Shoreline Retreat Research Articles

Decoupling processes and scales of shoreline morphodynamics

Behavior of coastal systems on time scales ranging from single storm events to years and decades is controlled by both small-scale sediment transport processes and large-scale geologic, oceanographic, and morphologic processes. Improved understanding of coastal behavior at multiple time scales is required for refining models that predict potential erosion hazards and for coastal management planning and decision-making. Here we investigate the primary controls on shoreline response along a geologically-variable barrier island on time scales resolving extreme storms and decadal variations over a period of nearly one century. An empirical orthogonal function analysis is applied to a time series of shoreline positions at Fire Island, NY to identify patterns of shoreline variance along the length of the island. We establish that there are separable patterns of shoreline behavior that represent response to oceanographic forcing as well as patterns that are not explained by this forcing. The dominant shoreline behavior occurs over large length scales in the form of alternating episodes of shoreline retreat and advance, presumably in response to storms cycles. Two secondary responses include long-term response that is correlated to known geologic variations of the island and the other reflects geomorphic patterns with medium length scale. Our study also includes the response to Hurricane Sandy and a period of post-storm recovery. It was expected that the impacts from Hurricane Sandy would disrupt long-term trends and spatial patterns. We found that the response to Sandy at Fire Island is not notable or distinguishable from several other large storms of the prior decade.

Accommodation space controls on incised-valley sediment accumulation rates during the Holocene marine transgression (0–11 ka) in Grays Harbor, a large meso-tidal estuary, Washington, USA

Holocene deposits (0–11ka) that were drill cored (275 samples from 17 sites) in the meso-tidal Grays Harbor estuary were analyzed for 1) sediment texture and source components and 2) calibrated radiocarbon age. These data were used with basin 10m-contour interval volumes to establish relations between rates of increasing basin accommodation space and sediment volume accumulation. Rates of increasing accommodation space (0.3–2.1×106m3yr−1) are based on interval volumes and a sea level curve (0–11.5ka) for the Grays Harbor basin. Total sediment accumulation rates (0.41–1.4×106m3yr−1) are computed for the same 10m-depth intervals (0–60m depth MSL) and corresponding 10m-interval sedimentation durations (0.7–5.0×103ka) from a sediment level curve. The rates of basin increasing accommodation space and total sediment accumulation are positively correlated (R2=0.81), but sediment accumulation lagged behind increasing accommodation space (23–37%) during early- and middle-Holocene time (7–10ka). Slowing rates of sea level rise after 8ka, permitted combined river and littoral inputs of sediment to reduce basin subtidal water depths (~10m) to intertidal water depths (~1m). The ratios between sand and mud components in the analyzed Holocene deposits are negatively correlated to basin water depth. Shallowing tidal flats promoted the export of river mud out of the estuary and the import of littoral sand into the central estuary areas. Variable rates of increasing basin accommodation space 1) directly controlled the total sediment accumulation rate in the evolving basin and 2) indirectly controlled the relative proportions of mud and littoral sand accumulation, due to changing water depths in the basin. The late-Holocene accumulation of littoral sand (52% of total sediment volume) is used to compute the net littoral sand sink (~100×106m3) in the Grays Harbor basin from a potential 1.0m rise of future sea level following predicted global warming. The contribution of littoral sand from adjacent barrier sand spits (20km in length) to fill the increasing accommodation space in the Grays Harbor estuary could result in 0.5–1.0km of shoreline retreat following a 1.0m rise in sea level.

