Analysis of sediment transport processes during reservoir flushing using different numerical models.

Gabriele Harb, Josef Schneider, Gerald Zenz

Tuesday 30 june 2015

12:30 - 12:45h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 5A. Sediment - Erosion

The deposition of sediments due to the reduced flow velocities, turbulences and bed shear stresses is a problem in many reservoirs. In the case of Alpine reservoirs with a small storage volume compared to the annual inflow, such as reservoirs of run-off river power plants, the water depth are usually lower than in reservoirs of storage and pump-storage hydro power plants. A larger part of the suspended sediments is thus transported through the reservoir and deposition of bed load fractions is the main problem. The deposition of coarse sediments at the head of the reservoir may cause problems regarding flood protection by raising the bed level and thus, raising the water level too. This study focus on the sediment transport processes during a flushing event in an Alpine reservoir. The reservoir was built in 1981 and is approximately 4.5 km long with an initial storage volume of about 1.4 Mio. m3. However, echo-soundings performed in 2007 showed that approximately 890,000 m3 of sediments are already deposited in the reservoir, which corresponds to an annual sedimentation rate of about 6.1%. A flushing event was modelled using an open source one-dimensional and a three-dimensional numerical model. The calibration of the hydrodynamic part of the models was done using ACDP measurements performed at the prototype. Echo-soundings performed before and after the flushing event were used to validate the morphological changes calculated by the numerical models. Additionally an extensive sensitivity analysis was carried out and several sediment transport formulae were tested.