General Information
What is a flood?
Floods can be described as the occupation of water in zones or areas that under normal conditions are dry. They are produced due to the affects of a temporary increase in the level of rivers, lakes or other bodies of water. To a certain extent, floods can be events controlled by man, depending on the land use near rivers.
Flooding usually occurs due to prolonged, intense rains, such as those that occur during tropical storms and hurricanes, combined with drainage problems provoked by various causes, generally the negligent action of people.
Effects and relative importance of floods
As with most natural hazards, floods can result in the loss of lives and property damages, with a great impact on public health which can take some time to recover. From 1980 to 1985, there were approximately 160 events related to floods in the world, in which at least 120,000 people died or were injured and 20,000,000 lost their homes (2). In terms of physical losses, floods are responsible for 40% of property damages caused by all natural disasters (1). In the United States, the direct economic losses reached 4 billion dollars per year before the great floods of the mid west in 1993 (5). In the Mississippi river valley, losses from one single flood in that year exceeded 10 billion dollars (6), and the long term effects of contaminated wells, mental health problems and diseases transmitted by vectors, will require continued monitoring beyond this period.
Factors contributing to problems of flooding
Adequate weather predictions, opportune alert systems for floods, and mitigating practices, such as management plans, have greatly helped in the prevention or reduction of the effects of floods on the health and well-being of communities in recent years. However, despite improved levels of preparation, deaths, diseases and injuries continue to occur in affected communities.
There are several possible explanations for this situation. First, the topography of some areas signifies a constant risk for residents. For example, the inhabitants of Bangladesh, an alluvial delta formed by three rivers, are subjected annually to floods, a condition that is exacerbated during the rainy season by river run-off from higher areas and by the high tides of the Bay of Bengal (8). In Puerto Rico, the drainage ditches differ in length and grade. Where they are long with little gradient, deaths occur when motorists attempt to cross submerged bridges; people located near the short and inclined drains, experience unexpected surges of water and, they subsequently drown (9). Floods can also accompany other natural disasters such as tide swells during hurricanes or tsunamis. Landslides can be a secondary affect of flooding and can exacerbate the dangerous conditions previously described.
Types of Floods
According to their origin, floods are classified as: pluvial, fluvial and coastal.
a) Pluvial and fluvial floods
These floods occur as a result of precipitations that occur when humidity contained in the seas, oceans and other large bodies of water are transported by the wind towards land; when the water vapor rises and its temperature decreases, precipitation occurs, taking one of the following forms; rain, snow or hail. This process can originate from: hurricanes, normal winds, polar masses and convective processes.
b) Coastal floods
In addition to pluvial flooding (caused directly by rain) and fluvial flooding (overflowing of rivers), coastal areas can be affected by storm tides, particularly in the Gulf of Mexico, where the higher elevation of the average sea level causes the tide to pass inland, in some occasions, affecting very large areas.
The wave swell exacerbates these phenomena and causes very extensive damages, such as the undermining of foundations of coastal buildings, shipwrecks, the demolition and destruction of harbor facilities, the destruction of coastal defensive installations and the erosion of beaches and cliffs. The water swell does not only cause destruction in its advance but also when it pulls back towards the sea.
Mechanisms that produce precipitation
Even though the mechanisms that produce precipitation are generally combined, it is useful to outline their characteristics individually.
a) Hurricanes
Hurricanes transport large quantities of humidity and also occupy enormous areas, provoking storms of long duration (various days).
These phenomena provoke generalized flooding in the main river basins.
b) Normal winds
Normal wind currents transport humidity from the sea towards land, when they reach the mountain areas they rise and provoke significant precipitations which are concentrated on the region where the winds originate.
c) Polar masses
These consist of the displacement of cold fronts that come from polar zones and cause precipitations.
d) Convection
When humidity rises due to the differing air temperatures in the layers of the atmosphere, the process is called convection. It causes intense precipitation, which is of short duration and is concentrated in small areas.
Floods in urban zones on high plateaus are associated with convection rains.
What are the affects of floods?
-
Solid materials (such as cars) being swept away.
-
Extensive areas covered in water.
-
Intense erosion.
-
Proliferation of microorganisms.
-
Damaged homes.
-
Interruption to communication channels.
-
Loss of lives.
-
Loss of crops.
-
Sediment deposits.
As well, floods have secondary affects:
How to avoid floods
Structural measures
These consist of water infrastructure projects that are carried out to control floods. They can be classified as:
a) Regulation projects:
Generally, regulation projects consist of dams that allow for rising water to be temporarily stored in order to release it at a later time in a controlled manner.
As well, in previous years more attention has been given to the improvement of river basins (reforestation, terracing, etc.), which regulate draining, and at least partially, have countered the negative effects of urbanization.
b) Modification projects:
These projects help to facilitate the fast transport of water through channels. They include dredging the rivers to conserve their original depth and in this way, they decrease the possibility of river overflow. They generally constitute the modification of channels by increasing their inclinations by means of meander cuts.
c) Protection projects
Longitudinal borders
They are established on one or both sides of the river and their function is to maintain the water within the river channel.
Perimeter borders
Their function is to avoid floods reaching economically important populations or regions.
Non Infrastructural or Institutional Measures
These are Civil Security measures that are used in order to reduce the damages caused by floods. They can be permanent or implemented only when the threat of an important hydro-meteorological phenomenon is present.
Permanent measures
These constitute land-use regulations for an area that is zoned as a flood plain. The goal is to avoid economically and socially valuable property and goods being located in zones that are subject to flood risk.
Operational measures
All measures that are carried out in the presence of a hydro-meteorological phenomenon that can cause a flood, should be supported by forecast models through the different phases (precipitation forecast, its draining, flow, infrastructure, etc.)
Malilay, Josephine. Inundaciones. Pp. 287-302. En: Noji, Eric K., ed. Impacto de los desastres en la salud pública. Organización Panamericana de la Salud. 2000.
México. Centro Nacional de Prevención de Desastres (CENAPRED). Inundaciones. Fascículo 3. 1992
ECHO/DIPECHO; Movimiento por la Paz, el Desarme y la Libertad (MPDL); Estado Mayor Nacional de Defensa Civil de Cuba. Guía comunitaria para la gestión de riesgo por inundaciones. 2003
|
|
Back
Top 