River Diversion
Before construction can begin, the river must be diverted through constructed channels on the surface or underground. This drains the construction site for access while still providing water further downstream. A temporary dam called a cofferdam is built further upstream from where the main dam is being built to protect construction from floods.
Another set of temporary cofferdams is built on the other side of the river while the main dam is being built. In the final step, the original cofferdam will be reconstructed and the portion of the main dam behind the cofferdam is completed. Once the cofferdam is removed for the last time, the dam is complete, and water is stored in the reservoir.
In bankside or other off-stream reservoirs, the river will be diverted to the off-stream storage site after construction is complete.
Foundations
This is a critical part of the reservoir’s structural integrity, as well as reducing seepage and erosion.
The reservoir is excavated after the available water is drained away. All loose soils and sediment are dredged while excess greenery is removed until bedrock is exposed.
Dammed valley reservoirs, or any reservoir that utilizes natural rock walls, will need to have the valley walls tested and prepared before holding water. Hundreds to thousands of cubic meters of unsuitable rock may be removed to reach bedrock that is strong and impermeable enough for use.
The local geology will be surveyed for faults and cavities. Any cavities or cracks in the bedrock are filled with concrete or grout to increase the stability and prevent seepage. The foundation surface of the dam will be moistened with some water and with the use of a roller it will be compacted to increase strength, stiffness and stability, while reducing permeability.
Lastly, if there are any major structures going to be found within the reservoir, (like water intake towers in the case of hydroelectric plants), their foundations will also be implemented at this stage.
Construction
Most on-stream dams are made from concrete and reinforced steel. These dams use their own structure’s weight, anchors, and the underlying geography to support incredibly large amounts of water. The world’s largest dammed reservoir is the Three Gorges Dam and Reservoir in China, which stores around 26.43 million cubic meters of water and generates about 84.68 billion kilowatt hours per year.
Embankment dams are made primarily of soil or rockfill found nearby the construction zone or directly from the excavation. Erosion is a common issue for embankment dams. Vegetation growth and other methods of soil stabilization can assist in this.
Filling the Reservoir
Once the dam has been constructed and all the structures are complete, the land that will be submerged needs to be surveyed. Potential contaminants, trash, and debris are all removed while the surrounding area is barricaded.
On-stream storage will be filled by releasing the previously diverted river, or the off-stream basin will be filled via channels.
The site will need to be carefully monitored throughout the filling process. As loose dirt or soil becomes wetter and heavier, mudslides and landslides are possible.
Water quality is monitored at the inflow and outflow of the reservoir as well as the reservoir water itself. Water quality can be affected by the reservoir since the water that was formerly flowing is now still. Over time, nutrients and chemical build-up in the water can render it unsuitable for drinking or bathing.
Testing Valves and Floodgates
Valves are placed in the dam so that water can pass through the dam and go downstream, reducing pressure and the overall water level of the reservoir. Valves are tested to ensure that minimum required flows are met and that they can withstand the passage of higher flows if needed. Spillways are part of a dam that allow water to automatically flow over the dam during a flood event, and some larger dams will include floodgates that can be opened to release more water.
Monitoring the Dam
The weight of a dam will eventually press the structure into the ground, decreasing its overall height. This “settling,” should be considered during design for water capacity and the overall freeboard. It’s important to monitor the dam at this stage, paying attention to the structural and functional capacity and integrity of the reservoir and impounding walls.
The walls and surfaces of the embankments or impounding dams need to be monitored as well. Small cracks will quickly begin to allow water to pass through, at which point damage and required repairs can become rapidly more severe and emergent.
Stability of the dam’s structure will also be monitored through recording its location along the stream or river. Drifting down or upstream means the foundation of the dam isn’t stable. Its inability to stand upright will also be a major sign that the dam isn’t structurally sound; tilting is also a concerning development.
Water quality up and down stream of the dam, or inside and outside of the reservoir, should also be monitored during routine maintenance.
