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Koysha HEP – Physical model investigation of the middle level outlets

Lais, Adriano
Höck, Esther
Arnold, Romeo

Visualisation of the Koysha dam (Source: www.ethiopia.salini-impregilo.com)  
Visualisation of the Koysha dam (Source: www.ethiopia.salini-impregilo.com)

The Koysha Hydro Electric Project is located at the Omo River in Ethiopia. The plant has an installed capacity of 2200 MW and will produce 6500 GWh of electricity per year. The Ethiopian

Electric Power Corporation (EEPCo) is the owner and project execution institution.

The 178 m high roller compacted concrete (RCC) gravity dam impounds a storage volume of 6000 million cubic meters. The dam is crossed by three power waterways feeding an outdoor power house which is equipped with eight turbines located at the left bank riverside. A gated spillway is a separate and independent structure on the left bank that discharges the water downstream of the dam and the powerhouse back into the Omo River. Two middle level outlets are foreseen to be located in the central part of the dam.

Each middle level outlet has a diameter of 6 m and is around 100 m long. The maximum discharge is around 900 m3/s per outlet. The conduits are steel lined and have to withstand a static pressure of about 98 m WC at the maximum storage level. Each conduit is equipped with a bulkhead gate at the intake and with a wheel gate and a radial gate at the outlet.

Construction of the Koysha middle outlets  
Construction of the Koysha middle outlets

VAW is commissioned by the contractor Salini Impregilo S.p.A. to perform physical model test on the middle level outlets. The investigation is separated into three main parts. The first part is the optimization of the hydraulic behavior inside the conduits including the observation of vortex formation for low water levels and flow detachments, extensive pressure measurements and cavitation analysis. The second and third part comprise the extent of the jet trajectories and the formation of the scour in the downstream section.

Contact:

Lais, Adriano
Höck, Esther

Arnold, Romeo

Keywords: RCC dam, middle outlets, hydraulics, jet trajectories, scour
Commissioned by: Salini Impregilo S.p.A

Dachwehr Höngg

Höck, Esther
Schroeder, Alice

Dachwehr Höngg  
Bear trap weir Höngg in a raised position.

After nearly 35 years of operation the weir Höngg is subject to refurbishment. The present weir, located at the river Limmat in Zürich, is a bear trap weir with three weir fields, which was built from 1978 to 1982. Its main functions are energy production as well as the replenishment of the groundwater aquifer upstream.

In the course of the weir refurbishment, the existing fish pass, which does not satisfy the requirements, shall be replaced by a new one. Additionally, the construction of a residual water power station will be checked. These constructional changes will narrow the outlet cross-section of the weir. To evaluate the effect on the water levels and flood safety, VAW is commissioned by the Elektrizitätswerk of Zürich (ewz) to perform hybrid model tests. The physical model includes the close range of the weir at a scale of 1:40. To learn the correct approach flow conditions to the hydraulic model as well as the influence on the flood safety in the whole concession area, the investigation will be backed by numerical 2D simulation of a larger area.

In a second phase, the best solution will be checked more detailed in regard to operational conditions. The inflow and outflow field to the residual water power station and the fish pass will be optimized. This phase will be processed with a combination of the hydraulic model and a numerical 3D simulation.

Contact:

Höck, Esther

Schroeder, Alice

Keywords: bear trap weir, bed load, pressure distribution, flood safety
Commissioned by: Elektrizitätswerk der Stadt Zürich (EWZ)
 
 
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Thu Jun 29 00:24:17 CEST 2017
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