M.J.P.M. Riksen

Spatio-temporal variability in accretion and erosion of coastal foredunes in the Netherlands: regional climate and local topography.

Depending on the amount of aeolian sediment input and dune erosion, dune size and morphology change over time. Since coastal foredunes play an important role in the Dutch coastal defence, it is important to have good insight in the main factors that control these changes. In this paper the temporal variations in foredune erosion and accretion were studied in relation to proxies for aeolian transport potential and storminess using yearly elevation measurements from 1965 to 2012 for six sections of the Dutch coast. Longshore differences in the relative impacts of erosion and accretion were examined in relation to local beach width. The results show that temporal variability in foredune accretion and erosion is highest in narrow beach sections. Here, dune erosion alternates with accretion, with variability displaying strong correlations with yearly values of storminess (maximum sea levels). In wider beach sections, dune erosion is less frequent, with lower temporal variability and stronger correlations with time series of transport potential. In erosion dominated years, eroded volumes decrease from narrow to wider beaches. When accretion dominates, dune-volume changes are relatively constant alongshore. Dune erosion is therefore suggested to control spatial variability in dune-volume changes. On a scale of decades, the volume of foredunes tends to increase more on wider beaches. However, where widths exceed 200 to 300 m, this trend is no longer observed.

Adaptation strategies to maintain dunes as flexible coastal flood defense in The Netherlands

Coastal dunes play an important role in coastal defense along sandy shorelines of the world. The majority of the shorelines experience erosion and this erosion is expected to accelerate under anthropogenic climate change and subsequent sea level rise. This paper investigates the impact of climate change, sea level rise and current management for coastal dunes in the Netherlands. Furthermore the paper discusses the implications of climate change projections for adaptation strategies into the future. Recent climate change scenarios for the Netherlands highlight rising temperature and accelerated sea-level rise. Their combined effects on dune-building processes are expected to be manifested through an increase in erosive forces at the expensive of accretive forces. In the Netherlands, a negative sand balance and inland migration of the beach-dune system has been successfully counteracted in the last decades through the application of sand nourishments. These have enhanced accretion on the one hand and limited erosion on the other hand. Generally, coastal protection has improved despite rising sea levels. Important preconditions that make this sand nourishment strategy possible are: a readily available sand resource that makes exploitation technically and economically feasible; a sound monitoring system supported by solid science; political consensus and a good institutional structure to implement the strategy. In the Netherlands, the necessary preconditions are already in place to successfully adapt to sea level rise. Given the expected accelerated rise in sea level and its potential effects on the dune-beach sediment balance, the annual sand nourishment will need to be intensified to ensure the preservation and integrity of the coastal zone.

Soft engineering vs. a dynamic approach in coastal dune management: a case study on the North Sea barrier island of Ameland, the Netherlands

ABSTRACT De Jong, B.; Keijsers, J.G.S.; Riksen, M.J.P.M.; Krol, J., and Slim, P.A., 2014. Soft engineering vs. a dynamic approach in coastal dune management: a case study on the North Sea barrier island of Ameland, The Netherlands. Dunes act as flood defenses in coastal zones, protecting low-lying interior lands from flooding. To ensure coastal safety, insight is needed on how dunes develop under different types of management. The current study focuses on two types of coastal dune management: (1) a “soft engineering” approach, in which sand fences are placed on the seaward side of foredunes, and (2) “dynamic coastal management,” with minimal or no dune maintenance. The effects of these management styles on dune formation are examined for two adjacent coastal sections of the North Sea barrier island of Ameland, The Netherlands, where dynamic coastal management was introduced in 1995 and 1999, respectively. For each section, we analyzed cross-shore profile data from 1980 until 2010, deriving dune foot position, crest position, crest height, and foredune volume for each year and analyzing the situation before and after the change in management. We further assessed the effect of the management regime on dune vegetation. Other factors that could influence dune development were also taken into account, such as beach width and shape, water levels, wave heights, and nourishments. Results show that implementation of dynamic coastal management did not directly affect the volume of the foredune. Growth was occasionally interrupted, coinciding with high-water events. In periods between erosive storms, dune growth rates did not show a significant difference between management types (p = 0.09 and 0.32 for sections 1 and 2, respectively). The main effect of the change was on vegetation development. Dynamic coastal management, therefore, did not reduce coastal safety.

