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A review of flood event management in the European Union (EU) indicated
that most member states make a distinction between the organisations
that are responsible for flood event management and the organisations
that are responsible for emergency management and planning. The
responsibility for the development of emergency plans often takes place
at a local government level. Emergency plans are usually implemented by
the emergency services (e.g. the police, ambulance service), the army
and the local authority. Organisations responsible for flood risk
management such as the Environment Agency in the UK and Rijswaterstaat
in the Netherlands carry out the flood forecasting and warning. The
response to flood risk management in the EU has mainly focused on flood
defence asset management, and forecasting and warning to reduce flood
risk rather than on organised evacuation.
Aside from the Netherlands there would appear to be limited experience
within the European Union of employing systematic approaches and
modelling tools to develop evacuation plans for flood event management.
If the tools do exist then and are employed in practice they are often
very generic and only applicable at a macro-level. The cost of
evacuation is also not well documented. This is partly as a result of
the fact that organised evacuation is only used as form of emergency
response in Europe in very rare circumstances. In most of Europe the
authorities have no powers to forcibly make people evacuate from an
area at risk of flooding.
A review of user requirements in France, the Netherlands and the UK
indicated that in terms of planning evacuation of an area at risk and
the emergency response the following were important:
- The forecast of the flood extents, depths
and velocities would assist
with emergency response;
- Knowledge of the road network, location
of the vulnerable receptors
(e.g. old people, hospitals) and evacuation times are key to emergency
planning.
In countries mass
evacuations have been carried out in the past (e.g.
the Netherlands in 1995 when 250,000 people were evacuated) the cost of
evacuation is an important factor that needs to be taken into account.
It is also important for emergency planners to understand what parts of
the area at risk or what buildings can be used as shelters.
| Evacuation modelling for flood event
management
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It has been found that there are three main scales at which evacuation
models are employed for flood event management as follows:
- Micro – This corresponds to a scale where
each individual receptor at
risk is modelled;
- Meso – This corresponds to a scale that
is between a micro and
meso-scale;
- Macro – A macro scale model provides an
initial estimate
of the evacuation time for a large area. (e.g. at a regional scale).
The type of
evacuation model that is appropriate will depend on the
level of risk and the processes which the evacuation modelling is
seeking to inform. A densely populated urban area where the scale of
potential evacuation is large may require a detailed simulation model
where the traffic and flood hazard is modelled in a truly dynamic way.
| Application of evacuation models in the
Thames Estuary
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Three evacuation models were tested in the Canvey Island and Thamesmead
areas of the Thames Estuary. These were:
- BC Hydro Life Safety Model (LSM) – micro
level model;
- A non-linear optimisation technique –
meso scale model;
- A simple spreadsheet based model – macro
scale model.
The following
conclusions were reached on each model.
- The LSM allows dynamic
interaction between the receptors and the flood hazard;
- The LSM requires asignificant
amount of data including:
- The location of individual
properties, vehicles and people;
- Flood depths and velocities
from a two dimensional hydraulic model;
- Details of the road network and
other pathways;
- It has potential to be used to
inform emergency plans for heavily defended areas (e.g. London,
some coastal areas) and dam risk assessments in the UK.; The effect of
the following can be
assessed for different flood events:
- Location and the number of safe
havens;
- The effect of road closures;
- Rate of dissemination of
warning;
- The time of day and the
location of the population;
- The model also provides
scientifically robust method to estimate fatalities and injuries during
a flood
event;
- In its present state the LSM is
not “user friendly” enough to be used by organisations such as the
Environment
Agency or the Emergency services without a significant amount of
training;
- The LSM has the potential to be
adapted in the future to provide estimates of the fatalities and
evacuation
times for other hazards (e.g. chemical or nuclear accidents).
| Non-linear optimisation based evacuation
model
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A non-linear
optimisation-based model for evacuation has several advantages for use
at a meso-scale:
- It is relatively quick to set up such a
model for small areas;
- It can be simply modified;
- Large areas can be modelled using
relatively simple networks;
- A number of different evacuation routes
and locations of safe havens can be tested quickly.
However, the
non-linear optimisation models requires many assumptions to be made,
although it could prove useful to establish initial evacuation times
for relatively large areas of the UK e.g. the Thames Gateway.
A simple
spreadsheet model was simple to set up to assess evacuation times.
However, it can only usefully be employed at a macro-scale as it makes
several simplifying assumptions such as assuming that all the traffic
flows freely. However, if geo-referenced census road network data are
readily available then a first order estimate of the evacuation time
can be made very quickly.
| Application of evacuation models in the
Schelde Estuary
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Three existing traffic or evacuation models were applied in the Schelde
pilot are as followst:
- Macro scale - Evacuation Calculator (EC);
- Macro scale - ESCAPE DSS;
- Meso scale - INDY
The aim was to draw
conclusions on the reliability of the model results and the suitability
of the models to support the planning of an evacuation. This study
focused on the technical aspect of evacuation management. The models
were compared with regards to:
- Scale they can be applied on;
- Suitability for evacuation planning;
- Resulting evacuation times;
- Reliability of the resulting evacuation
times.
