.
Erik de Haas, M.Sc.
Ministry of Transport, Public works and Water Management, Road and Hydraulic Engineering Division. P.O. Box 5044, 2600 GA Delft, The Netherlands
telephone: +31(15)2518279 telefax: +31(15)2518568
e-mail: E.dHaas@dww.rws.minvenw.nl
Martine Jak, M.Sc.
Ministry of Transport, Public works and Water Management, Road and Hydraulic Engineering Division. P.O. Box 5044, 2600 GA Delft, The Netherlands
telephone: +31(15)2518532 telefax: +31(15)2518568
e-mail: M.Jak@dww.rws.minvenw.nl
Abstract: In this paper we present the Flood Management
System, developed by the Dutch Government to support her safety policy.
The aim of the
system is to provide clear, up-to-date information on flood situations:
n
it enables an
assessment to be made of the potential seriousness of a given situation on the
basis of information about the weakest parts in the flood defence system;
n
it indicates
the potential effects of a flood;
n
it indicates
what action can be taken to minimise the risks. As a policy-planning instrument,
it can be used for drawing up evacuation plans, designating overflow areas and
spatial planning.
Keywords: river floods, flood management system,
risk-assessment
The Netherlands
is situated on the delta of three of Europe’s main rivers: the Rhine, the
Meuse and the Scheldt. As a result of this, the country has been able to develop
into an important, densely populated nation. But living in the Netherlands is
not without risks. Large parts of the Netherlands are below mean sea level and
the water levels, which may occur on the rivers Rhine and Meuse. High water
levels due to storm surges on the North Sea or due to high discharges of the
rivers are a serious threat for the low-lying part of the Netherlands.
Construction, management and maintenance of flood defences are essential
conditions for the population and further development of the country.
Without flood
defences much of the Netherlands would be regularly flooded. The influence of
the sea would be felt principally in the west. The influence of the major rivers
is of a more limited geographic impact. Along the coast, protection against
flooding is principally provided by dunes. Where the dunes are absent or too
narrow or where the sea arms have been closed off, flood defences in the form of
sea dikes or storm surge barriers have been constructed. Along the full length
of the Rhine and along the parts of the Meuse protection against flooding in
provided by dikes.
In determining
the required height of dikes, the traditional method in the Netherlands used
well into this century was to take the highest known water level, plus a margin
of 0.5 to 1 metre. After the large 1953 flood disaster in the Netherlands the
design method of flood protection was improved considerably by the Delta
Commission (see for example [1] and [2]). The starting point as proposed by the
Delta Commission after 1953 was to establish a desired level of safety for each
area that has to be protected against sea- or riverfloodings. This safety level
would need to be based on the costs of construction of dikes and on the possible
damages which would be caused by flooding. This economic analysis has been used
to differentiate the safety standard according to the expected damages in the
various areas. In 1996 these standards were laid down in legislation when the
Flood protection Act (Ministry of Transport, Public works and Water Management
1996) came into effect.
.
Fig.1 The optimal safety approach in economical terms.
The total costs (straight line) is the sum of flood damage
(dashed line) and costs of dike construction (dotted
line)
In 1993 and 1995
extremely high discharges occurred (about 12000 m3/s for the Rhine
and 3000 m3/s for the Meuse, while the design discharges for these
rivers are 15000 m3/s and 3870 m3/s). During that period,
no serious flooding occurred in the areas protected by dikes although there was
a severe possibility of dike
failure and river water levels exceeding dike levels. Almost 250,000 people were
evacuated for a considerable period of time. These events gave rise to a series
of new investigations in damage modelling, safety rules, dike stability, warning
systems and other related issues.
On the side of
the safety rules, the Dutch Ministry of Transport, Public Works and Water
Management is thinking about the possibility to develop a more risk – based
flood protection policy. The concept of flooding risks offers the possibility of
introducing other measures than building dikes. For example, it will be possible
to decide whether it would be more useful to invest in spatial planning of an
area or in strengthening the dikes. At present the tools required for such a
policy are being developed.
