Biogeomorphic feedback drives dune development along nourished coastlines

Résumé

Dutch coastal safety policy prioritizes creating favorable conditions for natural dune development, as over 75% of the Dutch coastline relies on its vegetated foredunes (the most seaward facing dune ridge parallel to the coastline) for flood defense against the sea. Coastal dunes are formed through a complex interaction between wind, waves, sand and vegetation and have the capacity (1) to reduce hydrodynamic impact from storm surges and (2) to keep up with sea-level rise by accumulating and stabilizing wind-blown sand. An essential element to this building-with-nature coastal strategy in the Netherlands is the utilization of European marram grass (Ammophila arenaria) to help stabilize and build up the foredunes. Compared to other (native) coastal pioneers that thrive on sandy soils, this beach grass is most effective at dune building because not only can it trap high amounts of wind-blown sand between its leaves and keep it in place within its roots, but it will in fact grow much more vigorously because of regular burial in sand. This introduces a reinforcing biogeomorphic feedback crucial to coastal dune development: trapping of wind-blown sand encourages marram grass to grow, which in turn enhances the capacity of marram grass to trap sand and build dunes.

As sea level rise accelerates, it is anticipated that sand nourishments need to increase in volume or frequency to combat erosion and maintain the sandy coast. Within this context, an unprecedented mega-scale beach nourishment of 21.5Mm3 termed Zandmotor (or 'Sand Motor'), was constructed in 2011 just south of the city The Hague. The overall purpose of this mega-nourishment experiment is to test whether its benefits in terms of coastal safety, spatial quality and ecological values outweigh the extra costs involved and to determine to what extent such an approach can help cope with expected changes in the global climate. In line with the building-with-nature approach, natural dynamics are encouraged to redistribute the sand of the Zandmotor along the coastline, thereby broadening the adjacent foredunes and beach. Because of its novel and experimental nature, however, the long-term effects of such a mega-scale beach nourishment on dune-building dynamics is unknown.

This objective of this research is to provide field-based evidence and insight into how reinforcing biogeomorphic feedback drives dune development, on the Zandmotor and along nourished coastlines in general. Insight into prevailing coastal aeolian dynamics is hereto essential, as wind-blown sand exerts direct control on foredune development. This insight, however, is challenged by a high degree of uncertainty in characterizing aeolian transport dynamics in coastal environments. This is foremost due to a lack of consensus about whether steady-state saltation (the principal mode of transport) is best described by a quadratic or cubic dependence to wind shear velocity (expression for the erosive force of the wind). But the saltation mass flux may often also not be in steady-state due to unsteady wind conditions or not be saturated to expected capacity due to meteorological and surficial conditions that limit the supply of sand to the aeolian transport system.

Furthermore, relatively little empirical data are available on the mutually reinforcing dune-building interaction between marram grass and sand deposition. Positive geomorphic feedback between the growth response of marram grass and burial by wind-blown sand is well documented and recognized to be fundamental to coastal dune development in temperate regions around the world. Field data, however, on plant response to sand burial, optimal and maximum burial levels and the capacity to deal with adverse conditions, are scarce. As a result, studies that model coastal dune development by incorporating positive feedback may be hampered by burial response growth functions that are not validated by empirical measurement.

Using a newly developed automated sand trap in conjunction with detailed wind forcing measurements, a synchronous long-term record of saltation mass flux and shear velocity measurements has been constructed. Nonlinear quantile regression analysis indicates that the saltation mass flux may be better explained by a quadratic than a cubic dependence to shear velocity. Fitting the quadratic and cubic function to the median response distribution indicates that saltation rates generated in (nourished) coastal environments may typically be two (for quadratic dependence) or three (for cubic dependence) orders of magnitude lower than predicted by transport equations of parameterized during wind tunnel experiments. The under-prediction of the saltation mass flux decreases when the quadratic and cubic functions are fitted to the observations made under optimal field conditions, but the still lower observed saltation rates indicate that coastal aeolian dynamics on the Zandmotor may be characterized by a persistent impact of supply-limiting conditions.

European marram grass possesses two traits that are uniquely advantageous to dune-building: (1) a very high tolerance to burial by wind-blown sand, and (2) more vigorous growth due to positive feedback to sand burial. Using high-resolution geospatial data  acquired with an unmanned aerial vehicle (UAV) and nonlinear quantile regression analysis, it is demonstrated that the growth response of marram grass to sand burial by wind, expressed by changes in Normalized Difference Vegetation Index (NDVI) and vegetation cover, can be described by a Gaussian response model. The regression curves indicate an optimal burial rate for marram grass of 0.31 meter of sand per growing season and suggest a maximum burial tolerance between 0.78 and 0.96 meter of sand per growing season.

While positive biogeomorphic feedback between marram grass and aeolian dynamics may ultimately be the most important driver for coastal dune development, it is often also the least considered component of the coastal aeolian transport-dune system. Putting more focus on the dune-building capacity of marram grass is advantageous to coastal management in the Netherlands: dune development is directly related to the growth response of marram grass to sand burial, thus maximizing the potential of marram grass to grow maximizes the potential of coastal dunes to develop and provide coastal safety.