by Ercan Ayboga
Full Decommissioning / Removal
The removal of a dam re-establishes fully the free flowing conditions in a river because all physical obstacles will be abolished. There will be no structure remaining and the river’s continuity restored. The dam removal allows for the regular occurence of floods and low water flows.
Partial Decommissioning / Partial Removal
Under certain circumstances the partial decommissioning of a dam can be more suitable. Partial decommissioning of a dam can be implemented for several reasons. The structure can be very old (up to 150 years or more) and regarded in terms of historical significance which should be preserved, at least a part of the dam. Another reason could be the conservation of an existing wetland or the creating of a lake/wetland. Another reason is to retain storage capacity in an effort to manage flooding. Additionally, there can be the need to hold back sediments trapped behind the dam.
Here, issues surrounding flow conditions are divide principally between two cases. In the first instance, free flowing conditions are re-established, i.e., there is no impoundment and no real fragmentation. In the second case, there is still a dam reservoir what can be an obstacle for fish and benthos migration, as well as continued sediment entrapment.
Changes to Dam Operations (Modifications)
In cases in which full or partial dam removal is not considered a viable option, there are opportunities to change dam operation and to implement new water use planning without significantly limiting or abolishing the social and economic benefits of the dam and/or hydroelectric power plant operation. For several years this has happened with many dams in the industrial/western countries (particularly North America, Europe and Australia) after the adoption of new laws like the European Water Framework Directive.
Establishing continuity: By means of fish passages, the opportunity is created for many fish species to pass the dam downstream and upstream to reach former spawning locations. This can positively impact migrating fish populations.
Modification of water release: This action aims to create more natural flow and sediment regimes. The increase of the bottom/instream flow (during critical times such as spawning season) can be an option if there is the demand for more flow water for the ecology, flora and fauna (fish, zoobenthos and animals at the riparian zone/terrestrial animals) of the river downstream.
There are many cases of smaller dams where the big part of the water is diverted into a channel for hydroelectric energy production. Here the instream flow in the river channel can be increased to a level where the flow conditions become more natural. There is the possibility of increasing the average flow over the year and/or allow for flooding at critical times. These measures improve conditions for existing downstream of the dam.
Aeration and temperature modification: This action can improve downstream water quality.