Impacts of the Deepwater Horizon oil spill on the salt marsh vegetation of Louisiana

The coastal wetland vegetation component of the Deepwater Horizon oil spill Natural Resource Damage Assessment documented significant injury to the plant production and health of Louisiana salt marshes exposed to oiling. Specifically, marsh sites experiencing trace or greater vertical oiling of plant tissues displayed reductions in cover and peak standing crop relative to reference (no oiling), particularly in the marsh edge zone, for the majority of this four year study. Similarly, elevated chlorosis of plant tissue, as estimated by a vegetation health index, was detected for marsh sites with trace or greater vertical oiling in the first two years of the study. Key environmental factors, such as hydrologic regime, elevation, and soil characteristics, were generally similar across plant oiling classes (including reference), indicating that the observed injury to plant production and health was the result of plant oiling and not potential differences in environmental setting. Although fewer significant impacts to plant production and health were detected in the latter years of the study, this is due in part to decreased sample size occurring as a result of erosion (shoreline retreat) and resultant loss of plots, and should not be misconstrued as indicating full recovery of the ecosystem.

Optimal index related to the shoreline dynamics during a storm: the case of Jesolo beach

Abstract. The paper presents an application of shoreline monitoring aimed at understanding the response of a beach to single storms and at identifying its typical behaviour, in order to be able to predict shoreline changes and to properly plan the defence of the shore zone. On the study area, in Jesolo beach (northern Adriatic Sea, Italy), a video monitoring station and an acoustic wave and current profiler were installed in spring 2013, recording, respectively, images and hydrodynamic data. The site lacks previous detailed hydrodynamic and morphodynamic data. Variations in the shoreline were quantified in combination with available near-shore wave conditions, making it possible to analyse the relationship between the shoreline displacement and the wave features. Results denote characteristic patterns of beach response to storm events, and highlight the importance of improving beach protection in this zone, notwithstanding the many interventions experimented in the last decades. A total of 31 independent storm events were selected during the period October 2013–October 2014, and for each of them synthetic indexes based on storm duration, energy and maximum wave height were developed and estimated. It was found that the net shoreline displacements during a storm are well correlated with the total wave energy associated to the considered storm by an empirical power law equation. A sub-selection of storms in the presence of an artificial dune protecting the beach (in the winter season) was examined in detail, allowing to conclude that the adoption of this coastal defence strategy in the study area can reduce shoreline retreat during a storm. This type of intervention can sometimes contribute to prolonging overall stability not only in the replenished zone but also in downdrift areas. The implemented methodology, which confirms to be economically attractive if compared to more traditional monitoring systems, proves to be a valuable system to monitor beach erosive processes and provide detailed indications on how to better plan beach-maintenance activities. The presented methodology and the proposed results can therefore be used as a basis for improving the collaboration between coastal scientists and managers to solve beach erosion problems, in locations where data are scattered and sporadic.

Erosion and heavy mineral depletion of a placer mining beach along the south-west coast of India: part III—short- and long-term morphological changes

The Chavara coast of south-west India is well known for its rich beach placer deposits which are being commercially mined by two public sector firms. The erosion of the beach and innershelf and the depletion of heavy mineral content of this coast have attracted a lot of attention of late, and an investigation of the mechanisms that drive these changes was taken up recently. The results of the investigation are presented in three parts. This paper, which is the last in the series, presents a study of the morphological changes of the beach and innershelf and integrates the results from the other two papers with an analysis of multi-dated shoreline and bathymetry data. The analysis of the multi-dated data showed an overall retreat of the shoreline and a relative deepening of the innershelf. The shoreline retreat has been quite alarming at the mining sites with one of the mining sites showing a retreat of nearly 400 m. The sediment deposition adjacent to the breakwater (built during 2000–2007) at the northern inlet has defused to some extent the high erosion observed earlier in the northernmost sector, and the presence of well-maintained seawalls has nearly maintained the shoreline south of the mining site. The erosion of the beach is accompanied by a deepening of the innershelf which is more pronounced in the shallower portions up to 10 m depth. The observed changes in the beach–innershelf morphology are analysed with respect to the nearshore sediment transport regime and heavy mineral distribution of the coast. It is observed that the combined intake of sediments by the two firms during the past one and a half decade is much above the sustainable mining level. Another contributing factor is the 2004 tsunami which drained off a sizable quantity of innershelf sediments rich in heavy minerals to the hinterland regions. The breakwaters at the two inlets bordering this coast have virtually compartmentalized this coast from the rest of the south-west coast making it a sediment sub-cell. In addition, the breakwaters act as groins causing imbalances in the erosion/accretion pattern. The recent spate in construction of shore protection structures like groins and seawalls is another contributing factor to the morphological changes. The study points to the urgent need for regulating the mining volumes to the sustainable levels as well as controlling the introduction of hard structures without proper impact analysis.

