978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

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.
• 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.
• 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.
• 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.
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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.