Modeling the biogeomorphic evolution of coastal dunes in response to climate change

Coastal dunes form in many parts of the world the first flood defense line against the sea. To study effects of climate change on coastal dune evolution, we used a cellular model of dune, beach and vegetation development (DUBEVEG). The model was calibrated and validated against field measurements of the Dutch coast, showing good performance for 10 year simulations. A sensitivity analysis showed that dune size and morphology are most sensitive to the rate of aeolian input and wave dissipation. Finally, 100-year climate-change scenarios were run to establish the impacts of sea-level rise and changes in vegetation growth rate on dune evolution. The results are in good agreement with conceptual models of dune evolution. Sea-level rise largely determines the direction of dune evolution: the rate of rising controls whether dunes are able to preserve their height or sand volume while migrating landward. The effect of changing vegetation growth rates, resulting from climate change, is most manifest in dune response to large disturbances. If vegetation is removed halfway into the simulation, vegetation growth rate determines whether a foredune will re-vegetate and recover its height. Low vegetation growth rates result in mobile dunes that lose sand. The good agreement between observations and predictions indicates that the model successfully incorporates the suite of biogeomorphic and marine processes involved in dune building.

Embryo dune development drivers: beach morphology, growing season precipitation, and storms

For development of embryo dunes on the highly dynamic land–sea boundary, summer growth and the absence of winter erosion are essential. Other than that, however, we know little about the specific conditions that favour embryo dune development. This study explores the boundary conditions for early dune development to enable better predictions of natural dune expansion. Using a 30 year time series of aerial photographs of 33 sites along the Dutch coast, we assessed the influence of beach morphology (beach width and tidal range), meteorological conditions (storm characteristics, wind speed, growing season precipitation, and temperature), and sand nourishment on early dune development. We examined the presence and area of embryo dunes in relation to beach width and tidal range, and compared changes in embryo dune area to meteorological conditions and whether sand nourishment had been applied. We found that the presence and area of embryo dunes increased with increasing beach width. Over time, embryo dune area was negatively correlated with storm intensity and frequency. Embryo dune area was positively correlated with precipitation in the growing season and sand nourishment. Embryo dune area increased in periods of low storm frequency and in wet summers, and decreased in periods of high storm frequency or intensity. We conclude that beach morphology is highly influential in determining the potential for new dune development, and wide beaches enable development of larger embryo dune fields. Sand nourishment stimulates dune development by increasing beach width. Finally, weather conditions and non-interrupted sequences of years without high-intensity storms determine whether progressive dune development will take place. Copyright

Measuring and modeling coastal dune development in the Netherlands

In the past couple of years, new coastal-dune research has sprung up in the Netherlands. In this paper, we give an overview of ongoing projects at Wageningen UR, Deltares, TU Delft and UTwente: how these are connected and what type of questions are addressed. There is an increasing demand for the understanding and prediction of coastal dune dynamics, both on the short (year) and long (100 years) term. We approach this from a variety of angles: scientific and applied, short-term and long-term, data-driven and model-based, biotic and abiotic, process-based and rule-based, and focused on components and integrated. We give examples of results and end with a discussion of the benefits of this integrated approach.

Connecting aeolian sediment transport with foredune development

Foredune volume is an important factor for coastal safety and depends on the balance between erosion through wave attack and sediment input via aeolian transport. Dune erosion can be simulated with good accuracy, but predictions of aeolian sediment transport into the foredunes are still difficult to make. As part of a larger project that aims to model foredune development over decades, the goal of this study is to improve the understanding of the temporal variability in sediment transport at the beach. Measurements of aeolian sediment transport at the beach of barrier island Ameland show that within events, wind velocity and rain are dominant controls. After aggregating wind and precipitation into a single meteorological index, it was found that these controls alone are not sufficient to explain the year-to-year variability in foredune growth rates. In contrast to the volume changes, the variability in the amount of elevation change of the foredune slope can be related to the wind climate and precipitation.

Accommodation space indicates dune development potential along an urbanized and frequently nourished coastline

With densely populated areas well below mean sea level, the Netherlands relies heavily on its dunes to ensure coastal safety. About half of the sandy coastline, however, is subject to structural marine erosion and requires frequent sand nourishment as a counteractive measure. A key component of present-day coastal safety policy is creating favorable conditions for natural dune development. These conditions essentially involve (1) a steady supply of wind-blown sand towards (2) a wide accommodation space where sand can accumulate and dunes are sheltered from frequent storm surge impacts. This paper examines to what extent an experimental mega-scale beach nourishment (termed Zandmotor in Dutch) has contributed to creating accommodation space favorable for dune development. Using publicly available airborne lidar data and Sentinel-2 satellite imagery, favorable accommodation space is identified by comparing recent changes in coastal morphology against dune vegetation-cover dynamics. With a focus on European marram grass (Ammophila arenaria) as the most prominent dune-building species, this paper demonstrates that the Zandmotor supports an especially high potential for incipient (embryo) dunes to develop as most of its favorable accommodation space is located on the beach. However, considering the conditions required for successful marram grass establishment as well as persistent anthropogenic disturbances arising from recreation and nature management practices, it is not likely that dune development along this urbanized coastline reaches its full potential.