Vegetation Controls on the Maximum Size of Coastal Dunes and the

Vegetation Controls on the Maximum Size of Coastal Dunes and the Role of Biophysical
Interactions in Determining Dune/Island State
Laura J. Moore and Orencio Duran
Duran and Moore (2013) recently extended the model of Hermann et al. (2008) to create
an aeolian eco-morphodynamic model (the Coastal Dune Model, CDM), which resolves the
coevolution of topography and vegetation in response to both physical and ecological factors to
simulate the formation of coastal foredunes. We find that foredune growth is eventually limited
by a negative feedback between wind flow and topography. Model results suggest that the
distance from the shoreline that dune-building vegetation becomes established is the primary
factor controlling the maximum size of foredunes. We also find that aeolian sand supply to the
dunes determines the timescale of foredune formation. These results offer a potential explanation
for the empirical relation between beach type and foredune size, in which large (small) foredunes
are found on dissipative (reflective) beaches. Higher waves associated with dissipative beaches
increase the disturbance of strand species, which shifts foredune formation landward thus leading
to larger foredunes, suggesting that plants play a more active role in modifying their habitat and
altering coastal vulnerability than previously thought.
Extending the model of Duran and Moore (2013) to include dune erosion by storm events
(Duran and Moore, 2015) we find island response to be intrinsically bistable and controlled by
previously unrecognized dynamics: the competing, and quantifiable, effects of storm erosion,
sea-level rise, and the aeolian and biological processes that drive dune recovery. When the
biophysical processes driving dune recovery dominate, islands tend to be high in elevation and
vulnerability to storms is minimized. Alternatively, when the effects of storms dominates, islands
may become trapped in a perpetual state of low elevation and maximum vulnerability to storms,
even when storms are mild. When sea-level rise dominates, islands become unstable and face
possible disintegration. These findings are supported by data from the Virginia Barrier Islands,
USA and provide a broader context for considering island response to climate change and the
likelihood of potentially abrupt transitions in island state.
In a current effort we are collaborating with U.S. and international partners to merge the
Coastal Dune Model with XBeach (a modelling approach to dune erosion, overwashing and
breaching; Roelvink et al., 2009) and AeoLiS (which simulates the influence of supply-limiting
factors on aeolian transport initially developed by De Vries et al. 2014). In addressing both
subaqueous and subaerial sediment transport and erosion during storms as well as inter-storm
evolution of subaerial topography, the resulting coupled model will allow, for the first time,
process-based simulation of event- and decadal-scale co-evolution of nearshore, beach and dune