Uncertainties in Sandy Shorelines Evolution under the Bruun Rule Assumption

In the current practice of sandy shoreline change assessments, the local sedimentary budget is evaluated using the sediment balance equation, that is, by summing the contributions of longshore and cross-shore processes. The contribution of future sea-level-rise induced by climate change is usually obtained using the Bruun rule, which assumes that the shoreline retreat is equal to the change of sea-level divided by the slope of the upper shoreface. However, it remains unsure that this approach is appropriate to account for the impacts of future sea-level rise. This is due to the lack of relevant observations to validate the Bruun rule under the expected sea-level rise rates. To address this issue, this article estimates the coastal settings and period of time under which the use of the Bruun rule could be (in)validated, in the case of wave-exposed gently-sloping sandy beaches. Using the sedimentary budgets of Stive (2004) and probabilistic sea-level rise scenarios based on IPCC, we provide shoreline change projections that account for all uncertain hydrosedimentary processes affecting idealized coasts (impacts of sea-level rise, storms and other cross-shore and longshore processes). We evaluate the relative importance of each source of uncertainties in the sediment balance equation using a global sensitivity analysis. For scenario RCP 6.0 and 8.5 and in the absence of coastal defences, the model predicts a perceivable shift toward generalized beach erosion by the middle of the 21st century. In contrast, the model predictions are unlikely to differ from the current situation in case of scenario RCP 2.6. Finally, the contribution of sea-level rise and climate change scenarios to sandy shoreline change projections uncertainties increases with time during the 21st century. Our results have three primary implications for coastal settings similar to those provided described in Stive (2004) : first, the validation of the Bruun rule will not necessarily be possible under scenario RCP 2.6. Second, even if the Bruun rule is assumed valid, the uncertainties around average values are large. Finally, despite these uncertainties, the Bruun rule predicts rapid shoreline retreat of sandy coasts during the second-half of the 21st century without strong reductions of greenhouse gas emissions.

Morpho-sedimentary features and sediment transport model of the submerged beach of the ‘Pineta della foce del Garigliano’ SCI Site (Caserta, southern Italy)

ABSTRACTIn this paper, we present the results of a detailed geomorphological and sedimentological study of a coastal sector of southern Italy, the Pineta della foce del Garigliano SCI (i.e. Site of Community Importance), which is largely affected by shoreline retreat and the degradation of dune habitat. The analysis of shoreline evolution demonstrates that severe erosion processes have occurred over the last 50 years. They caused the complete dismantling of the foredune, whereas the anomalous and advanced position of the secondary dune promoted a progressive loss of vegetation habitat of high environmental value such as juniper (Juniperus oxycedrus ssp. macrocarpa). Morpho-sedimentary data and hydrodynamic models suggest that the main climate events promoted a net longshore transport toward the South. Our analyses confirm that erosion processes are linked to natural factors but several negative human practices have contributed to the acceleration of shoreline retreat and degradation of the dune habitat. In addition to its scientific value, the map and data here presented represent an important tool for beach management purposes.

