Dynamic dune management, integrating objectives of nature development and coastal safety: Examples from the Netherlands

Geomorphology 199 (2013) 205–213

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Dynamic dune management, integrating objectives of nature development and coastal safety: Examples from the Netherlands Sebastiaan M. Arens a, b,⁎, Jan P.M. Mulder c, d, Quirinus L. Slings e, Luc H.W.T. Geelen f, Petra Damsma g a

Arens Bureau for Beach and Dune Research, The Netherlands Delft University of Technology, Faculty Civil Engineering and Geosciences, The Netherlands Deltares, Marine and Coastal Systems, Delft, The Netherlands d Twente University, Water Engineering and Management, Enschede, The Netherlands e nv PWN Drinking Water Company, Velserbroek, The Netherlands f Waternet, Vogelenzang, The Netherlands g Rijkswaterstaat, Centre for Water Management, Directorate General for Public Works and Water Management, Ministry of Infrastructure and the Environment, Lelystad, The Netherlands b c

a r t i c l e

i n f o

Article history: Received 3 February 2012 Received in revised form 19 October 2012 Accepted 22 October 2012 Available online 16 November 2012 Keywords: Nature management Coastal defence Dune mobility restoration Foredunes Aeolian processes Nourishment

a b s t r a c t This paper discusses and compares results of management interventions to remobilise dunes and obtain more autonomous changes in foredunes resulting from a change in coastal defence policy. In recent decades, nature conservation managers tried to restore aeolian dynamics and dune mobility landward of foredunes to maintain threatened, rare pioneer species. Results indicate that destabilisation activities yielded an important increase of blowing sand and its effects on ecology but with a limited effect on the desired integral remobilization of dunes. Roots remaining in the sand after removal of vegetation and soil is one of the main problems. Follow up removal of roots for 3 to 5 years seems to be essential, but it is not clear whether the dunes will remain mobile in the long term. In 1990 the Dutch government decided to maintain the position of the coastline by artificial sand nourishment. An intensive management of the foredunes was no longer required. Consequently, natural processes in the foredunes revived, and the sediment budget of the beach–dune system changed. Two main types of responses are visible. In some areas, increased input of sand resulted in the development of embryonic dunes seaward of the former foredunes, leading to increased stabilisation of the former foredunes. In other areas, development of embryonic dunes was insignificant despite the increased sand input, but wind erosion features developed in the foredunes, and the environment was more dynamic. The reasons for the differences are not clear, and the interaction between shoreface, beach and dunes is still poorly understood. Until now, attempts to mobilise the inner dunes were independent of changes made to the foredunes. We argue that an integrated, dynamic approach to coastal management, taking account of all relevant functions (including safety and natural values) and the dune–beach system as a whole, may provide new and durable solutions. An integrated approach would ideally provide fresh sand to the system by sand nourishment; define a wide safety zone, which enables the transition zone of beach to foredunes to develop freely; reserve space for natural processes without restrictions; and stimulate natural redistribution of sand within the system and restore inland transport of sand by removing vegetation behind the foredunes. A long time scale (several decades) is needed for this approach to be successful. © 2012 Elsevier B.V. All rights reserved.

1. Introduction 45,000 ha of coastal dunes in the Netherlands comprise multifunctional landscapes. With a hinterland situated below sea level, the importance of dunes with respect to protection against flooding is obvious. Also, large parts of the dunes are used for the production of drinking water. The importance of these functions always has been acknowledged in the past. As a result, the coastal dune landscape is ⁎ Corresponding author at: Iwan Kantemanplein 30, 1060RM Amsterdam, The Netherlands. Tel.: +31 20 3670258. E-mail address: [email protected] (S.M. Arens). 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2012.10.034

relatively undeveloped and presently represents important values for nature conservation and recreation. Past management of coastal dunes in the Netherlands was primarily concerned with erosion control (Klijn, 1990). The traditional strategy to counteract erosion and land loss was to stabilise foredunes as much as possible, and to suppress dynamic processes which would mobilise dunes and cause sand “loss” from the foredune system. The result was a rather artificial dune ridge (Arens and Wiersma, 1994), separating a more or less natural but mostly stabilised dune system from the dynamics of beach and sea. Because of this artificial obstruction, the exchange of sand between the beach and the more extensive dune environment was limited or non-existent. The stabilisation strategies

