The state of the coast

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Toestand van de kust

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The state of the coast

- Toestand van de kust -

Case study: South Holland

1206171-003

Alessio Giardino Giorgio Santinelli

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27 June 2013, final

1 Introduction

The Netherlands is a low-lying country, where approximately 26 per cent of the territory is located below mean sea level and 59 per cent is prone to flooding (PBL Netherlands Environmental Assessment Agency).

Protection against flooding is traditionally the primary objective of coastal policy in the Netherlands. However, since 1990 coastal policy has been subject to a number of modifications. New objectives such as the preservation of values and functions in the dune area and the sustainable maintenance of safety have been added to cope with the structural erosion problems of the Dutch coast. To fulfil these new objectives, the yearly volume of sand for nourishments was set at 6 *106 m3 in 1990 and increased to 12 *106 m3 in 2001. According to predicted sea level rise scenarios, higher volumes of sand might be necessary in the future for the sustainable maintenance of the safety levels, implemented by maintaining the sand in the entire coastal foundation.

On the other hand, the effect of the global economic crisis is pushing coastal managers to the development of optimal efficient and cost-effective nourishment strategies. Deltares has been commissioned by Rijkswaterstaat – WVL to develop the knowledge needed to carry out an effective nourishment strategy (spatially and temporally). Deltares organised this project

Kennis voor Primaire Processen – Beheer en Onderhoud van de kust (Knowledge for Primary

Processes - Coastal Management and Maintenance) in a number of sub-projects. In order to link the project results to the actual nourishment practice of Rijkswaterstaat, the subprojects focus on the validation of a number of hypotheses on which the present nourishment strategy is based. “Toestand van de Kust” (State of the Coast) is one of the sub-project of this multi-year program, with the aim of identifying the impact of nourishments for a number of indicators along the Dutch coast. During this second year of the project, the analysis has focused on the South Holland coast. The study will be extended to the entire Dutch coast during the next years.

This report summarizes the main findings from the study. In Chapter 2 the main objectives of the work are described. The assumptions on which this study is based are summarized in Chapter 3. The study area is described in Chapter 0, with focus on the different types of forcings influencing the morphological development of the coast: anthropogenic (nourishment, dune management and other man-made structures) and natural (storminess during the years, sea level rise and subsidence). In Chapter 5, the morphological development of different stretches of coast is described by means of indicators related to short term safety, medium term safety, long term safety, and nature and recreation. In Chapter 6, relations between nourishment volumes and indicators and between different indicators are described. In Chapter 0 a detail discussion on the effects of storms on a number of indicators is given. In Chapter 8, a first attempt of cost-benefit analysis is carried out by comparing the benefits identified by the changes in morphological indicators with the nourishments costs within different areas. Finally, Chapter 9 and 10 summarize the main conclusions from the study and put forward a number of recommendations for further work.

As additional deliverables to this work:

A complete database has been developed including the evolution of different indicators (in space and time) on a standard NetCDF format, according to the Open Earth philosophy (http://public.deltares.nl/display/OET/OpenEarth) (Appendix A). This database has been used as support tool in different KPP-B&O Kust subprojects (e.g. ijkswaterstaat Beheerbibliotheek).

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Moreover, support has been given for the development of the Wiki page: “The Kustviewer” with general explanations concerning the project and a collection of .kml files for on-line visualization: http://publicwiki.deltares.nl/display/KV/Kustviewer.

The external consultancy company HKV–Lijn in Water was also involved in the project with specific tasks described in the next chapters.

The present study is part of the project (KPP – Beheer en Onderhoud van de kust; Coastal Management and Maintenance). We would like to acknowledge comments and remarks from Gemma Ramaekers (WVL), which have resulted into an improved manuscript.

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2 Objectives

The objective of the present study is twofold:

• To support WVL in determining where to nourish.

This is achieved by indicating on which spots along the coast the sediment buffer is limited. This buffer does not only concern sediment volumes, but a wider range of coastal indicators. On spots that encounter limited buffers, the morphological development can be examined. If the buffer tends to get lower than a reference buffer and a (natural) increase in sediment volume is not expected on a short term, WVL can consider to nourish this part of the coast. In case financial state of affairs makes prioritizing urgent, the state of the coast can contribute to the prioritization process.

• To advise WVL on the most efficient nourishment strategy.

This is achieved by deriving the effect of the previous nourishment strategy (1990 till present). Learned lessons from the past can be used to improve future nourishment strategies.

In addition, the following hypotheses1 are validated in this study: Hypotheses

1) The nourishment strategy of the past years had lead to a positive2 development of a

number of “indicators” along the Dutch coast.

2) As a consequence, nourishments contribute to an increase of the safety level through a seaward shift of the erosion point.

By looking at the development of coastal indicators in the past, recommendations are derived to design the future nourishment programme at a time scales up to 10 years. The focus area of this report is the South Holland coast. During the next year, the same study will be extended to the whole Dutch coast.

To be able to achieve the objectives and to verify the hypothesis, a number of indicators have been defined. These indicators are representative of 1) the morphological development of the Dutch coast at different temporal and spatial scale and 2) related to policy objectives.

Moreover, the main causes inducing the morphological development of the coast (coastal management and storminess) were analysed, according to the scheme given in Figure 2.1.

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Background of the hyphothesis and the link with the present management choices are described in an integral report of the project KPP-B&OKust.

2

In this report, it is assumed that “positive” development corresponds to a decrease of the probability of breaching and a seaward shift of the MKL, dune foot, increase in beach width, and sediment volumes..

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27 June 2013, final a b io ti sc h b io tis c h

Figure 2.1 Functioning scheme of the coastal system: climate and weather (gray box) and management choices

(orange boxes), influence the hydrodynamics and morphological development of the coast (blue boxes). This has an impact on several functions such as safety, nature and recreation (green boxes). Other functions (e.g. extraction of drinking water) have not been considered in this report.

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3 Assumptions

A number of assumptions were defined to verify the basic hypothesis. These assumptions were the same as used in the analysis carried out for the North Holland coast (Giardino et al. 2012).

Assumption 1

The morphological development is dominated by the major human interventions: the nourishments, and by changes to the nourishment intensity. Therefore, the analysis was subdivided in three periods of time, corresponding to radical changes in the nourishment policy (Paragraph 4.2):

Period 1965 -1990: characterized by nearly no nourishment along the entire Dutch coast. The main objective of coastal policy was the defence against flooding.

Period 1991 – 2000: characterized by a nourishment scheme of about 6 millions of m3 of sand per year along the all Dutch coast. The main objective of the nourishment policy was the sustainable preservation of safety and of additional values and functions in the dune area by maintaining the coastline.