The Drowned Apostles: The Longevity of Sea Stacks over Eustatic Cycles

Bezore, R., Kennedy, D.M., and Ierodiaconou, D., 2016. The Drowned Apostles: The Longevity of Sea Stacks over Eustatic Cycles. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 592–596. Coconut Creek (Florida), ISSN 0749-0208.Cliffed rocky coasts are erosional environments, the remnants of which can be preserved as sea stacks as the shoreline retreats. These sea stacks form spectacular landscapes, such as the iconic Twelve Apostles in Victoria, Australia. However, they are ephemeral features formed on a centennial scale, continually eroding and collapsing, meaning that coasts characterised by sea stacks often have fewer features than when first described. The question arises then as to the longevity of such features and whether they can be preserved over eustatic cycles.The modern Twelve Apostles, of which 8 are still standing, are comprised of the Miocene Port Campbell Limestone and reach 45 m above sea level. Recent multibeam sonar data show five features around 6 km offshore, in 40–50 m water depth that appear to be relict sea stacks. Based on the morphology and geology of both the modern and drowned Apostles, it is inferred that the drowned and modern stacks evolved in a similar manner. While the modern sea stacks have an average height of 45 m, the drowned stacks have an average height of 4 m, suggesting a much greater age and also the possibility of multiple exposures to subaerial processes. The drowned stacks lay 655 m seaward of a drowned cliff averaging 14 m high which likely represents a former interstadial shoreline. This is much greater than the 91 m average distance between stack and cliff for the Modern Apostles, which may imply a more prolonged period of erosion along the drowned coastline.

Analysis of Sedimentation Problems at the Entrance to Mar del Plata Harbor

ABSTRACT Caceres, R.A.; Zyserman, J.A., and Perillo, G.M.E. 2016. Analysis of sedimentation problems at the entrance to Mar del Plata Harbor. Mar del Plata Harbor is an important center for the fishery industry in Argentina, and it has become an attractive site for international tourism because of the recent construction of a cruiser ship terminal. However, further development of this port halted several years ago because of the recurrent sedimentation problems in the access channel and basins. This site is characterized by advancement of the shoreline south of the harbor and a sandbank that develops across the access channel, while downdrift erosion and the associated shoreline retreat are due to partial blockage of the sediment transport caused by the harbor breakwaters. The mechanisms responsible for these sedimentation and erosion processes are examined using a coastal area morphological modeling system, MIKE 21/3 Coupled Model Flexible Mesh, developed by DHI. The morphological model was calibrated an...

Reconstructing and understanding the impacts of storms and surges, southern North Sea

AbstractCoastal barriers are ubiquitous globally and provide a vital protective role to valuable landforms, habitats and communities located to landward. They are, however, vulnerable to extreme water levels and storm wave impacts. A detailed record of sub‐annual to annual; decadal; and centennial rates of shoreline retreat in frontages characterized by both high (> 3 m) and low (< 1 m) dunes is established for a barrier island on the UK east coast. For four storms (2006–2013) we match still water levels and peak significant wave heights against shoreline change at high levels of spatial densification. The results suggest that, at least in the short‐term, shoreline retreat, of typically 5–8 m, is primarily driven by individual events, separated by varying periods of barrier stasis. Over decadal timescales, significant inter‐decadal changes can be seen in both barrier onshore retreat rates and in barrier extension rates alongshore. Whilst the alongshore variability in barrier migration seen in the short‐term remains at the decadal scale, shoreline change at the centennial stage shows little alongshore variability between a region of barrier retreat (at 1.15 m a−1) and one of barrier extension. A data‐mining approach, synchronizing all the variables that drive shoreline change (still water level, timing of high spring tides and peak significant wave heights), is an essential requirement for validating models that predict future shoreline responses under changing sea level and storminess. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Initial spreading of a mega feeder nourishment: Observations of the Sand Engine pilot project

