Consequences of floods: 2D hydraulic simulations for the case study area Central Holland

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1 Title: Consequences of floods: 2D hydraulic simulations for the case study area Central Holland Author: Dr. N.E.M. Asselman Institute: WL Delft Hydraulics Author: Ir. K. Heynert Institute: WL Delft Hydraulics Number of pages : 22 June 2003 Keywords (3-5) : Inundation, Flood, Modelling DC-Publication-number : DC Institute Publication-number (optional) : Q3289 WL Delft Hydraulics Report Type : Intermediary report or study : Final project report DUP-publication Type : DUP Standard DUP-Science Acknowledgement Exampletext: This research has been sponsored by the Dutch Ministry of Transport, Public Words and Water Management. Conditions of (re-)use of this publication The full-text of this report may be re-used under the condition of an acknowledgement and a correct reference to this publication. Other Research project sponsor(s):

2 Abstract Large parts of the Netherlands lie below sea-level, and the hazard of large scale floods leading to extensive damage and loss of life is always present. The Delft Cluster project Consequences of floods studies a range of possible consequences and aims at developing methods to quantify them. The developed methods are applied to a case study area in Central Holland. This report describes the hydraulic simulations that were carried out for this area in order to estimate water depths and flow velocities caused by failure of the river dike near Rotterdam and the coastal defence near Katwijk. Model simulations were carried out using the hydraulic SOBEK model. SOBEK is able to model overland flow in 2 dimensions using the SOBEK-Overland Flow module. Flow through channels and smaller water courses is simulated with the 1D component of SOBEK, called SOBEK-Channel Flow. The model simulations indicate that the most severe consequence are expected to occur after failure of the river dike near Rotterdam. The inundated area extents from the river dike near Rotterdam to the borders of the cities The Hague, Leiden and Gouda. Water depths decrease from 6 m in the central part to less than 1 m at the boundary of the flooded area. Maximum flow velocities follow a similar distribution with very high values of up to 7 m/s near the dike breach, decreasing to about 0.1 m/s at the boundaries of the inundated area. Inundation of the main part of the area occurs very rapid, i.e. within 5 hours. It takes about 5 days or more before places near the boundary of the flooded area are inundated. The total volume of water stored in the study area is m 3. When a total pumping capacity can be realised of 100 m 3 /s, it takes about 160 days to pump all water out. In case of failure of the coastal defence near Katwijk the total volume of water stored equals m 3. The time needed to pump all water out will be about 50 days with a pumping capacity of 100 m 3 /s. This difference is mainly caused by lower inflow rates due to the higher elevation of the land behind the dune area, which results in a smaller head difference, and because coastal water levels do not remain high for a very long time. Water depths vary between a few decimetres and about 2 m. Flow velocities vary from about 3 m/s near Katwijk to less than 0.5 m/s in the main part of the study area. PROJECT NAME: Flood consequences PROJECT CODE: BASEPROJECT NAME: Flood consequences and acceptability BASEPROJECT CODE: THEME NAME: Risk of flooding THEME CODE: 02 Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 2

