Image of the Carnarvon floods (2010)
A flood is the partial or complete covering of usually dry land areas with water from the unusual and/or rapid accumulation or runoff of surface waters from any source.
Floods are often caused by excessive heavy rainfall but may also be caused by such as storm surges, high tides, tsunamis, dam failure and more. Three common causes of flooding are:
Riverine flooding: when rivers or streams do not have the capacity to transport excess water and water flows out of the channel and onto the surrounding land.Storm surge: a rise above the normal water level associated with a storm or cyclone. This surge can inundate coastal areas.Flash flooding: a sudden flood event due to relatively short but intense rainfall.
Typically in the northwest, flooding will occur in summer related to cyclones and storms; whereas in the southwest flooding is commonly associated with winter storms. However, flooding can occur anywhere in WA at anytime of the year with the most flood prone areas being low-lying areas adjacent to waterways and coastal areas.
The Bureau of Meteorology delivers flood forecasting and warning for Australia based on rainfall and catchment conditions. The type of prediction is dependent on the quality of real-time rainfall and river level data. Flash flooding forecast are more difficult to provide due to their rapid onset.
Floods can cause significant damage to buildings and property, infrastructure such as roads, railways, power lines, water pipes and communication networks. Bridges and their abutments are often damaged (or submerged) by the fast flowing of floodwaters.
Damage to buildings and infrastructure can lead to significant economic losses. Indirect impacts such as disruption of business, industry and loss of livelihoods also contribute to economic loss. Downturns in tourism is often experienced after flooding events. The repair of damage however can lead to positive economic effects.
Flooding in agricultural and horticultural areas can lead to crop damage, fencing and loss of livestock. The natural environment can also be impacted through destruction of ecosystems and contamination of waterways. Nevertheless uncontaminated floodwater can rejuvenate the environment through increased soil fertility and recharge of water resources, and is a necessary natural process for many parts of WA.
Most flood-related deaths result when people attempt to drive, walk, swim or play in floodwaters. Depth and current are often misjudged and floodwater will sweep away and submerge even very large vehicles. Health conditions may also deteriorate due to contamination and waterborne diseases.
Monitoring: Early detection of potential floods using real-time rainfall and river level gauges.Land use planning: Appropriate land use planning can prevent important structures (residences, public buildings) from being built where they could be flooded.Floodplain management: Management of the floodplain such that it can be used appropriately without increasing risk.Physical controls: The use of dams or retention structures to minimise flood levels downstream.Risk awareness: Community awareness of floods and their impacts and how to prepare for and recover from flood events.