Sand nourishments are a widely applied technique to increase beach width for recreation or coastal safety. As the size of these nourishments increases, new questions arise on the adaptation of the coastal system after such large unnatural shapes have been implemented. This paper presents the initial morphological evolution after implementation of a mega-nourishment project at the Dutch coast intended to feed the surrounding beaches. In total 21.5millionm3 dredged material was used for two shoreface nourishments and a large sandy peninsula. The Sand Engine peninsula, a highly concentrated nourishment of 17millionm3 of sand in the shape of a sandy hook and protruding 1km from shore, was measured intensively on a monthly scale in the first 18months after completion. We examine the rapid bathymetric evolution with concurrent offshore wave forcing to investigate the feeding behaviour of the nourishment to the adjacent coast. Our observations show a large shoreline retreat of O (150m) along the outer perimeter of the peninsula, with locally up to 300m retreat. The majority (72%) of the volumetric losses in sediment on the peninsula (1.8millionm3) were compensated by accretion on adjacent coastal sections and dunes, confirming the feeding property of the mega nourishment. Further analyses show that the morphological changes were most pronounced in the first 6months while the planform curvature reduced and the surf zone slope flattened to pre-nourishment values. In the following 12months the changes were more moderate. Overall, the feeding property was strongly correlated to incident wave forcing, such that months with high incoming waves resulted in more alongshore spreading. Months with small wave heights resulted in minimal change in sediment distribution alongshore and mostly cross-shore movement of sediment.

The evolution of a thermokarst-lake landscape: Late Quaternary permafrost degradation and stabilization in interior Alaska

Thermokarst processes characterize a variety of ice-rich permafrost terrains and often lead to lake formation. The long-term evolution of thermokarst landscapes and the stability and longevity of lakes depend upon climate, vegetation and ground conditions, including the volume of excess ground ice and its distribution. The current lake status of thermokarst-lake landscapes and their future trajectories under climate warming are better understood in the light of their long-term development. We studied the lake-rich southern marginal upland of the Yukon Flats (northern interior Alaska) using dated lake-sediment cores, observations of river-cut exposures, and remotely-sensed data. The region features thick (up to 40m) Quaternary deposits (mainly loess) that contain massive ground ice. Two of three studied lakes formed ~11,000–12,000calyr BP through inferred thermokarst processes, and fire may have played a role in initiating thermokarst development. From ~9000calyr BP, all lakes exhibited steady sedimentation, and pollen stratigraphies are consistent with regional patterns. The current lake expansion rates are low (0 to <7cmyr−1 shoreline retreat) compared with other regions (~30cmyr−1 or more). This thermokarst lake-rich region does not show evidence of extensive landscape lowering by lake drainage, nor of multiple lake generations within a basin. However, LiDAR images reveal linear “corrugations” (>5m amplitude), deep thermo-erosional gullies, and features resembling lake drainage channels, suggesting that highly dynamic surface processes have previously shaped the landscape. Evidently, widespread early Holocene permafrost degradation and thermokarst lake initiation were followed by lake longevity and landscape stabilization, the latter possibly related to establishment of dense forest cover. Partial or complete drainage of three lakes in 2013 reveals that there is some contemporary landscape dynamism. Holocene landscape evolution in the study area differs from that described from other thermokarst-affected regions; regional responses to future environmental change may be equally individualistic.

Drones as tools for monitoring beach topography changes in the Ligurian Sea (NW Mediterranean)

The aim of this study was to evaluate topographic changes along a stretch of coastline in the Municipality of Borghetto Santo Spirito (Region of Liguria, Italy, north-western Mediterranean) by means of a remotely piloted aircraft system coupled with structure from motion and multi-view stereo techniques. This sector was surveyed three times over 5 months in the fall–winter of 2013–2014 (1 November 2013, 4 December 2013, 17 March 2014) to obtain digital elevation models and orthophotos of the beach. Changes in beach topography associated with storm action and human activities were assessed in terms of gain/loss of sediments and shifting of the wet–dry boundary defining the shoreline. Between the first and second surveys, the study area was hit by two storms (10–11 November 2013 and 21–22 November 2013) with waves approaching from the E–NNE, causing a shoreline retreat which, in some sectors, reached 7 m. Between the second and third surveys, by contrast, four storms (25–27 December 2013, 5–6 January 2014, 17–18 January 2014 and 6–10 February 2014) with waves propagating from the SE produced a general advancement of the shoreline (up to ~5 m) by deposition of sediments along some parts of the beach. The data also reflect changes in beach topography due to human activity during the 2013 fall season, when private beach managers quarried ~178 m3 of sediments on the emerged beach near the shoreline to accumulate them landwards. The results show that drones can be used for regular beach monitoring activities, and that they can provide new insights into the processes related to natural and/or human-related topographic beach changes.

