Risk management in dam operation: Aapproaches to improve flood control capabilities of existing dams

Needs Address

When floodwaters exceed a dam’s designed capacity and the reservoir fills to its limits, operators have no choice but to release a volume of water equivalent to the volume of inflow. In such cases, it is essential to take every possible measure to avoid any flooding that may occur downstream.

Adaptation effects

• During emergencies–such as when floodwaters exceed the dams designed capacity, or when floodwaters are expected to significantly exceed the discharge capacity downstream of the dam–flood damage downstream along the river can be reduced by reducing the maximum flood release. 
• More time can be secured for evacuation by extending the amount of time available for flood control at the dam.

Overview and Features

• The storage capacity of a multipurpose dam includes both “water use capacity” and “flood control capacity.” In cases where a flood is expected to fill the reservoir above the designed capacity, a larger than the planned flood-control volume is secured by advance discharge of a certain volume of service water (prior flow) to bring the water level as low as possible. 
• For water utilization dam, when a flood is expected to significantly exceed the discharge capacity downstream of the dam, the reservoir level is lowered in advance to the lowest level expected to have a flood-control effect. 

Cost

• Compensation for losses in the event of an early release of water, if the “service-water volume” cannot be restored after the flood. 
• Costs of upgrading of water level indicators and dam management control process equipment, etc., in the event they become necessary if water is allowed to fall below the dam’s planned level. 

Note: The approach to covering the costs mentioned above should be decided in advance through discussions with water users. 

Energy source

Same as during regular operations. 

Ease of maintenance

Same as during regular operations. It is critical to exercise caution when inspecting slopes around the reservoir during periodic patrolling, as the water level will have been lowered to the lowest possible level for the purpose of risk management operations by the dam. 

Considerations

• For service-water dams, in cases where risk management operations are conducted at the request of the river administrator, it is important to clarify who bears the responsibility for warnings and notifications to river users. 
• In cases where predictions under the risk management operations turn out to have been wrong, service-water volumes may be slow to recover, resulting in a shortage of water. 
• Because different regions may have different impacts from flooding and water shortages, it is important to coordinate interests and opinions among stakeholders. 
• Depending on the status of rainfall forecasts and discharge, etc., it is not always possible to achieve a reduction in water level. 
• There are limits to the flood control benefits of water level drawdown. For example, in the case of a prefecture-operated dam in Japan’s Wakayama Prefecture, the benefits of drawdown by the maximum discharge volume in a flood would be from zero to about 10 percent, which would mean an increase of flood control time of between zero and two hours. 

Co-benefit, suitability for developing countries

When carrying out the early release by operational measures, because advance notification can be given to the relevant agencies, municipalities can take measures for early evacuation, to prepare for possible river inundation downstream from the dam. 

Information Resources

• Information Office, Wakayama Prefecture, website of prefectural governor (in Japanese)
  http://www.pref.wakayama.lg.jp/chiji/press/240411/index.html#d240411_1
• Information Office, Wakayama Prefecture (River Division, River and Sewer Bureau, Prefectural Maintenance Section)
  Notifications about new river operations (in Japanese)
  http://www.pref.wakayama.lg.jp/prefg/080400/dam/zizennhouryuu/damoshirase.html