The Impact of Replenishing Flow and Sediment on the Riverbed Repairing of Minjiang Estuary and Water Level Rising Under Shuikou Dam

Abstract

Since the completion of Shuikou hydropower station, the process of flow and sediment has undergone profound changes in Minjiang estuary. Under the combined action of hydrodynamic force and sand dredging, the riverbed of Minjiang estuary was cut down sharply. The water level under the hydropower station dam which was 110 km away from Guantou section of Minjiang estuary decreased from 8.80 m in October 1991 to 5.40 m in April 2009, and it also caused shipping suspend during the dry season. Due to the fact that the tail water can not meet the suction height requirement of turbine, the normal operation of the unit has been affected. In addition, the decline of the water level has also led to salinity intrusion, which threatens the safety of urban water intake. The measure presented in this chapter is to replenish flow and sediment of the estuary by means of discharging flood through the combined use of the bottom hole and the surface hole of the hydropower station, so as to restore the riverbed and raise the water level under the dam of Shuikou hydropower station. The flow and sediment numerical model shows that under the condition of 3570 m3/s discharge flow, 2-day duration, and using synchronous measurement of tide level process in the estuary, when the sediment concentration of the discharge is 0.625, 6.25 and 28 kg/m3, the riverbed elevation under the dam of the hydropower station will rise by 0.26–0.32, 0.99–2.16 and 4.71–8.24 m, but as the distance is longer than D20 section which is 7.38 km away from Shuikou hydropower station, the riverbed elevation is of no obvious change. This means that the short-term measure of replenishing flow and sediment only has the repairing effect on the short-distance riverbed of the downstream of the hydropower station. Under the condition of 10-day duration, 6.25 kg/m3 sediment concentration and 308 m3/s basic discharge flow, the lowest water level under the dam of the hydropower station will rise by 7.77 m. Meanwhile, within the distance of about 12.62 km which is located at D1 section to D33 section of the downstream of the hydropower station, the lowest water level will rise by 1.91–7.77 m; among the section of about 27.44 km, which is located at D1 section to D64 Hongwei section, the average riverbed elevation will rise by 4.72 m. The results show that the water level under the dam of Shuikou hydropower station can be restored to the original design level through the measures of replenishing flow and sediment with 0.625 kg/m3 sediment concentration, 3570 m3/s discharge flow, and 4.38-day discharge duration.