Introduction

Hydropower dams provide a great service to society. Their number continues to increase because of growing demands for water and electricity. However, they disturb river morphology and ecology. The reservoirs behind the dams suffer from loss of storage and contamination due to sedimentation. The reduction of storage capacity poses challenges to dam operators. The requirements of minimizing storage losses while caring for the ecosystem downstream leads to investigations into optimum operational rules. Sediment management practices for this purpose, such as sluicing and flushing, may generate undesired morphological changes in the reach downstream of the dam. The gate opening pattern is expected to influence this. We developed a modelling approach to simulate flow regulation through a dam to understand the effect of different gate opening patterns on the morphodynamics. The aim is to seek improvements of the flushing process and to understand how this process affects the morphological changes downstream of the dam. We built a 2D morphodynamic model using Delft3D 4 open-source software, capable of simulating the time- and space-varying hydrodynamics and morphological changes in the reservoir and the river reaches. Delft3D software can be coupled with the real-time control toolbox (RTC) to control different gate openings in time. RTC is open-source software too. We apply the approach to Funagira reservoir, located in the Tenryuu River in central Honshū, Japan. The river downstream of the dam suffers from bank erosion and side bar formation due to the existing pyramid-shape opening pattern of the gates. Equal-shape openings are introduced as a new pattern. We analyze and compare both patterns in terms of flushing efficiency and sediment distribution downstream the dam.

Motivation

Sediment management in river systems with hydro-power dams is of crucial importance, both for economic as ecologic reasons. Sedimentation in reservoirs upstream the dams may lead to loss of active storage and blockage of the inlets and consequently loss of economic value. For the ecological value (e.g. fish habitat) a proper mix of sediment is important. Many hydropower companies around the world are therefor interested to improve the sediment management of the reservoirs. Together with J-Power, the largest electric utility in Japan, we are carrying out the current research project to jointly develop knowledge and techniques to improve the gate operation for better management of the sediment trapped in reservoirs and spread of sediment downstream.

Goal

The goal of this research project is to expand the knowledge on sustainable sediment management in rivers with reservoirs and generate generic tools, like numerical models (Delft3D). With these knowledge and tools reservoir operators can be advised on how to optimize their sediment management. The project is subdivided into two projects:

Project 2016 (DEL026)

In the current project the Funagira dam will serve as a case study on how the gate operation can help to improve the sediment management. By setting up a numerical model (Delft3D), including the gate operation (Real Time Control Module (RTC)), the gate operation pattern can be optimized.

The case study will be elaborated to investigate:

• the mobilization of sediment in the reservoir in the upstream part (lifetime of the reservoir).
• the influence of the jet of water coming across the dam on sediment transport downstream (erosion sedimentation problems downstream)
• behaviour and sorting mechanisms of graded sediment (ecological purposes)

Results

1. Literature survey on sediment management in reservoir

2. Case study Funagira dam: Numerical model setup and runs to study the influence of the gate operation pattern on the sediment flow through the dam and the sediment behaviour downstream
3. Development: Improve the numerical tools by including the dam operation (Delft3D and RTC) for modelling the behaviour of sediment up- and downstream of hydropower dams)
4.  Research output:  A paper has been prepared out of this research project under the name of " The effect of different gate opening patterns on reservoir flushing and morphological changes downstream a dam".

Project 2017 (DEL045)

2016  project is extended for another year to improve model performance for reservoirs. More development to  Delft3D software to include PID controller (A proportional–integral–derivative controller) is needed. In addition,  testing the software in different reservoirs and dams .Furthermore ,the effects of using  those sediment management practices to the ecosystem  shall be evaluated. More information is available in the new project page  (DEL045).