Rocky Shore Platforms Research Articles

Numerical investigation of the refractive properties of near-horizontal rocky shore platforms and their effects on harmonic and stationary wave patterns

Near-horizontal shore platforms display highly irregular plan shapes, but little is known about the way in which these irregularities influence the significant wave height (Hŝ) on the platforms and the frequency components of the nearshore wavefield. As ocean waves share akin refractive properties to light rays, it can be assumed that, similarly to optical lenses, shore platforms can separate waves according to their frequency depending on their geometry. Thus, we use a non-linear Boussinesq wave model to conduct harmonic and bispectral mode decomposition analyses to study the control of concave and convex platform edges over wind waves (WW: 0.125–0.33 Hz), swell waves (SW: 0.05–0.125 Hz) and infragravity (IG: 0.008–0.05 Hz) waves frequencies. For breaking and non-breaking waves, increasing the platform edge concavity intensified wave divergence and subsequent attenuation of SW and IG across the outer platforms, reducing Hŝ by up to 25 %. Increasing the platform edge convexity intensified focusing and amplification of SW and WW over the outer platforms, increasing Hŝ by up to 18 % and 55 % for breaking and non-breaking waves. In the presence of breaking, IG amplification was affected by wave divergence across the inner platform, a condition determined by a critical convex curvature threshold (K=1.8) balancing wave focusing from refraction and defocusing from breaking. We find that convex curvature can determine the relative dominance of WW, SW and IG across platforms. Alongshore, coherent wave interactions governed IG stationary patterns defined by a node near the platform centreline and two antinodes on either side of concave edges. A node was generated at the platform centreline, and two antinodes were observed on either side of the convex edges for K>1.8, with the opposite pattern observed for K<1.8. Such patterns likely result in alongshore variations in wave-generated currents and erosion shaping rock coasts.

Wave runup and inundation dynamics on a perched beach

Sandy beaches perched over rocky shore platforms are common globally, yet their mixed sand and rocky morphology present challenges for quantifying and predicting wave runup and inundation. For typical linear beach profiles, simple relationships can be made between vertical runup and horizontal inundation based on beach slope. However, as topographic irregularities increase, substantial deviations from these relationships can occur. Shore platforms often experience a range of beach states, from full sand coverage (accreted) to complete shore platform erosion (exposed). As a result, existing methods for quantifying runup and inundation on purely sandy beaches are generally not directly applicable to these coastlines. To advance our understanding of runup and inundation on perched beaches, the aim of this work is three-fold: (1) to provide a method for quantifying beach slope and runup across a range of perched beach profiles, (2) to assess the relationship between vertical runup and horizontal inundation observations, and (3) to understand how different hydrodynamic mechanisms (i.e., setup, swash) contribute to runup during different beach states. To achieve this, we conducted an 8-month field study along a perched beach in southwestern Australia that experiences large seasonal variations in beach state and wave climate. A method was developed to measure runup and beach slope when only the topography shoreward of the shore platform edge was known. Using this method, an approximately linear relationship between inundation and runup was identified by incorporating beach slope. Our observations suggest that the components that dominate runup on perched beaches were primarily dependent on beach state (i.e., accreted versus exposed). Runup was dominated by swash in the infragravity band when in an accreted beach state, and setup when in an exposed beach state. These results aid our understanding of coastal processes on perched beaches, while providing new methods applicable to a range of perched beach profiles.

Anomalously large marine potholes on a submerged relict shore platform: The Eastern Cape shelf of SE Africa