Period 2001 – now: characterized by a nourishment scheme of about 12 millions of m3 of sand per year along the all Dutch coast. The main objective of the coastal policy was extended with the objective of the preservation of volumes within the entire coastal foundation in order to keep up with the sea level rise.

The choice of the three time windows depending on changes to the nourishment strategy is however arbitrary.

Assumption 2

Morphological changes within each time window can be described by “linear” trends. Assumption 3

This assumption is related to the spatial scale at which the analysis has been carried out: at first the Jarkus transect level, and in second place a larger scale defined in the report as sub-area. Sub-areas have been defined as stretched of coastline characterized by a homogeneous nourishment policy (e.g. mainly beach nourishments, shoreface nourishments, no nourishment), and a similar autonomous trend (erosive or accretive) (Table 3.1). The autonomous trend was defined by looking at the MKL time variation before 1990. In the same table (last column) the main findings from Wijnberg (2002) on morphological trends for different areas are also given updated with the last anthropogenic interventions. The location of the different areas is also shown. The Figures with the nourishment volumes in time for the different sub-areas are shown in Appendix 0. In this way, possible hypothesis could be drawn on the relation between different nourishment policies, and the consequent morphological development.

Assumption 4

Given the fact that the Jarkus transect alongshore resolution defines the alongshore resolution of the analysis, we assume that one Jarkus transect is representative of an alongshore stretch of about 250 m.

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Table 3.1 Definition of sub-areas with homogeneous nourishment strategy and autonomous trend. Area code Limit sub-region (Jarkus number) Length (m) Nourishment strategy / Coastal type Autonomous trend before 1990 Wijnberg (2002) + info WVL Rijnland

1 5625 - 6025 4000 No nourishment Accreting – under

the effect of the Ijmuiden jetties

Small interannual fluctuations. Bar system (2-4 bars) with period of four

years.

2 6050 - 6350 3000 Beach + shoreface Erosive

3 6375 - 7025 6500 Mainly shoreface Alternating

4 7050 - 7275 2250 No nourishment Alternating

5 7300 - 7975 6750 Only shoreface Alternating

6 8000 - 8300 3000 Beach + shoreface Alternating

7 8325 - 9375 10500 Mainly shoreface Alternating

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Delfland

1 9740 - 9975 2350 Mainly beach Erosive Small natural

fluctuations of the shoreline. Mainly characterized by anthropogenic interventions: - jetties Scheveningen (km 102) and Hoek van Holland (km 119); - extensive beach nourishment (km 116 - km 119 in 1971 ) - dune compensation (km 102 – km 117 in 2008-2009) - Zandmotor (km 106 – km 111 in 2011). 2 9997 - 10140 1430 Mainly beach - groins Erosive 3 10193 – 10611 4180 Only dune reinforcement + Groins + Jetties at Scheveninghen Alternating3 4 10623 - 11394 7710 Beach + shoreface + dune reinforcement + groins Alternating2 5 11412 - 11825

4130 Mainly beach and

dune reiforcement + groins + jetties at Hoek van Holland

Accreting4

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The definition of autonomous trend is only based upon observations of MKL trends between 1965 and 1990. However, this stretch of coastline has been protected by groins built during the 18th and 19th century. Therefore, the definition of “alternating” should be perhaps replaced with “erosive” in case groins had not been built (Figure 4.3).

4

The definition of autonomous trend is only based upon observations of MKL trends between 1965 and 1990. However, this stretch of coastline has been nourished starting from 1971. Moreover, it is under the direct effect of the jetties of Hoek van Holland. The definition of “accreting” should be perhaps replaced with “erosive” if nourishments had not been built and no jetties were present in the south.

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Assumption 5

The last assumption concerns the choice of the indicators, which best describe the coastal morphological evolution and can be related to policy objectives. These indicators were divided in four different categories according to the time scale at which they are functioning: indicators for short term safety, medium term safety, long term safety and nature and recreation (Table 3.2).

Table 3.2 Indicators chosen for describing the morphological development of the South Holland coast.

System function

Time scale ( years)

Policy objective Indicator

Short term safety

1 Maintenance of safety

Cross-shore erosion length (Section 5.1.1) Probability of breaching (first dune row) (Section 5.1.1) Medium term safety 10 Suistainable maintenance of safety

TKL (Toetsen KustLijn) (Section 5.2.1) MKL (Momentane KustLijn) (Section 5.2.1) BKL (Basis KustLijn) (Section 5.2.1) MDL (Momentane DuneLijn)5

Long term safety

100 Sand volumes at different water depths (Section 5.3.1) Nature and recreation 1 Suistainable maintenance of dunes

Dune foot position (Section 5.4.1) Beach width (Section 5.4.1)

5. The values of this indicator were computed and added to the Open Earth database. However, this indicator is not further described in the report.

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4 Study area: morphology, anthropogenic and natural forcing

4.1 Morphological characterization of the study area

The South Holland coast is a sandy, microtidal, wave-dominated coast. This stretch of coast has a length of about 62 km, bounded in the north by the jetties of Ijmuiden (km 56 from Den Helder) and in the South by the jetties of Hoek van Holland (km 118 from Den Helder). This stretch of coast is further divided in two sections: Rijnland in the North (Figure 4.1) and Delfland in the South (Figure 4.2). Two different drainage basins define the border between these two areas: the one of the old Rhine river in the north, and of the old Maas river in the South.

The coastline has a curve shape with an orientation of about 20 in the North increasing to about 40 in the South. Historically, in particular the Holland coast, has been characterized by erosion (Figure 4.3) and different measures have been implemented to counteract this trend (Section 4.2).

Figure 4.1 Plan view of the Rijnland coast. Different sub-areas are shown in yellow. Jarkus number and limits between different sub-areas are indicated in blue.

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Figure 4.2 Plan view of the Delfland coast. Different sub-areas are shown in yellow. Jarkus number and limits between different sub-areas are indicated in blue.

Figure 4.3 Development of the coastline between Scheveningen and Hoek van Holland (Van Rijn, 1995; referring to Wentholt, 1912 and Ligtendag, 1990).

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Tides are semidiurnal with a tidal range ranging approximately between 1.3 m at neap tide and 1.8 m at spring tide, and slightly increasing from north to south.

Waves mainly approach the coast from southwesterly and northnorthwesterly directions. The wave climate is quite homogeneous along this stretch of coast, and characterized by a mean monthly wave height ranging between 1 m during the summer month up to 1.8 m during the winter months, and with a mean annual wave height of about 1.3 m (Wijnberg, 2002).