3 Executive Summary Introduction Large parts of the Netherlands lie below sea-level, and the hazard of large scale floods leading to extensive damage and loss of life is always present. The Delft Cluster project Consequences of floods studies a range of possible consequences and aims at developing methods to quantify them. The developed methods are applied to a case study area in Central Holland that consists of dijkring 14 (dike ring area number 14). This report describes the hydraulic simulations that were carried out for this area in order to estimate water depths and flow velocities caused by failure of the river dike near Rotterdam and the coastal defence near Katwijk. Study area Dijkring 14 is located in Central Holland and includes major cities like Amsterdam, Rotterdam and The Hague. Elevation varies from 6 m below mean sea level in the south eastern part near Rotterdam to more than 25 m above mean sea level in the dune area near the coast. The area is completely surrounded by water: the North Sea canal in the north, the Amsterdam-Rhine canal in the east, the North Sea in the west and the rivers Hollandse IJssel and Nieuwe Maas in the south. Although floods may occur as a result of failure of the dikes along any of these water bodies, this study has focused on only two of them: a coastal flood due to failure of the sluices near Katwijk (the Katwijk-case) and a river flood due to a dike break near Rotterdam (the Rotterdam-case). Modelling software Model simulations were carried out using the hydraulic SOBEK model. SOBEK is able to model overland flow in 2 dimensions using the SOBEK-Overland Flow module. Flow through channels and smaller water courses is simulated with the 1D component of SOBEK, called SOBEK-Channel Flow. Model development The overland flow model consists of a 2D elevation model with grid cells of 250x250 m². Due to the limited detail in the available elevation data, secondary dikes are not schematised. Flooding is expected to occur under design water level conditions of 5.75 m above mean sea level at Katwijk and 3.3 m near Rotterdam. These water levels form the boundary conditions of the hydraulic model. Results Rotterdam-case The model simulations indicate that the most severe consequence are expected to occur after failure of the river dike near Rotterdam. The inundated area extents from the river dike near Rotterdam to the borders of the cities The Hague, Leiden and Gouda. Water depths decrease from 6 m in the central part to less than 1 m at the boundary of the flooded area. Maximum flow velocities of 7 m/s occur near the dike breach and decrease to about 0.1 m/s at the boundaries of the inundated area. Inundation of the main part of the area occurs very rapid, i.e. within 5 hours. It takes about 5 days or more before places near the boundary of the flooded area are inundated. The total volume of water stored in the study area is m 3. When a total pumping capacity can be realised of 100 m 3 /s, it takes about 160 days to pump all water out. Results Katwijk-case Failure of the coastal defence near Katwijk also results in a large flooded area, but the water depths remain much less. In the main part of the study area, water depths vary between a few decimetres and about 2 m. Flow velocities vary from about 3 m/s near Katwijk to less than 0.5 m/s in the main part of the study area. The total volume of water stored equals m 3. The difference in inflow and Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 3

4 storage between the Rotterdam-case and the Katwijk-case is mainly caused by different inflow rates due to differences in elevation of the land behind the coast and the river dike, and because coastal water levels do not remain high for a very long time. Storm surges usually last for 6 to 12 hours, whereas river discharge can remain high for about two weeks. The time needed to pump all water out will be about 50 days with a pumping capacity of 100 m 3 /s. PROJECT NAME: Flood consequences PROJECT CODE: BASEPROJECT NAME: Flood consequences and acceptability BASEPROJECT CODE: THEME NAME: Risk of flooding THEME CODE: 02 Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 4

5 Table of contents Consequences of floods: 2D hydraulic simulations for the case study area Central Holland...1 Abstract...2 Executive Summary Introduction The software: SOBEK General information Boundary- and initial data Meteorological effects Other features of the proposed software Simulation of transport of pollutants Case study area Model development Overland Flow model Boundary conditions Results of the simulation Conclusions References...20 General Appendix: Delft Cluster Research Programme Information...21 List of Figures Figure 1 Topography of the study area dijkring Figure 2 The surroundings of Rotterdam are flooded for the rescue of Leiden (1574)...7 Figure 3 The ship twee gebroeders positioned in the gap that started to grow after failure of the river dike near Nieuwerkerk...7 Figure 4 Schematisation of the Hydraulic Model: a) Combined 1D/2D Staggered Grid; b) Combined Continuity Equation for 1D2D Computations...9 Figure 5 Elevation of the study area in m above mean sea level (NAP)...11 Figure 6 Water level fluctuations after failure (a) Nieuwe Maas near Rotterdam, (b) North Sea near Katwijk...14 Figure 7 Maximum water depths (in meters) computed with SOBEK (Rotterdam-case)...15 Figure 8 Maximum flow velocities (in m/s) computed with SOBEK (Rotterdam-case)...16 Figure 9 Time of inundation in hours after failure of the river dike near Rotterdam...16 Figure 10 Maximum water depths (in meters) computed with SOBEK (Katwijk-case)...17 Figure 11 Maximum flow velocities (in m/s) computed with SOBEK (Katwijk-case)...18 Figure 12 Time of inundation in hours after failure of the coastal defence near Katwijk...18 Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 5