Marine potholes are common features along rocky shore platforms from around the world. Their occurrence can be used as a palaeo-sea level indicator, however, little attention has been given to submerged marine potholes found on relict shore platforms below the depth limits of SCUBA (≤50 m). This paper examines (n = 238) anomalously large potholes found on a submerged aeolianite shore platform on the outer shelf of the Eastern Cape, South Africa. Using a combined ultra-high resolution seismic reflection, side scan sonar and multibeam bathymetry approach, together with remote observation vehicle dives, the pothole occurrences are mapped.Potholes are either individual or occasionally conjoined, form on sediment-free aeolianite platform and cluster mostly between water depths of 85–95 m. Their sizes are an order of magnitude larger than previously documented, and larger than their fluvial equivalents. Pothole strings are common, associated with the high points between NNW-SSE striking bedding planes of the aeolianite that crop out at the seabed. Potholes are often elongated and aligned with these bedding planes, or in a coast parallel fashion, associated with joints and micro-cliffs in the aeolianite.The potholes possibly formed during several slowstands or stillstands surrounding the Last Glacial Maximum (LGM, 25,000 to 20,000 yr BP) and the Bølling-Allerød interstadial (B-A, 14,500 yr BP). Given their sizes, we ascribe part of their development to chemical weathering of the platform, especially during the periods where sea spray could pond to form dissolution pans in the supratidal part of the aeolianite platform. A wet and warm B-A climate exacerbated chemical weathering, with karst processes initially dominant, followed by mechanical weathering with shoreline migration. Once submerged, only horizontal enlargement ensued. Geological contingencies (joints, bedding planes and platform lowpoints), modified by early weathering-dominant and later wave-dominant processes, were responsible for the evolution of the shore platform morphology.

Using UAV and Structure-From-Motion Photogrammetry for the Detection of Boulder Movement by Storms on a Rocky Shore Platform in Laghdira, Northwest Morocco

The detachment and mobilization of boulders from rocky shore platforms by waves involves complex geomorphic and hydrodynamic processes. Understanding these processes requires precise information on the rates and patterns of movement of these megaclasts scaled against the wave conditions that generate boulder mobility. Repeat photogrammetry and structure-from-motion (SfM) models commonly used in geomorphic analyses are an interesting option for monitoring boulder dynamics. In this study, we used unmanned aerial vehicle (UAV)-based digital photogrammetry and SfM differential models to identify recent boulder movements over a rocky shore platform in Laghdira, Morocco. Combining these results with data on storm occurrence in the study area allowed us to identify storm waves as the unique driver of the dislodged and mobilized boulders. The identified storm event had a significant wave height of 5.2 m. The UAV models were built from imagery captured in September and December 2019 using a DJI MAVIC PRO PLATINUM, and we used QGIS to produce 2D and 3D model outputs. The exploitation of the 2D model differentials allowed us to appreciate the response of the boulders to the storm waves and to determine platform volumetric changes and, therefore, boulder mobility. The 3D models were valuable in determining the mode of transport of the boulders. Mobility patterns included sliding, overturning with no further mobility, and rotation and saltation, as well as boulder breakup. Storm waves did not have a preferential impact on any particular boulder shape, size category, or position at the outer edge of the platform. These results highlight the utility of combining UAV surveys with identified storm events, which are much more frequent than tsunamis, in determining observed boulder initiation and mobility.

Challenges and new strategies in assessing multidecadal shore platform sandy beach evolution from aerial imagery

The world's coastlines are dominated by rocky coasts, where embayed and shore platform beaches develop. The intrinsic characteristics of these environments make them extremely vulnerable to changes in oceanographic forcing. Assessing long-term beach evolution can be a challenging task because coastline indicators were developed for wide and low-lying sandy beaches. For example, as most shore platforms beaches are limited by a cliff or structure, they have a static landward boundary (coastline), thus inhibiting the use of any coastline indicator as a proxy of beach volume changes. Also, as shoreline position is highly dependent on short-term sea-level fluctuations, the use of shoreline indicators is frequently subject to high uncertainty.This work presents an unconventional way to evaluate shore platform beach evolution by proposing a new indicator - the beach toe (BT) - to assess changes in sand retention for shore platform/perched beaches. BT position, together with a profile model, provide an effective tool to assess beach volume over time.The proposed beach indicator was tested at 12 shore platform beaches located at Cascais coast (Portugal), which can be considered illustrative of this type of beach.Results show that the new approach provides an objective indicator to assess both the beach configuration and changes in volume, being easily implemented using common vertical imagery. The comparison of Beach Toe and High-Water Line indicators with field data show a better performance of the BT indicator, supporting the use of the proposed indicator on shore platform beaches. The proposed analysis can be easily replicated to other similar rocky coasts, thus representing a much-needed advancement in the study of beaches perched on platform shores.