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were amplified by numerous other environmental stresses (Fig. 1) over the last century. The Dutch coastal dunes, as many other dune systems in northwestern Europe (Kooijman, 2004; Provoost et al., 2011), suffered from overstabilisation, resulting in a serious decrease in pioneer stages and biodiversity in general. Many of the factors that favour dune mobility (Fig. 1) have declined in importance over the last century, whereas factors favouring stability became dominant (Arens et al., 2007, in press). Since the 1970s, an increasing awareness of ecological values of young and dynamic systems led to a changed focus in nature management (e.g. van Dorp et al., 1985; Geelen et al., 1995; Van Boxel et al., 1997; Martinez et al., 2004). By the 1980s, most of the dunes were fully covered by vegetation. Awareness grew that these circumstances were a threat for biodiversity, especially since pioneer stages became rare and vegetation development in many regions approached climax conditions (van Dorp et al., 1985). Interest for remobilisation of dunes increased, and, in the 1990s, the decreasing ecological value of the dunes contributed to the first development of plans for restoration of dune mobility (Geelen et al., 1995; Terlouw and Van der Bijl, 1999). Nowadays, nature managers believe that remobilising dunes is an important key to maintain biodiversity, especially concerning the species of young and dynamic landscapes (e.g. Martinez et al., 2004; Arens et al., 2005, 2007; Rhind and Jones, 2009; Heathfield and Walker, 2011; Jackson and Nordstrom, 2011; Provoost et al., 2011). Essential is that at the upwind part mobile dunes create a bare surface by deflation, whereas at the downwind side sand deposition covers and destroys climax vegetation. This is not easy to accomplish. A huge management effort is required to overcome the hysteresis effect (Tsoar, 2005), i.e. that much more energy is needed to remobilize stabilised dunes, than to maintain dunes in a mobile state. The main intervention is to remove soil and vegetation. At issue is whether this action will restore mobility at the landscape scale. In 1990, the national policy of “Dynamic Preservation” (MIN V&W, 1990) was adopted, and the approach to coastal erosion management changed from reactive to pro-active (Hillen and Roelse, 1995; Van Koningsveld and Mulder, 2004; Mulder et al., 2011). Objectives of the policy are a sustainable preservation of safety against flooding and enhancement of values and functions in the dune area by combatting structural erosion by means of artificial nourishments. Since 1990, a yearly average of 6 million cubic metres of sand has been added to the Dutch coastal system (350 km). Since 2001, the yearly nourishment volume has been 12 million cubic metres, because the scope of the policy was extended to maintain the sand volume of the entire coastal and dune system (coastal foundation) to counter future sea level rise. Currently, in many countries, nourishment operations are conducted either for shore protection or restoration (e.g. Nordstrom et al., 2000; Hanson et al., 2002; Nordstrom et al., 2009).