One of the
important instruments that can contribute to such a policy, is the Flood
Management System (in Dutch: HIS, Hoogwater Informatie Systeem). The development
of HIS was a direct reaction to the floods in 1993 and 1995. The HIS is a
computerised management system containing information that can be used both to
prepare policy for and to respond to flood situations. In the project-organisation
all public authorities that take part in flood management and policymaking are
represented.
In this paper we
present the state-of-the-art of the Flood Management System.
The main purpose
of the system is to be a useful tool for flood managers to plan responses to
flood conditions and coordinate activities during floods. The main users are
provincial authorities, water authorities and the Ministry of Transport, Public
Works and Water Management.
The aim of the
system is to provide clear, up-to-date information on flood situations:
n
it enables an
assessment to be made of the potential seriousness of a given situation on the
basis of information about the weakest parts in the flood defence system;
n
it indicates
the potential effects of a flood;
n
it indicates
what action can be taken to minimize the risks. As a policy planning instrument,
it can be used for drawing up
evacuation plans, designating overflow areas and spatial planning.
For the moment
the system is focused on the river area.
The functions of
the HIS can be divided in four different groups, according to four different
needs for information: Detailed information on a regional level versus global
information on a nationwide level, and information during a flooding versus
information for policymaking.
The four
functions of the HIS are shown in the diagram below:
|
|
regional
(HIS-R) |
nationwide
(HIS-NL) |
|
high-water
situations |
active
monitoring |
optimal
information |
|
policy
planning |
preparation
of regional contingency plans |
national
scenarios and risk assessment |
To be able to
cope with the required objectives a great deal of flexibility is necessary. A
modular approach was introduced in the development stage. The system’s design
is “open” allowing new modules to be developed and attached easily in the
future. The HIS comprises the following modules, which correspond to the
functions in the diagram.
This function
gives quick access to detailed, geographically based information on measured and
anticipated water levels, data on water defences and flood conditions in
forelands. This provides a basis for assessing the potential danger of a
flood-situation. Needless to say, the information in the system has to be kept
up to date. The database is linked directly to monitoring networks and the river
flood reporting service. Data are presented in the form of topographical images
(GIS).
The monitoring
module is for use in practical situations. It contains more or less fixed data
on areas, and combines them with variable data (such as water levels and
updates) on flood situations.
The monitoring
module supports the monitoring function and the logbook function. It does not
forecast water levels, but uses them as an input to make the impact on the river
and the situation around the water defences visible. The module presents
information very clearly against a geographical background (GIS). It enables
some analyses to be made.
The monitoring
module (MM) can be used to keep an eye on real-time water-level measurements and
forecasts. Actual and forecast
water levels can be input from measuring stations. Users can generate lines of
inclination for each kilometre, access historic, real-time or forecast
water-level data presented as maps, tables or graphs, calculate and show
inundation outside the dike.
Fig.2 Screenshot from the monitoring module.
Shown on this image is part of the Rhine river.
Threatened sections of the dike are colored red.
This function is
designed to ensure a single, clear record of reports received from co-ordination
centres during flood situations. Such reports can contain a wide range of
information, such as the flooding of homes or commercial premises, seepage,
shipping reports, etc.
The flooding
module centers around a hydrodynamic model, which can tell the user what will
happen when a dike bursts. Having been fed information on current water levels,
the size of the breach and the rate at which it is expanding, the model will
provide answers to questions such as “how fast will a polder be flooded?”,
“what pattern will flooding follow?” and “how high will the water
rise?”. The module gives insight into the effectiveness of any measures that
might be taken.
A digital model
is used to present a graphic image of the area for which the calculations apply.
Major water corridors, storage basins and rivers can be introduced as lines. The
flood module is capable of making combined one-dimensional and two-dimensional
calculations of the water flow. Effectively reducing the grid size by modelling
rivers and channels as one-dimensional elements and the flood areas in a
two-dimensional grid.
