Floods and Flood Control

Types of Flooding

Of the two broad types of flooding, stream flooding impacts more people than coastal flooding because of the higher concentration of human populations in river valleys. The world's great river valleys—the Nile, Tigris-Euphrates, Indus, Ganges, Huang He, Yangtze and Mei Kong were not only cradles of human civilization but also have extremely high population densities. Natural causes for stream and river floods include seasonal snow melt or precipitation, glacial slip and/or landslides in the headwaters. Glacial slip happens when a glacier in the headwaters of a stream breaks off and slips into the main channel, causing temporary damming of the channel. The dam inevitably breaks from the pressure, causing a wall of water to flow down the channel. Glacial slip and landsliding can cause some of the most sudden and devastating floods.

Sudden, high-level flooding is also called flash flooding, and can also be caused by intense precipitation. Flash floods have the most adverse impact for life and property. If the ground is fully saturated as a result of prolonged precipitation, even very small amounts of precipitation on saturated ground in a watershed can cause intense flood events.

Beyond the natural causes of flooding, human impacts on watersheds, such as urbanization, deforestation and agricultural activity further accelerate flood peaks. Smaller amounts of precipitation in a watershed naturally flows downhill as surface runoff. A considerable amount of precipitation is interrupted by a well-vegetated watershed, and is stored in the plants or reevaporated into the air, percolated into the ground, or directed into the water table. Urbanized surfaces with concrete and asphalt cause precipitation to flow out of the watershed as surface runoff, causing higher flood peaks downstream. Furthermore, agricultural or deforested surfaces lack vegetative cover to intercept the precipitation, or have been compacted to the extent that their absorptive capacity is much lower than a natural surface, causing similar impacts as urbanized surfaces. Human modification of watersheds all over the world is becoming a more important cause of downstream flash flooding.

Manipulation of Waterways

Human manipulation of streams for irrigation, flood control and navigation have also impacted the pattern of flooding in world's streams and rivers. [Page 684]Increasingly, it is difficult to find streams or rivers that have not been manipulated. Part of the problem is increasing human occupancy of floodplains. The spatial extent of a floodplain is by definition the area along the stream channel, which may be flooded in the normal rhythm of flooding in a stream. Humans have occupied floodplains since the dawn of civilization because of the availability of fertile soils, hydropower, and river transport. Many preindustrial societies were well adapted to the cycles of flooding along their streams, but with higher population levels, urbanization, and industrialization, modern societies are increasingly inflexible in the face of cycles of nature. Consequently, modern societies have increasingly tried to control and tame streams and rivers instead of trying to adapt to their rhythms, with disastrous results.

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