The average slope of the shoreface in front of the Holland coast varies roughly between 1:140 and 1:450 with a flattening of the shoreface corresponding to the locations of the jetties of IJMuiden and Hoek van Holland (Wijnberg, 2002).

The median grain size (D50) on the foreshore ranges approximately between 170 m and 250 m as shown in Figure 4.4 (TAW, 1984). The alongshore grain size in the 80’s, when the last measurements were collected, shows that the biggest grain size was found in proximity of the Jetties of Ijmuiden and Hoek van Holland. Consistent changes in grain size distribution might have occurred in the last years due to the effect of nourishments.

Figure 4.4 Median grain size (in µm) along the Holland coast (TAW, 1984).

A alongshore bar systems exists, with a number of bars equal to 1-2 in the south up to 3-4 in the middle part and 2-3 close to IJmuiden. The cycle time of the bars is about 4 years, and which differs from the bar system north of Ijmuiden where it is of approximately 15 years (Wijnberg, 1996).

The alongshore sediment transport along the Holland coast has been derived by several authors using different models, verified by few field measurements (Kleinhans and Grasmeijer, 2005). A comparison between these results is given in Van Rijn (1995, 1997) (Figure 4.5). Despite the wide spreading between results, the general trend is northward directed transport between Hoek van Holland and Ijmuiden with yearly net longshore

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transport up to 500.000 m3/year and reduced nearly to 0 at the jetties in the north and in the south.

Time variation in sediment volumes at different water depths were computed by several authors (e.g. Van Rijn (2010), Vermaas (2010)) based on field measurements (Figure 4.6, Figure 4.7). The general trend shows a large increase in sediment volumes along the Holland coast from the period 1964-1990 to the period 1990-2006, mainly related to the nourishment works. Those last values would be even much bigger if the large nourishment and dune compensation works carried out during the last years especially at Delfland would be included in the analysis.

Figure 4.5 Computed alongshore sediment transport rates between -8 and +3 m NAP, along the whole Holland coast according to different authors (Van Rijn, 1995).

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Figure 4.6 Variation in sediment volumes along the Holland coast, between -8 m and -3 m, for the period

1964-1990 (left panel) and 1964-1990-2006 (right panel) (Unit: m3/year). The red colour indicates a decrease in sand

volume, while the blue colour indicates an increase (Van Rijn, 2010).

Figure 4.7 Variation in sediment volumes along the Holland coast, between -3 m and +3m, for the period

1964-1990 (left panel) and 1964-1990-2006 (right panel) (Unit: m3/year). The red colour indicates a decrease in sand

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4.2 Anthropogenic forcing

4.2.1 Nourishment policies over the years

The coastal policy has been undergoing several modifications in the last 20 years. Traditionally, coastal policy in The Netherlands has always put its primary focus on momentary flood safety. Strengthening of the dune was performed in the beginning through local nourishments placed on the dunes and on the beach, eventually combined with hard structures (Giardino et al., 2010). Once the safety criteria were established in the second half of the 20th century, other criteria and functions, such as ecology, were included in the decision making. This new way of thinking led to the formulation of the policy of ‘Dynamic Preservation’ in 1990. Besides safety against flooding, it was decided to include the preservation of values and functions in the dune area as policy objective. These principles were implemented through the decision of maintaining the coastline position approximately to that of 1990. The baseline position was defined as Momentary Coastline computed approximately by means of the volumes between the dune foot position and the -5 m line (Figure 4.8). Whenever the coastline would retreat more than this baseline position, sand nourishments were applied (Van Koningsveld and Mulder, 2004, after Rijkswaterstaat 1991). Besides beach nourishments, more economically attractive shoreface nourishments started becoming common practice. The average annual nourished volume between 1990 and 2000 was increased to about 6*106 m3/year for the all Dutch coast.

Figure 4.8 Definition of the Momentary Coastline. The yellow area indicates the volume under one Jarkus profile,

used to compute the momentary coastline position at one specific year (left panel). In the right panel, the different points refer to different MKL positions at different years, from which a linear trend is derived to predict the position for the next year (TKL).

Evaluation of this policy between 1995 and 2000 pointed out that the maintenance of the coastline was achieved. However, this policy did not consider the morphological development at larger scale, induced for example by sand losses at larger water depth and by sea level rise (Mulder et al., 2011). The hypothesis is that sand losses at deeper water could, in the long term, lead to a loss of sediments also in the upper shoreface. This would result in extra future afford for maintaining the coastline. Therefore, this policy was considered not to be sustainable at longer time scale. A new concept was developed: the compensation of loss of sediments due to sea level rise including the whole Coastal Foundation. The Coastal Foundation was defined as the area between the dune position and the -20 m depth contour. Nourishment volumes were defined by multiplying the expected sea level rise by the area of the Coastal Foundation. The assumption was that no transport occurs through the -20 m water depth. The sea level rise was estimated in 1.8 mm/year. In view of this new concept, the nourished volumes for the all Dutch coast were increased from 6*106 m3/year up to about 12*106 m3/year.

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Besides maintenance nourishments, which fall under the Dynamic Preservation policy, additional volumes of sand have been supplied along the Holland coast for additional purposes. The main ones are mentioned hereafter:

The “Van Dixhoorn Driehoek” nourishment executed at Hoek van Holland in 1971-1972 of the size of about 19 * 106 m3 of sand, and built with sand coming from the harbour extension works near Rotterdam. In order to maintain this nourishment, an average yearly nourishment of about 2 * 105 m3 was placed between 1990 and 2005 at this location with sand extracted from the entrance channel to the Rotterdam harbour.

The dune compensation project (Section 4.2.2) built in the period 2009 - 2011 between Hoek van Holland and Ter Heijde and with an approximate volume of 5.1 * 106 m3 of sand.

The “Zwakke Schakel” (“Weak links”) reinforcement, with the purpose of reinforcing the coastline at places where the coastal defences might not be sufficient to withstand future hydrodynamics boundary conditions derived for the year 2050. As an example, the “Zwakke Schakel” project for Delfland consisted of reinforcement with a total volume of sand of about of 10.1 * 106 m3. The sand volumes which have been used for the Zwakke Schakel project at Rijnland are not yet available.

The “Zandmotor” (Sand Engine) built in 2011 between Ter Heijde and Kjikduin and consisting of about 21.5 * 106 m3 of sand.

The total nourishment volumes applied along the whole South Holland coast for the three different periods are shown in Figure 4.9. A number of observations can be derived based on this figure. At first, the huge nourishment volumes applied in Delfland stand out in the figure. As described above, this is due to the fact that besides regular nourishments that fall within the Dynamic Preservation policy, a lot of other nourishment projects with different purposes have been implemented at this area.