INTRODUCCIÓN AL PROBLEMA DE LA EROSIÓN LITORAL EN URABÁ (SECTOR ARBOLETES- TURBO) COSTA CARIBE COLOMBIANA

Shoreline retreat has been the net dominant historical trend along the 145 km-length littoral between Arboletes and Turbo (Southern Caribbean of Colombia). For the last four decades, there were identified in this littoral shoreline retreats of about 50-100m in several places (Uveros, Damaquiel, Zapata, Turbo) and a maximun of 1.6 km in the Punta Rey-Arboletes area, where land losses were of 4.5 k m2, at exceptional rates of 40m/year. The synthesis of the available information suggest that the general “susceptibility” to erosion between Arboletes and Turbo could be related primarily to relative sea level rise, associated to tectonic movements as well as to the effects of mud diapirism and hydroisostacy. In the more critical areas (Arboletes, Turbo), the natural erosive trends were accelerated by anthropic actions, including river diversion (Turbo), beach mining, and inadequate (or total absense) practices for controlling residual and natural waters. Up to August 2000, there were invested about $ Col 10.000 billions in 155 engineering defences (groins, sea walls and rip-rap which totalize 6.2 km of total length and a volume of materials of 37.000 m3). With few exceptions, groins have not been successful and are now part of the problem, accelerating shore erosion along the adyacent sectors. In the short term, the littoral erosion between Arboletes and Turbo is caused both by marine and by subaerial factors. It is facilitated by the poor lithological strengths of cliffs and marine terraces, mainly composed of highly fractured and weathered claystones and mudstones (with stratification and weakeness planes dipping toward sea) and nonconsolidated, easily liquefacted, fine sediments; both conditions facilitate the occurrence of rocks falls, slides and mud flows that result in high figures of cliff retreat (3 to 4m), specially during the first 15 days of the summer-winter transition (april) and in high waves periods. The case of the littoral erosion between Arboletes and Turbo illustrates well how the natural erosive trends of an area can be accelerated by human interventions and the urgent need for coordinating efforts to cope with littoral land losses.

Cenozoic rivers of northeast Africa: Evidence of trans-Saharan drainage

Radar imagery of the Sahara of Egypt and eastern Libya has dramatically changed previous concepts in regard to long term aridity of this region, revealing an extensive network of buried fluvial channels documenting the river systems that developed in the proto-Sahara during the Late Paleogene and Neogene, following the early Eocene regression of the Mediterranean Sea, and prior to Pleistocene aridification. The fall in base level associated with the end-Miocene Messinian desiccation of the Mediterranean led to deep down-cutting and extension of river systems throughout much of North Africa and South Europe. The deep, infilled canyons beneath the modern Rhone and Nile are well known, but the radar imagery identifies other deep canyons under the North African Sahara that are now invisible. In eastern Libya, drainage from Serir Tebisti (Tebisti Massif) into the Mediterranean via the Sirt Basin was channeled by two main rivers, the Sahabi in the west and the Kufra in the east; these were subsequently united in the Early Pliocene when the latter captured the former. The Gilf River was another important channel that drained north from the Uweinat-Gilf highland in southwest Egypt, in a system that may have been connected with the Kufra during the Neogene and enabling fish to move from the presently isolated Chad Basin to northern Libya and Egypt. The interplay of these north-flowing rivers and the constant northward retreat of the Mediterranean shoreline from Oligocene onward resulted in deposits of fluvial and marine sedimentation in every possible combinaton that have been described by many authors in the region between the Tebisti and the present Mediterranean shoreline. Prior to the establishment of the Nile, a southwestward drainage, the Qena River, was initiated by Oligocene uplift and lateral faulting along the Red Sea shoulder of the East African Rift. This river discharged into a distributive network known as the Radar Rivers near latitude 23° 30'N in the Southwestern Desert. It has been suggested that waters from the Qena River continued to flow southwestward to the Atlantic Ocean, in what is known as Trans African Drainage System (TADS).

