When it rains or snows, some of the water is retained by the soil, some is absorbed by vegetation, some evaporates, and the remainder, which reaches stream channels, is called runoff. The amount of runoff depends on the soil type, catchment steepness, drainage characteristics, agriculture and urbanisation as well as the wetness of the catchment.
The rivers both convey the runoff to the sea, and store water temporarily when the level rises. When water can no longer be removed or stored by the river, flooding occurs and the excess water is stored until it can be removed. Periodic floods occur naturally on many rivers, forming an area known as the flood plain.
In upland rivers, the flooding is usually temporary. The river capacity is increased by overland flow along the flood plain as well as the increased depth of the main channel, and the flood begins to subside as soon as the peak runoff has passed. A flood that rises and falls rapidly with little or no advance warning is called a flash flood. Flash floods usually result from intense rainfall over a relatively small area.
More prolonged flooding can occur in lowland rivers, where the gradient is much flatter, possibly only a few inches per mile. There, downstream flooding can prevent drainage of areas upstream, leading to ponding of the water discharging from higher up the catchment in the flood plain.
These events are natural. Rivers are naturally occurring drainage systems, which have evolved to convey the flows arising most of the time. Gradients are difficult to change over any distance, and dredging out the channel will result in siltation during normal flows. Increasing the river capacity will only move the floodwater downstream more quickly, and generally make things worse elsewhere. Flood protection measures, although they may help in one location, will remove flood storage and increase the overall problem.
Effects of Floods
Floods not only damage property and endanger the lives of humans and animals, but have other detrimental effects. Rapid runoff causes soil erosion as well as sediment deposition problems downstream. Spawning grounds for fish and other wildlife habitat are often destroyed. High-velocity currents increase flood damage; prolonged high floods delay traffic and interfere with drainage and economic use of lands.
Bridge abutments, bank lines, sewer outfalls, and other structures within floodways are damaged, and navigation and hydroelectric power are often impaired. Financial losses due to floods commonly amount to millions of pounds each year.
Many of our rivers are regulated to some extent, and this can assist in managing the discharge of runoff. This control is, however, limited in the face of extreme flood events.
The basic methods of flood control have been practised since ancient times. These methods include reforestation and the construction of levees, dams, reservoirs, and floodways (artificial channels that divert floodwater). Recently there has been renewed interest in sustainable urban drainage systems (SUDS) whereby the problem is tackled at source.
Before storm water run-off flows into sewerage systems various devices are employed to reduce the volume and rate of run-off. This can include infiltration trenches and basins, retention ponds, permeable road surfaces and reed beds.
It is ironic that increasing the effectiveness of drainage systems can increase the impact of flooding. If surplus water is stored across the catchment in manageable quantities, minimal inconvenience results. Serious disruption will occur if the floodwaters from a large catchment are concentrated in a small area, as we have seen recently.
Planners and developers must also accept that some areas are prone to flooding, and are unsuitable for development.