Evaluation of nourishment strategies Holland Coast HK4.1: Long-term sustainable strategies for the Holland Coast Cycle 1

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1 Evaluation of nourishment strategies Holland Coast HK4.1: Long-term sustainable strategies for the Holland Coast Cycle 1 Bas Huisman Arjen Luijendijk Deltares, 2010

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3 Content 1 Introduction Introduction Readers guide 2 2 Strategies Introduction Considered scenarios Strategy 1 : Uniform along the coast Strategy 2 : Coastal cells 4 3 Description of the Holland coast model Introduction Models applied for the Holland Coast Wave model set up Coastline model set up Validation of longshore transports along the Holland Coast 10 4 Analysis of results Introduction Results Strategy 1 : Uniform along the coast Strategy 2 : Coastal cells Interpretation of results Evaluation of strategies 20 5 Evaluation of the nourishment strategies Findings Updated strategies for Cycle References 23 Evaluation of nourishment strategies Holland Coast i

4 1 Introduction 1.1 Introduction The starting point for the research on long-term sustainable development of the coast is the report of the Delta committee (Rapport Veerman, 2008 and Nationaal Waterplan, 2009). This report states that the coast should be adapted to climate future change. The consequence of this requirement is that a large upscaling of the yearly nourishment (sand) and sand-mining volumes will be required. The report states that Research must also be conducted soon to determine how such large volumes can be distributed as efficiently as possible in terms of the ecology, economy and energy efficiency. Within the framework of the project Building with Nature (abbreviated as BwN), the HK4.1 work package was defined which aims at the development of long-term strategies for maintenance of the coast. This involves nourishing the coast as well as sand mining offshore. The methodology for this package has been elaborated in the work plan (Deltares, 2010). An iterative design process (with several cycles) is followed to develop and design such strategies (See Figure 1.1). The first cycle starts with the design of a strategy, which is evaluated on the basis of expert judgment, by means of analogues that exist in nature and by means of numerical modeling. The findings from the evaluation of the initial strategy are then used to develop a new strategy, which is the starting point for the next cycle. Stakeholders Update Strategy Strategy development development Evaluation and optimising Design dimensions of strategy Modelling Expert judgment Developments of knowledge & models Analogues Figure 1.1 Process of strategy development Evaluation of nourishment strategies Holland Coast 1 of 23

5 The complexity of the design of the strategy is increased in each of the cycles (see Figure 1.2). The first strategy is defined only by the variation of the longshore distribution of the nourishing volumes (cycle 1). Consecutively, the frequency of the nourishments (cycle 2) and the distribution in the cross-shore profile (cycle 3-?) are added. Optimising strategy Strategy 1 Strategy 2 Cycle1 continuous distribution alongshore dis-continuous MKL volume Migration rates Cycles 2 continuous 1-year distribution alongshore frequency dis-continuous 1-10years MKL volume Migration rates Dune strength Beach width Biodiversity Cycles3-? continuous 1-year shore face distribution alongshore frequency distribution cross-shore Figure 1.2 Strategy to optimise the design which model?? dis-continuous 1-10years underwater MKL volume Migration rates Dune strength Beach width Biodiversity Intertidal area Dune area This report focuses on Cycle 1 of this process and consists of a definition of the applied strategies (or nourishment scenarios), a description of the coastline model that was used for the analysis and an interpretation of the results of the coastline modelling. 1.2 Readers guide The definition of the applied strategies can be found in Chapter 2. Chapter 3 presents the set up and boundary conditions of the applied coastline model. The modelling results of the strategies are presented and evaluated in Chapter 4. On the basis of this evaluation conclusions are drawn and recommendations are provided (Chapter 5). 2 of 23 Evaluation of nourishment strategies Holland Coast

