Comprehensive Risk Assessment for Natural Hazards - Planat

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(b) the public;and
(c) those who may be in a position to implement mitigation
measures.
Ideally, the key organizations involved in disaster-relief
efforts will have the maps displayed permanently on a wall,
and will have studied the maps and instituted disaster planning.
Ideally, the public, community leaders and
government bodies will also study the maps and appreciate
that prevention is worthwhile, and implement appropriate
mitigation measures. Also, near full-scale disaster exercises
may be conducted periodically to maintain the readiness of
disaster relief and management organizations, and to keep
the public aware of the potential hazard.
3.11 RELATED PREPAREDNESS SCHEMES
Three main ways are available to reduce future flood damage
to buildings and their contents:
(a) reduce the flood through physical means (dams, levees,
reforestation);
(b) build buildings to withstand flooding with minimal
damage, for example, elevation of buildings above the
flood level; and
(c) restrict or prohibit development on flood-prone land.
Realistic consideration of the costs and benefits of these
options requires hazard assessment and at least a rough
assessment of the risks. Lack of data for quantitative hazard
and risk assessment,however,should not preclude taking
steps to reduce future damages or disasters. Unconstrained
growth in a flood-prone area can be a costly mistake.
Certain types of advanced planning have the potential
to reduce social, and in some cases, physical damage in the
event of a flood:
(a) public education and awareness;
(b) flood forecasting (prediction of flood levels hours or
days in advance);
(c) disaster response planning, including evacuation planning
and preparation of emergency shelter and
services.
The value of public education and awareness cannot be
overestimated. Experience has shown that many people tend
to ignore flood warnings. Effective public education should
warn of the existence of the hazard, provide information
about the nature of the hazard, and what individuals can do
to protect their lives and possessions. For example, coastal
residents should be aware that tsunamis are a series of waves
that may occur over a six-hour period; it is not safe to go
back after the first wave has passed. Motorists should be
aware that, at least in certain developed countries, most
people who die in flash floods do so in their cars; they
should never drive into a flooded area.
Flood forecasts can be made using a variety of techniques
ranging from simple approaches to complex
procedures. The selection of the technique to be used is
largely dependent on the needs of the community and the
physical setting. One approach is to use mathematicallybased
hydrological process models. Such models transform
the most recent conditions (e.g., rainfall, soil moisture,
snowpack state and water equivalence), upstream flow
conditions, and forecasted precipitation and temperatures
into hydrological predictions of streamflow. In larger river
systems, forecasts could be made through use of mathematically-based
hydraulic models, wherein existing conditions
upstream are projected downstream based on the physical
conditions of the river’s channels and the specific properties
of the flood wave. In some cases a combination of models
may be required. A common example results from tropical
and extratropical storms wherein high winds can cause
marine waters to rise above normal levels. These same
storms can carry large amounts of rain inland, resulting in
dramatically increased streamflow in river systems. In lowlying
areas, where the slope of the river may be very low, the
rising water level of the surge restricts the passage of freshwater,
combining in effect to increase the consequences and
gravity of the event. In such cases, flood forecasting would
comprise a combination of river-runoff process modelling,
river hydraulic modelling and coastal surge modelling in
order to provide projections of conditions at specific
locations.
The availability of a flood-forecasting programme
enhances the opportunity for taking protective action.
Protective actions include evacuation, moving possessions
to higher ground, moving hazardous materials to higher
ground, building temporary levees with sandbags, filling in
weak spots in existing levees, and mobilizing heavy equipment
for construction of temporary preventative measures
and for clearing flood debris.
Hazard and risk information can be used to design floodforecasting
systems that are more effective because they:
(a) forecast floods for the geographic areas with the greatest
hazards and risks; and
(b) are sensitive to the flow levels at which flood damage
commences.
Similarly, disaster planning should take into account the
nature of the flood hazards. For example, the effect of flooding
on roads and bridges should be taken into account when
selecting shelter facilities, evacuation routes and supply routes.
Because floods often contaminate water supplies, plans should
be made to obtain a safe supply of drinking water.
3.12 GLOSSARY OF TERMS
Chapter 3 — Hydrological hazards
* Definitions taken from (WMO/UNESCO, 1992)
** Definition taken from (UNDHA, 1992)
*** Definitions taken from (WMO, 1992)
Annual peak: The largest instantaneous flow rate in a given
year at a given river location.
Assessment:** Survey of real or potential disaster to estimate
the actual or expected damages and to make recommendations
for prevention, preparedness and response.
Bathymetry: Underwater or submarine topography.
Depth-duration-frequency curve:*** Curve showing the relation
between the depth of precipitation and the
frequency of occurrence of different duration periods.
Disaster:* A serious disruption of the functioning of society,
casing widespread human, material or environmental
losses which exceed the ability of the affected society to
cope using only its own resources.