A second observation relates to the general trend towards an increase over time of shoreface nourishments volumes with respect to beach nourishments. The sand volumes applied in South Holland at each Jarkus transect are shown in Appendix 0.

Figure 4.9 Nourishment volumes (* 107 m3) at Rijnland (left Figure) and Delfland (right Figure) coast for the three different periods: 1965-1990, 1991-2000, and 2001-2010. Note that the Sand Engine is not included in the figure as this mega nourishment was built in 2011.

4.2.2 Dune management in the years

Human intervention in the foredune area of the central Dutch coast dates back to the 15th century and intensified from 1850 onwards (Bochev-van der Burgh et al. 2011). Large-scale

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stabilization of the fore(dune) area was completed in the beginning of the 20th century. Nowadays, only a few kilometres of the foredunes along this part of the coast is considered to be in natural state. Arens and Wiersma (1994) made a classification of the foredunes along the entire Dutch coast based on aerial photographs from 1988. The foredunes were classified according to the most prominent type of intervention at that moment.

Figure 4.10 shows the dune management intensity along the Dutch coast North of Scheveningen between 1965-1990 and 1990-2004. The Figure shows how the foredune between Ijmuiden harbour and Scheveningen used to be highly managed. Adjustments of the seaward facing slope using ground moving equipment were carried out using ground moving equipment along the entire foredune area (Bochev-van der Burgh et al., 2012). Sometimes, foredunes were completely remodelled and reconstructed. The dune management intensity has decreased during the last decades. For example, the use of excavators for foredune adjustment has been very limited in more recent years.

South of Scheveningen, more precisely between Hoek van Holland and Ter Heijde, a large dune compensation work has been recently concluded. Between 2008 and 2011, about 25.000 hectares of new land have been created to compensate the loss in nature due to the construction of Maasvlakte 2, the new harbour extension of Rotterdam harbour. The additional objective of the project was the strengthening of this stretch of coast to account for a possible future rise in sea level. Figure 4.11 shows the morphological changes at this stretch of coast between 1997 and 2011. The extension of the dune area between the two periods (green colour) is clearly visible. Moreover, according to the principle of Building with Nature, the former groyne system used to defend the coastline (figure above) has now been replaced by a more natural and flexible sand beach (figure below).

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Figure 4.10 Dune management intensity between 1965 and 1990 (upper figure) and between 1990 and 2004 (lower figure) (Bochev – van der Burgh, 2012).

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Figure 4.11 Morphological changes in beach and dune height between 1997 (figure above) and 2011 (figure below). The colour scale indicates beach and dune height in m (above NAP).

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4.2.3 Other man-made structures

Along the South Holland coast several constructions have been made over time (Table 4.1). Groynes were built during the eighteenth and nineteenth century to counteract the structural erosion. Nowadays, these groynes are no longer visible since recently during the Zwakke Schakel reinforcement they have been covered by sand which is now used as soft adaptation option against erosion.

A number of jetties, part of different harbours, were also constructed and extended in time at Ijmuiden, Scheveningen and Hoek van Holland. According to Van Rijn (1995), the jetties of Hoek van Holland and Ijmuiden almost fully block the alongshore transport.

Jetties have a double effect, the first one on tidal flow (Figure 4.12), and the second one on wave induced longshore transport (Figure 4.13). The net northward directed tidal flow will go around the jetties, converging south and diverging north of the dams (Figure 4.12). In the south, this will result in an offshore directed current, north of the dams in an onshore directed current. Close to the dams both, north and south, sedimentation will occur in the shallow zone. South and north of the sedimentation areas, this will cause erosion.

Moreover, harbour jetties create a shadow area with lower wave energy, inducing an extra sedimentation (Figure 4.13). Because jetties block the northward alongshore sediment transport, strong erosion will take place especially at the north of the jetties, outside the shadow zone. Temporary erosion in the southern part of the jetties can take place due to differences in wave induced set up. Close to the jetties, rip currents might also generate leading to offshore sand transport and possibly by-pass around the jetties.

Table 4.1 Overview of the man made sructures along the South Holland coast (Wijnberg, 2002).

Type of structure and location

Activity Period Spatial scale

Groins (km 98 – km 118) Construction 1776 – 1896 (now hidden in nourished sand) Harbour jetties

Ijmuiden (km 55 – km 56) Construction 1865 - 1879 1.5 km (cross-shore) Extension 1962 - 1967 Southern jetty: + 1.5 km

Northern jetty: + 1 km Scheveningen (km 102) Construction 1900 - 1908 0.3 km

Extension 1968 - 1970 + 0.5 km = 0.8 km Hoek van Holland (km 118) Construction 1864 - 1874 2 km cross-shore

Extension 1968 - 1972 Northern jetty +2.7 km = 4.7 km Discharging sluice

Katwijk (km 86) Construction 1807 Increase

discharge

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Figure 4.12 Converging and diverging tidal currents near jetties (Van Rijn, 1995).

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4.3 Natural forcing 4.3.1 Meteorological forcing

Besides the anthropogenic intervention (nourishments, dune managements, man-made structures), nature plays also a main role into the coastline morphological development. A wide number of researchers have tried to link different meteorological parameters to the coastline morphological development. Ruessink and Jeuken (2002) defined a storminess parameter related to the maximum annual water level to describe the dune foot dynamics of the Holland coast. Van Puijvelde (2010) linked the foredune development to a storm flood frequency parameter, defined as the number of hours in which the water level was above the mean water level for that period.

In this study, a number of morphological indicators were defined. Given the complexity of the sediment transport processes in the nearshore area, a unique relation between storminess and the different indicators does not exist (Giardino et al., 2012a). Therefore, different storminess parameters were defined, which better relate to the different morphological indicators (Giardino et al., 2012a; Vuik et al., 2012). Those relations are described in Chapter 0.

Moreover, the interference of the anthropogenic action and especially of the large nourishment volumes deposited on the beach, dune and breaker bars during the years make even more difficult to distinguish between natural and anthropogenic processes. An attempt to distinguish between effect of nourishments, natural forcing and long term trends on the morphological indicators has been done in Vuik et al. (2012).

4.3.2 Climatological and geological forcing

Besides the yearly variation in storminess, other external natural factors have an effect on the long term coastal morphology of the North Holland coast: the sea level rise and the subsidence. In 2006, the KNMI published four climate scenarios for the Netherlands, known as the KNMI'06 scenarios (KNMI, 2006). These scenarios estimate Sea Level Rise along the Dutch coast between 15-35 cm by 2050 and 35-85 cm by 2100.