Rauk - A Forgotten Witness of Holocene Sea-level Change and Development of Baltic Rocky Coasts: A Pilot Geomorphological Study in Lergrav Raukar Field

Strzelecki, M.C.; Duszyński, F.; Tyszkowski, S.; Zbucki, Ł., and Kasprzak, M., 2020. Rauk - Forgotten witness of Holocene sea-level change and development of Baltic rocky coastal zone: A pilot geomorphological study in Lergrav raukar field. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 659–663. Coconut Creek (Florida), ISSN 0749-0208.The Holocene history of Baltic Sea evolution is definitely one of the most interesting and intriguing stories of environmental change in Europe. To date, the key information on changes in sea-level and shifts in the environment associated with climate fluctuations were found in coastal lakes, marshes and wide stripe of barrier-lagoon systems of southern Baltic. Only limited research was conducted along rocky coasts, so characteristic features for Baltic islands from Bornholm, through Oland, Gotland, Fårö, Hiiumaa, Saarema to thousands of isles of Åland Archipelago. Among the most thrilling natural wonders found along Baltic coasts are rauks or raukars, unique limestone stacks—often with humanlike features—that tower above rocky shore platforms in northern Gotland and on Fårö, Gotland's sister island. Raukar coasts are one of the greatest touristic attractions in Sweden and the mythical coastal landscape of northern Gotland and Fårö had inspired many artists, including Ingmar Bergman. Surprisingly, what attracts tourists and artists, have been neglected by geomorphologists and those fascinating landforms still wait for detailed explanation of their origin and preservation. This paper provides insight into pilot geomorphological study of raukar coast in Lergrav, NE Gotland. Schmidt hammer rock tests were applied to characterize the degree of weathering of rock surface of well-preserved notches. The results of notch rock surface resistance and elevation above present sea-level are discussed with recent relative sea-level curves developed in the region.

Submerged Marine Terraces Identification and an Approach for Numerical Modeling the Sequence Formation in the Bay of Biscay (Northeastern Iberian Peninsula)

Submerged sequences of marine terraces potentially provide crucial information of past sea-level positions. However, the distribution and characteristics of drowned marine terrace sequences are poorly known at a global scale. Using bathymetric data and novel mapping and modeling techniques, we studied a submerged sequence of marine terraces in the Bay of Biscay with the objective to identify the distribution and morphologies of submerged marine terraces and the timing and conditions that allowed their formation and preservation. To accomplish the objectives a high-resolution bathymetry (5 m) was analyzed using Geographic Information Systems and TerraceM®. The successive submerged terraces were identified using a Surface Classification Model, which linearly combines the slope and the roughness of the surface to extract fossil sea-cliffs and fossil rocky shore platforms. For that purpose, contour and hillshaded maps were also analyzed. Then, shoreline angles, a geomorphic marker located at the intersection between the fossil sea-cliff and platform, were mapped analyzing swath profiles perpendicular to the isobaths. Most of the submerged strandlines are irregularly preserved throughout the continental shelf. In summary, 12 submerged terraces with their shoreline angles between approximately: −13 m (T1), −30 and −32 m (T2), −34 and 41 m (T3), −44 and −47 m (T4), −49 and 53 m (T5), −55 and 58 m (T6), −59 and 62 m (T7), −65 and 67 m (T8), −68 and 70 m (T9), −74 and −77 m (T10), −83 and −86 m (T11) and −89 and 92 m (T12). Nevertheless, the ones showing the best lateral continuity and preservation in the central part of the shelf are T3, T4, T5, T7, T8, and T10. The age of the terraces has been estimated using a landscape evolution model. To simulate the formation and preservation of submerged terraces three different scenarios: (i) 20-0 ka; (ii) 128-0 ka; and (iii) 128-20 ka, were compared. The best scenario for terrace generation was between 128 and 20 Ka, where T3, T5, and T7 could have been formed.

