• Floods

The human factor

Flooding is defined as the accumulation of water within a water body and the overflow of excess water onto adjacent floodplain lands.

The floodplain is the land adjoining the channel of a river (Fig. 2), stream, ocean, lake, or other watercourse or water body that is susceptible to flooding.

Riverine floodplain and causes of flooding

Flooding is the most common environmental hazard. It regularly claims over 20,000 lives per year and adversely affects around 75 million people worldwide. The reason lies in the widespread geographical distribution of river flood plains and low-lying coasts, together with their long-standing attractions for human settlement.

• riverine flooding
• fluctuating lake levels
• local drainage or high groundwater levels
• coastal flooding (or inundation) including storm surges and tsunamis

The appearance of flood hazard is dominantly limited to the prevailing weather system and geomorphological and topographical features of a given area.

Inland flooding, as distinct from coastal flooding, is generally caused by the overflow of watercourses as a result of intense rainfall or of a reduction in waterway area by landslide or debris damming (which themselves may be triggered by natural events such as earthquakes).

Coastal flooding can, in addition, be caused by extreme winds leading to storm surges, by off-shore earthquake induced tidal waves (known as tsunamis) or the subsidence of coastal land. Human manipulation of watersheds, drainage basins, floodplains and the effects of deforestation, soil erosion, silt carriage have increased volume and speed of runoff.

Landscape susceptible to flood development

The most flood-susceptible landscape settings are:

• Low-laying parts of active floodplains and river estuaries. In their natural state, these settings will suffer the most frequent inundation - coastal flooding.
• Low-laying inland shorelines, that become unprotected due to erosion of barrier islands, sand dunes or bluffs, so the wind-driven wave attacks may inundate and cause damage to buildings and other immovable shoreline facilities.
• Small basins subjected to flash floods. Flash floods are formed in arid and semi-arid zones where there is a combination of steep topology, little vegetation and high intensity, short-duration connective rainstorms. They can also occur in narrow valleys and heavily developed urban settings.
  Alluvial fens can create a special type of flash flood threat, especially in semi-arid areas where the fens support urban development. The braided drainage channels can meander unpredictably across the relatively steep slopes, bringing very high velocity flows of 5-10 metres per second, which are highly charged with sediment and produce significant hydrodynamic forces capable of destroying built structures. Velocity as low as 0.5 metres per second are capable of sweeping victims off their feet.
• Areas below unsafe or inadequate dams. When the foundations of the Malpasset Dam (France, 1959) failed, 421 people died. In 1963 a landslide created a major flood surge behind the Vaiont Dam (Italy). Although the structure withstood soundly, the overtopped water surge killed 3,000 people downstream. In the coal mining valley of Buffalo Creak (West Virginia, USA, 1972), 125 people were killed and 4000-5000 were made homeless when a poorly maintained dam burst.

Flood forecasting is based on seasonal patterns, capacity of the drainage basin, flood mapping, and surveys by air and land. Warnings can be issued far in advance for seasonal floods, but only shortly before onset in cases of storm surge, flash flood and tsunamis.

In its simplest form flood hazard is depicted in plan by lines that represent the areal extent of water-surface (Fig. 1). The longitudinal profiles of water-surface should also be shown (Fig. 2). Those water-surface profiles are determined both for floods of special frequency of occurrence and for historically recorded or estimated events of catastrophic magnitude. When provided with sufficient and relevant annotation, flood hazard plans serve to indicate the potential severity of inundation, thus provide relevant data for planning the organisation and resources for effective and efficient emergency response.

Can floods be predicted?

New Meteorology is able to predict all types of floods.