However, the Delta Commission has recently presented new and more drastic figures with sea level rise scenarios (including subsidence) between 0.65 and 1.3 m by 2100 (Deltacommissie, 2008). Nevertheless, it is important to point out that the actual nourishment policy does not consider yet those scenarios but a constant sea level rise equal to 1.8 mm/year, which multiplies the area of the whole coastal system (Section 4.2.1).

A map showing the possible predicted subsidence by year 2050 is shown in Figure 4.14. The map indicates a possible value for subsidence up to 2 cm by year 2050 for the South Holland coast, somehow smaller than in other regions of the Dutch coast. Sea level rise and relative subsidence are not further analysed in the report as, although important for long term effects, have at the time scale of the available measurements a secondary effect on the morphological indicators.

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5 Morphological development of the South Holland coast

5.1 Short term safety

Short term (or momentary) safety is mainly related to the probability of failure of the dune system caused by episodic events which may cause in a few days large erosion of the dune area, beach and upper shoreface.

5.1.1 Probability of breaching and erosion length

As indicators for the short term (or momentary) safety, two indicators have been selected: 1) the probability of breaching of the first dune row and 2) the cross-shore erosion length. The indicators were computed by ARCADIS (Van Santen and Steetzel, 2011) and HKV (Van Balen et al., 2011) for the entire Dutch coast, for the years between 1965 and 2010. Two different modelling approaches have been followed for the computation: the VTV model used by ARCADIS (formal present method for 6 yearly testing procedure) and the PC-Ring model used by HKV (used for the project Veiligheid Nederland in Kaart - VNK). Only the probability of breaching has been analyzed within this study, since the two variables are closely interrelated6. This is the probability that would lead to the failure of the first dune row (Xr landward than Xc in Figure 5.1). More details on the methodology followed for the dune erosion computation can be found in Van Balen et al. (2011).

Figure 5.1 Definition of erosion length (Xr) and critical erosion point (Xc) (Van Balen et al., 2011).

Vuik et al (2012), carried out a statistical comparison of the values computed using the two methods. Though the absolute values computed using the two methods are quite different, the trends derived using the two datasets are well comparable. For South Holland, the agreement in trends scores between reasonable and excellent at 80 % of the transects (Figure 5.3; Vuik et al., 2012) .

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Figure 5.2 Time development of the probability of breaching at transects 3300 (left figure) and transect 4300 (right figure), computed using PC-Ring (HKV) and VTV (Arcadis) models (Vuik et al., 2012).

Figure 5.3 Classification of agreement in trend of probability of breaching for values computed using PC-Ring (HKV) and VTV (Arcadis) models (Vuik et al., 2012).

The difference in absolute value between safety levels computed using the two methods is due to the fact that the erosion length derived using the VTV model is calculated with respect to a fixed norm erosion point for a Jarkus profile in 1990, so closer to the actual safety levels. Moreover, boundary conditions are fixed for a certain return period. On the other hand, safety levels computed with the VNK method are determined based on a residual strength for a Jarkus profile based on a geometrical description of the profile. Moreover, changes to the profile are accounted for in the computation of the residual strength and boundary conditions are derived according to a probabilistic computation. Those differences do not have any considerable impact in our analysis, since the analysis is carried out based on trends and not on values for a certain year.

5.1.2 Analysis

The time variation of the short term safety was at first analyzed at Jarkus transect level, in relation with the amount of sand nourished.

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Figure 5.4, Figure 5.5, Figure 5.6 show a number of examples of the analysis carried out for three different transects. All the figures for each Jarkus profile can be found on the public wiki of the project (Appendix A).

In the first figure, the time evolution of the probability of breaching at one transect in area 1 of Delfland (eroding coast with mainly beach nourishments) is shown. The second transect is part of area 3 of Rijnland (coast characterized by erosion and deposition spots, and with mainly shoreface nourishments). The last figure refers to area 1 of Rijnland (accreting coast under the effect of the Ijmuiden jetties and no nourishments). Figure 5.4 and Figure 5.5 clearly show that nourishments led to a ‘positive’ effect (decrease in probability of breaching) in the stretch of coast where they were applied, confirming our start hypothesis (Chapter 2). For clarity, 10-5 is the limit admitted by law for wave load during testing of the coastal defences at the Holland coast. At transect 5750 the large decrease in probability of breaching of nearly 4 orders of magnitude is related to the building out of the coastline in relation to the Ijmuiden jetties, which block the longshore transport to the North. At this stretch of coast, sand volumes in the shoreface and in the foreshore were already increasing before 1990 (Figure 4.6, Figure 4.7), leading to an improvement of the safety levels not related to nourishments.

A trend analysis was carried out using as a basis the three time windows defined in Chapter 3: 1965 – 1990, 1991 – 2000, and 2001 and 2010. Linear trends were computed within each time window for each transect. The trends defined at each transect for the three periods are given in Appendix 0, together with the relative confidence interval. Transects which are part of the same area, as defined in Chapter 3, are shown in the same figure. The trend analysis confirms the previous observation that in general nourishments have led to an improvement of safety (change in trend from positive to negative values). A relative change in trend of 0.1 year-1 on these plots, correspond to a change of one order of magnitude in safety, for a time window of 10 years.

In different areas with similar autonomous morphological behaviour and nourishment strategy, the average trend within each sub-area of Rijnland and Delfland was computed (Figure 5.7 and Figure 5.8). A value equal to zero in those figures indicates that in average no change occurs in that specific sub-area. The average trends also show a substantial change towards an average decrease in probability of breaching especialy for the last time window. This is particularly evident at area 6 of Rijnland, and areas 4 and 5 of Delfland, which were all mainly characterized by beach nourishments and dune reinforcement during the last period. Also, the change in safety appear to be larger in general at Delfland than at Rijnland, due to the fact that nourishments volumes, especially on the beach and dunes, have been at Delfland much larger than at Rijnland (Figure 4.9).

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Figure 5.4 Change in probability of breaching (black dots) and nourishment volumes (orange bar) at Jarkus transect 9770, located in Delfland. The black lines indicate the trend lines for the three periods: 1965-1990, 1991 – 2000, and 2001-2010.

Figure 5.5 Change in probability of breaching (black dots) and nourishment volumes (orange and blue bars) at Jarkus transect 6575, located in Rijnland. The black lines indicate the trend lines for the three periods: 1965-1990, 1991 – 2000, and 2001-2010.

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Figure 5.6 Change in probability of breaching (black dots) at Jarkus transect 5750, located in Rijnland.

Figure 5.7 Averaged trend of probability of breaching within each of the eight areas of Rijnland characterized by homogeneous autonomous trend and nourishment strategy.