As a consequence of the shift in coastal erosion management, the formerly intensive foredune management in several areas became redundant, and foredunes could develop spontaneously without further interventions. After 20 years, it has become clear that this change in policy resulted in important changes in the physical characteristics of the coastal environment (e.g. van Koningsveld et al., 2008; Arens et al., 2010; Van der Meulen et al., in press). With respect to aeolian dynamics, the issue is how these spontaneous changes in foredunes compare to the results of management interventions in the landward dunes and whether conditions for maintenance of biodiversity are better. Does linking the two compartments of the dune intensify the effects of interventions in the inner dunes, and can a revival of aeolian processes in foredunes be used to address sea level rise and improve safety against flooding? We use case studies from the Netherlands to answer these questions and discuss possible advantages of an integrated system approach. 2. Methods, climate and study areas All study areas are located in the Netherlands (Fig. 2). Section 3 focuses on two areas dealing with restoration projects in the inner dunes (The Van Limburg Stirum Area and Verlaten Veld) and Section 4 presents some examples of changes in foredunes (Heemskerk and Terschelling). Details of the study areas are given in the presentation of the case studies. Climate in the Netherlands is temperate humid, with strong seasonal contrasts. The storm season extends from October to March with a prevailing south-westerly to westerly wind. On the Wadden Islands in the North, north-westerly onshore winds are dominant in foredune development. Stormy periods alternate with cold periods, when the wind blows mostly from easterly directions. Since 1990, the number of storms has decreased slightly (KNMI, 2008), whereas temperature and rainfall increased during recent decades. We used digital geo-referenced full colour air photographs, recorded between 1995 and 2010, to study the development of bare or vegetated surfaces and erosion features. Height data were provided by Rijkswaterstaat, derived from Lidar data in a 5 × 5 m 2 grid and are part of the JARKUS-database. We used data for 1998 and 2012 mainly to calculate and visualise the changes in foredune height. We also used foredune cross sections from the JARKUS-database with development of height between 1979 and 2010 to calculate changes in foredune volumes. In one of the case studies (Verlaten Veld), we measured cross-profiles by means of GPS and laser theodolite. 3. Restoration of dune mobility behind the foredunes In the last two decades, more than 15 restoration projects were performed to restore aeolian dynamics and landscape building processes (Fig. 2 shows a selection). Some projects tried to reactivate smaller scale blowouts or to mobilise parabolic dunes. Others attempted landscape restoration by removing or adapting artificial landscapes (canals, golf course, and sand dykes). We focused on two projects (Van Limburg Stirum and Verlaten Veld), where large scale destabilisation occurred to restore aeolian dynamics. The longest data set is available for these projects, so they give the best insight to more than a decade of development after intervention. 3.1. Van Limburg Stirum

Fig. 1. Impact of environmental stress factors on dune mobility and stability, with indication of change (+ or −) over the last centuries. Adapted from Arens et al. (in press).

The Van Limburg Stirum area (VLS) is situated in the Amsterdam Water Supply dunes. A canal was excavated here to extract drinking water in the late 19th century. The sand was deposited along the edges of the canal. In 1995, the managers decided to restore the original landscape by replacing the sand in the original dune forms (Geelen et al., 1995). No stabilisation measures were taken, resulting in a bare sand area of 35 ha. A secondary goal of the project was to

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Fig. 2. Restoration projects, aiming at aeolian processes, in the Netherlands. Air photograph courtesy of Aerodata International Surveys.

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reactivate aeolian processes, giving room for dune remobilisation and long term (decades) landscape rejuvenation by deflation and dune building to create opportunities for pioneer species (mostly dune slack species). Details are provided in Arens and Geelen (2006). The intervention was expected to greatly extend the bare sand area, with extensive sand burial and possible development of mobile dunes. The question was how fast the destabilised surface would expand. Immediately after the intervention, aeolian processes enlarged the bare sand surface. This resulted in a maximum dynamic surface area in about 2–4 years. At the same time, colonisation of portions of the bare surface started either by the re-growth from roots or colonisation by seedlings. After 10 years, the stabilised surface equalled the aeolian active area in square metres (Fig. 3). After 15 years the area has transformed into a mosaic of small but still dynamic features. The features are often blowout-like, small deflation areas with groundwater near the surface. They are still expanding, often in windward direction, but also against the prevailing wind (and in response to groundwater level fluctuations). Nebkha dunes occur as well as stabilised surfaces and vegetation with some influence of sand burial (Fig. 4). Wind erosion is the dominant process. Surfaces that are eroded a few cm per year mostly remain bare. The impact of deposition is much less, and mostly results in a change in vegetation, not its destruction. The anticipated development of mobile dunes did not occur, but the remnant mosaic is ecologically valuable, giving room to the survival of rare (dune slack) species and pioneer environments. It is clear that the project did not result in long lasting, large scale mobilisation, but in a revival of morphodynamic processes on a time scale of 15–20 years, with probably much longer ecological effects. 3.2. Verlaten Veld The Verlaten Veld (VV) in Zuid-Kennemerland is part of the drinking water area of the Province of North-Holland (PWN). Currently no drinking water is extracted there. In 1998 the managers planned to remobilise a parabolic dune by removing soil and vegetation (a pine forest) from the