6 2 Strategies 2.1 Introduction This chapter describes the two strategies that were defined in the half-yearly BwN HK4.1 meeting in April The strategies differ with respect to the longshore distribution of nourishment along the coast. Two strategies were defined: Uniform along the coast Coastal cells The effect of these strategies was assessed for up to four yearly nourishment volumes to investigate the effect of larger nourishments on the coast. It is noted that this report assesses only the impact of the nourishment strategies with the Holland Coast coastline model. The sand mining strategies are assessed in a later stage of the HK4.1 project as the modelling approach has not yet been determined. 2.2 Considered scenarios Table 2.1 presents for the two strategies the considered scenarios of yearly nourishment volumes. The volumes are in million m 3 per year. Table 2.1 Scenarios for the considered strategies Uniform Coastal cells The following assumptions apply to the presented shoreline modeling results: placed in the active part of the cross-shore profile (evenly distributed) applied continuously in time (every year) considered period is 50 years sea level rise is not included yearly-averaged wave climate based on last 30 years tidal effects are not included. Evaluation of nourishment strategies Holland Coast 3 of 23

7 2.3 Strategy 1 : Uniform along the coast The first strategy distributes the nourishment of sediment uniformly along the coast (see Figure 2.1). Two nourishment volumes were modeled (3 and 12 million m 3 /year). Figure 2.1 Sketch of the scenario Uniform (small yellow dots indicate the nourishment locations; orange lines indicate the harbour moles of Scheveningen and Ijmuiden,and the Hondsbossche Zeewering) 2.4 Strategy 2 : Coastal cells In the second strategy, the sediment is nourished at a limited number of locations along the coast (see Figure 2.2). A strategy is modelled with three hot spots, which are located in a separate coastal cell (respectively Delfland, Rijnland and Noord-Holland), as well as a strategy with six hot spots. The applied radius of the area with the nourishment is 2 kilometer. Four different total yearly nourishment volumes were applied in the model, which are respectively 3, 6, 9 and 12 million m 3 /year. Sea level rise has not been included in this stage. Figure 2.2 Sketch of the scenario Coastal Cells (yellow dots indicate the nourishment locations; orange lines indicate the harbour moles of Scheveningen and Ijmuiden, and the Hondsbossche Zeewering) 4 of 23 Evaluation of nourishment strategies Holland Coast

8 3 Description of the Holland coast model 3.1 Introduction This chapter describes the Holland Coast coastline model that was set up for the assessment of the morphological development of the strategies mentioned in the previous chapter. First the characteristics of the applied coastline model UNIBEST-CL+ are described (Section 3.2). Sections 3.3 and 3.4 then provide details on the set up of respectively the wave model and coastline model. Section 3.5 provides some insight in the performance. 3.2 Models applied for the Holland Coast Two models were used for the set up of the Holland coast model: Wave model : SWAN Coastline model : UNIBEST-CL+ Wave model SWAN is a model that computes the transformation of waves towards the coast (Holthuijsen et al., 1993). The wave propagation includes shoaling, refraction due to current and depth, frequency shifting due to currents and non-uniform depth. Other physical processes that are included in the model are wave generation by wind, three- and four-wave interactions, whitecapping, bottom friction, depth-induced breaking and transmission through and reflection (specular and diffuse) against obstacles. Coastline model The 1D coastline model UNIBEST-CL+ has been applied for the assessment of the impact of the nourishment strategies. This model computes wave driven sediment transport along the coast and coastline changes due to spatial gradients in the transport (see Figure 3.1). It assumes that the transport develops instantaneously (assuming that the longshore current has sufficient time and space to develop), which is the case for longer stretches of coast. Note that cross-shore sediment transports are not included in the computations. Figure 3.1 Illustration of sediment transport due to obliquely incoming waves (note that gradients in sediment transport can cause erosion, which is illustrated in orange, or accretion ) Evaluation of nourishment strategies Holland Coast 5 of 23

