The effects of climate change on coastal management in the Hondsbossche Dunes

Abstract

In 2003, it was established that the sea dike between Petten and Camperduin was insufficiently safe.
To ensure the safety of the hinterland against flooding, this part of the Dutch sea defence system needed to be strengthened.
During the design process, the most favourable variant was to construct a dune and beach system in front of the sea dike, which was constructed soon after, and finished in 2015.
This changed the 'hard' sea defence to a 'soft' one, linking the beaches and dunes in the north and south with each other.
Thereby not only increasing the safety of this part of the coast, but also providing a new area reserved for recreation and nature.

The change from a hard to a soft coastal system fundamentally changes the characteristics of the area.
Where the old sea dike was a static structure, the newly constructed sandy beach and dune system is very dynamic in nature.
The change in properties of this coastal defence stretch brings the question how the HD (Hondsbossche Dunes) can best be maintained to ensure its functions as a sea defence now and in the future.
The answer to this question requires insight in the processes relevant to aeolian sediment transport in general, and for the HD in specific.
With the knowledge of what processes are important for coastal dune growth in the HD, a model can be selected to make predictions on the likely development of the HD in the future.

Aeolian sediment transport, much like hydrodynamic sediment transport, can be described as a balance between forcing and resistance against forcing.
The most important factors which govern the dynamics of dune growth as a consequence of aeolian sediment transport are: wind speed and direction, grain size, humidity, sediment availability, beach slope, and vegetation.
Especially vegetation (marram grass) is important, as it stabilizes the dunes with its roots and rhizomes below ground, and by locally decreasing the wind velocity with the biomass above ground.
The above ground component of the marram grass not only stabilizes the present sediment, it also accommodates a sheltering effect for any incoming sand grains.
Dune growth rate is the sum of incoming and outgoing aeolian sediment transport rates.
For a healthy dune system, this must be a positive value, as this process negates the effect of dune erosion as a consequence of storm events.

In the period between 2015-2018, positive dune growth rates have been observed.
An alongshore dune growth velocity gradient has been found, with the highest growth rate being in the south.
This is due to a combination of difference in vegetation health (being poorer in the north), and the orientation of the dunes in relation to the average wind direction (more onshore directed in the south).
The dune growth rate has decreased over time, but has remained positive over the regarded period.
This is due to the eroding beach, which decreases the sediment availability for aeolian transport.
Another factor is the coarsening of the average grain size as a consequence of aeolian transport on the constructed coast, increasing resistance to aeolian transport.
This largely explains the decreasing growth rates of the dunes, along with the fact that beach armoring starts to develop in the HD after construction.

The cellular automaton model DuBeVeg, developed at the University Wageningen, is used to model aeolian sediment transport, and the effect of vegetation in the HD.
This model is chosen over other models like AeoLiS and Aeolus, as DuBeVeg is capable of modeling vegetation growth both in surface area and effectiveness against erosion as a result of vegetation density.
Several improvements have been made to the model.
Non-erodible elements have been implemented, which allows the user to model the effects of the presence of objects such as the old sea dike behind the dunes, or pavement on top of the dunes.
Mass balance has been implemented in the model, with an option to define a sediment flux in the marine area of the model as a consequence of cross-shore and/or long-shore sediment transport.
A dynamic erosion profile on the beach is implemented, based on the erosion profile of Vellinga.
Climate change in the form of sea level rise and changes in wind velocity have been modeled.

Sea level rise affects aeolian transport rate and coastal dune development by influencing the sediment availability on the beach.
Higher water levels increase the reach of hydrodynamic processes, which result in higher beach erosion rates.
To ensure sediment availability for aeolian sediment transport, nourishments are an effective measure.
Higher sea level rise asks for a shorter interval between nourishments to ensure sediment availability.
Higher and lower wind velocities generally result in higher and lower aeolian sediment transport rates respectively.
Higher aeolian sediment transport rates don't necessarily result in higher dune growth rates, as the stabilizing effect of vegetation against aeolian erosion must be present to prevent aeolian transport out of the dune area.
Marram grass has an optimum accretion rate at which the plant grows best, and therefore stabilizes the dune most efficiently at this dune growth rate.
Therefore, an optimum dune growth rate exists.

To ensure the HD can maintain its functions as a sea defence, nature reserve and recreational area, it is important for the dunes to grow under average conditions.
To ensure this, sediment needs to be available on the beach which is ready for aeolian sediment transport toward the dunes.
Enough vegetation needs to be present on the dune surface to protect the dunes against aeolian erosion by sheltering, lowering the local wind velocity at the surface. Thereby inhibiting aeolian transport in the dune itself.
The sediment availability can be ensured by nourishing the beach or foreshore.
The presence of enough healthy vegetation can be ensured by providing the right abiotic conditions for the vegetation to thrive.
Increasing the biodiversity in the area makes the HD more robust to changes in the climate, such as sea level rise and changing wind climates which results in different rates of accretion in the dunes.

Increasing the biodiversity enhances the HD's other functions as a nature reserve and recreational area as well.
Biodiversity helps with the robustness of the ecosystem, and makes the area more enjoyable for recreational purposes.
The nourishments ensure the presence of a wide beach, ensuring enough room is available for a recreational day on the beach.