surface of the parabolic dune and adjacent deflation slack over an area of 13 ha (Terlouw and Van der Bijl, 1999; Terlouw and Slings, 2005). The plan was mainly experimental, to explore the possibilities of dune remobilisation at a larger scale, as opposed to previous small scale blowout reactivations (e.g. Van Boxel et al., 1997). The geomorphic setting is different from the van Limburg Stirum area, with a clear parabolic dune and adjacent deflation slack, but the response is comparable (Arens et al., 2004). Initially, the area dominated by aeolian processes expanded, whereas parts of the surface became colonised by vegetation, especially in the dune slack. Stabilisation is faster than at VLS (Fig. 3), but the aerial photo of 2010 indicates a comparable level of bare and dynamic surfaces after only 11 years (Fig. 5). Removing vegetation from the dune slack in front of the parabolic dune results in a massive supply of sand to the upwind slope and prevents wind erosion of the slope. Removal of vegetation from the crest results in a lowering of the crest of several metres and transfer of sand over the lee slope farther downwind. Consequently, the parabolic crest disintegrated partly. More detailed data from profile measurements (Fig. 6) reveal that the intended remobilization was not successful. The dune slack clearly expanded because of wind erosion. Apparently, the creation of fresh dune slacks from deflation works quite well, as also shown in the VLS area. On the parabolic dune crest, considerable sand transport occurred for five years, but then stagnated. The crest was severely eroded and moved downwind and considerable deposition occurred at the lee, but the lower part of the upwind slope remained remarkably stable. The desired process of dune mobility, resulting in erosion and dune slack formation at the upwind side and destruction of vegetation by sand burial on the downwind side, appeared to be decoupled. The question remains why the upwind slope did not erode, despite the complete removal of vegetation. 3.3. Implications of inner dune case studies These projects indicate that a huge increase of sand supply and consequent burial of vegetation for a number of years does not lead

Fig. 4. Part of the van Limburg Stirum area in 1996 and 2010. The area extends 1300 m from south to north, ca. 500 m from the coastline. Air photograph 1996 courtesy of Eurosense, 2010 Aerodata International Surveys.

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Fig. 5. Verlaten Veld area in 1999 and 2010. The area extends 700 m from south-west to north-east. Air photograph 1999 courtesy of Hansa Luftbild, 2010 Aerodata International Surveys.

both projects, we see an extension of wet dune slacks and often scattered blowouts that remain active for a longer time. To ensure full remobilisation, a follow up management of removing roots for 3–5 years, which currently is practised in some new projects, is probably necessary to start-up the ‘sand engine’. Other important conclusions from the VV experiment can be drawn. In a more natural setting, the upwind slope of the parabolic

to large scale vegetation decay. Vegetation growth apparently is sufficient to withstand considerable sand burial, even if it exceeds 0.5 m in a year. Also, remnant roots pose severe problems, either because of regrowth from the roots causing stabilisation or from sheltering the surface like a lag deposit, preventing wind erosion. Consequently, after some major deflation, the transfer of sand from the deflation zone to the depositional zone stagnates, and burial of vegetation ceases. In

