RIZA - Institute for Inland Water Management
and Waste Water Treatment
Dept. WST
Van Leeuwenhoekweg 20
3316 AV Dordrecht
The Netherlands
p.jacobs@riza.rws.minvenw.nl
Abstract
Chapter
1 Introduction
Chapter
2 Study area
Chapter
3 Regime of high river discharge: impact of storm duration and intensity
on flood protection
3.1
Introduction
3.2
Future scenario’s
3.3
Discussion
Chapter
4 Regime of low river discharge: consequences for salt intrusion
4.1
Introduction.
4.2
The reference situation: salt intrusion under average conditions.
4.3
Future scenario’s: impact of decreased summer discharges and sea level
rise.
4.4
Sensitivity to the estimate of sea level rise.
4.5
Discussion.
Chapter
5 Conclusions and recommendations
Acknowledgements
References
Management of the Rhine-Meuse delta in The Netherlands involves, among a great number of other aspects, three major fields of interest: protection against flooding, fresh water supply and shipping. The water system in this area is mainly driven by the combined effects of tide and river discharge. To be able to anticipate changes in these driving mechanisms, so that the functions of the aforementioned fields of interest are guaranteed, there is a need for detailed predictions of climate scenario’s. In particular, quantitative estimates of future changes in sea level rise, average storm duration and intensity, and in the occurrence and duration of extremely low and high river discharges are needed.
At RIZA, a tool has been developed with which the combined effects of changes in boundary conditions and management interventions (mainly of morphological conditions) on the hydrodynamical characteristics of the delta area can be calculated. This DSS (Decision Support System), which is based on a one-dimensional numerical model, has been used to investigate the effects of climatological changes on two management aspects: risks of flooding due to a combination of peak river discharges and storm surges and salt intrusion due to a combination of low river discharges and storm surges. The third aspect, the effects of climatological changes on shipping, has not been examined, since it is expected that the mean water level in a large part of the area will increase.
For the first aspect, estimates of mean sea level rises and increases of peak river discharges are used to asses the increase in risks of flooding. The accuracy of the model results is mainly impeded by the uncertainty in the estimates of temperature changes, sea level ris and increase of peak discharges of the rivers Rhine and Meuse, and of the characteristics of the storm surges: the intensity, duration and frequency of occurrence of storm wind fields. Furthermore, there is a need for more insight into the extent of dependency of the occurrence of these storm set-ups in combination with peak discharges, since the method used in the DSS is based on the assumption that all stochastic variables are independent.
To estimate the extent
of salt intrusion into the delta area, periods of low river discharges
are simulated. It is expected that these periods will occur more often
in the future, and that their average duration will increase. However,
detailed estimates of these changes are not yet available. The effects
of an increase in average water level at the sea boundary of the delta
are investigated as well as the combination of a change in low river discharges
and sea level rise. Finally, the sensitivity of the results with respect
to the estimate of sea level rise is discussed and remaining questions
to climatologists are addressed.
The influence of climate on the water systems of river deltas such as found in The Netherlands manifests itself in various ways. First of all there is the direct influence of the amount of precipitation in the delta area itself as well as in the entire catchment area (which determines the river discharge) and the influence of the wind field (which induces a change in the net inflow or outflow of seawater and river water, respectively). These climate aspects are expected to change significantly over the next century or so, mainly due to the greenhouse effect (whether man-induced or not). Furthermore, climatic changes on a global scale are expected to result in an increase of the average water level at sea, which forms an indirect way in which climate influences the hydrodynamics of a river delta water system.
Some of the climatic parameters that are expected to change over the next century are further investigated here to estimate their impact on the water system of the Rhine-Meuse delta in The Netherlands. This approach was also followed in a study in the 80’s and early 90’s (WL|Delft Hydraulics, 1990), in which a physical model of the Rhine-Meuse estuary was used to determine the effects of changing sea level and river discharge on the water levels and salt intrusion in this area. However, with this physical model it was hardly possible to apply some statistical methods that are used to determine the design levels of primary dikes and dams (RIZA/DBW, 1987a). In this paper, a statistical method is used both for estimating the effects of climatic changes on flood protection and salt intrusion in the Rhine-Meuse delta.
