Flooding

A flood event is usually triggered by heavy rainfall on all or a portion of a sub-catchment. During a flood event, rain falls at a rate faster than the soil and vegetation can absorb, and surface run-off enters streams and rivers where it increases streamflow, and sends a pulse of water down the river. If rainfall persists, the amount of water flowing downstream continues to increase, eventually exceeding the capacity of the river channel. A flood event in which water rises in the river channel until it overflows the riverbank and spreads across the floodplain, is known as bankfull discharge. Flood waters often recede quickly, as water in the floodplain is absorbed or drains back into the river channel as the flood water flows downstream.

Flow variability is increasingly recognised as an important factor in the health of riverine aquatic ecosystems (Poff et al. 2007). Extreme floods are important because many of the processes that shape the river occur during the largest floods, also known as reset events. Extreme low-flow periods are also important and can impact species selection. This is especially true in southern Africa where prolonged drought and erratic rainfall can stress individual species and whole ecosystems. Natural variability of river flow (discharge) in a river system, including extreme events, is part of the hydrologic regime that creates and maintains a healthy river system. The plants and animals of a river system are generally adapted to the natural pulses of the hydrologic regime of streams or rivers, and the growth and migration patterns of organisms are closely linked to availability and quality of water.

While floods are generally perceived as bringing negative impacts, they can also have some positive benefits. They are a part of most natural hydrological systems, which support ecological function and renewal of the landscape. They bring nutrients to the soil that once the flood waters have dissipated, leaving behind more productive agricultural land. Furthermore, the inundation and saturation of the soil brings significant recharge to groundwater resources, and also clears away dead vegetation and redistributes surface sediments.

Flood Risk

Flood risk is increased when soil moisture saturation is already high from previous rainfall or flooding events, as the time taken to reach saturation and overland flow is reduced. Therefore, more of the water falling as rainfall contributes to the flood, rather than being absorbed into the soil. For more information on how soil moisture conditions contributed to the February 2000 floods in the Limpopo River basin, please refer to the Flooding section of the Hydrology of the Limpopo River basin.

Flooding and Cyclones

Flooding occurs following a large input of water into a river system. One such common input of water on the east coast of Africa is rainwater falling during tropical cyclones. The rainfall associated with tropical cyclones is often intense and sustained for significant periods of time, resulting in a rapid saturation of the soil, followed by a huge pulse of run-off into the drainage network, often causing flooding. The extent and duration of the flood is determined by the length of the rainfall event, the status of soil saturation and the speed of recharge and soil drainage.