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would be the most erosive part of the dune. That did not occur here. In future experiments, the dune slack, crest and lee slope will be left untouched to force the erosion of sand from the upwind slopes and deposition on the lee slope. Remobilisation of dunes is not simply just a matter of removing vegetation. Removing vegetation results in small scale features like blowouts, sand patches and small scale deflation. 4. Changes in coastal erosion management: effects on foredunes Because of the intensive nourishment programme since 1990, the position of the coastline has been maintained (van Koningsveld and Mulder, 2004), affecting the foredune system as well (Arens et al., 2010, in press). The sediment budget of foredunes is changing, resulting in the development of embryonic dunes in front of or against the foredunes in many locations. Calculations of sediment budgets indicate that many of the nourished sites show a positive trend in volume development of the foredunes. Fig. 7 shows an example for the Wadden island of Texel (Fig. 2). Apparently, a strong correlation exists between an enlarged beach sand source enhanced by nourishment and an increase in aeolian sediment transport to foredunes, but the effects vary considerably along the coast. The reasons for this are not clear. In many areas the sand is deposited in embryonic dunes in front of the former foredunes, similar to accreting systems. In other areas, sand is transported onto and over the foredunes, resulting in a dynamic foredune environment with the development of blowouts and carves (large blowouts that are connected to the beach). 4.1. Remobilisation of foredunes by wind and wave erosion

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Wind erosion in a dozen areas is no longer suppressed by managers since the change in erosion management. Blowouts start to develop and transform into carves when the upwind border encounters the beach. This results in spectacular changes at locations with some storm damage. Fig. 8 shows development near Heemskerk, along the Mainland Coast (Fig. 2). In 1996 sand fences and marram plantations dominate the foredune. By 2010 dynamic features developed and transformed into active parabolics. Locally, the amount of erosion exceeds 10 m, sand burial is slightly less extreme with burial just below10 m. This deposition is great enough to prevent colonisation by vegetation. The landward input of sand is increased considerably at the blowouts and carves, although no measurements are available yet. Dune erosion by waves is not an essential requirement for the development of blowouts, as shown in the top part of Fig. 8, but it helps. The number and surface areas of aeolian features are mapped for 18 study sites (in total 56.5 km of coastline spread over the different areas (Arens et al., 2010). The results indicate an important increase between 1996 and 2008 (Fig. 9).

0 1990 2000 2010 beach nourishment shoreface nourishment

Fig. 7. Development of foredune volumes in time. Example Texel (south-west). Nourishments in 1993/1994, 2000 and 2006 (beach) and 2003/2005 and 2007 (shoreface). Each km represents the average of 4 cross-sections. Source data: Rijkswaterstaat, Dutch Ministry of Infrastructure and Environment.

Experience with the restoration of aeolian processes in foredunes was gained during a project on the Wadden island of Terschelling (Fig. 2), where marram grass was bulldozered down from the foredunes between 1995 and 2000. The result was a nearly complete remobilisation of the foredunes and considerable sand burial of the area landward. Meanwhile, a new, smaller foredune developed near the former dunefoot (Fig. 10). Deposition (exceeding 0.3 m in 14 years) occurs more than 300 m downwind from the dunefoot. In the adjacent original foredunes deposition reached no farther than 80 m downwind. Despite some stabilising activities to reduce the extreme dynamics, the area still exhibits large surfaces with active deflation or extensive sand burial (Arens et al., in press). 4.2. Implications of foredune case studies Changes in foredunes, resulting from the nourishment policy and decreasing management efforts, indicate that large scale dune dynamics can be reactivated under current climatic conditions. Further study is needed however, of the interaction between shoreface, beach and foredunes to understand why in some situations the formation of embryonic dunes dominates, leading to stabilised former foredunes, whereas in other situations wind erosion and inland transfer of sand dominates, and new parabolic dunes start to develop. Apparently, dynamic features in the foredunes may develop spontaneously under the right conditions, resulting in the evolution of new parabolic dunes. This process has been suppressed for at least 150 years, but reoccurs when management interventions cease. Intervention in the foredunes is much more successful than the inner dune restoration projects in reinstating aeolian processes and long term development (> 10 years). The stress factors (wind, salt and sand from the beach) are much higher in the foredunes than in the inner dunes. Rates of wind erosion are much higher, but the rate of deposition is also much higher, and continues for a much longer time (in our foredune examples more than 10 years). The autonomous response of foredunes to erosion (either wind or waves) results in a revival of dune dynamics, which exceeds the infrequent autonomous changes in the inner dunes (mainly blowout development and water erosion) by at least an order of magnitude. Where the former foredunes tend to stabilise because embryonic dunes develop, intervention measures in the foredunes appear successful with respect to the initiation of wind erosion and transgressive dune development. 5. Discussion: combining erosion and dune management to connect beach and dunes Restoration projects in the inner dunes demonstrate that simply removing vegetation is not enough to remobilise dunes. The problems of heritage of roots must be solved, for example by a follow up management of removing roots until the surface is truly bare. Current experiments will show if this will be successful. Spontaneous developments in foredunes in nourished areas with minimal additional management interference indicate that large scale dynamics can be reinstated successfully. Remobilisation by simply removing marram grass from the foredunes also proved to be successful. Mobile dunes are often created in the foredunes, because of the release of large amounts of sand stimulated by wave erosion and cliff development, followed by wind erosion (e.g. van Dieren, 1934; McFadgen, 1985, 1994; Hesp, 2002; Beekman, 2007). Several authors stress the importance of a source of sand supply for dune mobility (e.g. Barbosa and Dominguez, 2004; Walker and Barrie, 2004). The next step in restoration of dune mobility is to connect the dune remobilisation projects to the management of coastal erosion using sand nourishment and to manage the coastal landscape as an integrated system. Incorporation of beach–dune dynamics in management of coastal nature reserves will be much more effective. In the harsher dynamic environment of the foredunes, colonisation of