6 1 Introduction Large parts of the Netherlands lie below sea-level, and the hazard of large scale floods leading to extensive damage and loss of life is always present. The Delft Cluster project Consequences of floods studies a range of possible consequences and aims at developing methods for quantification. The developed methods are applied to a case study area located in the Netherlands. The following criteria were used for the selection of the case study area: Flooding must be realistic; Floods should be large scale; Various types of flooding may occur (marine and fluvial); Different types of damage are expected with respect to the environment, economy, housing and casualties; The economic value of the infrastructure, industries and other services should be large and comprise a range of economic consequences when disrupted. Dijkring 14 in Central Holland (Figure 1) is the most densely populated area in the Netherlands. It includes large cities such as Amsterdam, Rotterdam, The Hague and Leiden. Also, the economic heart of the country is located here. The area is exposed to different water systems that may cause flooding. This, together with the low elevation and therefore the large expected water depths when flooded make this area an interesting case study. Figure 1 Topography of the study area dijkring 14 The present safety level of this area is among the highest in Holland, with the maximum probability of exceedance of the hydraulic design criteria for coastal and river defence systems being 1: The probability for flooding may be expected to be even lower. At first sight this high safety level makes the area of Central Holland an unrealistic case study for a flooding study. However, flooding of this area has occurred in the past. Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 6

7 In 1574 the area was inundated in an attempt to free the city of Leiden from the Spaniards, which had conquered the city earlier that year. On 30 July 1574 the States of Holland and the army of the Prince of Orange decided to break the dykes at several locations near Rotterdam (Figure 2) to flood the country, so that weapons and ordnance could be shipped to Leiden while the Spanish could not move theirs with horses. Unfortunately the water rose too slowly and the city of Leiden could not be reached. It was not until 18 September, when the weather grew worse, that the water began to rise. Finally, on 3 October the Dutch army entered the city with food: bread with cheese and herring. Nowadays 3 October is a day with many festivities, known as the Leidensontzet (rescue of Leiden). Figure 2 The surroundings of Rotterdam are flooded for the rescue of Leiden (1574) The last time the area was almost inundated was in During the flood of 31 January 1953 the dike along the river Hollandse IJssel almost broke near Nieuwerkerk (Figure 1). A large flood could only be prevented by directly placing a ship in the initial gap that had occurred in the river dike. Figure 3 shows the ship called Twee Gebroeders positioned in the river dike (see also Figure 3 The ship twee gebroeders positioned in the gap that started to grow after failure of the river dike near Nieuwerkerk Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 7

8 Methods that are developed as part of the Delft Cluster project to quantify the consequences of flood comprise assessment of the following: Direct and indirect economical damage; Damage to agriculture and the environment; Collapse of buildings and infrastructure; Casualties. All methods need information about the extent of the flood, water depths and flow velocities. As no data on past floods is available for this area, hydraulic simulations are required. It was decided to carry out these simulations with the hydraulic SOBEK model. This report describes the development and application of an inundation model for the case study area dijkring 14 or Central Holland. Attention is paid to: the applied software (chapter 2); the study area (chapter 3); development of the model (chapter 4); modelling results (chapter 5). Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 8