9 Wave driven longshore transport occurs close to the coast (in a strip of hundreds meters from the shoreline). It is caused by a longshore current that is generated by waves that come in obliquely to the coast (under an angle). The magnitude of the longshore current depends on the wave characteristics (wave height, wave period) as well as on the angle under which the waves come in. Tide can be included in the computations in a schematized way (limited number of conditions). 3.3 Wave model set up The following steps were performed to derive representative nearshore (normal) wave conditions: Selection of offshore data point with wave measurements; Schematisation of offshore wave data to a limited number of representative conditions; Transformation of waves from offshore to nearshore; For the derivation of representative normal wave conditions, data points with a long record of wave measurements are required. At some locations on the North Sea such wave records over a long period (tens of years) are available. Wave measurement data were used of three available offshore stations: Euro Platform (EUR), IJmuiden Munitiestortplaats (YM6) and Eierlandse Gat (ELD), for the period 1979 to See Figure 3.2 for the locations. Figure 3.2 Wave buoys along Holland Coast 6 of 23 Evaluation of nourishment strategies Holland Coast

10 First, the wave climate for YM6 was determined. For each wave condition in this climate, the local wave height, wave period and wave direction for the other two stations were derived from the waves that were measured simultaneously with the YM6 condition. This means that for all three stations, the climates consist of an equal number of wave climates with an equal probability of occurrence, but with locally derived wave height, period and direction. By doing so, a realistic offshore wave field is obtained, which accounts for the phenomenon that the mean wave height increases northward. The near-shore wave conditions were computed with SWAN, in which the three offshore conditions were imposed as a varying boundary condition. The wave roses for the three different wave buoys are presented in Figure 3.3. The schematised wave climate is representative only for the offshore location. Therefore, the propagation of waves towards the coast have been computed to attain normal wave conditions at relevant nearshore locations. For this purpose SWAN model simulations have been carried out. In the wave computations, a constant water level at mean sea level (excluding surge, tide and sea level rise) has been applied. The structures (e.g. the Ijmuiden breakwaters) have been schematised as closed obstacles. Figure 3.3 Wave roses at the three locations for the period ; from left to right: Europlatform, Ijmuiden, Eierlandse Dam Model computations were performed for a bathymetry that was based on data of the Dutch coast for 2004 (supplemented with 2003 data). Figure 3.4 presents the outline of the applied wave grid and the 2004 bathymetry. A typical computed wave field (height and direction) is presented as well. Evaluation of nourishment strategies Holland Coast 7 of 23

11 Bed level [m] H m0 [m] x 10 5 x 10 5 hsig wave vector (mean direction) 23-Nov :29: y coordinate 5 y coordinate (m) x coordinate x x coordinate (m) x 10 4 Figure 3.4 Wave grid outline and bathymetry (left) and typical computed wave field (right) 3.4 Coastline model set up The coastline model consists of a number of components. For each of the following components the set up of the model is described: Schematised coastline Hydraulic boundary conditions (waves and tide) Sediment characteristics Schematisation of harbour moles Schematised coastline The coastline has been set up in RD coordinates, for which data was used from approximately 450 measured cross-shore profiles of the coast (JARKUS database). For each of the cross-shore profiles the MKL position was used as the coastline point. A coastline was derived from the JARKUS profile data for the year Revetments were schematised in the model at the Petten sea defence and at Den Helder. Groynes were not included in the model. 8 of 23 Evaluation of nourishment strategies Holland Coast