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0 2003-2008 bare sand spot bare sand spot

Fig. 9. Increase of aeolian features for a number of sites (56.5 km) in the Dutch foredunes. Histogram: numer of features (left Y-axis), lines: surface area (right Y-axis).

bare surfaces by vegetation is more difficult, and only a few plants survive. The supply of sand by nourishment, if allowed to flow freely through and over the foredunes, ensures continuous disturbance of the surface and prevents rapid colonisation by vegetation. The best possibilities for restoring dune mobility will be in situations where the foredunes can develop without any interference of management (e.g. placement of sand fences, planting of marram grass, mechanical reconstruction of the seaward slope, and levelling of erosion cliffs), so carves and blowouts can develop freely while sand nourishment guarantees a continuous source of sand. Most of the debate on restoration of aeolian dynamics in dunes focuses on benefits for nature (e.g. Kooijman, 2004; Bonte and Hoffmann, 2005; Rhind and Jones, 2009; Jones et al., 2010; Heathfield and Walker, 2011; Bonte et al., 2012). Since the change in coastal defence policy, coastal erosion is no longer only a threat, but an opportunity for nature management. The reverse however, is also true: restoration of aeolian dynamics in dunes could benefit safety. By allowing natural processes

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Fig. 10. Foredune remobilisation after human interference on the island of Terschelling. For legend see Fig. 8. Distance between white dots at the coastline is 200 m. Air photograph courtesy of Aerodata International Surveys. Source Lidar data: Rijkswaterstaat, Dutch Ministry of Infrastructure and Environment.

in the foredunes, the transition zone between beach and dunes becomes wider, and sand can be released from the beach environment into the foredunes and landward dunes. With mobile dunes, the sand can even reach farther inland than previously. With respect to sea level rise, we need to think in terms of management over several decades. This means that nourishing the beach will eventually also result in nourishing the dune belt, leading to an enhanced accumulation of sand and a rise in surface level over a larger dune area. Allowing natural processes to occur in the dune area as a whole, contributes to the policy objective of sustainable safety by enabling the dunes to ‘grow-with-the-sea-level’ (NWP, 2009). Thus, the plea to stimulate more natural dynamics to favour habitat supports long term coastal protection, which, currently is the goal of coastal defence policy makers. An integrated approach of nature and coastal erosion management could include (1) providing fresh sand to the system by nourishment; (2) defining a wide safety zone as far from the coastline as possible, enabling the transition zone of beach to foredunes to develop freely; (3) reserving space for freely moving sand, not impeded by infrastructure or legal restrictions; (4) stimulating natural redistribution of sand within the system with a minimum of management interference, except active destabilisation of overstabilised surfaces in foredunes and connected inner dunes; (5) stimulating and restoring inland transport of sand by removing vegetation behind the foredunes; and (6) allowing for a long time scale of development. The transition from carves to parabolic dunes that move inland requires several decades. The six steps described above can be realised only in wide dune areas. Specific steps can be applied however, in a smaller dune belt with more urgent safety requirements (e.g. in the developed and spatially restricted scenarios in Nordstrom et al., 2011). For example, a certain amount of wind erosion can be tolerated on nourished beaches to allow inland transport. If a defined condition of the foredunes (volume or height) is exceeded, measures are taken to temporarily stop wind erosion. After a few years, when the size of the foredunes has increased, wind erosion could be allowed again.

