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Floods – Forecasting and Forewarning

 

 

Flood forecasting is the use of hydrological data – mainly anticipated precipitation and stream-flow data in rainfall-runoff and stream-flow routing models to forecast flow rates and water levels. The periods of forecast range from a few hours to days ahead, depending on the size of the watershed or river basin. A mere six inches of moving water is enough to knock a person off his feet, while two feet of running water can carry away small cars and two-wheelers. Hydrologists collect and use several types of data to predict where and when floods may occur, and with what severity. The most important tasks in flood forecasting which can help indicate the severity and immediacy of the threat are to monitor the amount of rainfall occurring on a real time basis, and the rate of change in river stage on a real time basis.

Advance warning and forecasting of floods depends on a reliable forecast of weather – in particular rainfall. Huge amounts of data in respect of variables like temperature, rainfall, humidity, wind speed and direction (which are continually changing), and terrain are processed for delineating areas of heavy rain. Computer models constructed using these data are used to forecast the extent, intensity and duration of rainfall in the next one to four hours. However, such forecasts give only a very short lead time for response.

Dramatic advances in data availability through ground radars and satellite based earth observing systems, and fast data processing techniques in terms of methods, and availability of fast computers have made it possible to make weather forecasts on the basis of increasingly accurate computer models of the atmosphere and ocean/atmosphere interactions. In some parts of the world, three-day-ahead forecasts of heavy rain are now as accurate as one-day-ahead forecasts were a decade ago.

The accuracy of climate and weather forecasts varies with lead time, spatial scale (or size) of the region of interest, the weather or climate variable being forecast (for example, rain, thunderstorm), as well as with latitude. Generally, temperature forecasts are more accurate than rainfall forecasts. The mid-latitudes are easier to forecast than the tropics (so north India has more accurate forecasts than south India). It is generally easier to forecast when the lead time of the forecast is relatively short – so a seven-day forecast is usually less accurate than a forecast of tomorrow's weather. Finally, it is generally easier to forecast rainfall over a large area (for example, a large catchment) than a small area (for example, over a reservoir). This is because the intensity of any rain system varies on small spatial scales, but the variation is somewhat averaged out over a large area. On account of this fact, forecasts of floods for specific locations (and their timing) are not altogether accurate.

Other Factors in Flood Forecasting:

For flood forecasting, land surface characteristics such as soil types, their moisture content, vegetation cover and terrain are now included in computer models along with data from satellites, radars and gauges, resulting in better predictions about how a certain location will handle a given amount of water. The biggest challenge is to estimate how much rain water the ground can soak up. The National Aeronautics and Space Administration (NASA) of the US has begun exploring the use of satellites to measure soil moisture over much larger areas than those currently monitored by rain gauges.

The risk of a major flood is an ever-present reality along most waterways. To understand the inherent potential for floods in any given watershed, we need to look at the physiographic features present in the catchment, which influence its rainfall patterns, snowmelt, and evapotranspiration.

Flood phenomena in a watershed are also influenced by its stream-flow which is in turn controlled by its shape, topography (slopes), soils, and vegetation. Topographical features like mountain fronts or regional plateaus can significantly increase the intensity of rainfall or snowfall in the watershed by preventing moisture-laden air form crossing over to the other side. Steep valleys and thin soil covers can move water downhill very rapidly, aggravating the floods. Another important factor in flood forecast is to understand how the rainwater will behave once it is on the ground. If the soil is saturated the runoff will be larger as compared to when the soil is dry. Also important from the prediction point of view will be the type and density of vegetation cover and the shape of the drainage basin. A sparse vegetation cover and a nearly circular drainage basin will result in the surface runoff reaching the stream outlet rapidly, thus enhancing the severity of the flood event.

In addition to these factors, we need to consider the influence of human activity in the watershed which may include altering its land use, as for example rendering the ground surface impervious through so-called developmental activities. Human activity may also result in construction of embankments, ponds and other flood control structures, which can mitigate the ill-effects of flood.  

Flood forecast also takes into consideration whether the watershed lies in proximity to large supplies of moist air, and whether wind and storm patterns interact with this moist air to produce very heavy rainfalls or snowfalls. Historical records of large, damaging floods in the region may also constitute an input for flood forecast. In India, the Central Water Commission is responsible for forecasting floods, and disseminating information for the general public.

Predicting Flash Floods:

A flash flood is a rapid flooding of low-lying areas like river banks, dry lakes and basins. Flash floods may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or melt water from ice or. Flash floods may occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam. Flash floods occur suddenly, leaving little time for warning.  They usually occur in the lower reaches of small streams with less than 50 square km of drainage area. Flash floods are known to have developed in less than a minute.

In order to predict flash floods, a hydrologist will need to know four things:

1.     Where is the ‘bulls eye’ of intense rain

2.     How quickly is it coming down

3.     How much rain is falling, and

4.     How saturated is the soil.

The hydrologist will need to have access to data from Radar and rain and stream gauges in order to estimate the rate of rainfall and the rate of rise of river waters. However, radar is limited in its ability to detect rain in mountainous areas. In addition, the father away you get from the radar, the less accurate the information.