The paper is set up as
follows: the study area will be introduced in chapter 2. After defining
a reference situation that is valid for the present day situation, firstly
changes in storm duration and intensity are considered: the results of
model calculations for the present and future situations are described
in chapter 3. Secondly, in chapter 4, the effects of changes in summer
river discharge and sea level on salt intrusion are simulated. Finally,
main conclusions and remaining questions are addressed in chapter 5.
Almost 50% of the land surface of The Netherlands lies below average sea level, reason why water is a continuous threat. This certainly holds for the area that contains the Rhine-Meuse estuary, which forms the transition zone between the fluvial and marine water systems. The area is bounded on the eastern side by the rivers Lek , Waal (both downstream branches of the Rhine) and the Meuse (in Dutch: Maas) and on the western side by the North Sea (see figure 1). Like many delta areas, the Rhine-Meuse estuary region serves for a large number of functions and activities, like living and working in the urban area of Rotterdam-Dordrecht, the harbour of Rotterdam (one of the largest in the world), agriculture, fishing and recreation. A number of these activities depend heavily and directly on the water system of the estuary: recreation, shipping and fresh water supply for industrial, agricultural and drinking water purposes.
…Figure 1
…Rhine-Meuse
delta region
The hydrodynamical characteristics of the area are governed mainly by the river discharges of Rhine and Meuse as well as by the water level at the sea boundary. The wind field also plays an important role: it influences the water levels, both at the sea boundary and in the estuarine water system itself. These three boundary conditions vary in space and time, making the hydrodynamics of the system indeed very dynamic.
Apart from the more natural influences on the Rhine-Meuse estuary mentioned above, man also has a considerable impact. The rivers Rhine and Meuse are regulated by sluices far upstream, while in the lower part of the estuary, where three contributing rivers have split into a rather complicated network of branches with interconnecting channels (see figure 1), the Haringvliet sluice gates are used as a way of regulating the water flow. The control of the Haringvliet sluice gates, which at present are opened only during low tide, is targeted at maintaining the river discharge along the Northern branch at a sufficiently high level to minimise the salt intrusion through the Rotterdam Waterway. Furthermore, there are three storm surge barriers in the area: the so-called Maeslant barrier in the Rotterdam Waterway, the Hartel barrier in the Hartel Canal and the barrier at the mouth of the Hollandsche IJssel.
The results of the effects of the aforementioned climatic changes on the hydrodynamical characteristics of the Dutch delta area are most important for two main items of (fresh) water management: continued protection against flooding and limitation of the increase of salt intrusion into the delta to guarantee the availability of sufficient fresh water supplies. At present times, the safety against flooding is guaranteed by sufficiently high dams, river dikes, storm surge barriers as well as the Haringvliet sluices.
Together with increased
protection from storm surges, the closing of the Haringvliet also created
a large fresh water reservoir, which is used until present day to serve
the demand of drinking water companies as well as providing water for the
surrounding area for irrigation purposes. In fact, there are numerous inlet
points for drinking and agricultural water in the entire Rhine-Meuse delta
region. Some of these lie in that part of the delta which is, every now
and then, influenced by sea water (see also figure 1). It is clear that
timely anticipation on climatic changes that influence the Rhine-Meuse
delta is vital for the several functions of this water system.
Chapter 3 Regime of high river discharge: impact of storm duration and intensity on flood protection
In the major part of the study area, land is protected against flooding by a combination of dunes, dikes and storm surge barriers. For dimensioning the dike height an acceptable inundation frequency is defined. The design water level, which is the level occurring with this frequency, is calculated by simulation of 54 combinations of river discharges and storm set-ups (an increase in water level at the sea boundary above the value due to the astronomical tide only – see figure 2) with a one-dimensional hydrodynamical model (RIZA, 1991).
The method also accounts
for the chance of failure of storm surge barriers in the area. Under the
assumption that river discharge, storm set-up and the failure of storm
surge barriers are independent phenomena, the chance of each combination
can be calculated and so the water level frequency distribution can be
constructed (see table 1; DBW/RIZA, 1987a and RIZA, 1995). In table 2 the
acceptable inundation frequencies and matching water levels are given for
6 locations in the study area (see also figure 1). Note that the average
water level in most of the study area is about equal to mean sea level.
…Figure 2
…Change of sea water
level at Hoek van Holland for six storm set-up situations (Source: RIZA,
1999).
…Table 1
…Occurrence
of Rhine discharge and sea water levels (wind set ups) in the winter months
(discharge: excess frequency per day; sea water level: number of times
per high tide) (Source: RIZA, 1999).