12 Figure 3.5 Schematised coastline of Holland coast model Hydraulic boundary conditions The sediment transport was computed at 113 locations along the coast. The cross-shore profiles at these locations were derived from the JARKUS database. The active height of the profile was set at 10 meters. For each of these locations the nearshore wave climates (at NAP-5m) from a SWAN model for the coast of Holland were used as a boundary condition in the UNIBEST-CL+ model. The wave climate consists of a total number of 269 conditions. Tide was not included in the computations. Sediment characteristics A gradation of sediment with a median grain diameter (D50) of 200 m was used for the transport computations. For the tenth and ninety percentile values of the grain diameter (D10 and D90) a size of 120 m and 300 m were respectively used. Sensitivity runs were made with smaller and larger sediment. These runs were performed with sand with a median grain diameter (D50) of 150 m and 300 m. The 10 th and 90 th percentile values were scaled linearly with the D50. For the current project, the sediment transport formula of Van Rijn (2004) was applied. Schematisation of harbour moles Near to harbour moles the wave climate is disturbed, as the breakwaters shield part of the wave energy. Special attention was therefore paid to these areas. The local wave climate at IJmuiden was derived from a local SWAN model, which included the sheltering by the breakwaters. While for the harbour of Scheveningen the sheltering of the waves and diffraction were computed by means of a formulation by Kamphuis (1992). Four separate wave climates were computed at respectively 100, 300, 500 and 700 meter from the harbour moles (each of course with a different shielding angle of the waves). The bypass at the considered harbour moles was derived from literature and hindcasts of the morphological development at IJmuiden (which are performed in the framework of other studies). For IJmuiden a bypass of 90,000 m 3 /yr was applied in the model, while a bypass of 200,000 was applied at Scheveningen. Evaluation of nourishment strategies Holland Coast 9 of 23

13 Implementation of nourishments in shoreline model The considered nourishments were placed in the active part of the cross-shore profile (evenly distributed between -7m and +3m); so, an active height of 10 meters was applied. The nourishments were applied continuously in time; at every numerical time step. In the uniform strategy, the distance between the sources was approximately 600 meters. In the model, this was schematise by applying sediment sources at every 600 meter. 3.5 Validation of longshore transports along the Holland Coast The computed longshore sediment transports of the Holland Coast model can be compared to literature data (van der Rest, 2004 and Van Rijn, 1995). Figure 3.6 shows the computed sediment transport for three sediment diameters (D 50 =150, 200 and 300 m) and two sediment transport formulae (Top: Van Rijn, 2004, Bottom: Bijker, 1977). The upper panel shows that the computed transports with Van Rijn (for D 50 = 200 and 300 um) match quite well with literature data (* in the plot), while for Bijker (lower panel) only the transports for the D50=300 case match well. Figure 3.6 Computed sediment transport along the Holland coast for three sediment diameters (D 50=150, 200 and 300 m) and two sediment transport formulae (Top: Van Rijn, 2004, Bottom: Bijker, 1977) 10 of 23 Evaluation of nourishment strategies Holland Coast

14 It should be noted that for the area close to Den Helder, influenced by the Marsdiep tidal inlet, the transports are not representative. This area is highly influenced by the complex geometry and corresponding tidal currents in the flood and ebb channels. This area is indicated in lightyellow (blanked) in the graphs. The new Holland coast model contains more detailed cross-shore profiles and coastline features than older models like the Planstudie Veiligheid (PSV, 2004) model. Furthermore, the sediment transport formulations (Van Rijn, 2004) are well validated and more complex than the Bijker formulation in the previous models. Although the presented shoreline model of the Holland Coast has not yet been calibrated, the comparisons with the longshore transport distribution from literature data indicate that the present model is well able to reproduce the longshore transport distribution (Van Rijn formulation). Given the model performance on the longshore transports, the model is well suited to be applied for evaluating the long-term nourishment strategies. Evaluation of nourishment strategies Holland Coast 11 of 23

