Delft3D4 is open source software package, developed by Delft Hydraulics (now called Deltares) in collaboration with Delft University of Technology, is a model system (2D and 3D) that consists of a number of integrated modules which together allow for the simulation of hydrodynamics, transport of water-borne constituents (e.g. heat and salinity), sediment transport and morphological change(Mulatu, 2007).
The RTC toolbox is used to control hydraulic structures like weirs, pumps, and hydropower and water intakes (http:// oss.deltares.nl/web/RTC-Tools) .This tool is coupled with Delft3D4 software to simulate the dam operation and morphological and hydrodynamics processes within the model domain.
The two software mentioned above are coupled together to simulate the reservoir, the dam and the downstream reach of the dam.
Modelling approach
The high flow velocities generate high bed shear stresses and let the water carry a high amount of sediment to the downstream reach of the dam. In order to mimic this behaviour, a 2D modelling approach is considered to capture all morphological and sediment sorting processes. Furthermore, a real-time control module (RTC) is coupled with Delft3D to operate the gates. The Delft3D modelling system is capable of catching these processes as shown by the previous Delft3D model studies on the Tenryuu River (Becker, 2015 ; Sloff, 2009; Sloff, 2001; Yossef, 2010). Both softwares’ are open source (Mulatu, 2007).
The two gates operation techniques are set up for three different peak discharges to grasp or calibrate, firstly, the hydrodynamic behaviour of the model and then investigate the bed load transport corresponding to the different gate operations. The upstream bed topography is expected to have an influence on the downstream morphological changes. Flushing in one year may result in lower reservoir bed topography in the subsequent year. Therefore, two-bed topographies for the reservoir are used to explore the sensitivity of sediment transport through the dam to the reservoir bed topography. In view of model uncertainties, results will be interpreted in a qualitative way.
The gate operation is simulated by using the Barrier Function of Delft3D. The barrier is a combination of a movable gate and a quadratic friction term which is added to the momentum equation. The depth-averaged flow rate through the barrier is calculated using the following equation:
(1)
Where:
Q is the discharge in m3/s, is the barrier contraction coefficient (0 < 1), A is the area of the barrier (the width of the gate times the gate opening) in m, g is the gravitational acceleration in m/s2, H1 is the reservoir water level in m and H2 is the downstream water level or in some cases the dam overflow crest level, in m.
The contraction coefficient is used in the Delft3D model to acquire the equivalent energy loss coefficient () in both velocity directions (U and V) at the barrier. According to a depth-averaged analysis, the energy loss coefficient is related to the barrier contraction coefficient as
(2)
However, it can be used as a calibration coefficient to ensure that the barrier discharges the correct amount of water through the corresponding gate opening (Deltares, 2016).