of responses to this change occurred. In many areas an increase of embryonic dunes resulted, which is a fundamental change compared to the original coastal development, with a negative sediment budget and a retreating coastline. This has implications for the rate of sand burial landward of foredunes and consequently for species diversity. In many other areas, an increase of aeolian dynamics occurred, in some cases with severe erosion and deposition (height changes up to ±10 m in 14 years). This resembles a restoration of the former dynamic dune landscape, with a revival of aeolian processes and a positive effect for rare species but with a relatively stationary coastline. About the year 1995, dune nature-conservation managers began attempts to restore dune mobility landward of the foredunes by active intervention, removing vegetation and soil. Most projects are partly successful on a timescale of 10–20 years, but not in remobilisation of dunes. To achieve full remobilisation, follow up management is required. Future remobilisation interventions should focus on the windward slopes of dunes, leaving adjacent dune slack, crest and lee slope vegetated. With respect to ecological development, the interventions led to an increased surface of wet dune slacks and a diverse mosaic of stable and dynamic parts with increased biodiversity. Erosion dominates in the areas of intervention and is most effective to retain bare sand. Bare surfaces gradually decline by colonisation; buried surfaces usually stabilise faster when the buried plants grow through the deposition. The increase in aeolian dynamics in many locations, since the change in policy in 1990, suggests that foredune dynamics should be incorporated into future programmes of dune remobilisation. Remobilisation is more successful in the harsher foredune environment than in the inner dunes, even without intervention. Linking nature management to coastal erosion management can be rewarding with respect to maintaining conservation values and biodiversity and long term (decades) sustainable preservation of safety against flooding. Ample sand nourishment combined with a stimulation of natural dynamics, enhances the landward sand transport and contributes to the objective to let the dunes ‘grow-with-the-sea-level’.

6. Conclusions Acknowledgements Since 1990, the Dutch changed their coastal erosion management approach from reactive to proactive. By interfering on a larger scale, they managed to stabilise the coastline by means of nourishment. As a result, the foredune sediment budget has changed to mostly positive. Two types

Many of the ideas in this paper were developed during several projects under the programme “Duurzame Verstuiving”, financed by the drinking water companies Dunea, PWN and Waternet. We thank Harrie