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Figure 5.8 Averaged trend of probability of breaching within each of the eight areas of Delfland characterized by homogeneous autonomous trend and nourishment strategy.

Vuik et al. (2012) computed the efficiency of shoreface and beach nourishments in improving the safety levels in time. The efficiency of a nourishment was evaluated as the slope of the correlation line between nourishment volumes and change in probability of breaching. The higher the (absolute value of the) slope, the more efficient the nourishment is. The outcomes of these calculations for Rijnland and Delfland are summarized in Figure 5.9. In this Figure the ratio between the slopes of the two correlation lines for shoreface and beach nourishments is shown. As an example, the ratio between shoreface and beach nourishment efficiency at kustvak 7, after one year, is equal to 49%. This means that a certain shoreface nourishment volume will lead to a reduction of the probability of breaching 0.49 times the reduction given by a beach nourishment with the same volume.

More in general, it is clear that, while after one year the effect of a beach nourishment is much bigger than that one of a shoreface nourishment, those two effects tend to become similar when considering a time scale of about 8 years. A value higher than 100 % has physically no meaning as shoreface nourishments will most likely not have a higher impact on the probability of failure than a beach nourishment. Therefore, values higher than 100 % can only be explained considering that changes in the value of the indicator are not specifically related to the volumes nourished on a certain kustvak but for example to volumes of sand transported from other areas or due to different effects (e.g. man-made structures). Remarkable differences between different areas are also clearly visible in the Figure. Several aspects might explain this difference: the different share of shoreface and beach nourishment volumes nourished at a specific kustvak, different hydrodynamic conditions, and the effect of other factors different from the nourishments (e.g. man-made structures).

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Figure 5.9 Ratio between shoreface and beach nourishment efficiency in relation to changes in probability of breaching at North Holland (kustvak 7), Rijnland (kustvak 8) and Delfland (kustvak 9), at 1, 4, and 8 years after the nourishment was built (Vuik et al., 2012). A value of 100 % represents a nourishment efficiency of shoreface and beach nourishment with exactly the same value.

5.2 Medium term safety

Medium term safety mainly refers to safety at areas affected by structural erosion and which, in the longer term, might lead to problems of short term safety as well. The time scale under consideration has the order of years.

5.2.1 MKL, TKL and BKL

Variations in medium term safety due to nourishments were evaluated by computing MKL (Momentary Coastline) and TKL (Testing Coastline) trends at each Jarkus transect. These indicators define the coastline position as a function of the volumes of sand in the near shore zone, approximately between the dune foot (+3 m NAP) and -5 m NAP (Van Koningsveld and Mulder, 2004). The values are compared to the BKL (Basal Coastline) position, corresponding with the position of the coastline in 1990, and occasionally adapted in later years.

5.2.2 Analysis

As for the short term safety, the time variations in MKL, TKL and BKL positions were computed at each transect and for all the years between 1965 and 2010. Figure 5.10, Figure 5.11, and Figure 5.12 show three examples of this analysis, respectively for a transect where

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mainly beach nourishments were applied, a transect mainly characterized by shoreface nourishments, and a transect at which no nourishments have been built. In particular, Figure 5.10 and Figure 5.11 clearly show a change in trend from erosive to accretive from the moment at which nourishments started being applied. Once again, the seaward shift in MKL position shown by Figure 5.12 is not due to nourishments but to the blocking of northward directed longshore transport by the jetties of Ijmuiden.

The difference between MKL value averaged within each time window and BKL position, as defined for the year 2011, is shown transect by transect in Appendix D.1. The difference MKL - BKL provides the value of the sand buffer for momentary safety, which guarantees boundary conditions for safety on the medium-term: in particular a positive value means a coastal stretch in a relative ‘safe’ situation, which does not strictly need to be nourished. At most of the transects, this average value has been positive for the last decade. And even though sometimes the average buffer has been negative, as for example shown at sub-area 5 of Rijnland (approximately between transects 7300 – 7500), the strong positive seaward trends of the last decade (Appendix D.2) has brought also those locations to a relative ‘safe’ situation. Very positive trends are especially visible in areas 4 and 5 of Delfland due to the very large nourishments: average shifts in MKL position up to 20 m/year for a time window of 10 years, which would correspond to a total local shift of 200 m, can be observed.

The averaged differences between MKL and BKL position within each area and each time window both for Rijnland and Delfland are given respectively in Figure 5.13 and Figure 5.14, showing a positive buffer at all areas for the last decade. In particular, the average shift of the MKL between the first and the last time window for the whole Delfland is more than 50 m. The average trends are shown in Figure 5.15 and Figure 5.16, suggesting that the already ‘safe’ situation is moving towards an even ‘safer’ status, more sustainable in the long term and able to cope with possible sea level rise effects.

Figure 5.10 Change in MKL, TKL and BKL, and nourishment volumes (orange bar) at Jarkus transect 9770, located in Delfland. The black lines indicate the trend lines for the three periods: 1965-1990, 1991 – 2000, and 2001-2010.

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Figure 5.11 Change in MKL, TKL, and BKL, and nourishment volumes (orange and blue bars) at Jarkus transect 6575, located in Rijnland. The black lines indicate the trend lines for the three periods: 1965-1990, 1991 – 2000, and 2001-2010.

Figure 5.12 Change in MKL, TKL, and BKL at Jarkus transect 5750, located in Rijnland. The black lines indicate the trend lines for the three periods: 1965-1990, 1991 – 2000, and 2001-2010.

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Figure 5.13 Difference between MKL and BKL position of the year 2011 at Rijnland, averaged within each of the areas.

Figure 5.14 Difference between MKL and BKL position of the year 2011 at Delfland, averaged within each of the areas.

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Figure 5.15 Averaged trend of MKL within each of the eight areas of Rijnland characterized by homogeneous autonomous trend and nourishment strategy.

Figure 5.16 Averaged trend of MKL within each of the eight areas of Delfland characterized by homogeneous autonomous trend and nourishment strategy.

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Vuik et al. (2012) also computed the relative efficiency in time of shoreface and beach nourishments in relation to changes in MKL position (Figure 5.17).

As an example, the ratio between shoreface and beach nourishment efficiency at kustvak 7, after one year, is equal to 33%. This means that a certain shoreface nourishment volume will lead to a seaward shift of the MKL position 0.33 times the shift provided by a beach nourishment with the same volume.

