Omsack Pongkhao
Vientiane Times
October 30, 2012
Scientists and international experts fine-tuning the design of the Xayaboury dam have unveiled a scale model of the planned run-of-river structure to ensure the most efficient level of sediment flushing.
The 1:120 scale model simulates actual hydrologic flows and water velocities in the Mekong River at Xayaboury, aiming to test whether the dam design functions as intended so that optimum levels of river water, nutrients and an abundance of fish will flow through the site year round.
Lao government officials and media personnel from Vientiane visited the Asian Institute of Technology (AIT) in Bangkok last week to witness the model in operation.
Dr Sutat Weesakul, Senior Research Engineer at AIT’s Water Engineering and Management Programme, and Laboratory Supervisor Mr Arturo G. Roa briefed Lao officials on the results of the initial round of testing.
Mr Roa said that testing at AIT has led to “a much improved version of the old design”. Hydraulic model testing has already resulted in key changes to the spillway gates that regulate how much sand and sediment are transported at any time. The modifications made as a result of the tests will enable engineers to flush sediments to achieve “riverlike conditions”.
“In the old design, the sand flushing outlet was only located at the powerhouse and now we have the facility or device for flushing out sediment at the spillway area. Since the outlet is constructed very low all the sediment load should be able to pass through it.”
The modification means the water flowing through the dam will be the same as the quality of water upstream, in terms of nutrients and aquatic life.
“As far as I know,” said Mr Roa, “during operation the upstream headwater level will be reduced because of the opening of the low level outlets. So in this case, we are simulating very close to natural conditions because we have to lower down the upstream [water] levels. So if the sediment is under natural conditions and is flowing fast downstream then there is no reason why it cannot flush out with the new design.”
Mr Reza Rezaeeha, a civil eng ineer from international hydrologic consulting firm Poyry, confirmed that the studies actually started in 2009, and in the period since, some changes and modifications h ave been made to the design.
“We made some changes and we updated the model. At that time, the end of 2010 actually, we had some modifications to the low level outlet, actually for preventing sedimentation in the carriage. Now the result is this one.We are using this model because we want to see the performance of the dam is safe,” he said.
“We checked the performance of the hydraulic system and checked the condition because we are scaling the prototype with this model. We see the behaviour of the discharge and also the structure and the discharge of each of the gates of the spillway and the low level outlet. After that for the first stage we will check the low level outlet and spillway, and for the second stage we will check the low level outlet and the spillway with the powerhouse.”
Deputy Minister of Energy and Mines Mr Viraphonh Vi ravong explained that “the engineers say they have a good design and the requisite experience but to prove to other people that the design actually works it is necessary to do a hydraulic modeling test.
Mr Viraphonh said the experts told him they are pleased with the test results and modifications of the design. “It’s nothing abnormal. It’s something that has been accepted for the last 40-50 years of developing hydropower projects,” he stressed.
As always the emphasis is on both operational and environmental considerations of what will be a run-of-river project designed and built by some of the most experienced hydropower experts in the region and the world. The end result of the tests and modifications will benefit the people of the Lao PDR and the lower Mekong.
Construction of a scale model, which replicates natural river flows, is necessary to satisfy the concerns of any critics, Mr Viraphonh explained. Technically this kind of test is recognised by experts worldwide as being more accurate than abstract mathematical modeling.
The project developers have invested 17 million baht (more than 4.4 billion kip) to develop the scale model, which mimics natural conditions in the Mekong as close as is possible, and gives a much better projection of the actual operation of the dam.
The hydraulic modeling study undertaken by the Asian Institute of Technology at its hydraulics laboratory is now 50 percent complete. AIT scientists say they should complete the testing programme by December.
Xayaboury will be the first run-of-river dam to be built on the lower Mekong River, with a capacity of 1,285MW. It will require investment capital of US$3.8 billion and is expected to take eight years to complete. A run-of-river dam has the advantage that outflow equals inflow so there is less impact on the environment and the lifestyle of people who live along the river.