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...Table 2
...Acceptable
inundation frequencies and matching high water levels at 6 locations in
the study area (Source: Dienst Weg- en Waterbouwkunde,1996)
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Climate changes will affect
the high water level frequency distribution in the Rhine-Meuse delta. An
increase of the peak discharges of the rivers Rhine and Meuse as well as
a rising (mean) sea level are predicted. Little is known on the effects
of climate change on the storm intensity and duration frequency distribution.
This chapter deals with the effects of climate changes on high water levels
in the study area and the uncertainty in the predictions of effects of
climate change resulting from uncertainty on the effects of climate changes
on storm intensities and duration.
To investigate the effects
of climate changes on the high water levels 3 scenario’s are distinguished.
The sea level rise and increase in the river peak discharges are listed
in table 3.
...Table 3
...Three scenarios
for relative sea level rise and increase of the peak river discharges of
Rhine and Meuse in 2050 (with the year 2000 as a reference) (Source: Projectteam
NW4, 1997).
| Scenario |
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discharge (> 10,000 m3/s) of the river Rhine |
discharge (> 2,000 m3/s) of the river Meuse |
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(no increase in temperature) |
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(T + 1 oC) |
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(T + 2 oC) |
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In figure 3 the increase
of the high water levels in the study area for scenario C is presented.
The increase is presented for levels occurring with a frequency of 1/2000
year-1. The high water level increase related to climate changes
varies from less than 0.1 m to more than 0.7 m. The increase is lowest
in the Rotterdam area. Here the high water levels are more or less controlled
by storm surge barriers. The increase is highest in the eastern part of
the study area, where the high water levels are determined by the peak
river discharges.
...Figure 3
...High water
level increase for scenario C. Shown is the increase in water levels occurring
with a frequency of 1/2000 year-1 with the situation in the
year 2000 as a reference.
As stated before little
is known on the effects of climate change on storm intensities (wind speeds)
and duration (storm lengths). At present, a standard duration of 29 hours
is used for the latter. This is (based on historical information on the
storm duration frequency distribution) approximately the mean storm duration
in the present situation. The storm surge frequency distribution is also
based on historical information of wind speeds and storm surges. To illustrate
the effect of an increase in the (mean) storm duration an increase of 4
hours was simulated. To investigate the effects of an increase in the storm
intensities an increase of 10% of all wind speeds above 10 m/s was assumed.
The effects of this simulated increase in storm intensity and duration
are illustrated in figure 4.
...Figure 4
...High water
level increase in 2050 (with 2000 as a reference) for a frequency of 1/2000
year -1 due to sea level rise and an increase of the river discharges
according to scenario C and due to an increase in the storm duration and
intensity. The results are presented in a cumulative way: the effects of
an increase in storm duration are added to the increase due to climate
scenario C only; the increase due to a higher storm intensity is added
to the combined increase due to scenario C and to an increase in storm
duration.
Figure 4 shows that a
rather small increase in the storm duration, but even more in storm intensity
has major effects on the simulated increase of high water levels. The effects
are small in the eastern parts of the study area, but large in the western
part of the area. The effects of an increase in storm intensity and duration
are related to an increase of the frequency of situations in which the
discharge to the sea is limited by high water levels at sea.
The simulation results show that climate changes might have large effects on the high water levels in the Rhine-Meuse delta. The high water level increase to be expected in the coming decades is however rather uncertain. This is partly due to uncertainties in the temperature rise and resulting sea level rise as well as in the expected peak river discharges. Simulations for a hypothetical increase of storm intensities and duration show that rather small changes in these variables have large effects on the simulated high water level increase, especially in the western part of the delta. These simulations illustrate the necessity of reliable estimates of the effects of climate changes on wind speeds en storm lengths.
In previous paragraphs
it was mentioned that the high water level calculations are based on the
assumption that river discharges and storm surges at sea are independent.
The method used only distinguishes a winter and summer season with two
different sets of frequencies for specific discharge an storm surge values.
Climate changes will possibly lead to a higher correlation between river
discharge and wind speed. More information on the probable changes in the
relation between wind and river discharges is therefore needed.
