Operational rules for sustainable sediment management of dam cascades

Application to Mekong hydropower dams in Lao PDR

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Hydropower plants have proven to generate electricity reliably and predictably, store water to meet water demands during dry periods, improve navigability and reduce flood risk. The reservoirs of these hydropower plants face ongoing sedimentation, negatively effecting the electricity generation, water storage volume, navigability and flood risk. Obtaining an equilibrium of the in- and outgoing sediment flux reduces these negative effects.

The incentive for this report is the planned run-of-river hydropower dam cascade in the main stream Mekong river in Lao PDR. The Mekong river is characterised by the distinct low and high flow seasons and a unique ecosystem. At the location of the planned cascade, the river is located between mountains and the river bed consists of bedrock with alluvial bed forms at the banks and between rock outcrops.

Long, narrow and relatively shallow reservoirs are created due to the construction of the planned dams in the main stream Mekong. Flushing operations have proven to be an effective method to recover reservoir volume from rivers with a high season flow with long, narrow and shallow reservoirs. The Mekong river in Lao PDR has all those properties.

This report attempts to determine operational rules for a run-of-river hydropower dam cascade to increase the sediment flux by flushing operations at the dams, whilst still maximizing the electricity generation.

To do so, two steps are taken. Firstly, a set of experiments on the Mekong river and flushing events at a dam is performed in a Delft3D-FM software with the Real Time Control module. The output of these experiments are used as input for a bucket model with volume conservation to test different operational rules.

Using the Delft3D-FM software with the Real Time Control module, a one-dimensional representation of the Mekong river is made. This model is used for two purposes. First, the reduction of the sediment flux is determined by comparing the sediment flux during a representative year for the situation pre-dam and post-dam construction. Secondly, this model is used to test the influence of the draw down rate, water level set point and duration of flushing operations on the cumulative sediment flux through a dam for four different discharges. The results of the flushing experiments will then be used as input for the second step of the research, the bucket model.

Different simulations are performed using operational rules with different threshold discharges for initiating flushing events, different draw down rates and different frequencies of flushing a reservoir. Using random sampling the influence of these three variables is tested. Additionally, two predetermined strategies are tested and compared to the operational rules tested using random sampling. In the first strategy the dams in the cascade are flushing progressively from the most upstream dam towards the downstream dam. The second operational strategy flushes the reservoirs progressively starting from the downstream dam towards the most upstream dam.

Obtaining free flowing conditions during a flushing event increases the sediment flux significantly compared to flushing events that do not reach free flowing conditions, especially for lower discharges. Increasing the draw down rate proves to have limited influence on the sediment flux further upstream in the reservoir, while the flood wave released at the dam increases. Extending the duration of free flowing conditions after draw down proves to be an effective method to increase the sediment flux at the lowest reducing in electricity generation.

Performing a flushing event with 48 hours at free flowing conditions during the flood season increases the yearly cumulative sediment flux on average by a factor three. The cumulative electricity generation is reduced by just 2.5 %. Extending the duration of the free flowing conditions in a flushing event increases the sediment flux per lost GWh electricity generation.

Current operational rules at the planned cascade in the main stream Mekong do not consider flushing at all. It is recommended to include flushing events in the operational rules which are initiated if the discharge is above the design discharge of the turbines and below extreme discharges to increase the sediment flux and minimize electricity generation loss.