FAQ – Flow

Will Don Sahong reduce or change flows going downstream?
Will the operation of DSH power station prevent flows from reaching the Phapheng Falls?

No. Flow of the Mekong River will remain unchanged. The project will not abstract or divert any water to or from the river. It has virtually no storage potential, and the numerous other existing channels that bypass the project will act as natural spillways. DSHPP will operate as a run-of-river station, which means the river flow entering the Hou Sahong flows like a natural river through the headpond (residence time 3-4 hours) and is discharged through the turbines. Water is not stored in the headpond like a conventional hydro storage scheme. With run-of-river, turbine flows are adjusted to follow the flow and water level variation occurring naturally in the Mekong River, and no water is removed from the river by the scheme. As a result, the total river flow downstream of the scheme will always be the same as the total river flow upstream. With DSH operating, some of the flow that would previously have passed over the Phapheng Falls will be re-distributed to the Hou Sahong for generation. These flows then recombine at the outlet of the Phapheng channel. There is therefore no effect on total river flow downstream.

DSH will be operated so as to always provide a minimum flow over Phapheng Fall of 800 cumecs as a priority over generation flows. This will be achieved by regulating or reducing the flow through the turbines when necessary to ensure the minimum flow is met. There will be an automatic river flow monitoring station on the river just upstream of the Falls which will be linked with the power station control room and will automatically adjust turbine flow when necessary to ensure flow over the Falls is not reduced below the guaranteed minimum.

How did you measure the flow in Hou Xang Pheuak? The level seems high as Hou Xang Pheuak is mainly dry from February to April.
Is there any other project that has proven the proposed method to guarantee 800m3/sec at the falls?

We measured the flow at the Hou Sahong outlet to Hou Xang Pheuak and at the Hou Xang Pheuak outlet to the Mekong, and then calculated the flow in Hou Xang Pheuak by difference. The flow measurement methods are all described in the Project Hydraulics Report. Further baseline flow monitoring is ongoing.

Yes. There are two principles as 1) Controlling flow through the turbines, 2) Recording water level in the river channel 1. Water flow through hydro turbines can and has been able to be controlled in modern hydro turbines for more than a century. 2. Water level recorders are used throughout the world for various monitoring and control applications, many of which are employed in much more demanding roles than measuring the slowly changing water level in a river channel.

Will the channel modification in Hou Xang Pheuak change the flow rate through the channel?
Flow in SHG before the project is 5% and after is 15% of the total Mekong flow per year. So where does the extra water in Sahong come from?

No. The flow rate is determined by the channel depth at the Hou Xang Pheuak inlet, whereas the modifications will be to constrictions midway down the channel.

The additional water will be diverted into the Sahong channel upstream of the Phapheng channel, but this diversion will only occur when the total flow over the Phapheng Falls is above 800 m3/s. This minimum flow over Phapheng will be part of the operating conditions for the project.

Will the 5m deep excavation at the inlet prevent flow from reaching the Phapheng Falls?
Will the additional flow for Don Sahong mean that flows in the Hou Xang Pheuak are reduced ??? especially in the dry season?

The Hou Sahong inlet requires deepening by 5m so that the station can generate at its normal capacity during the dry season. This excavation cannot cause the flow over the Falls to be reduced to unacceptable levels because; (a) the main river channel immediately upstream of the inlet is between 15-20m deep ??? much deeper than the 5m deepened Sahong inlet, and (b) the turbines will control the flow into the Hou Sahong ??? the excavation at the inlet will not control the flow.

No, flows into the Hou Xang Pheuak will not be affected by operation of the Power Station, as the inlet to the HXP is upstream of the Hou Sahong inlet and will therefore see the same flows in the main river as at present. Also the HXP inlet is about 1m higher than the inlet to the Hou Sahong.

With no control structures at the Hou Sahong inlet, how can you be confident that you will achieve the required diversion flow for the power station?

The 2D hydraulic modelling reported in the published information has since been further extended to CFD (3D) modelling, specifically to verify the conditions for water diversion into the Hou Sahong, and to check that the inlet excavation as-designed will provide the necessary capacity. The CFD modelling has now provided a high degree of confidence in showing that the diversion will provide the necessary capacity without the need for hydraulic structures. The results of the CFD modelling will be published as well, once the reporting is completed. In addition, the contractor???s engineer will undertake independent verification of the diversion as the contractor is required under the contract to guarantee the diversion capacity.