Erosion of rocky shore platforms by block detachment from layered stratigraphy

AbstractThe majority of shore platforms form in rocks that are characterised by layered stratigraphy and pervasive jointing. Plucking of weathered, joint and bed bounded blocks is an important erosion process that existing models of platform development do not represent. Globally, measuring platform erosion rates have focused on microscale (&lt; 1 mm) surface lowering rather than mesoscale (0.1‐1 m) block detachment, yet the latter appears to dominate the morphological development of discontinuity rich platforms. Given the sporadic nature of block detachment on platforms, observations of erosion from storm event to multi‐decadal timescales (and beyond) are required to quantify shore platform erosion rates. To this end, we collected aerial photography using an unmanned aerial vehicle to produce structure‐from‐motion‐derived digital elevation models and orthophotos. These were combined with historical aerial photographs to characterise and quantify the erosion of two actively eroding stratigraphic layers on a shore platform in Glamorgan, south Wales, UK, over 78‐years. We find that volumetric erosion rates vary over two orders of magnitude (0.1‐10 m3 yr‐1) and do not scale with the length of the record. Average rates over the full 78‐year record are 2‐5 m3 yr‐1. These rates are equivalent to 1.2‐5.3 mm yr‐1 surface lowering rates, an order of magnitude faster than previously published, both at our site and around the world in similar rock types. We show that meso‐scale platform erosion via block detachment processes is a dominant erosion process on shore platforms across seasonal to multi‐decadal timescales that have been hitherto under‐investigated. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd

Bedrock hardness values and morphological zonation of a shore platform in South Africa

Shore platforms found along rock coasts worldwide are often interpreted as wave abrasion acting on uniform bedrock surfaces. Whilst this viewpoint may apply at the macroscale, there are complex feedbacks among bedrock properties (rock type, mineralogy, rock structures), weathering and erosion processes and rates, and microtopography at the meso- to microscale. In turn, these influence wave runup, backwash processes, and the extent of wave-splash and physical, chemical, and biological weathering processes that result. This study investigates the relationships among bedrock hardness properties, microtopography, and shore platform denudation processes from a site on the Indian Ocean-facing coast of South Africa. Three shore-normal transects (45–57 m in length) were surveyed across the sandstone platform using a differential global positioning system (GPS). These data were integrated with rock surface hardness measurements along the transects taken using a Proceq Equotip instrument. Results show that platform morphology and hardness values vary considerably, corresponding to the most dominant denudation processes found at different positions in the tidal frame. The lowest hardness values (most extensive rock weathering) are found on the landward third of the platform but away from the land margin. The highest and greatest variability of hardness values are found on bedrock highs immediately above the highest astronomical tide level, in the lower third of the profile. This position suggests a dominant role of wave-splash and wetting/drying. Variations in hardness values and microtopography through the transects were used to develop an evolutionary model linking rock hardness values to specific weathering and erosion processes of the rocky shore platform.

WAVE OVERWASH ON A ROCK PLATFORM: REMOTE SENSING AND PRESSURE SENSOR OBSERVATIONS

Open ocean rocky shore platforms are typically exposed to high wave energy and are often the location of recreational activities from sightseeing and walking to fishing (Kennedy et al. 2017). The exposure of these environments, combined with the use for recreation, results in a high level of risk for those who use the rock platform. In Australia, for example, 19% of coastal fatalities occur on rock coasts, most commonly when individuals fall from microtidal semi-horizontal platforms into the ocean (SLSA, 2014a,b). Managing the hazards and resultant risk on rocky shore platforms requires a different approach to that taken for sandy beaches as the sites are typically remote. Here we explore the wave overwash hazards on a remote but high visitation rocky shore platform 40 km south of Sydney, Australia.

Automated Sensing of Wave Inundation across a Rocky Shore Platform Using a Low-Cost Camera System

Rocky coastlines are frequently used for recreation, however, they are often highly exposed and hazardous environments resulting in high risk to visitors. Traditional approaches to managing human safety in coastal settings (such as the surf lifesaving clubs that have proven effective on beaches) are not necessarily transferable to rock platforms due to their often remote and fragmented distribution and the different recreational uses. As such, a different approach is required. To address this, we present a low-cost camera system to assess the wave hazard on a high-visitation rocky shore platform: the Figure Eight Pools Rock Platform, New South Wales, Australia. The camera system is shown to be highly effective and allows identification of both the distance and frequency of wave inundation on the platform using a novel pixel analysis technique. Nearshore wave height is shown to be the primary factor driving inundation frequencies along the cross-platform transect investigated with some influence from wave period. The remotely sensed camera data are used to develop a preliminary overwash hazard rating system, and analysis of the first month of data collected suggests that the platform is highly hazardous to visitors. Future work will expand this hazard rating system, developing a predictive tool that estimates the overwash hazard level based on forecast wave and tide conditions to improve visitor safety at the site.

