Types of flooding
Council considers four types of naturally occurring flooding that can affect the Moreton Bay Region:
River, creek and urban flooding (Flood)
Council uses the term “Flood” to describe the occurrence of flooding where the source is rising waters from a river, creek or urban drainage system.
Flooding is caused by heavy rainfall, typically lasting between one hour to a day or more falling over large parts of a catchment or several catchments. This heavy rainfall causes the water levels in a river, creek or urban drainage system to rise and exceed the capacity of the main channel or pipe network. It may take several hours for these floodwaters to rise, reach a peak and then subside. This type of flooding can occur in both the coastal and hinterland parts of our region and in both rural and urban areas.
Council’s overland flow mapping represents the inundation of gullies and depressions where stormwater runoff may flow in areas upstream of river, creek and urban flooding map extents.
Overland flow is generally caused by localised short intense bursts of rainfall (typically less than one hour in duration) and will usually dissipate shortly after the rainfall has stopped. Overland flow can occur when the capacity of a watercourse or drainage system is exceeded, resulting in excess stormwater flowing through nearby natural and man-made gullies and depressions on its way to a downstream river, creek or drainage system. Overland flow can also occur in elevated areas and/or a significant distance away from a watercourse.
Tidal inundation can occur on very low lying coastal land during naturally occurring large high tides.
Large tides occur on a seasonal basis each year and are sometimes referred to as "king tides". The Highest Astronomical Tide (HAT) is the highest predicted tide that can occur in response to the gravitational effect of the sun and moon. Typically, king tides occur 2-3 times a year resulting in a tide level that closely approximates HAT. These tides can inundate very low-lying areas along our coastline.
Storm tide inundation can occur on low-lying coastal land when extreme weather conditions produce a storm surge resulting in sea levels above normal tide levels.
A storm surge can occur when a tropical cyclone or intense low pressure system, and any associated strong onshore winds, cause sea levels to rise above normal levels. A storm surge when combined with a normal tide and increased wave action is called a storm tide. Storm tides can result in inundation of low-lying coastal areas.