9 2 The software: SOBEK 2.1 General information The hydraulic simulations were carried out using the SOBEK model developed by WL Delft Hydraulics (see SOBEK Overland Flow consists of a 2-dimensional modelling system based on the Navier-Stokes equations for depth-integrated free surface flow. All equations are solved through a fully implicit finite difference formulation for all terms in the Navier-Stokes equations, based upon a staggered grid. The special way in which the convective momentum terms have been formulated allows for the computation of mixed sub- and supercritical flows. Based upon this formulation it is also possible to compute the behaviour of standing and moving hydraulic jumps. For these computations to be robust and accurate, there is no need to introduce artificial viscosity. In combination with the 2D modelling system, SOBEK is able to handle 1D elements such as (small) water courses and hydraulic structures. In this 1D-2D combination, the 2D overland flow, including the obstructing effects of embankments and natural levees, is simulated through the 2D equations of SOBEK Overland Flow, while the sub-2d grid gullies and the hydraulic structures are modelled with SOBEK Channel Flow. Both modelling systems produce implicit finite difference equations, which are also linked through an implicit formulation for joint continuity equations at locations where both modelling systems have common water level points, as shown in Figure 4. a b Figure 4 Schematisation of the Hydraulic Model: a) Combined 1D/2D Staggered Grid; b) Combined Continuity Equation for 1D2D Computations The main advantages of combination of flow in the 1D and 2D domain are: 2D grid steps can usually be significantly larger, as no refinement of the 2D grid is required for the correct representation of hydraulic structures and gullies; as a result, the simulations will run much faster for a comparable level of accuracy; a wide variety of hydraulic structure descriptions can be used. Robustness and accuracy The Overland Flow and the Channel Flow modules of SOBEK are based upon the same numerical principles and both allow for extremely stable and robust computations. In the first place this is based upon the properties of the numerical schemes applied. In the second place, a number of checks are made at every step in the computation to prevent physically unrealistic results, such as negative water depths. If such a constraint is not satisfied, the time step will be reduced. Such a procedure is also applied in the flooding and drying of cells in the Overland Flow module. Every time only one neighbouring computational cell can be wetted or dried, otherwise the time step will be reduced to satisfy this criterion. As discussed before, the robustness and accuracy of the numerical schemes follow to a large extent from the particular way in which the convective momentum terms have been discretized. The Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 9

10 formulation implemented also suppresses the development of oscillating velocity directions at irregular model boundaries. Here the finite difference scheme offers the same robust behaviour as models which follow irregular boundaries with their grid contours, such as curvilinear grids and finite elements. For proof of accuracy, comparison of results has been made with experimental studies, both with published data and obtained through own laboratory experiments. Of particular interest is the strict volume conservation. This feature is of particular importance in the simulation of transport of pollutants. 2.2 Boundary- and initial data The software offers a wide range of possible boundary conditions, such as water levels or discharges given as a function of time. At boundaries discharges can be specified which are spread over a selected number of grid cells as a function of the individual cell conveyance. The modeller does not have to predefine such distributions. At downstream boundaries the user can also specify rating curves. Also here, the discharge can be distributed over a selected number of grid cells as a function of the individual cell conveyance. Initial data can be given as dry bed, water depth or water level. The model will adjust automatically the correct initial state as a function of boundary data supplied. Water levels can also be specified along line elements in order to follow gradients along rivers and channels. A hot-start functionality is available allowing the continuation of a simulation from a previously computed state. 2.3 Meteorological effects Both the Overland Flow and the Channel Flow modules of SOBEK allow for the inclusion of meteorological effects, such as wind, precipitation and evapotranspiration. In the case of a flooding event caused by a dike break these processes can, however, be neglected. 2.4 Other features of the proposed software Also, both the Overland Flow and the Channel Flow modules of SOBEK allow for the specification of spatial variations in roughness. Hydraulic roughness can be specified as Manning, Chézy or White Colebrook values. Every grid cell can have its own roughness value, which can be modified through import from GIS or through the editor available in the user interface. 2.5 Simulation of transport of pollutants Delft modelling systems are connected to the common water quality module DELWAQ. DELWAQ imports velocity and water level (volume) data from the hydrodynamic modules and uses these as a basis for the simulation of the transport of pollutants and the description of water quality processes. DELWAQ contains a library of more than 200 chemical and (micro)biological processes, which can be activated by the modeller. At the beginning of the Delft Cluster project Consequences of floods DELWAQ could not be used in combination with the Overland Flow module. This new development was accomplished during the project and is treated separately. Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 10

