Dynamic dune management, integrating objectives of nature development and coastal safety: Examples from the Netherlands
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 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 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, Arens et al., 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, Arens et al., 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.
Section snippets
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
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
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, Arens et al., 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
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.
Conclusions
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 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
Acknowledgements
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 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
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