15 4 Analysis of results 4.1 Introduction This chapter provides the results for the two considered strategies and an analysis of the characteristics of these strategies. Section 4.2 presents the results, which are analysed in Section 4.3. The effectiveness of the strategies is evaluated in Section Results Strategy 1 : Uniform along the coast The first strategy consists of a nourishment that is uniformly distributed along the coast. Model runs were performed with total yearly nourishment volumes of 3 and 12 M m 3 / year. Two aspects are considered: the time development of the nourishment and the effect of a very large increase in nourishment volumes. Figure 4.1 shows the (relative) impact of the nourishment on the coastline over time, after 10 and 50 years of continuous nourishments of 3 M m 3 / year. This figure shows the difference between the autonomous situation (due to redistribution) and the situation with the nourishments. In this way only the impact of the measure is shown. Scheveningen IJmuiden Hondsbossche zeewering Scheveningen IJmuiden Hondsbossche zeewering Figure 4.1 Computed impact of uniform sediment distribution after 10 and 50 years for a scenario with 3 million cubic meters of yearly nourishments. Top : Impact on coastline development after 10 years; Bottom: Impact on coastline development after 50 years. These graphs show that the sediment is distributed quite uniformly, except for the part in front of the Petten sea defence. The sediment that is nourished in front of the Hondsbossche zeewering is transported to the North as the Hondsbossche zeewering extends seaward from Evaluation of nourishment strategies Holland Coast 13 of 23

16 the coast. This explains the large accumulation of sediment on the northern (right) side of the Hondsbossche zeewering. It is, in reality, expected that a (smaller) part of the sediment will be transported to the South. The average accretion of the coast is in the order of 2.5 meters per year, assuming that the sediment is distributed over a height of approximately 10 meters (which is a fixed value in the model). This is expected to be realistic for the short term (order 10 s of years), but might overestimate the accretion for the long-term. On the basis of expert judgment it is estimated that the long-term (longer than 10 years) rate of accretion for this strategy (3 M m 3 / year) is more in the order of 1 to 1.5 meters a year. There are two reasons for a slow down in the accretion rates over longer timeframes. On the long-term the sediment has more time to redistribute to the lower parts of the cross-shore profile and to the dunes. Therefore the active height over which the sediment distributes, which is fixed at 10 meters in the model, should be larger for longterm simulations. Very large nourishments will protrude over a large distance in seaward direction. The water depth is larger here, which will require more sediment and result in a slow down of the accretion. This holds especially for locations along the coast where the tidal channels are close to the coast, which is the case for the beaches at Den Helder. Besides the time development, the impact of a very large yearly nourishment volume can be studied. The impact of a very large nourishments (12 M m 3 / year) is presented in Figure 4.2. Sediment nourished at Hondsbossche Zeewering that was transported to the North as long as the Hondsbossche zeewering extended from the coast. Accumulation of sediment at Hondsbossche Zeewering from the moment that the coastline position of the adjacent coast was leveled with the Hondsbossche Zeewering. Additional sediment bypass at Scheveningen will occur if the coastline accretes significantly. Consequently, there is less accretion in the impacted situation South of Scheveningen. This effect is especially visible for the fine sediment (D 50=150 m which has a wider crossshore distribution. Figure 4.2 Computed impact of uniform sediment distribution after 50 years for a scenario with 12 million cubic meters of yearly nourishments. Left: Coastline change in 50 years (red line). Top right: Impact on coastline development; Bottom right: impact on sediment transport. 14 of 23 Evaluation of nourishment strategies Holland Coast