Shoreline dynamics and environmental change under the modern marine transgression: St. Catherines Island, Georgia, USA

The current study evaluated shoreline dynamics and environmental change at St. Catherines Island, Georgia, with attention to the two major controls of barrier island formation and modification processes. The major controls are the rate of sea level rise for the Georgia Bight which has remained constant in twentieth and twenty-first century tide gauge data and dynamically changing rates of sediment supply based on anthropogenic modifications to land cover and fluvial systems that are reflected in sediment mass flux to the southeastern US coast. The island is dependent on the net longshore transport of sediment from the north and the closest fluvial system with appreciable discharge is the Savannah River. The evaluation of anthropogenic modifications to the rate of sediment supply within the Savannah River watershed indicates that in spite of significant changes in sediment flux rates of +300 % in 1859–1951 (the pre-dam era) and −20 % in 1951–2014 (the post-dam era), shoreline retreat on St. Catherines Island was continuous during the study period with a significant acceleration of shoreline retreat observed with spit and berm landforms during the modern or post-dam era. The apparent acceleration in the shoreline retreat rates for the spit and berm landforms is indicated as an inflection point in time-series plots of data. The change in shoreline retreat rates for the spits and berms correlates with the timing of major disruptions in the rate of sediment supply to the coast of Georgia, specifically the placement of the initial impoundments on the major rivers of the southeastern US.

Long-term responses of sandy beach crustaceans to the effects of coastal armouring after the 2010 Maule earthquake in South Central Chile

Earthquakes and tsunamis are large physical disturbances frequently striking the coast of Chile with dramatic effects on intertidal habitats. Armouring structures built as societal responses to beach erosion and shoreline retreat are also responsible of coastal squeeze and habitat loss. The ecological implications of interactions between coastal armouring and earthquakes have recently started to be studied for beach ecosystems. How long interactive impacts persist is still unclear because monitoring after disturbance generally extends for a few months. During five years after the Maule earthquake (South Central Chile, February 27th 2010) we monitored the variability in population abundances of the most common crustacean inhabitants of different beach zones (i.e. upper, medium, and lower intertidal) at two armoured (one concrete seawall and one rocky revetment) and one unarmoured sites along the sandy beach of Llico. Beach morphology changed after the earthquake-mediated uplift, restoring upper- and mid-shore armoured levels that were rapidly colonized by typical crustacean species. However, post-earthquake increasing human activities affected the colonization process of sandy beach crustaceans in front of the seawall. Lower-shore crab Emerita analoga was the less affected by armouring structures, and it was the only crustacean species present at the three sites before and after the earthquake. This study shows that field sampling carried out promptly after major disturbances, and monitoring of the affected sites long after the disturbance is gone are effective approaches to increase the knowledge on the interactive effects of large-scale natural phenomena and artificial defences on beach ecology.

Detection and analysis of historical variations in the shoreline, using digital aerial photos, satellite images, and topographic surveys DGPS: case of the Bejaia bay (East Algeria)