Systematic analysis of rocky shore platform morphology at large spatial scale using LiDAR-derived digital elevation models

Much of the existing research on rocky shore platforms describes results from carefully selected field sites, or comparisons between a relatively small number of selected sites. Here we describe a method to systematically analyse rocky shore morphology over a large area using LiDAR-derived digital elevation models. The method was applied to 700km of coastline in southwest England; a region where there is considerable variation in wave climate and lithological settings, and a large alongshore variation in tidal range. Across-shore profiles were automatically extracted at 50m intervals around the coast where information was available from the Coastal Channel Observatory coastal classification. Routines were developed to automatically remove non-platform profiles. The remaining 612 shore platform profiles were then subject to automated morphometric analyses, and correlation analysis in respect to three possible environmental controls: wave height, mean spring tidal range and rock strength. As expected, considerable scatter exists in the correlation analysis because only very coarse estimates of rock strength and wave height were applied, whereas variability in factors such as these can locally be the most important control on shoreline morphology. In view of this, it is somewhat surprising that overall consistency was found between previous published findings and the results from the systematic, automated analysis of LiDAR data: platform gradient increases as rock strength and tidal range increase, but decreases as wave height increases; platform width increases as wave height and tidal range increase, but decreases as rock strength increases. Previous studies have predicted shore platform gradient using tidal range alone. A multi-regression analysis of LiDAR data confirms that tidal range is the strongest predictor, but a new multi-factor empirical model considering tidal range, wave height, and rock strength yields better predictions of shore platform gradient (root mean square error of predictions reduced by 5%). The key finding of this study is that large-scale semi-automated morphometric analyses have the potential to reveal dominant process controls in the face of small-scale local variability.

Understanding coastal processes to assist with coastal erosion management in Darwin Harbour, Northern Territory, Australia

Sand transport pathways in Darwin Harbour, Northern Territory, Australia, are being investigated to assist with coastal management. Coastal erosion, which threatens public and private infrastructure, is one of the major problems along the harbour beaches. A study of sediment transport is essential to identify the challenges encountered by the stakeholders in coastal management. Darwin Harbour, located in the tropical, cyclone prone area of Australia, was, until recently, considered a near pristine estuary. A semi-diurnal macro-tidal embayment, the tidal variation in the harbour reaches up to 8 m with a mean tidal range of 3.7 m. The beach morphology consists of sandy pocket beaches between coastal cliffs, sandbars, rocky shore platforms, tidal flats and mangrove fringes. A two-dimensional depth averaged finite-element hydrodynamic model (RMA-2), coupled with a sediment transport model (RMA-11) from Resource Modelling Associates, has been used to infer the sources and the depositional areas of sand in the harbour. Grain size distributions and geochemical analysis are also used to characterize the sand and its source(s). Initial results show that the beach sand is mostly of offshore origin with small sand input from the rivers. Potential supplementary sand sources are the eroded materials from the shore platforms and the rocky cliffs. Due to the rapid development in Darwin Harbour, this study is fundamental in understanding coastal processes to support decision making in coastal management, particularly in a macro-tidal, tropical estuary.

