Flash Flood Risk Management – A Training of Trainers ... - ReliefWeb

Flash Flood Risk Management – A Training of Trainers Manual
• subsidies targeted at reducing flash flood risks for the protection of forests, pastures, rangelands, and
water bodies, including subsidies to community forestry user groups or to targeted farmers to encourage
environmentally friendly farming practices.
Mitigation strategies
Mitigation strategies can reduce the intensity, frequency, and impacts of flash floods. These strategies can
include the following.
Reducing discharge through natural retention. Various measures can be adopted to promote the natural
retention of water.
• Suitable areas for water retention need to be identified based on a land-use plan. If these areas fall
within farmers’ private properties, the farmers must be compensated for the loss they incur due to reduced
farming revenues;
• Passive flood control measures (as advised by hydraulic engineering experts) can be incorporated into
regional development programmes and construction plans so that retention basins can be identified and
used for flash flood mitigation;
• Natural retention areas need to be identified and improved.
Monitoring, warning, and response systems (MWRSs). MWRSs can contribute to the mitigation of flash
floods and can be a very effective non-structural measures (IDNDR 1997; UNISDR 2005). MWRSs include
systems for data collection, monitoring, and transmission; forecasting; warning; dissemination; disaster
management; and response. When any of these components fails to function, the effectiveness of the whole
system can be hampered.
session 10
• Data collection, monitoring, and other associated systems. Collection and monitoring of
hydrometeorological data, such as rainfall, temperature, and stream flow, is essential since these data are
used in simulations and forecasting that help planners prepare for future scenarios and choose between
possible alternatives. Ground-based observation networks and satellite-based precipitation estimations are
both good sources of data. Ground-based networks are a tried and true means of collecting rainfall and
other meteorological data. Where ground-based observation networks are not available, satellite-based
estimates of precipitation may be the only source of rainfall data. Combined satellite and ground-based
rainfall estimates provide the best input for flash flood forecasting and early warning systems.
It is important that data be transmitted efficiently from the site to the centre where the data are analysed.
The centre can then issue forecasts and warning messages to end users in a timely manner. A wide range
of data transmission systems are available; these can include, for example, automated gauge readers that
use VHF radio or telegraph to transmit data digitally to the centre.
• Forecasting systems. A forecasting system consists of models (e.g., hydrological, hydraulic) that
can predict potential flash flood events; these models are used to closely follow the evolution of key
parameters that could trigger a flood event. Forecasting can be very accurate, but if the computation time
is too long they are not relvant. Flash floods, as their name implies, are rapid processes and typically the
lead time is very short. Since time is of the essence, it is sometimes preferable to forego elaborate (and
time consuming) forecasting models in favour of simpler models, such as flash flood guidance tables,
which can give immediate results.
• Warning systems. It is often difficult for the general public to grasp the meaning of quantitative flash flood
forecasts; for general dissemination, qualitative warnings are much more useful (Box 9). An example of
such a qualitative warning is the flash flood guidance; this is the volume of rainfall during a given time
(e.g., 1 to 6 hours) over a given small catchment area. Even the general public can easily interpret this
number knowing that rainfall in excess of the flash flood guidance number is considered a flash flood
threat.
• Dissemination systems. Flood forecasting and warning information needs to be disseminated effectively. In
the majority of cases, even good forecasts fail to prevent damage and loss of life because they are poorly
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