.6 mg·l~(-1). The average sediment concentration in wet season of Jarum river ranged from 102.4 mg·l~(-1) to 248.8 mg·l~(-1); while the baseflow concentration range was 6.8 mg·l~(-1) to 128 mg·l~(-1). The estimated sediment yield for 2001 was 310.2 t·km~(-2)·yr~(-1)and 92.3 t·km~(-2)·yr~(-1) at Pelarit and Jarum, respectively. The trend in sediment discharge is significantly affected by the rainfall and runoff in the catchment area. Pelarit river have a much higher rainfall-runoff compared to Jarum river. The total amount of runoff at Pelarit was 1,093 mm (62% of rainfall) and only 574 mm (36% of rainfall) at Jarum. The high sediment concentration and sediment load are reflected in the gross sedimentation rate in the reservoir. The gross sedimentation rate varies from 42.3 g·m~(-2)·d~(-1) in the deepest part of the reservoir, to about 142.5 g·m~(-2)·d~(-1) and 85.0 g·m~(-2)·d~(-1) near the inlet of Pelarit and Jarum river, respectively. Sedimentation rate near the river mouth is directly related to the sediment discharge of the river. Sedimentation rate decreases southward, as the distance from the inlet increases towards the outlet.