Page - 331 - in Book of Full Papers - Symposium Hydro Engineering

changes in the transboundary river. The Manwan HPP is the most typical case of reservoir sedimentation problems of the Lancang River. Its construction was started in 1986 and put into operation in 1993. As the first hydropower plant on the main stream of the Lancang River, it has accumulated lots of measured data on sedimentation over the years. The Manwan HPP stands on the middle reach of the Lancang River, the backwater of which reaches the Xiaowan HPP, and it neighbors the Dachaoshan HPP downstream. Its dam is a concrete gravity dam with a crest length of 418m and a maximum dam height of 132m as well as a crest elevation of 1002.0m. Its normal water level is 994m, total capacity of reservoir is 920 million m3, dead storage capacity is 663 million m3, and the regulating storage capacity is 257 million m3. It is a seasonally regulating reservoir. The annual mean sediment runoff is about 40 million tons, the measured maximum sediment charge is 14.3kg/m3, and the mean sediment charge is 1kg/m3. The limiting level during the flood season (June - September) for desilting was set for the Xiaowan HPP before it was put into operation, which was 988m. When the Xiaowan HPP was put into operation, the Manwan HPP’s operating water level for desilting during the flood season was deactivated. Within 20 years after the Manwan HPP was put into operation, the reservoir was mainly operated in the method of “storing clear and releasing muddy”, where the operating water level was lowered prior to and after the flood season every year and the sediment was flushed through the special holes at the bottom of the right and left banks to effectively clear away the mud at the water intake and reduce the regulating capacity loss. In 2010, after the operation commencement of the Xiaowan HPP, its regulating ability as the leading reservoir began to function. To fully exert the power generation capacity of the hydropower plants, the Manwan HPP has been operating at a high water level all year round since then. The change in the operation mode has led to the change in sedimentation in the reservoir. 2. SURVEY OF SEDIMENTATION IN THE MANWAN RESERVOIR 2.1. UNDERWATER TOPOGRAPHIC SURVEY PLAN FOR THE MANWAN RESERVOIR According to the current situation and characteristics of the reservoir area of the Manwan HPP, the following underwater topographic survey plan shall be implemented: A multi-beam sounding system shall be used to collect the cloud data of underwater high-density points, which consists of a SONIC 2024 high- performance multi-beam sounding instrument from R2Sonic Company of America, Octans fibre-optical rhumbs from France and a Trimble R7 GNSS receiver from America; GPS-RTK real-time positioning technology shall be used for the underwater data collection with the assistance of Hi Target HD27 single beam echo sounder in the shallow area and the total station and the digital echo 331