Flooding in Victoria
Generally in Victoria major regional floods occur about every 5 to 10 years. Larger floods are less frequent, but it is not unknown for more than one major flood to occur in a catchment in successive years.
Flooding can be simply described as an overflowing or accumulation of a body of water onto normally dry land by excessive rainfall or storm surge in coastal areas. While flooding is usually a natural event, and one which is important to maintain biological diversity, it becomes a serious problem for the community when water accumulates where it is not intended and damages property, endangers lives and causes loss of amenity.
In Victoria there are three main causes of flooding, which act either independently or in combination: prolonged heavy rainfall, storm surge and high tides. Tsunamis and dam failure are other potential causes, but the combination of low frequency of occurrence, small impacts (in the case of tsunamis), and good safety management (for dams) means the flood risk from these is very small.
Floods are usually caused by heavy prolonged rainfall, which produces surface runoff which then flows overland into rivers and streams. When there is a large volume of runoff, water is unable to be contained within the river banks and it overflows onto the adjacent floodplain, which is the low lying land bordering a watercourse. This is known as riverine flooding.
Flooding can also affect urban areas through overland flows which occur following heavy rainfall, when the capacity of drainage systems is exceeded.
Coastal flooding can be caused by high tides or storm surges, which result in higher water levels along the coastline, often in combination with riverine flooding. A storm surge is a temporary increase in the coastal sea level caused by falling atmospheric pressure and severe winds during a storm.
The management of floods and floodplains brings together the resources of various agencies, authorities, municipal councils, emergency services and the flood affected community to take appropriate and timely action in relation to flood prevention, response and recovery.