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van der Hagen (Dunea) for his contributions. The research on the impacts of sand nourishment on dunes was financed by Rijkswaterstaat and by “OBN”, the network of the Bosschap, Board for Forests and Nature, linking nature managers, scientists and policy makers. We thank Evert Jan Lammerts, Anton van Haperen and Carleen Weeber of the OBN-network and Marieken van der Sluis-Meijerink of Rijkswaterstaat for their contributions. Bert van der Valk and Frank van der Meulen of Deltares are acknowledged for their support during several phases of the research. Finally we want to thank Karl Nordstrom and Nancy Jackson for offering us the chance to write this paper. We also thank them and two anonymous reviewers for critically reviewing the paper. Karl Nordstrom also did a fabulous job by changing our Denglish into English. Part of the work for this paper (JPMM) took place in the framework of the Kennis voor Klimaat (Knowledge for Climate) Programme, project ‘Climate proof flood risk management’. References Arens, S.M., Geelen, L.H.W.T., 2006. Dune landscape rejuvenation by intended destabilisation in the Amsterdam water supply dunes. Journal of Coastal Research 23, 1094–1107. Arens, S.M., Wiersma, J., 1994. The Dutch foredunes; inventory and classification. Journal of Coastal Research 10, 189–202. Arens, S.M., Slings, Q.L., De Vries, C.N., 2004. Mobility of a remobilised parabolic dune in Kennemerland, The Netherlands. Geomorphology 59, 175–188. Arens, S.M., Geelen, L.H.W.T., Slings, Q.L., Wondergem, H.E., 2005. Restoration of dune mobility in the Netherlands. In: Herrier, J.-L., Mees, J., Salman, A., Seys, J., Van Nieuwenhuyse, H., Dobbelaere, I. (Eds.), Proceedings: Dunes and Estuaries 2005. International Conference on Nature Restoration Practices in European Coastal Habitats, Koksijde, Belgium, 19–23 September 2005: VLIZ Special Publication, 19, pp. 129–138. Arens, S.M., Slings, Q.L., Geelen, L.H.W.T., Van der Hagen, H.G.J.M., 2007. Implications of environmental change to for dune mobility in the Netherlands. Universidad de Cantabria, Proceedings of the International Conference on Management and Restoration of Coastal Dunes, October 3–5, 2007. Universidad de Cantabria, pp. 115–122. Arens, S.M., Van Puijvelde, S.P., Brière, C., 2010. Effecten van suppleties op duinontwikkeling; geomorfologie (Effects of nourishments on dune development; geomorphology). Bosschap Report OBN142 DK by order of the Ministry of Agriculture, Nature Management and Food Quality (141 pp. + annexes). Arens, S.M., Slings, Q.L., Geelen, L.H.W.T., Van Der Hagen, H.G.J.M., in press. Restoration of dune mobility in the Netherlands, in: Martinez, M.L., Gallegos-Fernández, J.B., Hesp, P.A. (Eds.), Restoration of Coastal Dunes. Springer Series on Environmental Management. Barbosa, L.M., Dominguez, J.M.L., 2004. Coastal dune fields at the São Francisco river strandplain, Northeastern Brazil: morphology and environmental controls. Earth Surface and Processes 29, 443–456. Beekman, F., 2007. De kop van Schouwen onder het zand. Duizend jaar duinvorming en duingebruik op een Zeeuws eiland. Uitgeverij Matrijs, Utrecht. Bonte, D., Hoffmann, M., 2005. Are coastal dune management actions for biodiversity restoration and conservation underpinned by internationally published scientific research? In: Herrier, J.-L., Mees, J., Salman, A., Seys, J., Van Nieuwenhuyse, H., Dobbelaere, I. (Eds.), Proceedings: Dunes and Estuaries 2005. International Conference on Nature Restoration Practices in European Coastal Habitats, Koksijde, Belgium, 19–23 September 2005: VLIZ Special Publication, 19, pp. 165–178. Bonte, D., Breyne, P., Brys, R., De la Pena, E., D'hondt, B., Ghyselen, C., Vandegehuchte, M.L., Hoffmann, M., 2012. Landscape dynamics determine the small-scale genetic structure of an endangered dune slack plant species. Journal of Coastal Research 28 (4), 780–786. Geelen, L.H.W.T., Cousin, E.F.H.M., Schoon, C.F., 1995. Regeneration of dune slacks in the Amsterdam Waterwork Dunes. In: Healy, M.G., Doody, J.P. (Eds.), Directions in European Coastal Management. Samara Publishing Limited, Cardigan, Wales, pp. 525–532. Hanson, H., Brampton, A., Capobianco, M., Dette, H.H., Hamm, L., Laustrup, C., Lechuga, A., Spanhoff, R., 2002. Beach nourishment projects, practices and objectives — a European overview. Coastal Engineering 47, 81–111. Heathfield, D.K., Walker, I.J., 2011. Analysis of coastal dune dynamics, shoreline position, and large woody debris at Wickaninnish Bay, Pacific Rim National Park, British Columbia. Canadian Journal of Earth Sciences 48 (7), 1185–1198. Hesp, P.A., 2002. Foredunes and blowouts: initiation, geomorphology and dynamics. Geomorphology 48, 245–268.

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