How Is A Hydroelectric Dam Built?

Hydroelectric dams use the natural flow of water to generate clean and renewable electricity. This article will provide an overview of the major steps involved in building a hydroelectric dam from start to finish.

We will cover the planning and approval process, site preparation, river diversion, excavating the dam foundation, constructing the dam structure and spillway, installing the turbines and generators, reservoir filling, testing and commissioning, and finally operating and maintaining the dam.

Understanding how hydroelectric dams are constructed provides insight into this important source of renewable power that provides electricity to millions of people worldwide.

Planning and Approval

Before construction of a hydroelectric dam can begin, extensive planning and approval processes must take place. The first step is usually a feasibility study, which examines the costs and benefits of the proposed dam. Engineers will analyze the topography of the area to identify optimal locations and dam heights. Environmental consultants conduct studies to assess the impacts on wildlife, vegetation, water quality, and more. These initial investigations help determine if the project is economically and environmentally viable.

The next phase is the environmental impact assessment (EIA), a thorough report documenting all the potential environmental, social, and health effects. The EIA will outline mitigation strategies to minimize harm. Extensive public consultations are also required at this stage to address community concerns. The final feasibility study and EIA are submitted to various government agencies for review and approval.

Since large dams inevitably alter rivers and ecosystems, the approval procedure is comprehensive. Permits regulating water resources, wildlife, heritage sites, and more, must be obtained before construction can proceed. Negotiations with indigenous groups may also be necessary if treaty lands are impacted. The planning and approval phase for a major hydroelectric project can take 5-10 years to complete before the builders receive final consent.

Site Preparation

Once the dam site has been selected and approved, the area must be cleared and prepared for construction. This involves clearing trees, vegetation, topsoil and any existing structures from the foundation area and planned reservoir. Explosives may be used to break up and remove any large rock outcroppings. Access roads to the site are also constructed to allow equipment and materials to be brought in. An encampment area for workers is prepared, along with facilities like concrete batch plants and workshops. Drainage channels are dug around the perimeter to divert water away from the construction site. Any archaeological or historical artifacts are recovered before they become inundated by the reservoir. Environmental mitigation measures may also be implemented, such as relocating wildlife or replanting vegetation in other areas.

Site preparation requires heavy construction equipment such as bulldozers, excavators, dump trucks and front-end loaders. The cleared area must be large enough for the dam structure, spillways, penstocks and powerhouse. Sufficient rock and soil material must also be available to produce concrete aggregate and fill the dam’s rock-and-earth embankments. The foundation rock or soil is extensively surveyed to identify any faults, fractures or weak zones that could affect the dam’s seismic stability.

River Diversion

Prior to building the dam structure, the river’s flow must be diverted and controlled. This is a critical step that allows the foundation and other dam components to be constructed “in the dry” rather than underwater.

There are a few main ways to divert a river’s flow:

• Tunneling – Tunnels can be drilled through the rock on either side of the dam location to reroute the river’s flow around the construction site.



• Cofferdams – Watertight temporary dams built upstream and downstream of the construction site. These block and divert the river into bypass channels.
• Staged construction – Build the dam in sections across the river, allowing the river to flow through openings in the partially built dam.

The method chosen depends on the river’s size and flow, the geology, and how much flow needs to be diverted. Proper control structures like gates and valves are used to regulate the diverted flow. Monitoring systems track flow rates and water levels.

River diversion is a major undertaking, but it is essential for accessing and dewatering the foundation and construction site. Careful planning ensures the natural flow is maintained downstream throughout the build.

Foundation Excavation

Excavating and preparing the foundation is a critical step in dam construction. The foundation is the base on which the entire dam structure rests, so it must be dug deep and wide enough to properly support the dam. Proper foundation prep also ensures the dam structure integrates well with the surrounding geology.

The first step is to excavate down to bedrock level. This requires clearing away layers of soil, loose rock, sand, and gravel until you reach solid bedrock. Digging down to bedrock provides a stable, uniform foundation on which to build the dam. Dynamite and other explosives may be used to break up and remove stubborn rock layers.

Once the bedrock is exposed, it must be prepared through cleaning, grouting, and consolidation. Workers clean the rock surface to remove loose debris. Next, grout (a cement mixture) is pressure-injected into cracks and voids in the bedrock. This waterproofs the foundation and binds it together. Finally, dental concrete, rock bolts, and netting are added to further strengthen and reinforce the bedrock.

The foundation is dug wider than the base of the dam itself. This extended foundation provides structural support and prevents water from seeping underneath. Excavators dig drainage channels, galleries, and outlets along the dam’s footprint to control water flow. The entire foundation prep process ensures the bedrock can withstand the enormous weight of the dam and water pressure from the reservoir.

Dam Structure

The dam structure itself is the most crucial component of a hydroelectric dam. The dam wall holds back and impounds the reservoir of water that will power the turbines and generators to produce electricity.