11 3 Case study area The case study area consists of dijkring 14, located in Central Holland (Figure 1). Major cities are Amsterdam, The Hague, Rotterdam, and Leiden. The total number of inhabitants equals 3.6 million. Elevation within the study area varies from less than 6 m below mean sea level in the area north east of Rotterdam to more than 25 m above mean sea level in the dune area near the coast (Figure 5). The main part of the study area is located 1 to 2 m below mean sea level. With respect to the two cases Rotterdam and Katwijk it is important to notice that the area near Rotterdam has a much lower elevation than the area near Katwijk (i.e. north of the Hague). Figure 5 Elevation of the study area in m above mean sea level (NAP) As can be seen in Figure 1 the area is surrounded by various water bodies that may cause floods. The North Sea is located to the west of dijkring 14. Erosion of the dunes or failure of one of the hydraulic structures along the coast may cause flooding. However, given the relatively high elevation of the land near the coast, inundation will proceed relatively slowly. The eastern and northern borders of the area are defined by the Amsterdam-Rhine canal and the Noordzeekanaal. Both canals have relatively low water levels and discharges. The most disastrous flood is expected to occur as a result of failure of the river dikes along the Hollandse IJssel or the Nieuwe Maas near Rotterdam. Supply of water to these locations by the river Rhine is large and the difference in water level during periods of high discharge is large. For instance, the design water levels that are used for the design (height) of the river dikes equals about 3.3 m above mean sea level (Rijkswaterstaat, 2001), whereas the elevation of the land behind the dike locally is as about 9.5 m lower. After initial failure, the gap in the dike is expected to grow rapidly because of this large slope in water level. Inflow through the gap will continue for a long time because of the absence of a river flood plain. At other locations along the Rhine distributaries in the Netherlands inflow through the gap will stop as soon as water levels in the river drop below the elevation of the flood plain in front of the dike. This usually is the case within a few weeks. Here, floodplains are absent, as a result of which water can flow through the gap even when water levels at the river drop to a very low level. Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 11

12 Two cases have been studied for the Delft Cluster project. One case simulates flooding of the area after failure of the sluices near Katwijk, along the North Sea coast north of The Hague. The other case simulates inundation after failure of the river dike near Rotterdam. The most disastrous case, i.e. failure of the river dike near Rotterdam, is used as case study in all other sub-projects to determine the consequences. The Katwijk-case only is used in few sub-projects for comparison. Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 12

