Coastal Dunes

Flow dynamics and sediment transport responses over a large, vegetated foredune at Prince Edward Island,
Canada, during an offshore wind event are examined. Data were collected along an instrumented transect that
extended from the dune crest, down the lee-side (seaward) slope of the dune, across a wave-cut scarp, and on
to the back-beach. When the wind direction at the dune crest was approximately crest-normal (less than
about 15° deviation), the mean near-surface flow directions along the dune slope and on the back beach were
generally onshore, indicating reversed (onshore) flow relative to the regional (offshore) wind direction.
Although flow patterns were consistent with a lee-side recirculation eddy, large excursions in flow direction
were also prevalent, suggesting that the eddy was unstable and alternated with highly turbulent wake flow. As
wind direction at the crest veered to greater than 20° from crest-normal, lee-side winds shifted toward strongly
alongshore flowwith minimal directionally variability. On the dune slope, the wind vectorswere slightly offshore
whereas on the back-beach they were slightly onshore.
Wind speeds and sediment transport were greatest at the foredune crest and declined rapidly downslope due to
flow expansion and deceleration in the wake zone as well as to the influence of a sparse vegetation layer. Mean
particle counts (averaged over a 15-min interval) derived from laser sensors positioned at the crest were large
(7.76 per second) in comparison to those measured in the immediate lee of the crest (0.52 per second) and
farther down the dune slope (b0.13 per second). In contrast, the values were as large as 25.62 per second on
the middle of the back-beach, declining rapidly to a value of only 0.24 per second at the dune toe. Transport
intensity was highly variable with the largest Activity Parameter (AP = 0.5) values at the dune crest and on
the back-beach, and with the smallest values (AP b 0.1) on the lee-side dune slope down to the top of the
scarp. Calculations of sediment (particle) flux divergence between instrument stations show that deposition
was significant immediately downwind of the dune crest but negligible across most of the lower dune slope.
Deposition was also prevalent on the dune ramp below the scarp.
These results demonstrate that sediment transport across the beach–dune system was spatially discontinuous
during this offshore wind event. Rebuilding of the dune ramp at the toe of the scarp occurred quite independently
of, and with a different sediment source than, the broadening of the dune crest, which was fed with sediment
from the landward side of the foredune. Such process ‘decoupling’ is an example of the complexity by which
foredunes evolve or are maintained, and as with previous research reinforces the importance of offshore and
alongshore wind events to beach–dune morphodynamics.

Despite widespread recognition that landforms are complex Earth systems with process-response linkages that span temporal scales from seconds to millennia and spatial scales from sand grains to landscapes, research that integrates knowledge across these scales is fairly uncommon. As a result, understanding of geomorphic systems is often scale-constrained due to a host of methodological, logistical, and theoretical factors that limit the scope of how Earth scientists study landforms and broader landscapes. This paper reviews recent advances in understanding of the geomorphology of beach-dune systems derived from over a decade of collaborative research from Prince Edward Island (PEI), Canada. A comprehensive summary of key findings is provided from short-term experiments embedded within a decade-long monitoring program and a multi-decadal reconstruction of coastal landscape change. Specific attention is paid to the challenges of scale integration and the contextual limitations research at specific spatial and/or temporal scales imposes. A conceptual framework is presented that integrates across key scales of investigation in geomorphology and is grounded in classic ideas in Earth surface sciences on the effectiveness of formative events at different scales. The paper uses this framework to organize the review of this body of research in a 'scale aware' way and, thereby, identifies many new advances in knowledge on the form and function of subaerial beach-dune systems. Finally, the paper offers a synopsis of how greater understanding of the complexities at different scales can be used to inform the development of predictive models, especially those at a temporal scale of decades to centuries, which are most relevant to coastal management issues. Models at this (landform) scale require an understanding of controls that exist at both 'landscape' and 'plot' scales. Landscape scale controls such as sea level change, regional climate, and the underlying geologic framework essentially provide bounding conditions for independent variables such as winds, waves, water levels, and littoral sediment supply. Similarly, a holistic understanding of the range of processes, feedbacks, and linkages at the finer plot scale is required to inform and verify the assumptions that underly the physical modelling of beach-dune interaction at the landform scale.

Coastal dunes are transitional systems which offer manifold ecosystem services, including material and
intangible benefits, whose relevance for human society has been little investigated to date. In order to
plan a sustainable use of these threatened ecosystems, proper indicators to quantify ecological services
and functions over time should be provided. Since the functionality of coastal dunes is affected by the
specific contiguity among different dune habitat patches, in this paper we propose the use of landscape
integrity, expressed by the number and length of boundary types among the aforementioned habitats, as
an ecosystem functionality indicator. In particular, we aim at properly expressing the trend of coastal
dune mosaic integrity in two Italian LTER sites over the last sixty years by applying a generalized diversity
function on the number and length of boundary types. Such functions, unlike a traditional diversity
index, allow a complete summarization of landscape diversity and structure. The proposed
method has proved to be effective in the considered context, because it has adequately underlined the
different landscape integrity trend recorded in the two sites. In particular, generalized diversity functions
showed to efficiently express both rough and moderate anthropogenic transformations, which affected
spatial heterogeneity and functionality of the considered coastal dune mosaic. Further applications at
different scales and across different ecosystems are encouraged.

The form, height and volume of coastal foredunes reflects the long-term interaction of a suite of nearshore and aeolian processes that control the amount of sand delivered to the foredune from the beach versus the amount removed or carried inland. In this paper, the morphological evolution of more than six decades is used to inform the development of a simple computer model that simulates foredune growth. The suggestion by others that increased steepness of the seaward slope will retard sediment supply from the beach to the foredune due to development of a flow stagnation zone in front of the foredune, hence limiting foredune growth, was examined. Our long-term data demonstrate that sediment can be transferred from the beach to the foredune, even with a steep foredune stoss slope, primarily because much of the sediment transfer takes place under oblique rather than onshore winds. During such conditions, the apparent aspect ratio of the dune to the oncoming flow is less steep and conditions are not as favourable for the formation of a stagnation zone. The model shows that the rate of growth in foredune height varies as a function of sediment input from the beach and erosion due to storm events, as expected, but it also demonstrates that the rate of growth in foredune height per unit volume increase will decrease over time, which gives the perception of an equilibrium height having been reached asymptotically. As the foredune grows in size, an increasing volume of sediment is needed to yield a unit increase in height, therefore the apparent growth rate appears to slow.