The Impact of Dams and Reservoirs: Costs, Benefits, and Management Strategies

Posted on Sep 2, 2024 in Geology

Result from Construction of Dams and Reservoirs

Increased Water Storage and Evaporation

Reservoirs, as artificial stores with larger surface areas than rivers, increase the amount of water stored on the surface. This leads to greater water loss through higher evaporation rates, resulting in more water vapor in the atmosphere. As this vapor rises, it cools and condenses into water droplets, potentially increasing precipitation.

Groundwater and Discharge

Seepage into groundwater increases base flow. Dams regulate water flow, leading to less extreme variations in discharge downstream. This results in fewer floods with longer delays and lower peak flows. However, decreased discharge downstream can cause environmental problems, such as reduced water for irrigation.

Sediment Transport and Erosion

Increased velocity downstream, due to water carrying less sediment, increases the erosive power of the river. Changes in sediment transport downstream of dams can lead to clear water erosion.

Costs of Dams and Reservoirs

Financial Costs

Construction and maintenance of dams and reservoirs can be very expensive. For example, the Three Gorges Dam in China cost $39 billion to build.

Loss of Land

Reservoir flooding results in the loss of large areas of land, displacement of populations, and loss of cultural heritage. The Aswan High Dam in Egypt, for instance, led to the forced migration of 100,000 Nubian people.

Disease

Dams and reservoirs create large bodies of stagnant water, which can become breeding grounds for disease-carrying organisms. The Aswan High Dam in Egypt has seen an increased rate of Bilharzia, a disease caused by freshwater snails that can lead to diarrhea and an increased risk of stomach cancer.

Loss of Nutrients

Dams trap large amounts of sediment, reducing nutrient-rich alluvial deposits downstream. This can decrease farmland fertility and impact farmers’ livelihoods, as seen with the Three Gorges Dam.

Geopolitical Conflict

Dams store large quantities of water and reduce downstream flow, potentially causing tension between affected downstream states and the state where the dam is constructed. The Mekong River dams, such as the Xayaburi Dam in China, have reduced river flow in Laos and Cambodia.

Benefits of Dams and Reservoirs

Hydroelectric Power (HEP)

Dams can generate renewable energy by harnessing fast-flowing water. The Three Gorges Dam in China, the world’s largest HEP station, has a capacity of 22,500MW and generates power for nine provinces.

Water Supply

Reservoirs can store large quantities of water for domestic and agricultural use. For example, 60% of the water from the Aswan Dam is used for irrigation.

Flow Regulation

Dams and reservoirs can help prevent floods by maintaining river water levels and reducing extreme seasonal irregularities in flow. During high rainfall, they can store excess water, and during low flow periods, they can release stored water. In 2011, the Three Gorges Dam released 5 billion m3 of water during a severe drought in China.

Navigation and Transport

Reservoirs raise water levels upstream and slow down the flow rate, improving navigation conditions for ships and boats.

Recreation and Tourism

Large reservoirs can support recreational activities like fishing and water sports, attracting tourism and creating jobs. The Contra Dam in Switzerland, for example, was used by commercial bungee jumping operators.

Case Study: The Akosombo Dam in Ghana

Constructed between 1961 and 1966, the Akosombo Dam and its reservoir, Lake Volta, cover 3.6% of Ghana’s total land area.

Benefits

  • Created 2,500 jobs in construction and maintenance.
  • Provided irrigation water, improving crop yields.
  • Provides affordable electricity to Ghana and neighboring countries Togo and Benin.

Costs

  • Cost $130 million to construct.
  • Displaced 80,000 people into resettlement villages with inadequate livelihoods.
  • Reduced sediment deposition on Togo’s beaches, leading to increased erosion.

Floodplain Management and Erosion

Erosion Processes

  • Hydraulic action: The force of water loosening material and widening cracks in the river bank.
  • Abrasion: The scraping action of transported material.
  • Attrition: The breaking of stones when they collide with other material.
  • Corrosion: The dissolving of rock (e.g., limestone) by acid in the water.

Transportation Processes

  • Traction: The rolling of stones and rocks along the river bed.
  • Saltation: Stones/rock bouncing, usually smaller rocks due to the energy required.
  • Suspension: Very small particles carried along in the river current.
  • Solution: Dissolved material.
  • Floatation: Leaves and twigs floating on the surface.

Deposition

Deposition occurs when the river loses energy and can no longer transport its load. This can be caused by reduced rainfall, increased evaporation or abstraction, friction, narrow channel sections, and the river’s mouth.

Oxbow Lakes

Oxbow lakes are formed through erosion and deposition. Lateral erosion, caused by the corkscrew motion of water flow, concentrates on the outer bank of a meander. During floods, the river can break through and create a new channel, leaving the old meander to become an oxbow lake.

Human Modification of Floodplains and Flood Effects

  1. Urbanization: Reduces percolation to groundwater and increases surface runoff.
  2. Deforestation: Decreases interception and infiltration, increasing runoff.
  3. River management strategies: Can increase flood risk by altering channel width and flow.
  4. Draining of wetlands: Increases land available for agriculture but reduces natural flood control.

Stream Management Strategies

Hard Engineering (Structural) Approaches

  1. Artificial levees: Expensive but can stop floods and increase storage.
  2. Channel straightening: Increases downstream gradient and flood risk but can remove excess water.
  3. Flood relief channels: Restrict urban growth but create habitat and controlled water release.
  4. Dams and reservoirs: Expensive with socio-economic impacts but provide storage and HEP.

Soft Engineering (Non-structural) Approaches

  1. Land-use zoning: Limits flood damage by restricting development in flood-prone areas.
  2. Do nothing (wetlands): Allows natural floodplains to absorb and regulate water flow.
  3. Afforestation: Increases infiltration and interception to reduce flood risk.