Modelling foredune dynamics in response to climate change

Summary

Coastal dunes are prominent features along many of the world’s sandy shorelines. They are valued for their contributions to flood protection, biodiversity, fresh water supply and recreation. The most seaward dune ridge or foredune is the most dynamic part, showing fluctuations in size and morphology in response to erosion by the sea and subsequent recovery by interactions between wind-blown sand and vegetation. Given their dependency on multiple natural processes, coastal dunes may be particularly sensitive to the effects of climate change, including sea-level rise (SLR) and changes in temperature and precipitation.

To mitigate anticipated coastal erosion in the next decades, the Dutch sand nourishment regime will be intensified to raise the beach profile proportionally to the SLR. However, it is not clear how the added sand is distributed within the foredune system and whether this enables foredunes to keep up with sea-level rise. In addition, possibilities for dune re-mobilisation are investigated to enhance landward transport and biodiversity. However, effects of this intervention on foredune dynamics and the dune landscape are not entirely clear.

This thesis has examined yearly to decadal scale foredune dynamics and the impacts of climate change and management options on these dynamics.

Which factors control year-to-year variations in dune growth on the Dutch coast?

Dunes depend on aeolian transport for sand supply. While measurements of aeolian transport show complex spatio-temporal variations, we find that the yearly sand supply to dunes along the Dutch coast is relatively constant, between 10-20 m3/m irrespective of the beach width (Chapter 2). This means that a wider beach does not necessarily provide more sand to dunes and beach width is not a limiting factor in sand supply to the dunes.

In contrast to the sand input, the amount of sand lost during a storm surge does depend on the beach width. Wider beaches are able to dissipate more of the incoming wave energy and thus protect the dune better than narrow beaches. On a term of decades, this gives rise to steady dune growth on wider beaches and irregular, frequently interrupted growth on narrow ones.

How do biogeomorphic interactions control foredune shape?

The distribution of sand over the foredune, and therefore the morphological evolution, is strongly tied to vegetation patterns (Chapter 3). It was found that deposition patterns across foredunes show a characteristic distribution, starting with a sharp increase upon crossing the seaward vegetation limit, reaching a maximum between 5-20 m further landward and then gradually decreasing inland of the crest. The deposition pattern is further modified by the general vegetation pattern. On a timescale of years, there is no correlation between density of vegetation cover and the amount of accretion. However, by accounting for the gradual depletion of the sand load over the foredune, an empirical relationship can be defined between vegetation cover and its sand trapping efficiency. For fully covered surfaces, sand trapping efficiency is around 50%, indicating that sediment can pass densely covered foredunes.

Although literature suggests a relation between the level of plant burial and plant growth, we found no evidence for enhanced vegetation growth in high-deposition zones. A gain in vegetation cover was found to occur for burial between 0 m/year and 1 m/year, which indicates that lower and upper tolerance limits of burial have not been exceeded. Other growth limiting factors are likely to be of similar importance, masking any possible dependency of vegetation growth on sand accretion.

What are the effects of climate change on meso-scale evolution of coastal dunes?

The results on yearly erosion/accretion and sedimentation patterns were implemented in a computer for dune evolution called DUBEVEG, developed in Wageningen (Chapter 4). Algorithms for aeolian transport and vegetation growth were taken from existing models and combined with a new module for wave action and dune erosion. The model was calibrated and validated against field measurements. The good agreement between observations and predictions indicates that the model successfully incorporates the suite of biogeomorphic and marine processes involved in dune building.

Model simulations show that the evolution of a dune strongly depends on the sequence of storms and quiet periods. During quiet periods, dunes are able to build seaward at several metres per year as vegetation colonises the area near the dune foot, leading to dune accretion. Following the dune-foot position through time, we find an irregular pattern of seaward advance and regression. However, the average of a large number of runs with varying storm sequences reveals a clear trend. For a given wave climate and beach profile, we find that the model predicts a certain seaward limit to which the foredunes may build, or equilibrium position at which erosion and accretion are balanced. If the momentary position of the dune foot is seaward of this limit, seaward movement can be rapid. If, in contrast, the momentary position is at or seaward of the limit, periods of minor seaward growth are followed by periods of landward retreat, resulting in an oscillation around the equilibrium.

Climate scenarios, consisting of SLR and a gradual change in vegetation growth, were developed to examine climate-change effects on dune dynamics. Sea-level rise largely determines the direction of dune evolution by forcing the dune-foot landwards. The rate of rising controls whether dunes are able to preserve their height or sand volume while migrating landwards. 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 re-stabilise: a value below the threshold will preclude complete recovery and the dune remains bare.

What management options are available to mitigate climate-change effects on coastal dune evolution?

Sand nourishments are effective to mitigate the effect of SLR on coastal dunes. Model results show that by raising the beach proportionally to SLR, dunes are able to preserve their dunefoot position, height and volume. sHowever, the associated landward retreat is often not feasible.

A reduction in vegetation cover, related to either (1) artificial remobilisation, (2) dunefoot erosion or (3) climate change promotes landwards transport and therefore contributes to the long-term preservation of a wider dune zone. If vegetation growth is reduced as a consequence of increasing summer drought, re-mobilisation becomes more effective, with high rates of landwards transport persisting for several decades.

On the long term, it is recommended to use a combination of sand nourishments and remobilisation efforts to preserve the coastline, promote landwards transport and make benefit of a dune’s natural self-regenerating capacity. Under the precondition that safety requirements are met, these natural processes enable long-term preservation of flood protection, biodiversity and dynamic landscapes.