Sites of Flood occurrence
Riverine flooding occurs in relatively low-lying areas adjacent to streams and rivers. In the extensive flat inland regions of Australia, floods may spread over thousands of square kilometers and last several weeks, with flood warnings sometimes issued months in advance. In the mountain and coastal regions of Australia flooding can happen rapidly with a warning of only a few hours in some cases.
The Great Dividing Range which extends along the length of eastern Australia provides a natural separation between the longer and slower westerly flowing rivers and the shorter, faster easterly flowing coastal rivers. In some cases natural blockages at river mouths, including storm surge and high tides, also may cause localized flooding of estuaries and coastal lake systems.
Flash floods can occur almost anywhere there is a relatively short intense burst of rainfall such as during a thunderstorm. As a result of these events the drainage system has insufficient capacity or time to cope with the downpour. Although flash floods are generally localized, they pose a significant threat because of their unpredictability and normally short duration.
A flood typically occurs when a river (or other body of water) overflows its banks. As you can read Physical Geography: The Global Environment, third edition, annual floods can even be a normal part of a floodplain’s development. These floods deposit sediments that build a river’s natural levees, broad ridges that run along both sides of the channel. Figure F-3 shows the relationship between floods and natural levee development. As the river spills out of its channel, the coarsest material it is carrying is depostied closest to the overflow, hence along the levees. When the river contracts after the flood, it stays within its self-generated levees.
Image from Physical Geography However, not all floods are so regular and productive. Infrequently—perhaps once in a century—a river may experience a flood of such magnitude that its floodplain is greatly modified. Water up to several meters deep may inundate the entire floodplain, destroying submerged levees, eroding bluffs, and disrupting the entire system. These sorts of floods have cost millions of lives in the densely populated floodplains of Asia’s major rivers. They also occur in the Mississippi Basin of the central United States, where the damage, too, can be enormous. No reinforcement of natural levees or construction of artificial levees can withstand the impact of such a powerful “100-year” flood.