The extreme 2013/2014 winter storms: hydrodynamic forcing and coastal response along the southwest coast of England

AbstractThe southwest coast of England was subjected to an unusually energetic sequence of Atlantic storms during the 2013/2014 winter, with the 8‐week period from mid‐December to mid‐February representing the most energetic period since at least 1953. A regional analysis of the hydrodynamic forcing and morphological response of these storms along the SW coast of England highlighted the importance of both storm‐ and site‐specific conditions.The key factor that controls the Atlantic storm wave conditions along the south coast of southwest England is the storm track. Energetic inshore wave conditions along this coast require a relatively southward storm track which enables offshore waves to propagate up the English Channel relatively unimpeded. The timing of the storm in relation to the tidal stage is also important, and coastal impacts along the macro‐tidal southwest coast of England are maximised when the peak storm waves coincide with spring high tide. The role of storm surge is limited and rarely exceeds 1 m.The geomorphic storm response along the southwest coast of England displayed considerable spatial variability; this is mainly attributed to the embayed nature of the coastline and the associated variability in coastal orientation. On west‐facing beaches typical of the north coast, the westerly Atlantic storm waves approached the coastline shore‐parallel, and the prevailing storm response was offshore sediment transport. Many of these north coast beaches experienced extensive beach and dune erosion, and some of the beaches were completely stripped of sediment, exposing a rocky shore platform. On the south coast, the westerly Atlantic storm waves refract and diffract to become southerly inshore storm waves and for the southeast‐facing beaches this results in large incident wave angles and strong eastward littoral drift. Many south coast beaches exhibited rotation, with the western part of the beaches eroding and the eastern part accreting. © 2015 The Authors.Earth Surface Processes and Landformspublished by John Wiley &amp; Sons Ltd.

Storm Swash Deposition On An Embayed Rock Coastline: Facies, Formative Mechanisms, and Preservation

Abstract Storm swash terraces mark the most distal, landward deposition of storm wave events on high-energy rock coasts. They form a potential, but as yet unexploited, sedimentary archive of past storminess. This paper describes storm swash terraces at Morgan Bay, Eastern Cape, South Africa. They form as steeply seaward dipping (30–60°), partially vegetated terraces comprising alternating beds of organic-rich sand, mud, and gravel. The terraces are up to 5 m thick, and the basal elevations range from 0.5 m amsl in the embayment, to 6.5 m amsl on the headland margins. Regardless of location, they are always fronted by a rocky shore platform on which most storm wave energy is attenuated. Six sedimentary facies are delineated on the basis of their lithological and paleontological characteristics and the nature of each bounding surface. Deposits in the most exposed headland locations are thicker and coarser-grained than those formed in the embayment. Composition of the depositional terrace reflects the fronting shore platform, indicating a short source-to-sink transport distance. The thickest storm deposits form where the backing topography is high and accommodation is limited. Storm swash terraces have a high preservation potential with successive storms depositing material in an aggradational manner: reworked material is incorporated into subsequent depositional units. The spatial controls on facies partitioning in each terrace appear to be a function of position in the embayment, the availability of material to seaward, and the presence of accommodation space. Areas of particularly limited accommodation space produce aggradational storm sequences that form especially good archives of marine storminess.

Rock‐protecting seaweed? Experimental evidence of bioprotection in the intertidal zone

AbstractBiogeomorphological processes are an important component of dynamic intertidal systems. On rocky shores, the direct contribution of microorganisms, plants and animals to weathering and erosion is well known. There is also increasing evidence that organisms can alter rock breakdown indirectly, by moderating temperature and moisture regimes at the rock–air interface. These influences have been purported to represent mechanisms of bioprotection, by buffering microclimatic fluctuations associated with weathering processes such as wetting and drying and salt crystallization. However, virtually nothing has been done to test whether microclimatic buffering translates to differences in actual rock breakdown rates.Here we report a preliminary laboratory experiment to assess how an artificial canopy (chosen to represent seaweed) affects mechanical rock breakdown. Using a simplified and accelerated thermal regime based on field data from a rocky shore platform in southern England, UK, we find that breakdown (mineral debris release) of mudstone covered with a canopy is reduced by as much as 79% relative to bare rock after around 100 thermal cycles. Reduction in rock surface hardness (measured using an Equotip device) was also greater for bare rock (17%) compared to covered rock (10%) over this period. Measurements of salt crystal formation indicate that the mechanism driving these differences was a reduction in the frequency of crystallization events, via moisture retention and shading of the rock surface. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Sand Dynamics as a Tool for Coastal Erosion Management: A Case Study in Darwin Harbour, Northern Territory, Australia