The conclusions are
summarised below.
Scale
The evacuation models EC and ESCAPE are not very detailed, being
applied at a macro-scale. INDY presents an improvement in detail, being
applied at a meso-scale. None of the models model individual movement
of persons, instead they model the flows of total number of inhabitants
from each postal code zone.
Suitability
The EC can be used for a quick estimation of the evacuation time, if
the whole area is to be evacuated. However, due to the amount of
parameters that must be estimated by the user, it is advisable to
validate the EC with the INDY model.
ESCAPE is not very user-friendly, as it gives little insight in the
calculation algorithms. The order in which the evacuees leave their
homes is derived within the simulation. The model does not give insight
in this. Therefore the results could not be compared with the other
models.
INDY is the most realistic of the three models and gives the
most reliable results. Besides evacuation time estimates, it gives
insight in the site-specific causes of congestion. However, the
duration of the model set-up and simulation run-time are considerably
longer.
Resulting evacuation times
In EC some scenarios combine the above-mentioned assumptions: a fast
departure profile, no reduction factor (this means no congestion at the
outflow) and optimal traffic management. This result of this scenario,
which was an evacuation time of approximately 12 hours, is considered
highly underestimated and unreliable.
Because ESCAPE algorithms are not understood in detail, it could not be
properly compared to EC and INDY. However, because it assumes an
optimal departure profile and an optimal traffic management (no
congestion), ESCAPE is also expected to underestimate the evacuation
time.
All models predict that it will take at least 22 hours to evacuate the
whole study area, when the evacuation is managed and no unexpected
events (like accidents) happen and people leave very efficiently. While
this is an underestimation of the actual evacuation time, it is still
too long to complete the evacuation within the lead time of the
event-forecast, which is usually 6 hours in this region.
Reliability
Because the model results were not compared with actual
evacuation times from a historical event, it is difficult to conclude
which model produces the most reliable results. It seems intuitive that
INDY would produce more reliable results than EC and ESCAPE, because
INDY simulates the actual road network in a dynamic way, whereas EC
simulates the network based on assumptions regarding the departure
profile and evacuation velocities, and ESCAPE assumes an optimal
departure profile and fixed road capacities. All three models are
expected to underestimate the evacuation time due to the assumptions
that no congestion occurs from cross flow traffic, that no accidents
will occur, and that residents will respond to an evacuation call in an
ideal and optimal way.
| Development of a prototype flash-flood
warning system for roads in the Gard Region |
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A prototype flash-flood warning system for the Gard Region of France
was developed to carry out the following:
- Identify road inundation potential based
on local geographical information.
- Develop a rainfall-runoff models on the
catchments located upstream the identified points prone to flooding.
- Combine the flooding potential of roads
with the estimated flood discharges, to provide an integrated tool able
to forecast in real time the flooding of roads.
Based on the
geographical and hydrological information the sections of the road
network that are most sensitive to flooding in the Gard Region pilot
area were identified and their vulnerability to flooding rated. The
proposed method has been calibrated on the Gard Region pilot area. Some
information is required on the past road inundations in order to
develop a road vulnerability assessment tool.
A simple distributed rainfall-runoff has been calibrated and validated
on the hydrological database available in the region. The validation
results appear relatively satisfactory and provide an acceptable method
to estimate peak discharges for the ungauged catchments in the Gard
Region.
Both model outputs, i.e. vulnerability index and estimated discharges,
have now to be combined to compute submersion risk indexes for the
various vulnerable points of the road networks at various instants of a
storm event. The maps obtained were compared to the available maps of
observed road submersions for four recent flash floods (i.e. December
2002, November. 2003, December 2003 and September 2005).
The results
were far from perfect and the prototype should not be considered as a
decision support system but rather as a source of information, among
others especially field observations that can help the emergency
services during a flood event to improve their decision making. The
prototype provided a relatively accurate and hence useful source of
information. The emergency services in the Gard Region emergency
services are to be consulted on the usefulness of this tool in the near
future.
The key
recommendations that have come out of this research are as follows:
- There is a requirement for evacuation
models. However, to be useful there is a need for them to model the
individual receptors (e.g. people, houses, vehicles) and to be able to
give decision makes other information other than just evacuation times
(e.g. loss of life and injury estimates, effects of different
management plans);
- More work needs to be carried out to make
evacuation models more “user friendly” so that they are used more
widely;
- The spilt of responsibilities between
water management organisations and bodies that are responsible for
emergency planning means that in some cases neither organisation wishes
to be responsible for carrying out evacuation modelling. There is a
need for flood risk managers to incorporate evacuation modelling as
part of their flood event management work.
The following model
comparison analysis was performed in the case of Schelde pilot site.
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