17 The model computes a large seaward extension of the coast. The coast extends even beyond the Hondsbossche zeewering. Initially the nourished sand at the Hondsbossche zeewering was transported to the North, but after the moment that the adjacent coast became leveled with the Hondsbossche Zeewering the sea defence started to become buried with sediment. It should be noted that the very large accretion of 500 meters after 50 years (10 meters per year) is expected to be overestimated with the current model. It is estimated to be in the order of 2 to 5 meters a year (given that the active height is about two to three times the height in the model). The large accretion in the model results in additional bypass of sediment at the harbour moles of Scheveningen, which will result in more sedimentation in the harbour. This effect starts earlier for the fine sediment (D50=150 m) as the cross-shore distribution of the sediment is wider for this sediment, which will result in more bypass of sediment Strategy 2 : Coastal cells The second strategy consists of three or six hot spots where all of the sediment is nourished. Strategies with varying nourishment volumes are considered (3, 6, 9 and 12 M m 3 / year). In the model the sediment is nourished over a length of 4000 meter with a gradually decreasing nourishment rate towards the edge of this area. Figure 4.3 and Figure 4.4 respectively show the impact of the nourishments on the coastline and sediment transport after 10 and 50 years of continuous nourishments of 3 M m 3 / year (1 million for each hot spot). These figures show the difference between the autonomous situation (due to redistribution) and the situation with the nourishments. In this way only the impact of the measure is shown. The edges of the distribution areas of the nourishments are indicated with dashed lines. Figure 4.3 Computed impact of three coastal cells after 10 years for three types of sediment (D50=150, 200 and 300 m). Left: Coastline change in 10 years (red line). Top right: Impact on coastline development; Bottom right: impact on sediment transport. Evaluation of nourishment strategies Holland Coast 15 of 23

18 migration speed Figure 4.4 Computed impact of three coastal cells after 50 years for three types of sediment (D50=150, 200 and 300 m). Left: Coastline change in 50 years (red line). Top right: Impact on coastline development; Bottom right: impact on sediment transport. Besides the time development of the three nourishment hot spots, the influence of the magnitude of the nourishments can be evaluated. Figure 4.5 and Figure 4.6 respectively show the coastline impact of two nourishment volumes (respectively a total of 3 or 9 M m 3 / year) after 10 years. migration speed Figure 4.5 Computed impact of three coastal cells after 10 years for a strategy with 3 x 1 million cubic meter of nourishments a year. Left: Coastline change in 10 years (red line). Right: Impact on coastline development. 16 of 23 Evaluation of nourishment strategies Holland Coast

19 migration speed Figure 4.6 Computed impact of three coastal cells after 10 years for a strategy with 3 x 3 million cubic meter of nourishments a year. Left: Coastline change in 10 years (red line). Right: Impact on coastline development. Figure 4.5 and Figure 4.6 show that the influence area of larger nourishments is not much larger than for the smaller nourishments. Difference in volume mainly impacts the magnitude of the measures. A strategy with 6 hot spots was also considered because a larger part of the coastline can be influenced then. Figure 4.7 and Figure 4.8 show the impact on the coastline after 10 and 50 years of a nourishment strategy with 6 hot spots with a total volume of 3 M m 3 / year. Figure 4.7 Computed impact of three coastal cells after 10 years for a strategy with 3 million cubic meter of nourishments a year. Figure 4.8 Computed impact of three coastal cells after 50 years for a strategy with 3 million cubic meter of nourishments a year. From Figure 4.7 and Figure 4.8 it follows that the coastline impact for the six hot spots is similarly shaped to the larger hot spots, but much more distributed along the coast. Evaluation of nourishment strategies Holland Coast 17 of 23

