Several types of data can be collected to help hydrologists predict when and where floods might occur: •
Monitoring the amount of rainfall occurring on a real-time basis.
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Monitoring the rate of change in river stage on a real-time basis, which can help to indicate the severity and immediacy of the threat.
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Knowledge about the type of storm producing the moisture, such as duration, intensity and aerial extent, which can be valuable for determining the possible severity of the flooding.
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Knowledge about the characteristics of a river’s drainage basin, such as soilmoisture conditions, soil saturation, topography, vegetation cover, impermeable land area and snow cover, which can help to predict how extensive and damaging a flood might become.
In the UK the Met Office collects and interprets rainfall data and works with the Environment Agency to issue flood watches and warnings as appropriate.
Recurrence intervals refer to the probability of a flood occurring based on past flow states compiled over at least a 10 year period. Often people use them to infer magnitude where a 1 in 100 year flood will exceed that of a 1 in 40 year flood. Hydrologists determine the recurrence interval based on previous flow states and the probability that the discharge will exceed that able to be contained by the channel. A 1 in 100 recurrence interval refers to a 1 per cent probability that the river will reach a certain discharge for that river. Several 100 year floods could still occur within 1 given year as the data is based on averages. A 100 year storm over a catchment may not necessarily equate to a 100 year flood as many factors will influence the rate of drainage.
34 Hydrology and fluvial geomorphology
Causes of flooding Flooding can be classed as an inundation of water covering the land’s surface. Most commonly flooding is the result of excessive precipitation caused by low pressure depressions that bring storm clouds with great vertical extent. Flooding occurs when water exceeds the capacity of a river channel although it can be the result of a rising water table or coastal inundation. In situations where floodwater travels at great speed there is increased likelihood of damage. In the case of the Boscastle flood (2004), the extreme nature of the flood uprooted trees and carried cars into a narrow channel, further exacerbating the flood. Prediction: forecast and warning Floods are considered the most serious type of natural disasters in the world due to their frequency and intensity affecting widespread populations. On average flooding contributes to 10 000 deaths per year globally with projections showing an increase due to climatic instability and population growth. Much of modern flood prediction utilises technology and relies on computer models and simulation software that use algorithms (mathematical formulas) based on the characteristics of an area. The use of precipitation data as well as relief, land use and saturation rates may all be used to help forecast flow rates from a few hours to a few days. Due to recent technological advances such as greater computing capability, reduced errors and better physical modelling, more effective use of data, flood forecasting and warning has never been better. However, despite this, due to the unpredictable nature of our weather there is still a high percentage of risk in many areas. Satellites, radar and climate modelling have all helped to track global weather systems and statistical models are used with flood histories to try to predict the results of expected storms. In the UK the Environment Agency has thousands of monitoring stations across many major river networks. Most of the measurements used to make predictions are taken electronically by sensors in the river, stored on site and then automatically sent back to databases used by forecasting systems. River and seawater level measurements are now also sent from telemetry systems and published online. Despite this, due to the flashy nature of many of our river systems, many properties in England and Wales have less than six hours of flood warning time. In the case of Boscastle in 2004, the town had less than three hours’ warning. Scale and impact Large drainage basins often provide greater opportunity for warning as the water has further to travel, delaying its impact. In the case of the Brahmaputra and Ganges rivers that run into Bangladesh, bringing meltwater down from the Himalayas, settlements may have up to 72 hours to prepare for a flood event. However the extent of the flood has the potential to be more severe. In the 2007 Bangladesh flood 1000 people lost their lives and 9 million more were made homeless. Prevention and amelioration Extreme weather events only become hazardous when there is a population that may be affected. As the global population grows more and more people are marginalised and forced to live in hazardous areas simply due to a lack of space. This, combined with the greater frequency and intensity of some weather events, increases a population’s vulnerability and their capacity to cope. Often in Middle Income Countries (MICs) economic losses exceed social losses as more and more buildings are built on floodplains. Floodplains are desirable places to build because of their building potential as easily accessible flat land. However this is not without risk. Flood protection can take a number of forms, such as loss-sharing adjustments and event modifications. Loss-sharing refers to mechanisms designed to help cope with a flood. They include insurance payments and disaster aid, the latter of which may take the form of money, equipment and technical assistance. In MICs insurance is an important loss-sharing strategy though not all houses will be eligible for insurance and many homeowners underestimate the impact of flood damage.