Sediment load assessment of small embankment dams in southern regions of NWFP [Pakistan]

Siltation has a large impact in reducing the benefit obtained from sixty-two dams constructed in the southern region of NWFP. The United Nations development programme (UNDP) in its poverty reduction project concentrated on the sediment deposited in these dams to safeguards their service lives. A field study was conducted to find sedimentation in nine-sampled reservoirs of the embankment dams during August 2004 to September 2005 at Shakardara District Kohat, Pakistan. The suspended load through integrated approach and the total deposited sediment load cumulated in the reservoirs (By Grid survey) were determined separately. Staff and rain gauges for stage measurement and precipitation in the reservoirs and area were installed. Siltation life of each embankment dam was determined from rate of incoming sediment, trap efficiency and reservoir capacity. The intensity of rainfall observed during the study period (July 2004 to August 2005) ranged from 1.2 to 92.1 mm/h. Runoff from the catchment of the embankment dams ranged from 10.49 to 17.02% of the rainfall, with over all average of 13.12%. The result of the study showed that the overall suspended sediment load in runoff ranged from 1478 to 11.812 mg L-1 with overall average of 3699 mg L-1. It is also found that the average incoming, total sediment load in the selected embankment dams varied from 721 to 1875 tons-km2 year-1. The reservoir capacity of the sampled embankment dams ranged from 5.008 to 120.190 m2 and the siltation lives of the sampled reservoirs ranged from 11 to 99 years with overall average of 38 years. In general the siltation lives of the sampled embankment dams increased with increase in reservoir capacity. Embankment dams with capacity less than 40,000 m2 were found to have siltation lives of less than 40 years and their construction should be avoided.