Causes for Floods

Flooding occurs most commonly from heavy rainfall when natural watercourses do not have the capacity to convey excess water. However, floods are not always caused by heavy rainfall. They can result from other phenomenon, particularly in coastal areas where inundation can be caused by a storm surge associated with a tropical cyclone, a tsunami or a high tide coinciding with higher than normal river levels. Dam failure, triggered for example by an earthquake, will result in flooding of the downstream area, even in dry weather conditions.

In general, the factors which influence whether a flood will occur include:

Flooding occurs in both natural and developed watersheds. When the rate of rainfall or snowmelt exceeds the rate of infiltration to the ground, the excess water, called runoff, moves across the ground surface toward the lowest section of the watershed. As the surface runoff enters stream channels, stream levels increase. If the rate of runoff is high enough, water in the stream overflows the banks and flooding occurs. This area of over-bank flow is called the flood plain. All natural watersheds have flood plains. Structures located in these flood plains are subject to damage. In a natural watershed, flooding can be affected by ice jams, the accumulation of debris at channel constrictions, and even the dam-building activity of beavers.

Human activity has profound impacts on flooding. The two major activities which impact flooding are land use change and the building of flood control structures.

Land Use Change- Hundreds of years ago, the Delaware River Basin (USA) was covered by forests. This maximized the infiltration of rainfall and slowed the movement of runoff. As the land was cleared for agriculture, infiltration rates were reduced and runoff rates increased. The increase in runoff rates widened flood plains and stream channels in many of the basin's watersheds. With gradual urbanization and the increasing use of asphalt and concrete paving, in addition to densely spaced buildings, infiltration rates were further reduced with corresponding increases in runoff rates. Because of these land use changes, flood flow rates in many areas are much higher than they would naturally be for a given rain storm. Although some land that was formerly in agricultural use has been reforested, the runoff reduction benefits have been offset in many areas by continued urbanization.

The transportation network associated with land use change also affects flooding. In addition to the impacts of impervious paved surfaces, bridges and culverts usually constrict stream channels and flood plains. This aggravates upstream flooding, especially when the constrictions become clogged with ice or debris.

Flood Control Structures: The purpose of flood control structures is to physically constrain or to convey flood waters. Flood control structures include dams, levees, lined stream channels, and storm sewers. Dams and levees have been used for centuries to open flood plains to agriculture and settlement, and in the case of dams, to detain flood waters for gradual release or for use for water supply, recreation, and the generation of hydroelectric-power. Dams and levees are highly effective in flood loss reduction.

Though effective, one drawback to the use of dams and levees for flood loss reduction is that they are very expensive. Secondly, local cost sharing requirements and environmental issues have slowed construction of new facilities in recent years. Flood control dams and levees are not necessary where there is no flood plain development.