A Case Study of Sedimentation Management Plan of 243MW Warsak Power Station

Abstract

Warsak hydroelectric dam, located in North-West Pakistan, is subjected to massive sedimentation related
problems which has not only resulted in complete silting up of its reservoir but also frequent wear and tear of its 06No
power generating units. In this case study previous research work carried out by various agencies has thoroughly been
scrutinized. The studies revealed that an average annual sediment transport through the turbines of the plant is 75Metric
tons/year with silt contributing towards 50% of the total. In the absence of silt excluding infrastructure the only viable
option left is to manage the reservoir operation to effectively cater for this problem. Previous studies results has deduced
that no structural measures can do anything positive about the sediment transport problem, since they will always
encroach on the reservoir and reduce the available flow section, increasing flow velocity and the sediment concentration.
It has been found that the sediment transport capacity is exponential with most of the transport occurring in a small
period of time. It is thus possible to avoid a large percentage of the sediment intake to the turbines and to reduce the rate
of erosion of the turbines by stopping power generation during high flow conditions. If the powerhouse is shut down
whenever the river flow goes over 80,000 cusecs, the sediment intake reduces to 04 Mtons (33% reduction in sediment
intake), while the power production will go down to 1090 GWh per year (2% reduction in power production). During the
stoppage period, the available flow can be used to flush the reservoir, taking care of keeping the downstream sediment
concentration in acceptable values (8 to 16 g/L), and increasing settling capacity on the reservoir. Concerning the
hopper system, with 90% efficiency in terms of bed load capture, they will reduce the intake of coarse sediment (coarse
sand and pebbles) by 180,000 to 250,000 tons per year, depending on shutdown flow. These are large absolute values
that justify the construction of the hopper system. A prefabricated trash cleaning system with much higher cleaning
capacity with fixed rakes operating each of the six trash racks can solve the existing trash problems with greater
efficiency