Chapter 4 Regime of low river discharge: consequences for salt intrusion
With the current climate, it does not occur very often that, due to a combination of low river discharge and high water level at sea (due to a storm surge), the salt intrusion reaches the drinking water inlet points. Furthermore, the duration of the excess of the salt concentration is never long enough to create serious problems for the water companies, since they have ample storage capacity to endure the period of excess. However, climate models predict an increase in low river discharge periods in the summer; not only will they occur more often, but they will also last longer each time. It is clear that this will threaten the operationality of the water 'production' function that is attributed to some parts of the Rhine-Meuse delta.
We have investigated the effects of these changes on the salt intrusion by using a probabilistic method, similar to the one which is operational to determine the design water level on which the actual height of river dikes and dams is based (see also chapter 3).
The two most important external factors that determine the level of salt intrusion into the Rhine-Meuse delta are the upstream river discharge of the Rhine (measured at Lobith station) and the windspeed (including direction). The discharges are usually high in late winter and spring, and subsequently low during summer and autumn. To define the reference situation, we have used hydrological data of the discharge of the Rhine for the months May - October (inclusive). For the wind speed, we have used three years of daily averaged data measured at Hoek van Holland. These parameters have been divided into a number of classes (see table 4). The chance of occurrence of each class has been determined for each of the parameters. The results have also been included in table 4. Note that the parameters wind speed and direction have been converted into the wind set-up.
...Table 4
...Occurrence
of river discharge and wind speed (wind set-up) in the months May-October.
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The same one-dimensional
model as was used for the calculations presented in chapter 3, based on
the advection-diffusion equation, has been run for each of the possible
5*7=35 combinations. Since the chance of occurrence of each parameter is
known, so is the chance of occurrence of each of these combinations, providing
that the parameters are independent.
4.2 The
reference
situation: salt intrusion under average conditions
Salt intrusion in the Rhine-Meuse estuary can only occur via the Rotterdam Waterway, since, with the Haringvliet sluices closed at high tides, it is the only direct connection between the upstream rivers and the North Sea. Salt intrusion in this relatively narrow and deep channel manifests itself in the shape of a salt tongue that flows into an easterly direction underneath the (more or less continuous) fresh water outflow in the upper layer. In the one-dimensional representation used here, it is impossible to discern between the concentrations of the two layers, rather the model calculates an average concentration over the complete cross-section of the channel. Due to industrial activities, the background Cl- concentration of the river Rhine is of the order of 200 mg/l (the Cl- concentration of the North Sea is about 17,000 mg/l). Any increase of the concentration above the background level indicates the presence of the salt tongue, although it is impossible to say anything about the relative thickness of the underflow and of the concentration thereof.
Figure 5 shows the salt
concentrations (Cl- in mg/l) under average conditions (frequency
of occurrence 50 times per year) for the reference situation. In the Northern
branch, the effects of the saline underflow reach Rotterdam (approximately
36 km from the sea boundary via the route Rotterdam Waterway - Nieuwe Maas)
while the salt intrusion via the Oude Maas reaches somewhat further (beyond
the junction of Oude Maas and Spui).
...Figure 5
...Reference
situation: chloride concentration for the excess frequency of 50/year
In more extreme cases
(frequency once per year, see Figure 6), the salt tongue along the Nieuwe
Maas does not reach much further, while intrusion along the Oude Maas extends
until and beyond the southern mouths of the Spui and Dordtsche Kil rivers.
Although the Haringvliet sluice gates are closed during high tides, and
therefore no direct salt intrusion can occur via this connection to the
sea, salinification of the southern branch of the Rhine-Meuse estuary can
still take place via these two north-south branches. The main reason for
the observed differences between Nieuwe and Oude Maas is the tidal forcing
of the salt tongue along the trajectory Oude Maas - Dordtsche Kil (in fact,
the salt intrusion in this part of the estuary is strongly related to the
difference in water levels between Hoek van Holland and Moerdijk). The
salt intrusion in the Nieuwe Maas is relatively more related to the upstream
river discharge. This is partly due to the differences in bathymetry: while
the Nieuwe Maas becomes increasingly shallow further east (down to less
than 9 m towards the east of Rotterdam), the Oude Maas stays deep (approximately
14-18 m) until the junction with the Dordtsche Kil.
...Figure 6
...Reference
situation: chloride concentration for the excess frequency of 1/year
4.3Future
scenario’s: impact of decreased summer discharges and sea level rise
The first future scenario
considered here involves the effects on the distribution of the Rhine discharges
in the summer as a result of a global temperature increase of 2°
C (corresponding to the extreme scenario C for the year 2050 - see table
3). In table 5, the results of this global warming on the hydrological
regimes in the Rhine basin are shown, together with the change in occurrence
from the reference situation. These results have been obtained from the
RHINEFLOW model (Grabs et al., 1997).
