The dynamic nature of the Australian coast means that long-lasting droughts can be followed by heavy rains and flooding. These fluctuations are natural as they bring the environment back to life, but they can also pose considerable risk to floodplain-inhabiting communities. There are a few different flood categories, which take into consideration their type, size and duration:
- Catchment flooding is caused by prolonged or intense rainfall, for example severe thunderstorms and extratropical cyclones called east coast lows (ECLs). The massive amounts of water running off catchments can lead to:
- Banks of creeks and rivers breaking, as has happened with the Hawkesbury River, Erina Creek, Narara Creek, Ourimbah Creek, Wyong River and many others;
- Filling of coastal lakes and lagoons, which has occurred with Tuggerah Lakes, Wamberal Lagoon, Terrigal Lagoon, Avoca Lake, Cockrone Lagoon and Pearl Beach Lagoon;
- Overland flow across ordinarily dry land in both urban and rural areas on its way to waterways.
- Coastal flooding occurs due to tidal- or storm-driven coastal events, including storm surges and wind-induced waves in coastal waterways. Some known examples where this type of flood occurs:
- Brisbane Water and the lower reaches of Narara Creek and Erina Creek;
- Hawkesbury River;
- The coastal beachfront.
- A combination of both catchment and coastal flooding in the lower portions of coastal waterways can happen after one or a series of storms. There are a few factors that influence which of these two sources of flooding is dominant: the location and configuration between the catchment, floodplain and waterways, as well as the specifics of storm cells (air masses that contain updrafts and downdrafts). If there is a high tide or a storm happening at the ocean entrance at the time of rainfall, the lake flood level will be even higher.