As already observed for the nourishment efficiency with respect to the probability of breaching, also for the MKL position shoreface nourishments are much less effective than beach nourishments on the short time scale but their effects become comparable on the medium-term time scale (8 years). Once again, the value of 108% in the ratio of efficiency between shoreface and beach nourishment is physically not realistic and can only be explained considering that some of the changes in MKL indicator were not due to the volume nourished at that kustvak but perhaps coming from different kustvakken or due to other effects (e.g. presence of hard structures). Once again it is quite remarkable the different behaviour observed at Delfland and Rijnland, where the nourishment efficiency after 8 years of shoreface and beach nourishment is almost comparable, and the behaviour observed at Rijnland where shoreface nourishments reach a peak in efficiency after 4 years, after which this starts decreasing. Several factors might explain these differences (e.g. different shares of shoreface and beach nourishment volumes at this kustvak, different hydrodynamic conditions, presence of hard structures, transport of sand from other kustvakken).

Figure 5.17 Ratio between shoreface and beach nourishment efficiency in relation to changes in MKL at North Holland (kustvak 7), Rijnland (kustvak 8) and Delfland (kustvak 9), at 1, 4, and 8 years after the nourishment was built (Vuik et al., 2012). A value of 100 % represents a nourishment efficiency of shoreface and beach nourishment, with exactly the same value.

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5.3 Long term safety

Long term safety refers to safety at the time scale of decades/centuries. In general, sediments at the large temporal and spatial scale provide boundary conditions to the medium and short term safety. For example, a decrease in sediment volumes at the larger scale might induce a decrease in sediment availability for the medium term safety and, as a consequence, for the short term safety.

The relation between sediment volumes at different water depths and the exact time scale under consideration is a very difficult subject as it would involve measuring the net exchange in sediment transport, at different water depths, at the time scale of decades/centuries. Unfortunately, this is not possible as measurements are generally limited in time and also in space, especially in deeper water. Numerical models could support these measurements, but their accuracy is not yet so precise to be able to assign time scales in relation to morphological changes at different depths for such a long time scale. Moreover, a unanimous concept to define the relation between water depths, (significant) morphological changes, and the time scale does not yet exist. The definition of “closure depth” defining the depth seaward of which there is no significant change in bottom elevation and no significant sediment transport is often question of debate, and several equations exist in literature to define this depth.

In our study, we have defined different water depths which would correspond to safety at different time scales. The larger the water depth, the larger the time scale under consideration.

5.3.1 Sediment volumes

As a representation of the boundary conditions for long term safety, the trends in sand volumes within different water depths were computed. The volumes of sand within different intervals were kindly computed and provided by ALTERRA. The following water depths limits were used as boundary for the volume computation:

-1 / - 5 m -1 / -8 m -1 / -12 m -5 / -8 m -5 / -12 m -8 / -12 m

Very little information is available between the -12 m and the -20 m water depth line, defining the limits of the coastal foundation. Therefore, those volumes were omitted from the study. However, it is our advice that more measurements will be collected also at those depths, as this area is considered by policy makers for definining the nourishment volumes (Section 4.2.1).

Volumes were computed according to the procedure described in Figure 5.18. All volumes are determined using as a reference the Jarkus transect of the year 1990 (red line). Based on this transect, a landward and a seaward boundary are defined as intersect of the Jarkus profile year 1990 with the given water depths limits, defining the upper and lower boundary (in this case +0 and -2 m). The volume in the reference year corresponds to the red area. Using the same landward and seaward boundaries defined for that specific transect, the volume is computed for each year. When the profile between landward and seaward boundary goes below the lower boundary (i.e. for the profile given in blue), this part of volume is assumed to be negative and subtracted from the overall volume. In the same way, if the profile goes above the upper boundary as in the landward part of the profile, this part of the area is added up to the total volume, defining a new upper boundary.

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Figure 5.18 Definition of the boundaries for the volume calculation in case of hypothetical depth intervals: 0/ -2 m.

5.3.2 Analysis

As for the other indicators, the time variation in volumes within different depth intervals were computed at each transect and for all the years between 1965 and 2010. The average absolute values within water depths -1 ÷ -5 m, -5 m ÷ -8 m, and -8 m ÷ -12 m with respect to the boundaries defined for the year 1990 are shown in Figure 5.19 - Figure 5.24. The relative trends are shown in Figure 5.25 Figure 5.30. Both, absolute values and trends show a tendency towards an increase in volumes between -1 m and -5 m, at Rijnland and Delfland. At intermediate depths between – 5 m and -8 m sand volumes are quite stable at Rijnland, while showing big increases at Delfland, most likely due to the big nourishment works (Figure 4.9). For water depths between -8 m and -12 m it is quite difficult to draw final conclusions as many of the values are missing. Averaged values were in fact computed whenever at least 20 % of the measurements were available at that area. For this reason, all values are missing in Figure 5.24 for Delfland. On the other hand, Figure 5.21 shows relatively small changes in volumes at this depth, at least from what can be concluded from the values available for Rijnland. It is also interesting to note how the absolute values of sand volumes close to the jetties of Ijmuiden and Hoek van Holland, especially for water depth between -1 m and -5 m, is quite bigger than for the other areas (Figure 5.19 and Figure 5.22). This suggests milder slopes at this areas which are directly under the effects of the jetties.

Jarkus transect 1990

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Figure 5.19 Sand volumes between -1 m and -5 m, averaged within each time window, and for the different areas of Rijnland.

Figure 5.20 Sand volumes between -5 m and -8 m, averaged within each time window, and for the different areas of Rijnland.

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Figure 5.21 Sand volumes between -8 m and -12 m, averaged within each time window, and for the different areas of Rijnland. Missing lines refer to areas where less than 20% of the values are available for that period at that specific water depth.

Figure 5.22 Sand volumes between -1 m and -5 m, averaged within each time window, and for the different areas of Delfland.

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Figure 5.23 Sand volumes between -5 m and -8 m, averaged within each time window, and for the different areas of Delfland. Missing lines refer to areas where less than 20% of the values are available for that period at that specific water depth.

Figure 5.24 Sand volumes between -8 m and -12 m, averaged within each time window, and for the different areas of Delfland. Missing lines refer to areas where less than 20% of the values are available for that period at that specific water depth.

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Figure 5.25 Trends of sand volumes between -1 m and -5 m, averaged within each time window, and for the different areas of Rijnland.

Figure 5.26 Trends of sand volumes between -5 m and -8 m, averaged within each time window, and for the different areas of Rijnland. Missing lines refer to areas where less than 20% of the values are available for that period at that specific water depth.

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Figure 5.27 Trends of sand volumes between -8 m and -12 m, averaged within each time window, and for the different areas of Rijnland. Missing lines refer to areas where less than 20% of the values are available for that period at that specific water depth.

Figure 5.28 Trends of sand volumes between -1 m and -5 m, averaged within each time window, and for the different areas of Delfland. Missing lines refer to areas where less than 20% of the values are available for that period at that specific water depth.