20 4.3 Interpretation of results The results of the models have been interpreted to gain insight into the following issues: Shape of the impacted area Area of influence (length of impact) Effect of large nourishment volumes Shape of the impacted area The model results (Section 4.2) indicate that the impact area of local large nourishments on larger spatial scales (some kilometres from the nourishment) is fairly symmetrical. The accretion updrift (South) from the hot spots initially takes place due to the blocking of sediment transport (the sediment transport decreases towards the nourishment resulting in extra sedimentation), while the area downdrift (North) from the nourishment is fed with sediment from the nourishment. If the annual nourishment volume is sufficiently large, the direction of sediment transport may change after some years. Both situations are stable and lead to symmetrical impact. However, unstable and asymmetrical coastline shapes may occur if the volume of sediment added to the system exceeds the local capacity to transport sediment. In this situation typical spit like features can occur. The coastline model is not suitable for simulating complex unstable coastal features (phase 3) and results were therefore not obtained for these situations. These phases are illustrated in Figure 4.9. Phases Updrift (left) Updrift (right) Phase 1 : Reduction of transport t=1 Suppletion - Coast tends to equilibrium orientation - Sediment transport reduces - Accumulaton of sediment - Coast orientation further away from equilibrium orientation - Sediment transport increases - All sediment is transported downdrift Phase 2 : Only reached if annual nourishment volume is larger than local undisturbed sediment transport direction changes t=2 Suppletion Phase 3 : Only reached if impact of annual nourishment volume on transport is larger than maximum local transport capacity t=3 Suppletion max transport - Impact of nourishment on sediment transport larger than local sediment transport - Sediment transport not only reduces but also changes direction - Accumulation of sediment partly due to the nourishment and partly due to the reduction of the local sediment transport - Coast orientation further away from equilibrium orientation - Sediment transport increases - A part of the sediment is transported downdrift - Coastline exceeds orientation with maximum transport. - Sediment transport can not increase any more - Part of the sediment is not transported downdrift, resulting in extra accumulation and an unstable coastline shape. Figure 4.9 Mechanism behind the impact of nourishments in a coastline model. 18 of 23 Evaluation of nourishment strategies Holland Coast

21 It is noted that some asymmetry may in reality occur due to the influence of the tide. Especially for very large nourishments the sediment transport can locally be larger due to the impact of the contraction of the tide. Area of influence The model results also show that the sediment is distributed from the three or six hot spots over their coastal cells (e.g. Rijnland). From the results, it can be seen that fine sediment is transported more quickly than coarse sediment, which is related to the larger sediment transport capacity for fine sediment. The area influenced by the nourishment increases during the simulation. To get an idea of the rate at which this area expands, a migration rate of this impacted area is considered. The differences in migration rate for the three/six considered hot spots are small, which is shown in Figure This figure shows the length of impact of each of the three hotspots relative to their centre for the strategy with 3 M m 3 / year. It is noted that the first two kilometres should not be considered as the nourishment has a initial radius of two kilometres. Figure 4.10 Migration rates of three hotspots for strategy with total of 3 million cubic meters a year The rate at which the large scale nourishments migrate is about 200 to 600 meter a year, which is in line with findings for the MER of the large nourishment (Deltares, 2009) based on morphological Delft3D computations. It should be noted that tidal effects are not included and that the tidal currents may enhance the migration rate. The magnitude of the nourishment does not influence this number significantly, which is shown in Figure This figure shows the differences between the migration rates for four strategies with different nourishment volumes for one of the three hotspots (the one at the coastal cell of Rijnland). 3 Mm 3 6 Mm 3 9 Mm 3 12 Mm 3 Figure 4.11 Migration rates of the hotspot at Katwijk (central one of three hot spots) for strategies with a total of 3, 6, 9 and 12 million cubic meters a year Evaluation of nourishment strategies Holland Coast 19 of 23

22 As a remark it is noted that the transport and migration rates were derived for locations at some distance from the nourishments. The migration rates near to the hot spots can be much larger if the contraction of the tide is taken into account, but this plays hardly any role at larger distances from the hot spots (i.e. >5 km). Influence of nourishment magnitude From the model simulations with larger nourishment volumes it followed that the shape of the coastline impact is more or less similar for small and large nourishments. Consequently, the magnitude of the coastline changes scales almost linearly with the nourishment volumes (compare maximum coastline change of 500 and 1500 meter for the first hot spot). 4.4 Evaluation of strategies A number of aspects can be found from the model simulations and the interpretation of the effectiveness of the strategies. Strategy related Uniform nourishments will result in a quite even distribution of sediment, which provides a direct benefit for the coast from the nourished sediment. It will, however, be very difficult and costly to apply this maintenance strategy. Migration rates of mega-nourishments are in the order of 200 to 600 meter per year; however, tidal effects are not included. A strategy with only a three or six hot spots can not provide sediment at all locations along the coast within a timeframe of 50 years. As a consequence, more than 6 hot spots will be required if the whole of the coast needs to be nourished within 50 years. Very large nourishments may result in unstable coastline features like spits. Model related Strategies with very large nourishments volumes at a few locations are difficult to model with a coastline model as the local features can not be resolved and can make the coastline model instable. Contraction of the tidal current at very large nourishments may be of significant importance for the local sediment transport. This requires a field model. Unstable coastline features like spits can not be simulated with a coastline model. 20 of 23 Evaluation of nourishment strategies Holland Coast