The Bejaia coastline is one of the most beautiful and attractive coastlines in Algeria; this is due to its landscape and ecological diversity running along it. The eastern coast of Bejaia is formed by a wide stretch of sand, occupying the bottom of the bay. Its beaches are fed by sediments carried by two main wadis that debouch into the bay: Soummam in the west and Agrioun in the east. The undertaken work in this article focuses on the study of coastal erosion, which has a significant damage on the natural patrimony. This erosion risk in perpetual evolution, by location, is dramatically affecting the beautiful sandy beaches and various infrastructures over all the coast length of the Bejaia bay. In order to map and evaluate this risk, a methodological approach has been followed. This approach is mainly based on a diachronic variation analysis in the position of the shoreline over a period of 60 years; based on a series of ortho-rectified aerial photos, satellite images, as well as DGPS topographic surveys on the ground. The results revealed significant variations in the position of the shoreline during the last 60 years, especially on both sides of Soummam and Agrioun wadis. The most observed retreat of the shoreline is in the western part of the coast, where the rate of evolution reached −7.89 m/year (beach of Sidi Ali Lebhar), whereas the shoreline retreat reached −1.75 m/year in the center of the bay and −2 m/year toward the east.

Suspended sediment transport and beach dynamics induced by monochromatic conditions, long waves and wave groups

This study presents the analysis of the water surface elevation, velocity and suspended sediment concentration measurements obtained at a large wave flume under mobile bed conditions. The wave reproduced erosive and accretive conditions, and included monochromatic, short waves perturbed with a free long wave, bichromatic and random conditions. Each tested condition started from a handmade 1/15 slope and lasted for an approximate time period of 2.4h (6 runs of the selected wave condition), to compare the different beach profile developments and, in particular, the events that control sediment transport in the swash and surf zones. All erosive tested conditions produced a shoreline retreat and a bar at the breaking area whose development in time is directly correlated to the length of the breaking area. On the other hand, not all accretive conditions present a shoreward transport, and random conditions do not seem to alter the initial profile. The processed data show the suspended sediment event control produced by the existence or absence of wave–backwash interactions in the swash zone. The existence of these interactions, and their number within the wave group, will be a key parameter in controlling the sediment stirring, water velocity magnitudes and, therefore, the suspended sediment fluxes in the inner surf and outer swash.

Anthropogenic effects on shoreface and shoreline changes: Input from a multi-method analysis, Agadir Bay, Morocco

In many situations, the links between shoreline fluctuations and larger-scale coastal change embracing the shoreface are not always well understood. In particular, meso-scale (years to decades) sand exchanges between the shoreface and the shoreline, considered as important on many wave-dominated coasts, are rather poorly understood and difficult to identify. Coastal systems where sediment transport is perturbed by engineering interventions on the shoreline and shoreface commonly provide fine examples liable to throw light on these links. This is especially so where shoreface bathymetric datasets, which are generally lacking, are collected over time, enabling more or less fine resolution of the meso-scale coastal sediment budget. Agadir Bay and the city of Agadir together form one of the two most important economic development poles on the Atlantic coast of Morocco. Using a combined methodological approach based on wave–current modelling, bathymetric chart-differencing, determination of shoreline fluctuations, and beach topographic surveying, we highlight the close links between variations in the bed of the inner shoreface and the bay shoreline involving both cross-shore and longshore sand transport pathways, sediment budget variations and new sediment cell patterns. We show that the significant changes that have affected the bay shoreline and shoreface since 1978 clearly reflect anthropogenic impacts, notably blocking of alongshore sand transport by Agadir harbour, completed in 1988, and the foundations of which lie well beyond the depth of wave closure. Construction of the harbour has led to the creation of a rapidly accreting beach against an original portion of rocky shoreline updrift and to a net sand loss exceeding 145,000m3/year between 1978 and 2012 over 8.5km2of the bay shoreface downdrift. Shoreline retreat has been further exacerbated by sand extraction from aeolian dunes and by flattening of these dunes to make space for tourist infrastructure. Digital elevation models of part of the bay beach between 2012 and 2014 confirm this on-going sand loss. These changes have involved the establishment of two divergent longshore bay sediment cells instead of the original single unidirectional cell. A prospective view of these changes suggests that perturbation of longshore drift and the on-going bay sediment budget deficit will eventual directly pose threats to the harbour access and to coastal tourism on which the economic growth of Agadir has been built.