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5.4 Nature and recreation

5.4.1 Dune foot, dry and wet beach

In this study, changes to the coastal functions nature and recreation have been related to changes in position of the dune foot and beach width. The underlying assumption is that a seaward shift in dune foot position will provide a larger space for nature and recreation. In the same way, a larger beach is assumed to be a potential condition for developing recreation. Several definitions exist in literature to define the position of the dune foot. For this study, we decided to use the standard definition of dune foot position used in The Netherlands as the intersection between the dune profile and the +3 m NAP line. The dry beach is defined as the beach area between the dune foot position and the mean high water line, while the wet beach is the part of beach between the mean high water and mean low water line.

Relationships can be found in literature which relate changes in beach width to shift in dune foot position (De Vriend and Roelvink, 1989; Damsma, 2009). In general, a wider beach will lead to a seaward shift of the dune foot position. The adjusting time to equilibrium can be up to 50 years. In this report, we will mainly focus on changes related to the total beach width and of the dune foot position.

5.4.2 Analysis of beach width at mean low water (wet + dry beach)

The beach width was computed for all Jarkus transects for the period 1965-2010. The analysis was carried out for wet beach, dry beach and beach width at mean low water (dry + wet beach). Only the analysis related to the beach width at mean low water is described in the report. The averaged values of beach width, averaged within the different time windows and for the different Jarkus transects are shown in Appendix E.1. The trend computed for the different time windows are given in Appendix E.2. Averaged beach widths and trends for each area are shown in Figure 5.31 to Figure 5.34.

When looking at the absolute values of the average beach width, it is possible to see that the beach width is in general quite stable in time or with a tendency to a slight increase. The average beach width is about 120 m, widening up to about 500 m close to the IJmuiden jetties, as it was already suggested by the large sand volumes present between the beach or at low water depth, as described in Section 5.3.2. From a spatial point of view, oscillations are visible most likely due to the presence of sand waves (Appendix E.1).

A large increase in beach width is clearly noticeable at Delfland approximately between transects 10623 and 11163 due to the large nourishment works on the beach carried out between 2008 and 2009 (see Appendix 0).

Trends in beach width are also quite stable or with a tendency to a slight increase and with the superimposition of natural oscillations related to the presence of sand waves (Appendix E.2). It is remarkable the decrease in trend of beach width approximately between transects 5625 and 5750 and between transects 9947 and 10103, respectively located just in the south of the jetties of Hoek van Holland and just in front of Scheveningen. The first reduction in beach width is related to the construction of the Marina at IJmuiden in 1993-1994 and the creation of a seaward dune row with material coming from the harbour, and with the consequent reduction of the beach width of about 600 m. In the case of Scheveningen, this decrease is related to nourishment works carried out in 2004, which moved forward the dune front and with the consequent decrease of the beach width.

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Figure 5.31 Beach width at mean low water (dry + wet beach) averaged within each time window, and for the different areas of Rijnland.

Figure 5.32 Beach width at mean low water (dry + wet beach) averaged within each time window, and for the different areas of Delfland.

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Figure 5.33 Trends in beach width at mean low water (dry + wet beach) averaged within each time window, and for the different areas of Rijnland.

Figure 5.34 Trends in beach width at mean low water (dry + wet beach) averaged within each time window, and for the different areas of Delfland..

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5.4.3 Analysis of dune foot

Dune foot migration generally is assumed to be related to changes in beach width and storminess. A wider beach provides boundary conditions for a seaward movement of the dune foot (Paragraph 5.4.2), while a storm leads to a landward retreat (Chapter 0). As an example, a year with a storm with a maximum water level 1 m higher than a different year, can lead to an additional landward retreat of the dune foot of about 7 m in Delfland, and which can take years for a full recover.

The dune foot time variations between 1965 and now were computed for all transects, together with the position of mean high water and mean low water line (Figure 5.35). The averaged absolute values of dune foot position with respect to the 2010 position, within each time window are shown in Appendix F.1, while the trends are given in Appendix F.2.

In general, the dune foot appears to have shifted seaward at most transects. It stands out the extreme shift close to IJmuiden up to about 800 m in total. As observed in Paragraph 5.4.2, this was mainly related to the construction of a new dune row with material coming from the new Marina at IJmuiden built in 1993-1994.

A visible seaward shift is also noticeable in Delfland, approximately between transects 11394 and 11800 due to the large dune reinforcements and beach nourishments. The average absolute values and trends in dune foot position are shown in Figure 5.36 to Figure 5.39, confirming the observation of the general seaward shift in dune foot position with average trends of about 1 m/year at Rijnland, and 3-5 m/year at Delfland.

Figure 5.35 Change in dune foot position (green crosses),mean high water position (red crosses), mean low water position (blue crosses), and nourishment volumes (orange and blue bars) at Jarkus transect 6175 The black lines indicate the trend lines for the three periods: 1965-1990, 1991 – 2000, and 2001-2010.

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Figure 5.36 Dune foot position averaged within each time window, and for the different areas of Rijnland.

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Figure 5.38 Trend in dune foot position averaged within each time window, and for the different areas of Rijnland..

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The relative efficiency in time of shoreface and beach nourishments with respect to changes in dune foot position has been investigated in Vuik et al. (2012) (Figure 5.40).

As an example, the ratio between shoreface and beach nourishment efficiency at kustvak 7, after one year, is equal to 25%. This means that a certain shoreface nourishment volume will lead to a seaward shift of the dune foot position 0.25 times the shift provided by a beach nourishment with the same volume.

As already shown in Figure 5.9 and Figure 5.17, the efficiency of shoreface nourishments with respect to that one of beach nourishments appear to increase in time. Remarkable difference are once again observed between different kustvakken. Several factors might explain these differences (e.g. different shares of shoreface and beach nourishment volumes at each kustvak, different hydrodynamic conditions, presence of hard structures, transport of sand from other kustvakken).

Figure 5.40 Ratio between shoreface and beach nourishment efficiency in relation to changes in dune foot position at North Holland (kustvak 7), Rijnland (kustvak 8) and Delfland (kustvak 9), at 1, 4, and 8 years after the nourishment was built (Vuik et al., 2012). A value of 100 % represents a nourishment efficiency of shoreface and beach nourishment, with exactly the same value.

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The state of the coast - Toestand van de kust : case study South Holland

The state of the coast

Toestand van de kust

The state of the coast

- Toestand van de kust -

Contents

1

Introduction

2 Objectives

3 Assumptions

4 Study area: morphology, anthropogenic and natural forcing

5 Morphological development of the South Holland coast