Application of a three-dimensional hydrodynamic model to the Nam Theun 2 Reservoir (Lao PDR)

Abstract

The hydrodynamics and thermodynamics in large lakes are key factors governing water quality. In such ecosystems, assessments of water quality cannot be made without sufficient knowledge of water temperature and currents. In the case of the Nam Theun 2 Reservoir, this information was provided by a modelling approach using the Delft3D FLOW software.Between June 2008 and 2010, measured surface and bottom water temperatures ranged from 18.1 to 31.8°C and from 16.0 to 25.1°C, respectively. The mean absolute errors between the observed and the computed temperatures were between 0.54 and 1.02°C at the surface and between 0.67 and 0.86°C at the bottom. Larger errors were modelled when a high vertical temperature gradient was present but, the seasonal patterns were properly simulated, especially the stratification and destratification periods. The model was most sensitive to the vertical eddy viscosity and the Ozmidov length scale. Current velocities were also properly simulated both in absolute value, direction and profile shape. The maximum error (0.09ms-1) occurred close to the bottom of the Headrace Channel where the highest speed was measured (around 0.2ms-1), but the average error was less than 0.02ms-1. Until May 2010 (beginning of normal operation), currents were mainly oriented north-westward. Then, flows were diverted towards the water intake with very low velocities in the downstream part of the reservoir.Although used here for a large reservoir, a similar modelling approach can also be very useful to the understanding of natural large lake ecosystems.