Manual Local Flood Early Warning Systems - Planet Action

It is desirable to quantify risks. This is a substantial effort and to do it will require manpower
and resources. In case the stakeholders of the LFEWS decide not to focus on these calculations
the respective parts in Chapter 4 can be ignored.
The total risk is defined by the product of the hazard, the elements at risk and their vulnerability.
This means we have to look at these three determining factors one by one first and then at
their product.
Risk = Hazard * Elements at risk * Vulnerability
It is possible to calculate risks in a quantitative manner. This manual is not describing these
methods in detail but the principles and general examples are explained here 24 .
4.1. Hazards
The flood hazard has one main contributor, rain. Therefore the observation of rainfall is of
primary importance. In case you observe rainfall already with gauges in the river basin you
might like to add another technique, the estimation of rainfall with data freely available
in the internet (see Chapter 5.4.1). If the rainfall data of the LFEWS are not collected
completely or reliably the satellite data may help you considerably.
Under rare conditions an extraordinary amount of rain may fall on the watershed and
cause a flood that is much more severe than the usual/annual floods “One in a hundred
years flood”. If a LFEWS did not consider this yet it is advisable to ask expert advice on how
to estimate such floods.
The severity of floods is not only influenced by rains. Many factors may increase or decrease
the flood hazard. Collect the respective data and analyze them to reveal trends.
• Over the years the river may change its path and with this the flood-prone area will
most likely change. This might also be artificial as man is diverting rivers too.
• Siltation of the river may reduce the channel capacity and increase the chances of
flooding while dredging increases the capacity and reduce the danger of floods.
• Water from rivers is sometimes used for
irrigation. This might be small scale by a minor
channels or pumps, but NIA might also run a
big scheme and this might affect the flooding
behavior of the river.
• Drainage problems can aggravate flooding.
Elevated roads without sufficient (or clogged)
culverts or bridges with too narrow widths can
limit drainage and cause flooding.
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For details of these methods it is recommended to ask the National and Economic Development Authority for advice.
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