23 5 Evaluation of the nourishment strategies 5.1 Findings This Building with Nature HK4.1 work package aims at developing strategies for the maintenance of the Holland Coast. In this report, the first two strategies for maintenance of the Holland coast are assessed by means of model computations, as part of Cycle 1. These strategies differ only with respect to the longshore distribution of sediment volume. The following strategies have been investigated: Uniform along the coast Strategy with a yearly nourishment which is distributed uniformly along the coast. Coastal cells Strategy with yearly nourishments at three locations along the coast that are located in the coastal sections of Delfland, Rijnland and Noord-Holland. For this purpose, a coastline model has been set up for the coast of Holland with UNIBEST- CL+. The performance of the model has been checked by making a comparison with available literature data on longshore sediment transport along the coast of Holland. The comparison showed that the model performed satisfactory in terms of the net longshore transports. Scenarios of 3 million up to 12 million m 3 per year have been considered in the model. The nourishments are evenly distributed across the active cross-shore profile from -7 to +3 m. Sand volume is added to the model every year. First indicative model simulations with the uniformly applied nourishments show that this strategy will lead to a quite even distribution of sediment, which provides a direct benefit for the coast from the nourished sediment. It will, however, be very difficult and costly to apply this maintenance strategy. First indicative model simulations indicate that the migration rate of the impact length of nourishments is rather low (about m/year). As a consequence, more than three hot spots will be required when the whole Holland coast needs to be nourished within 50 years from the same locations. Strategies with very large nourishments volumes at a few locations are difficult to model with a coastline model as the local features can not be resolved and can make the coastline model instable. Locally a detailed model is required which can also resolve the effects of the contraction of the tidal current at very large nourishments, which can be of significant importance for the local sediment transport and migration rate. Evaluation of nourishment strategies Holland Coast 21 of 23

24 5.2 Updated strategies for Cycle 2 Discussions during the evaluation of the two strategies indicated the need for the two following items: New variable: frequency According to the work plan, Cycle 2 will include the variable: frequency. Where in Cycle 1 the frequency was every year, now the frequency will be varied. As an example, the volume to be nourished can be applied at three or six different locations, where the volume is applied in a fixed sequence. So, e.g. each of the six locations will be nourished every six years with a fixed volume. Another variant may be every 12 years, but with a doubled volume. Cross-shore parameters In the evaluation of the strategies, a need for information on cross-shore parameters was identified. The width of the beach and e.g. intertidal area is required to properly evaluate the ecological indicators. So, information on the coastal profile and slope should be provided in Cycle 2. Both items will be addressed in Cycle of 23 Evaluation of nourishment strategies Holland Coast

25 6 References Deltares, Morfologische berekeningen MER zandmotor. P.K. Tonnon, J. van der Werf, J.M. Mulder. November Deltares project number Deltares, Work plan for HK 4.1 submitted to Ecoshape. Holthuijsen, L.H., N. Booij and R.C. Ris, 1993, A spectral wave model for the coastal zone, Proceedings 2nd International Symposium on Ocean Wave Measurement and Analysis, New Orleans, Louisiana, July 25-28, 1993, New York, pp Veerman, Samen werken met water: een land dat leeft, bouwt aan zijn toekomst. Bevindingen van de Deltacommissie Evaluation of nourishment strategies Holland Coast 23 of 23