...Table 5
...Occurrence
of river discharge in the summer months for the future situation as a result
of a 2°
C global temperature change.
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A second future scenario
has been defined as the situation where the average sea level has risen
45 cm with respect to the reference situation (again corresponding to the
extreme climate scenario for 2050, IPCC, 1995). In a third series of calculations,
the two effects (decreased river discharge and sea level rise) have been
combined. The results of these three different scenario’s are shown in
table 6 in two different ways: firstly, the chloride concentrations corresponding
to an excess frequency of once per year (extreme salt intrusion) are given
and, secondly, we present the number of tides (per year) during which the
maximum chloride concentration at a certain station was higher than 250
mg/l (with an average duration of 12 hours and 25 minutes, there are 704
tidal periods in one year). This concentration has been chosen because
it is the upper limit that a number of water companies use for intake of
fresh water.
…Table 6
…Results of
scenario calculations for 6 stations (see figure 1 for locations).
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| Cl- concentration (mg/l) 1/year |
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| Station |
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| Lekhaven |
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| Beerenplaat |
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| Brienenoord |
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| Bernisse |
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| Alblasserdam |
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| Excess frequency of 250 mg/l | ||||||||
| Station |
(figure 1) |
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In general, the chloride concentrations increase at all stations due to decreased river discharges and, even stronger so, due to a sea level rise. The combined effects are, of course, strongest of all three scenario’s considered here. Note that the results of the combination are not simply the sum of the separate scenario’s of decreased summer discharges and sea level rise (i.e., the effects are not linear).
For each scenario, the effects of climatological changes are strongest at the stations distributed along the northern branch of the delta (along the Nieuwe Maas - stations 1,3 and 5). Stations 1 and 2, 3 and 4 and 5 and 6 are pair-wise roughly at the same distance from the junction of Oude and Nieuwe Maas. As indicated above, the salt intrusion in the reference situation reaches further along the Oude Maas than along the Nieuwe Maas (see also table 6 by comparing stations 1-2, 3-4 and 5-6). Not only is there a higher frequency of salt intrusion, the concentrations are higher as well. Further investigations of field measurements (Rijkswaterstaat, 1998) have shown that this holds for those cases that the chloride concentrations at stations 1 and 2 are above approximately 2000 mg/l. Changes in boundary conditions (sea level or river discharge) do make the cases of extreme salt intrusion stronger (and make them occur more often – see second part of table 6), but the relative effects are of course small in the Oude Maas, since the concentrations (at excess frequency of 1/year) are already high.
Figure 7 shows the difference
in the once per year concentrations between the scenario with decreased
summer discharges and sea level rise on the one hand, and the reference
situation on the other hand. The figure shows clearly the large effects
along the Oude and Nieuwe Maas. Furthermore, it demonstrates that an increase
in salt intrusion is expected in almost all of the river branches of the
Rhine-Meuse delta area.
...Figure 7
...Difference
in chloride concentration at excess frequency of 1/year between combination
of effects (2050) and the reference situation.
4.4Sensitivity
to the estimate of sea level rise
Since the effects of sea
level rise were in general almost one order of magnitude stronger than
those of decreased river discharges, we have carried out a test to determine
the sensitivity of the results to the estimate of the value of the rise
of sea level expected for the year 2050 in the extreme scenario. Calculations
with the combined climatological effects are repeated, but now with an
increase in average sea level of 50 cm rather than 45 cm (extreme scenario
+ 10%). Table 7 shows the results for the same stations as before. The
differences between the case of 50 cm sea level rise and 45 cm sea level
rise are generally smaller than the extra 10% increase on the sea boundary.
…Table 7
…Results of
calculations to determine sensitivity to estimate in sea level rise.
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| Cl- concentration (mg/l) 1/year |
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| Station | Label (figure 1) |
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| Lekhaven |
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| Brienenoord |
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| Bernisse |
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| Alblasserdam |
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| Excess frequency of 250 mg/l | |||||
| Station | Label (figure 1) |
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| Lekhaven |
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Calculations have shown that changes in boundary conditions due to climatological changes expected for the year 2050 will result in a large increase in salt intrusion in the Rhine-Meuse delta area. This increase manifests itself in a larger area that will be affected by salt intrusion, and consequently also in increased average chloride concentrations in those areas that are also in the present situation under the salt influence of the sea. This implies important consequences for the management of the water system in the area, in particular for the water inlet points for drinking and agricultural water production (compare the locations of the six salt reference stations in figure 1 with those of the inlet points). For a few of the chosen salt reference stations, the excess frequency for the 250 mg/l concentration increases by more than 50%.