With the
assistance of a geographically oriented user interface, various analyses can be
made on the basis of maps, film and tables – e.g. a map with flood levels in
the affected area at a given time. These data form the basis for further
analyses, e.g. to calculate the damage incurred and time needed to evacuate
residents.
Making a flood
calculation is a lengthy process. It is therefore not advisable to use the
module during crisis situations. At the policy preparation stage, however, the
one-dimensional and two-dimensional flood scenarios form a good basis for
disaster prevention plans. The module also yields information on the effects of
measures taken to limit both the number of victims and the damage caused by the
flood.
Flooding of a
dike ring causes enormous damage. The extent of this damage depends on the
nature of the flood, for example salt or fresh water, and the properties of the
area, for example depth and land use. Escape possibilities and early evacuation
can play a mitigating role here. In calculating the consequences of flooding,
the Ministry of Transport, Public works and Watermanagement developed an
instrument by which calculates the damage and victims for each area in a uniform
and practical manner.
Fig.3 Output form the flooding module
Input for the
assessment of damage includes the depth of water at each location, the speed of
the current and the rate at which the water is rising. This data is the standard output from the HIS
flooding module. It’s combined with the data from the flooded area: land use,
number of homes, infrastructure, population etc. The result of the damage
assessment is the total economic damage that can be expected, given that
particular scenario, and the number of fatalities. The information can be
presented either as a table or as a map (GIS).
The standard model
The module is
based on the standard damage and victim model, commissioned by the Ministry of
Transport, Public works and Watermanagement .
The formula
generally used to calculate flood damage is as follows:
Where ai = damage factor, category i,
depending on depth of water
Ni = number of units in category i
.Si = maximum damage per unit in category i e
The damage factor a is derived from the damage function. The damage
function is in principle different for each category. The factor represents the
influence of hydraulic conditions, such as depth of water and weather.
In establishing
the standard model, a number of choices were made:
·
damage
is determined by water depth, waves and speed of current
·
maximum
cost of damage is based on replacement value
·
the
damage functions are the same throughout the Netherlands. This means that, in
certain hydraulic conditions, the damage to a house in Limburg is the same as
the damage to an identical house in the central Netherlands.
·
Assessable
damage is damage that can be expressed in monetary terms – i.e. damage to
buildings, means of storage and production, damage caused by interruption to the
production process. The number of lives claimed is the only non-monetary damage
to be included.
·
No
distinction is made between damage caused by fresh or salt water
The model works
on the assumption that everyone was at home. It is possible for the user to
indicate what percentage of residents had been evacuated before the flood
occurred.
Fig.4 Illustration the standard model. It combines the data
from land use (top left) and
the results form a flooding scenario (bottom left) to form a map
in which the amount of damage is presented per grid cell.
This function is
an aid in preparing and implementing regional evacuation plans: it indicates the
time required, accessible routes, when action should be taken, reception centers
for evacuees The evacuation module is currently still under development. The
fire brigade of Nijmegen is, in
cooperation with several neighbouring cities, developing an evacuation
instrument. A study is now being carried out to investigate the possibility of
integrating this as a module in HIS.
Agreements on
the best way for the central database to meet this need are being made as part
of the HIS project. The regional parties (provincial water boards and municipal
authorities) require a system that includes information and scenarios for
coordinating activities during flood situations in their own areas. Central
government needs a nation-wide information system, which includes local
information and scenarios that can be used to make decisions affecting the
country as a whole.
References
[1] Jorissen, R. (1996). Safety, Risk and Flood protection
Policy. Ministry of Transport, Public Works and Water Management. Internal
report.
[2]
Jorissen, R. and Kok, M. (1997) ‘a physical-based approach to predicting the
frequency of extreme river waterlevel’: review from problem owner and
engineering perspective. In: Roger Cooke, Max Mendel and Han Vrijling,
Engineering Probabilistic Design and Maintenance for Flood protection. Kluwer
Academic Publishers, 1997.
[3] Road and Hydraulic Engineering Division, Flood Management System. Ministry of Transport, Public Works and Water Management. Brochure, june 1999.