13 4 Model development 4.1 Overland Flow model The most important item of the 2D overland flow model is the schematization of the elevation of the flooded area. Most models that are presently being developed use data from the AHN (actual height data bank of the Netherlands). This data base consists of detailed elevation measurements (minimum point density of 1 point per 4 m²) carried out using airborne laser altimetry. This data base allows the detection of smaller elements, such as railway dikes and highways, whose elevation exceeds that of the surrounding area, and that are important in obstructing the flow. Unfortunately, these recently collected elevation data have not yet become available for Central Holland. Therefore, an older and less detailed elevation model was applied. This elevation model consists of grid cells with a size of 250 x 250 m². Roads and railway dikes cannot be detected in this schematization. Another shortcoming of the applied elevation model is that elevations in the north western part of the model (near Amsterdam) are much higher than might be expected from topographical maps. However, as the extent of the flood is not expected to reach this far it is expected that this does not cause problems for the hydraulic simulations in this study. As failure of the coastal defence near Katwijk is assumed to take place as a result of failure of the sluices it can be expected that the local canals and other water courses in this area play an important role in the progress of the flood wave. To account for this effect these local water courses were schematised in the 1D channel flow model. 4.2 Boundary conditions The flood simulations that were carried out with the SOBEK model assume failure of the river dike near Rotterdam in the first case and failure of the coastal defence near Katwijk in the second case. In both cases, failure occurs under hydraulic conditions that result in water levels equal to the design water level of 3.3 m above mean sea level near Rotterdam and 5.75 m above mean sea level near Katwijk (Rijkswaterstaat, 2001). Graphs of water level variations near the gaps after failure are shown in Figure 6. It is assumed that the design water level near Rotterdam is caused by a combination of high river discharge and high coastal water levels due to a westerly storm. The gap width of the dike break near Rotterdam is 250 m. The location of the gap is such that the discharge capacity of the river branches near the gap do not pose significant limitations with respect to the inflow rate. Failure of the coastal defence near Katwijk is simulated as failure of the sluices. Inflow can take place through the canal. The width near the coast is about 110 m. In both cases it is assumed that the gap is closed after 10 days. The hydraulic conditions that are applied, the relatively wide gap near Rotterdam and the absence of obstacles to the flow, such as railway dikes, result in a worst case scenario. On the other hand, closure of the breach in the river dike after 10 days might have resulted in an optimistic scenario. When hydraulic information about the flooded area is needed to develop accurate evacuation plans, the assumptions with respect to the hydraulic boundary conditions and model assumptions need to be reconsidered. Also, a more accurate elevation model should be applied that accounts for obstacles to the flow (railway dikes, other secondary dikes) and that has more accurate elevation estimates for the north-west part of the study area. The model results, however, are suitable for the purpose of this study, i.e. testing and application of the developed models to quantify the consequences of floods. Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 13

14 water level (m above mean sea level) water level (m above mean sea level) time (hours after failure) time (hours after failure) a b Figure 6 Water level fluctuations after failure (a) Nieuwe Maas near Rotterdam, (b) North Sea near Katwijk Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 14

15 5 Results of the simulation The model results consist of maps of water depths and flow velocities for each modelling time step. The maps indicating maximum water depths, maximum flow velocities and time of inundation in hours after failure of the dike are shown here (Figures 7 through 12). Rotterdam-case In case of failure of the river dike near Rotterdam, a very large area is flooded. Water depths decrease from 6 m in the central part to less than 1 m at the boundary of the flooded area (Figure 7). Maximum flow velocities follow a similar distribution with very high values of up to 7 m/s near the dike breach, decreasing to about 0.1 m/s at the boundaries of the inundated area (Figure 8). The time of inundation in hours after failure of the river dike is given in Figure 9. The most important conclusion is that inundation of the south-east part occurs very rapid, i.e. within 5 hours. It takes about 5 days or more before places near the boundary of the flooded area are inundated. The total volume of water stored in the study area is m 3. This implies an average inflow of about 1500 m 3 /s. Under the assumption that a total pumping capacity is available of about 100 m 3 /s it takes at least 160 days to pump all water out of the area. Figure 7 Maximum water depths (in meters) computed with SOBEK (Rotterdam-case) Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 15

16 Figure 8 Maximum flow velocities (in m/s) computed with SOBEK (Rotterdam-case) Figure 9 Time of inundation in hours after failure of the river dike near Rotterdam Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 16

17 Katwijk-case Similar as for the Rotterdam-case, failure of the coastal defence near Katwijk results in a large flooded area (Figure 10). A major difference between the two cases is that failure of the coastal defence near Katwijk results in much lower water depths. Large water depths of more than 3 m only occur in the Grote Polder north east of The Hague and some polders north east of Leiden. In the main part of the area water depths vary between a few decimetre and about 2 m. The regular patterns visible in the water depths in Figure 10 result from inaccuracies in the digital elevation model for the area south-west of Amsterdam. Figure 10 Maximum water depths (in meters) computed with SOBEK (Katwijk-case) Flow velocities near the gap also are lower. In the Rotterdam case, maximum flow velocities near the gap reached values of 7 m/s. In the Katwijk-case, maximum flow velocities near the gap vary from 1 to slightly more than 3 m/s. In the main part of the flooded area maximum flow velocities are less than 0.5 m/s (Figure 11). Figure 12 shows the time of inundation in hours after failure of the sluice. The area closest to Katwijk is flooded within 4 hours. It takes more than 2 days before places near the boundary of the flooded area are inundated. The total volume of water stored in dijkring 14 equals m 3. For comparison, the total volume stored in the Rotterdam-case was m 3. This difference is mainly caused by lower inflow rates due to the higher elevation of the land behind the dune area and because coastal water levels do not remain high for a very long time. When a total pumping capacity can be realised of 100 m 3 /s, it takes about 50 days to pump all water out. Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 17

