Floods are among the most frequent and costly natural disasters. As much as 90 percent of the damage related to all natural disasters (not to include droughts) is caused by floods and associated debris flows. On average Floods cause $3.7 billion dollars worth of damage and kill 110 people per year. The number one reason is due to flash floods.
A simple definition of flooding is water where it is not wanted. Another, more comprehensive definition of a flood is: A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland or tidal waters from the unusual and rapid accumulation or runoff of surface waters from any source.
Floods can have both positive and negative impacts. They can bring welcome relief for people and ecosystems suffering from prolonged drought, but also are estimated to be the most costly natural disaster. 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 phenomena, 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.
During a flood situation and its aftermath, knowledge of how to protect your health and safety is vital. In winter and early spring, it is especially important to understand the health threats associated with exposure to cold weather and cold water.
Flash Floods are Deadly. More than half of the deaths occur with vehicles.
A raging creek bed can have water moving at thirty miles per hour. Just six inches can knock you off your feet. Once the water level reaches the side of the vehicle, it can move it sideways. At a depth of two feet, the water will wash most automobiles downstream. This even includes older heavy cars weighing two tons or more.
The raging water will pickup tree branches, fence posts, barbed wire, boulders and anything else the water can move. A 10 seconds rain storm can develop about six inches deep runoff, and in less than one minute the water could be several feet deep.
Incidentally, it doesn't have to be raining at the crossing. It could be raining intensely almost anywhere upstream.
What causes floods?
Flooding occurs in known floodplains when you have one of the following situations happening: prolonged rainfall over several days, intense rainfall over a short period of time, or an ice or debris jam causes a river or stream to overflow and flood the surrounding area. Melting snow can combine with rain in the winter and early spring; severe thunderstorms can bring heavy rain in the spring and summer; or tropical cyclones can bring intense rainfall to the coastal and inland states in the summer and fall.
Flash floods can occur within six hours of a rain event, or after a dam or levee failure, or following a sudden release of water held by an ice or debris jam, and flash floods can catch people unprepared. You will not always have a warning that these deadly, sudden floods are coming. So if you live in areas prone to flash floods, plan now to protect your family and property.
As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases runoff two to six times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements and viaducts can become death traps as they fill with water.
Several factors contribute to flooding. Two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts. Topography, soil conditions, and ground cover also play important roles. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms. Floods, on the other hand, can be slow- or fast-rising, but generally develop over a period of hours or days.
Due to the accumulation of excessive surface runoff.
Flood waters consigned to the flood plain immediately along a river/channel or in random low lying and topographically depressed areas.
Flooding is relatively shallow and there is no perceptive flow of water as when inundation is rapidly spreading to adjacent areas.
Due to overflowing of rivers and lakes, unexpected and serious breaks in dikes, levees and other protective structures or uncontrolled releases of dam water.
Coverage of a wide continuous area and rapid spreading to adjacent areas of relatively lower elevation.
Flooding is relatively deep in most parts of the flood-stricken areas. Currents of flowing flood water will be swift as the flood spreads to other areas.
Flood damage refers to all varieties of harm caused by flooding[iii]. It encompasses a wide range of harmful effects on humans, their health and their belongings, on public infrastructure, cultural heritage, ecological systems, industrial production and the competitive strength of the affected economy. Some of these damages can be specified in monetary terms, others – the so called intangibles – are usually recorded by non-monetary measures like number of lives lost or square meters of ecosystems affected by pollution. Flood damage effects can be further categorized into direct and indirect effects. Direct flood damage covers all varieties of harm which relate to the immediate physical contact of flood water to humans, property and the environment. This includes, for example, damage to buildings, economic goods and dykes, loss of standing crops and livestock in agriculture, loss of human life, immediate health impacts, and contamination of ecological systems. Indirect or consequential effects comprise damage, which occurs as a further consequence of the flood and the disruptions of economic and social activities.
This damage can affect areas quite a bit larger than those actually inundated. One prominent example is the loss of economic production due to destroyed facilities, lack of energy and telecommunication supplies, and the interruption of supply with intermediary goods. Other examples are the loss of time and profits due to traffic disruptions, disturbance of markets after floods (e.g. higher prices for food or decreased prices for real estate near floodplains), reduced productivity with the consequence of decreased competitiveness of selected economic sectors or regions and the disadvantages connected with reduced market and public services.
• Flood Watch: Flooding is possible. Tune in to NOAA Weather Radio, commercial radio, or television for information.
• Flash Flood Watch: Flash flooding is possible. Be prepared to move to higher ground; listen to NOAA Weather Radio, commercial radio, or television for information.
• Flood Warning: Flooding is occurring or will occur soon; if advised to evacuate, do so immediately.
• Flash Flood Warning: A flash flood is occurring; seek higher ground on foot immediately.