Sediment production during an unusually dry year in the steep forested Sungai Pangsun catchment, Selangor, Peninsular Malaysia

Sediment transport in a 2.6 km2 steep, undisturbed headwater rainforest Sungai Pangsun catchment in the Ulu Langat Forest Reserve in Selangor, Peninsular Malaysia was measured during a 12-month period. The rainfall and runoff recorded during the study period from July 1997 to June 1998 were 2052 and 866 mm, respectively. This was an unusually dry water year, affected by the El Nino/Southern Oscillation phenomenon; rainfall was about 30% below the long-term mean of 2514 mm. Suspended sediment yield computed from in situ readings taken at 15 min intervals was 21.8 t. Bedload yield, measured with a hand-held Helley-Smith sampler and by deposition in a stilling pool at the catchment outlet was 4.6 and 11.4 t, respectively. Due to difficulties in sampling bedload during storm discharge with the Helley-Smith sampler, biweekly measurements of deposition in the pool were considered the more reliable. Two major storm events, in October 1997 and May 1998, contributed 45% of the total bedload yield indicating the importance of peak flows on bedload transport in the catchment. Episodic bedload transport was also attributed to the influence of coarse woody debris, particularly in places where sediments were stored behind debris accumulations along the stream channel—about 60% of the 16 m3 of woody debris mapped in a 300 m reach upstream of the gauging site was mobilized during the October event. During the 12-month study period, suspended and bedload sediment comprised 66 and 34%, respectively, of the total sediment transport. The higher ratio for suspended load was largely due to the dry study period. It is envisaged that bedload would contribute a higher proportion of sediment during normal hydrologic years.