Although the results do
not seem to be very sensitive to the estimate of sea level rise, the applied
increase of 5 cm on the expectation of climate scenario C (+45 cm) has
to be put in the light of the uncertainty of the climate scenarios proper.
It is clear that estimates that range from 10 to 45 cm will have large
impact on the salt intrusion that can be expected.
Chapter
5 Conclusions and recommendations
Climate has a large influence on the water systems in delta areas such as the Rhine-Meuse estuary in The Netherlands. In the above, we have shown the extent of the effects of climatic changes that are predicted for the future in various scenarios. Two major features of the water system of the estuary are highlighted: safety against flooding and the use of the water system as a fresh water supply. The expected changes in average sea level, storm duration, storm intensity and river discharge (an increase in winter months and a decrease in the summer months) all create serious threats to the ways the water system is used at present times with respect to the above mentioned features.
The simulations described in chapter 3 highlight the need for reliable estimates of the effects of climate changes on sea level, peak river discharges, wind speeds en storm lengths. Furthermore, the dependency of some of these parameters on one and another is, in the present situation, assumed to be rather weak, but may increase in the future. It is clear that future calculations to obtain design levels for dams and dikes are in need of estimates that may shed more light on the change of these dependencies.
From the simulations of
future salt intrusion scenario’s it is clear that for this function of
the water system similar kind of reliable estimates of sea level rise and
changes in (summer) river discharge are needed too. So far, we have only
looked at the combined effects of these two controlling factors. If climate
has a discernible effect on the distribution of storms throughout the year
(and on their duration and intensity), then these aspects will also have
to be taken into account. At this moment, there is no view on possible
changes in the coincidence of periods of low river discharge and storms,
which of course will lead to a maximum in salt intrusion. If climatologists
are able to shed some light on these changes, then the timely anticipation
necessary for good management of the Rhine-Meuse delta will be aided enormously.
We thank Sacha de Goederen
for taking care of the calculations, Aad Fioole for producing some of the
graphs and Dik Ludikhuize for stimulating discussions and for carefully
reading the manuscript.
DBW/RIZA, 1987a, Het optreden van hoogwaterstanden in het noordelijk deltagebied, DBW/RIZA document no. 87.018, Lelystad, The Netherlands.
DBW/RIZA, 1987b, Effect variatie opzetduren op de hoogwaterstanden in het noordelijk deltabekken, DBW/RIZA document no. 87.054, Lelystad, The Netherlands
Dienst Weg- en Waterbouwkunde,1996, Hydraulische randvoorwaarden voor primaire waterkeringen, ISBN-90-3693-718-3, Delft, The Netherlands
Grabs, W., K. Daamen, D. Gellens et al., 1997, Impact of cklimate change on hydrological regimes and water resources management in the Rhine basin. CHR report no. I-16, CHR, Lelystad, The Netherlands.
IPCC (Intergovernmental Panel on Climate Change), 1995, Second Assessment Report of Working Group 1. Cambridge University Press, Cambridge, U.K.
Projectteam NW4, 1997, Klimaatverandering en bodemdaling: gevolgen voor de waterhuishouding van Nederland
Rijkswaterstaat, 1998, Zout in het Noordelijk Deltabekken in 1997, Directorate Zuid-Holland, document no. APS/98-211, Rotterdam, The Netherlands
RIZA, 1991, Gebruikershandleiding ZWENDL.
RIZA, 1995, Het optreden van hoogwaterstanden in het noordelijk deltagebied in de situatie met toekomstige infrastructurele aanpassingen, RIZA document no. 94.014X, Lelystad, The Netherlands
RIZA, 1999, Hydraulische verkenning IVB, Resultaten 2000-2050, Autonome variant, RIZA document no. 98.130X, Lelystad, The Netherlands
WL|Delft Hydraulics, 1990, Synthese waterloopkundig systeem-onderzoek Noordelijk Deltabekken, Report no. Z244, Delft, The Netherlands