The report prepared by English nature, the environment agency the department for environment, food and rural affairs and the forestry commission. 3rd chapter of the report clearly indicates on the wetland creation options for flood mitigation. Those are as follows (English Nature et al., 2003),
• Creation of wetlands on land previously protected from flooding to a high standard where an acceptable change in land use to a wetland allows the standard of protection to the land to be reduced then additional floodwater storage can be gained that provides flood management benefit.
• Creation of wetlands in existing or new wash lands, flood detention reservoirs and flood bypass channels maintaining a high water table and/or standing water in such areas reduces the storage available for flood management and will require the storage area to be “over-designed” to compensate for this
• Wetlands created alongside river channels by removing or setting back existing defenses or during widening of channels unless a very large area is involved this will have a neutral impact on storage processes within the floodplain but can increase the capacity of the channel to pass flood flows.
• Wetlands created by land management changes
These include measures such as removing/not maintaining under drainage, restoring ditches, reducing stock levels, restoring arable land to rough pasture and creating woodlands. Given these conditions wetlands will naturally re-establish themselves where they are sustainable. Land management changes that increase storage are nearly always beneficial to flood management
2.2 Reservoirs
Generally, a reservoir is constructed for multi-purpose performance. Among them the water storage is the major one. Except that a reservoir is used for water supply for drinking, irrigation, hydropower generation and flood controlling.
During peak flood, the reservoir stores all or portion of water within the reservoir and release it to downstream slowly. Small and medium floods fully captured by the reservoir and
large floods are only partially attenuated. Further a, reservoir is needed for flood mitigation only for few days or weeks in any particular year.The important function of flood retention reservoir is to decrease the flood peak by retention. This is done by keeping some portion of water and releasing it in a controlled manner. Therefore, peak flow is reduced at downstream places which are to be protected. The flood retention function of reservoir is not necessary to reduce the total volume of runoff but needed to reduce the peak flood by redistributing the inflow hydrographs (Harmanciogiu, 1994).
Dams and reservoirs have been classified as four groups considering their role in flood mitigating as follows (European group of flood 2010).
1. Reservoirs with the single purpose of regulation (water supply, irrigation, hydropower, navigation or environmental flows), in which the incidence in flood mitigation could be small.
2. Multipurpose reservoir in which flood mitigation is an important objective, but secondary to purposes associated with water storage.
3. Multipurpose, reservoir in which flood mitigation is the principle objective, combined with other secondary objectives.
4. Reservoirs with the single purpose of flood mitigation and reduction of downstream flood impacts and damages.
And furthermore, flood mitigation purpose is needed only for flood season which is small time period of particular year. So, reservoir operated for other purposes during rest of the year.
This is table shows us the flood mitigation of dams in Europe(European group of flood 2010).