It may be difficult to predict a flash flood, but one can look out for these factors:

•      Flash floods occur within six hours of a rain event.

•      Listen for news of dam or levee failures.

•      Watch for slow-moving thunderstorms that repeatedly move over the same area.

•      Hurricanes are another obvious source of intense rain.

•      Water collecting in pools are an indication that the ground is oversaturated with water.

•      Do not camp or park cars etc. near a river or on a street that is known to be flooding frequently.

 

FLOOD FOREWARNING:

India is the worst flood affected country in the world after Bangladesh and accounts for one fifth of global death count due to floods. According to the National Flood Commission, about 40 million hectares of land in the country is liable to floods. An average of 18.6 million hectares of land is affected annually. Floods devastate communities, distort the economy and set back development. Floods tend to do their worst, for obvious reasons, to the poorest communities. Many hazards associated with floods can be identified, and much of the damage they do can be pre-empted by timely warning issued for the benefit of communities living in vulnerable areas. Flood forewarning is aimed at reducing losses from flood and other associated disasters. This calls for risk assessment in the likely event of a flood, on the basis of which the forewarning model is based. The model is built upon risk indices in flood disaster system.

Risk assessment and forewarning of flood disaster has become a research hotspot in recent years. There are many risk assessment methods, involving mathematical statistics and evidence, vector graphics, fuzzy mathematics, gray correlation, evolutionary modeling, and so forth. Apart from mathematical/statistical factors considered, the terrain plays an important role in flood vulnerability.

In 2009, scientists simulated the effects of a tropical storm in the Philippines, and predicted that the cities of Cagayan de Oro and Iligan would be hit by flash floods. The scientists were conducting an exercise as part of a UN strategy for disaster reduction, to which 168 nations signed up in 2005. The scientists identified the possible effects of a storm induced flood hazard. At that time, the prediction was dismissed as alarmist, and the Philippine government had not yet enacted its disaster management plan – the two coastal cities remained without protection. Catastrophic flooding affected more than 300,000 people.

Three years before Hurricane Katrina devastated New Orleans in 2005, there were warnings about the city's vulnerability. Katrina still somehow surprised a complacent government. Knowledge is power only for those prepared to act upon it.

Floods can bring with them other disasters like landslides. Geological disasters due to floods can cause significant damage. Statistics issued by the Chinese Ministry of Land and Resources show that geological disasters triggered by floods kill more than 1000 people and cost millions of dollars in economic losses. There is therefore a need to issue early warnings for floods and flood related disasters in vulnerable areas.

The development of flood warning system is an essential element in regional and national flood preparedness strategies. Flood warning is being considered as an alternative to dealing with the actual flood problems, partly because these systems are less expensive compared to managing damages arising from the floods. The benefits of an early warning system can be calculated by assessing the possible savings on the quantity of flood damage to public and private assets resulting from the action taken in response to the warning.

The key elements of a flood warning system mainly consist of four essentials:

1.     Risk knowledge

2.     Monitoring and warning services

3.     Dissemination and communication

4.     Response capability

1.     Risk Knowledge: Risks arise when hazards and vulnerabilities appear together at a particular location. Assessments of risk require systematic collection and analysis of pertinent data and should consider the dynamic nature of hazards and vulnerabilities that arise from processes such as urbanization, rural land-use change, environmental degradation and climate change.

2.     Monitoring and warning services: There must be a sound scientific basis for predicting and forecasting hazards and a reliable forecasting and warning system that operate 24 hours a day. Continuous monitoring of hazard parameters and contributing factors is essential to generate accurate warnings in a timely fashion.

3.     Dissemination and communication: Warnings must reach those at risk in good time. Clear messages containing simple, useful information are critical for enabling proper understanding of warnings and responses in order to safeguard lives and livelihoods.

Information and Communication technology (ICT) is a key element in early warning. ICT plays an important role in disaster communication and dissemination of information to organizations in charge of responding to warnings and to the public during and after a disaster. Early warning communication systems are made of two main components:

a)     Communication infrastructure hardware that must be reliable and robust, especially during the natural disasters; and

b)     Appropriate and effective interactions among the main actors of the early warning process such as the scientific community, stakeholders, decision makers, the public, and the media.

Many communication tools are currently available for warning dissemination such as Short Messaging Service (SMS), email, radio, TV and web services.

4.     Response Capability: It is essential that communities understand their risks; respect and follow the warning and know how to react. Education and preparedness programs play a key role in reducing risks. It is also essential that disaster management plans are in place, resources allocated and standard procedures well practiced and tested. The community should be well informed on options for safe behavior, available escape routes, and how best to avoid damage and loss to property.

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S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

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email: farooq.amu@gmail.com