What is a flood mitigation dam: A flood mitigation dam typically comprises of a wall or embankment that provides a water storage or reservoir, an outlet or flood gates to control water flow and a spillway to pass flows that exceed the dams design capacity. Most dams in Australia are built for water supply however, some are designed for flood mitigation purposes and have a built-in storage area that is kept free for temporarily storing floodwater with the aim of reducing peak flood heights downstream.
A flood mitigation dam generally does not improve community access and recreational use. Why? Although some dams permit fishing, kayaking and non-powered boating, commonly dams restrict access for safety and environmental reasons particularly if water is used for supply. This can mean areas that you once use to visit for fishing and bushwalking etc. may no longer be accessable.
A flood mitigation dam usually causes equality issues and can impact individual members of the community. Why? A dam is typically a large structure requiring large amounts of land to provide sufficient water storage. As a result particularly in urban catchments, large areas of land need to be purchased causing the relocation and disruption of local community members.
Flood mitigation dams in general provide an additional level of flood protection to the community during flooding. Why? The dams “free” storage can be filled by flood waters and later released. This in effect reduces the flood water level downstream but increases the time that land is inundated. Please note: Sometimes community safety may not result from dams because: 1) the dam may be kept full for water security reasons particularly after drought periods, which in turn significantly diminishes the ability for the dam to temporarily store flood water during an event; 2) Dams usually only collect water from one major river (as it is not feasible to have dams on every major river) so rainfall on other streams and rivers may cause flooding; 3) Floods may generate much greater volumes of water than even large dams can store.
A flood mitigation dam can improve community awareness and understanding of the local flood risk. Why? Flood mitigation dams are usually seen as water supply areas, but with signage can prove to be valuable flood reminders.
Flood mitigation dams have significant negative environmental and ecological impacts. Why? A flood mitigation dam can cause the mass flooding of productive forests and habitats, have significant impacts on animal migration patterns, increase erosion and cause stream lowering downstream, and cause large releases of greenhouse gases from rotting organic material such as trees and vegetation found now at the bottom of the dam.
A flood mitigation dam causes negative water quality impacts. Why? Dams change the flow regime of a river both upstream and downstream. The impacts of this include: diminishing the capacity of the river to regulate its flows causing scour and erosion, changing temperature and oxygen content as reservoirs can be oxygen poor and cold, increases salinity downstream and reduces the ability for the lower floodplain to recharge.
Flood mitigation dams have major initial costs and are typically not economically viable if only used for flood mitigation purposes. Why? Dams require significant capital investment and range from hundreds of millions to billions. The construction of dams is relatively complex requiring numerous engineering design and feasibility studies, purchasing large amounts of land, providing environmental offsets, and then construction costs including material and labour.
Flood mitigation dams have moderate ongoing maintenance costs. Why? The maintenance of dams is critical due to the fact that failure of a dam causes catastrophic impacts downstream. Maintenance can include routine inspection for deficiencies such as cracking and movement, seepage, flood gate testing and operation, and ensuring operation plans are up to date.
Flood mitigation dams have the ability to significantly reduce flood related damages in large flood prone areas (NSW Government 2005). Why? As detailed previously Flood mitigation dams can substantially reduce damage costs particularly for more frequent flood events. Note: Failure however, can cause catastrophic downstream damages.
Flood mitigation dams cause adverse flood impacts to other areas. Why? As mentioned previously a dam is created by placing a wall or embankment in a river and then allowing the upstream area to flood. This therefore, has significant adverse impacts to the inundated land.