There are several designs and materials used to build dam walls, depending on the site conditions and project needs. Concrete gravity dams are the most common – they resist the force of the water through sheer mass and strength. Arch dams and buttress dams use less concrete but transmit forces through curved structures. Other types like embankment dams use earth, rock and tailings to form the structure.

To build a concrete dam, huge amounts of reinforcing steel and concrete are required. A network of steel rebar cages are embedded inside each concrete block poured. The rebar provides tensile strength while the concrete gives compressive strength. Cooling pipes may also be installed in some dams to regulate the concrete’s temperature as it cures.

The blocks are carefully joined together, with waterstops between them to prevent leakage. Construction takes place in stages, allowing the concrete to cure properly. The dam wall gradually takes shape from the bottom up, using cranes, buckets, pumps and other heavy machinery. The spillway and intake structures are also constructed during this phase.

Strict quality control and continuous monitoring is critical during the entire dam structure construction process. Concrete mix, placement and curing must adhere to the highest standards for strength and durability.

Turbines and Generators

Once the dam structure itself is mostly completed, it’s time to install the hydroelectric power generating equipment. This includes the turbines that will harness the power of the falling water, as well as the generators that convert that mechanical power into electrical energy.

Installing the turbine runner is a major milestone in the construction process. The runner sits inside the turbine housing and has curved blades that spin when hit by the force of the falling water. It is usually custom designed and manufactured specifically for that dam based on the water flow volumes and head height. The runner is lowered into place with a heavy lifting crane and secured firmly within the turbine housing.

After the turbine is installed, workers must install the generator stator and rotor. The rotor is attached to the turbine shaft and spins within the stator to produce electricity. The generator often weighs hundreds of tons, so it also requires heavy lifting equipment to put in place. Once fully assembled with the turbine, the generator undergoes testing to ensure it can produce electricity to specifications before the dam becomes operational.

Reservoir Filling

Once the dam structure, spillways, turbines, and other components are installed, the reservoir area behind the dam is allowed to fill with water. This is a gradual process that usually takes several months to over a year. The rate of filling is carefully controlled by opening valves and operating spillway gates.

Filling the reservoir too quickly can put excessive water pressure on the new dam structure and cause leaks or additional settlement. Engineers follow a schedule of incremental increases to the reservoir level. The filling stages are monitored using gauges, downstream water flows, visual inspections, and other data to ensure the dam remains safe.

As the water level rises, it submerges roads, bridges, trees, and structures that were in the original river valley. Boat ramps, marinas, and other lake recreational facilities may be constructed during reservoir filling. Fish are often stocked in the new reservoir prior to it reaching full capacity.

When the water level nears the top of the dam, filling proceeds slowly and stops below the maximum for the first year. The new dam is monitored for performance through its critical first year before allowing the reservoir to reach its intended water storage capacity.

Testing and Commissioning

Once the main components of the hydroelectric dam are built, extensive testing needs to be done before full operations can begin. The turbines, generators, control systems, and other equipment must be methodically checked to ensure everything is functioning properly and safely.

The turbines will be spun at increasing speeds, first with no water flow and then gradually with more water let through, to confirm they operate smoothly across their full operating range. Sensors are used to monitor vibration levels, temperatures, pressures, and other parameters. Tests may need to be repeated after any adjustments or repairs.

Generators are also run through a range of loads to verify their output. The voltage, frequency, and other electrical characteristics are carefully monitored. Tests help confirm the generators and electrical systems integrate properly with the grid infrastructure they will be supplying.

Control systems and safety mechanisms will be evaluated to ensure the dam’s equipment can be reliably and safely operated. Alarms, fail-safes, backup systems, and other protections are evaluated. The dam’s operators will be trained on proper procedures.

The commissioning process involves gradually ramping up operations. As testing proceeds without issues, the hydroelectric dam will start generating power at partial capacity. Only after thorough testing will full commercial operations begin.

Operation and Maintenance

Once the hydroelectric dam is fully constructed and operational, ongoing operation and maintenance is critical to ensure maximum efficiency, safety, and longevity of the dam. Dams have a typical lifespan of 50-100 years, but require constant monitoring and upkeep.

A team of dam operators work around the clock to monitor and control the flow of water through the dam. They operate spillway gates, valves, and turbines to control water levels in the reservoir and rivers. Real-time monitoring with sensors allows operators to respond to any anomalies or emergencies quickly.

Routine maintenance includes lubricating and maintaining turbine equipment, inspecting the dam structure and concrete for cracks or leaks, monitoring sediment buildup in the reservoir, testing warning systems, evaluating stability of surrounding slopes, and removing debris and blockages. Larger maintenance projects like resurfacing spillways or replacing turbines and generators may occur every few decades.

Strict safety protocols protect workers and the public during dam operations and maintenance. Dams have emergency action plans for flood events, earthquakes, sabotage, or other disaster scenarios. Ongoing dam safety inspections by regulators identify any deficiencies to address. With diligent maintenance and training, hydroelectric dams can operate safely for many decades.