18 Figure 11 Maximum flow velocities (in m/s) computed with SOBEK (Katwijk-case) Figure 12 Time of inundation in hours after failure of the coastal defence near Katwijk Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 18

19 6 Conclusions The results of the case studies are described in great detail in Chapter 5. From the results the following striking conclusions are drown: In both cases, inundation of a large area near the gap in the river dike or the coastal defence is flooded within several hours. This rapid progress of the flood wave minimises possibilities for warning. The inflow near Rotterdam is much larger than the inflow near Katwijk. This difference mainly is caused by differences in hydraulic head and the duration during which inflow can take place. As a result of the difference in inflow, water depths are larger in the Rotterdamcase than in the Katwijk-case. Depending on the total amount of water stored in the flooded area and the total pumping capacity that can be realized, it takes up to 6 months or more to pump all the water out. Both the Katwijk and the Rotterdam case study are based on a realistic, but relatively unfavourable hydraulic scenario. Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 19

20 7 References Rijkswaterstaat, Hydraulische randvoorwaarden 2001, Delft: Dienst Weg en Waterbouw, Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 20

21 General Appendix: Delft Cluster Research Programme Information This publication is a result of the Delft Cluster research-program (ICES-KIS-II), that consists of 7 research themes: Soil and structures, Risks due to flooding, Coast and river, Urban infrastructure, Subsurface management, Integrated water resources management, Knowledge management. This publication is part of: Research Theme : Risk of Flooding Baseproject name : Consequences of floods Project name : Consequences of floods Projectleader/Institute Prof. A.C.W.M. Vrouwenvelder Project number : Projectduration : Financial sponsor(s) : Delft Cluster TNO Ministry of Public Works, Road and Water Management Projectparticipants : GeoDelft WL Delft Hydraulics TNO Delft University of Technology Twente University Alterra CSO Delphiro Total Project-budget : Number of involved PhD-students : 2 Number of involved PostDocs : 0 Delft Cluster is an open knowledge network of five Delft-based institutes for long-term fundamental strategic research focussed on the sustainable development of densely populated delta areas. Keverling Buismanweg 4 Tel: Postbus 69 Fax: AB Delft info@delftcluster.nl The Netherlands Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 21

22 Theme Managementteam: Ground and Construction Name Organisation Prof. J.K. Vrijling Delft University of Technology Ir. E.O.F. Calle GeoDelft Prof. A.C.W.M. Vrouwenvelder TNO Projectgroup During the execution of the project the researchteam included: Name Prof. Ir. A.C.W.M. Vrouwenvelder Dr. Ir. P.H. Waarts Ir. J.E.A. Reinders Dr. E.E. van der Hoek Ir. S.N. Jonkman Ir. K. Heynert Prof. A. van der Veen Ir. L.C.P.M. Stuyt Ir. M. de Muinck Keizer Organisation TNO TNO TNO GeoDelft RWS-DWW WL Delft Hydraulics Twente University Alterra Delphiro/CSO Other Involved personnel The realisation of this report involved: Name Dr. N.E.M. Asselman Ir. K. Heynert Organisation WL Delft Hydraulics WL Delft Hydraulics Date: September 2003 Consequences of floods: 2D hydraulic simulations for the case study area Central Holland p. 22