<%namespace name="tools" file="tools.tex"/>

This test concerns the morphodynamic response of sandy dunes to extreme storm impacts at Assateague Island, Maryland, USA, which was analyzed before by \citet{Jimenez2006}. Two consecutive North Easters attacked the barrier island during late January and early February, 1998. The bathymetry was measured using LIDAR in September 1997 and again February 9th and 10th, 1998 after the two storms had subsided.

Three types of dunes were identified by \citet{Jimenez2006}, shown in \autoref{fig:Assateague_Island_profiles}. Profile A (upper left panel) is initially characterized by a steep faced dune, where the maximum run-up exceeded the dune crest height and the mildly sloped back of the dune. The morphological response is characterised by profile lowering, decrease of the beach face slope and landward barrier displacement, while retaining barrier width.

Profile type B is a double-peaked dune profile and has two different shapes. Profile B1 (upper right panel) is initially characterized by a primary and secondary dune, both of which are lower than the maximum run-up height and which are separated by a valley. Profile B2 (bottom left panel) initially has two peaks of which the seaward one is lower. The backside of the barrier of either type is therefore either characterized by a secondary dune line (profile B1) or a taller crest of the dune (profile B2) which prevents the eroded sand from being transported to the backside of the dune. The main morphological response for these profile types is a decrease of the beach face slope, outer shoreline retreat and narrowing of the barrier.

The height of the dune crest of profile C (lower right panel) exceeds the maximum run-up height and so little overwash is observed. The morphological response of this type of profile is crest lowering due to slumping, decrease of the beach face slope and retreat of the outer shoreline. The width of the barrier is seen to decrease.

The storm impact of the two North Easters on Assateague Island were modelled with XBeach for the four profiles described by \citet{Jimenez2006}. The profiles were extended with a shallow foreshore and a 1:100 slope in seaward direction till a water depth of 9 m below NAVD88. As XBeach has not been shown to accurately simulate morphological change during very long storm durations, the simulations were run for a total of 20 hours. The measured wave and surge conditions were parameterized for each storm by a constant surge level and a constant wave spectrum (Pierson-Moskowitz) (see \autoref{tab:Assateague_Island_conditions}). This approach assumes that two 72 hour storms with varying surge and wave conditions can be approximated by two 10 hour simulations with constant maximum surge and wave conditions following a similar approach as \citet{Vellinga1986}. This approach also facilitates further sensitivity studies into the effect of varying hydraulic forcing conditions. The calculation grid size varies from 18 m at the offshore boundary to 2 m on the islands. A morphological acceleration factor of 5 is applied. The final simulated bed profiles are shown in \autoref{fig:Assateague_Island_profiles}.

${tools.table('Assateague_Island/', 'conditions.tex', 'Hydrodynamic boundary conditions XBeach simulations')}

${tools.image('Assateague_Island/', 'profiles.eps', 'Pre-storm profiles (black dotted line), measured post-storm profiles (black solid line) and modelled post-storm profiles (red solid line). Upper left panel: profile A. Upper right panel: profile B1. Lower left panel: profile B2. Lower right panel: profile C. The seaward side is on the left in all panels. Note that the measured post-storm profiles contain only the sea surface and emerged topography and no submerged topography.')}

% if interpret:

The profile changes calculated by XBeach are largely consistent with the description of dune evolution given by \citet{Jimenez2006}. \citet{Jimenez2006} observed that profile A became flatter, with large quantities of eroded sediment deposited on the back side of the barrier island, due to the consistent wave over-topping. The model replicates this behaviour, except that the island is lowered more than in the measurements and that the seaward face of the island does not roll back as it does in the measurements.

The observed response of profile B1 was dune face retreat, overwash deposition in the dune valley between the primary and secondary dunes and narrowing of the island, \citet{Jimenez2006} also noted decrease of the beach face slope. It can be seen in \autoref{fig:Assateague_Island_profiles} that the morphological development of the island is well represented by the model. The simulated dune crest retreat corresponds closely to the measured retreat. Overwash takes place in the model and sediment is deposited in the valley between the primary and secondary dunes, although the magnitude of deposition is less than in the measurements.

The XBeach model of profile B2 shows a slope reduction on the seaward side and lowering of the seaward dune. The second dune crest retains its crest level as described in the work of \citet{Jimenez2006}. The beach slope decrease in the XBeach model is in line with the description given by \citet{Jimenez2006}, but differs from their measured profile. It is unclear why the measured profile shows almost no erosion of the beach face.

\citet{Jimenez2006} observed, in general, profile C to lower in height, the seaward dune slope to become smaller, and seaside retreat of the shoreline resulting in barrier narrowing. The XBeach model shows retreat of the upper dune face and a reduction of the seaward dune slope. The model over predicts the sedimentation at the base of the dune and under predicts the crest lowering.

% endif