Darwin Harbour, Northern Territory, Australia, is a semi diurnal macro-tidal embayment that is situated in a cyclone prone area. The tidal variations range up to 8 m with a mean tidal range of 3.7 m. The coastal area consists of mangrove fringes, sandy beaches, tidal flats, rocky shore platforms and coastal cliffs. The main morphological changes are movement of the sandbars and erosion of beaches and coastal cliffs. Sea level rise due to climate change and more intense cyclones and storm surges may exacerbate these processes with detrimental impacts on the coast and the adjacent city, particularly when occurring at high tide. To assist with coastal erosion management, a greater understanding of morphological changes is required. A two-dimensional depth averaged finite-element hydrodynamic model (RMA-2), coupled with a sediment transport model (RMA-11) from Resource Modelling Associates, have been used to deduce the sources and spatial patterns of sand erosion and deposition in the harbour. Geochemical analysis is also used to characterize the sand source(s). This paper presents hydrodynamic simulations focusing on culturally and recreationally significant beaches in Fannie Bay. Simulations indicate that the Cullen Bay sandbar is an indirect sand source replenishing Fannie Bay beaches. Respective geochemical results also show similar Rare Earth Element contents of the sand in the area. Considering the fast pace of development in and around Darwin Harbour, this study is essential in providing a fundamental understanding of coastal processes and to assist coastal and shoreline management in a tropical estuary.

Effect of grazing on distribution and recovery of harvested stands of Lessonia berteroana kelp in northern Chile

Understanding the ecological factors regulating exploited natural communities is important in establishing conservation and management strategies. Since the 1960s, artisanal fishermen have harvested up to 300 000 dry tons yr �1 of wild populations of Lessonia spp. kelps. Adult kelps form a key habitat on the rocky shores of Chile, and benthic grazers regulate kelp populations by grazing or bulldozing microscopic and juvenile stages. To establish the role of the grazer assemblage in the recovery of kelp stands following harvesting, we conducted a manipu- lative experiment simulating artisanal kelp extraction and then manipulating the presence of grazers. We followed community succession and spatial distribution of L. berteroana recruits in manipulated and control rocky shore platforms for ca. 12 mo. Inter-individual distances of recruits and spatial autocorrelation analyses showed that the presence of grazers determined a patchy dis- tributional pattern of L. berteroana. The aggregated spatial pattern of kelp recruits in the pres- ence of grazers was followed by numerous coalescence events between small holdfasts, but no coalescence events were observed between recruits in the grazer removal areas. Our results sug- gest that grazing and recruit coalescence play an important role in the recovery of kelp stands fol- lowing artisanal harvesting. Incorporating these processes into conservation and management strategies may bolster current strategies, which are based solely on the spatial structure of kelp stands.

Coastal towers and historical sea level change along the Salento coast (southern Apulia, Italy)

The perimeter of Salento Peninsula (southern Apulia, Italy) is studded by numerous coastal towers built in the XVI century aiming to create an “early warning system” against pirates' attacks. At present, two of them, Fiumicelli tower on the Adriatic coast and Pali tower on the Ionian one, have bases a few decimeters below mean sea level. The unusual position of both coastal towers can be explained by taking into account geomorphological features of the coastal area and the relative sea level change during the last 500 years.Geomorphological analysis reveals the occurrence directly in front of the two coastal towers of a wide rocky shore platform to about 1 m depth. If a mean sea level about 0.6–1.0 m below present position is supposed at the time of coastal tower construction, the wide shore platform would be slightly above sea level and would be able to protect the towers from sea storms. Furthermore, by comparing these data with the results of available glacio-hydro-isostatic models for southern Apulia during the late Holocene, it is possible to postulate tectonic subsidence of the Salento coastal area during the last five centuries.The collated data establish the Quaternary tectonic history of Salento Peninsula. It experienced a period of general subsidence during the Lower Pleistocene that was interrupted by rapid uplift during the Middle Pleistocene. Uplift stopped at MIS 9.3 (about 330 ka), replaced by a phase of substantial stability which lasted until the Late Pleistocene–Holocene. Finally, active tectonic subsidence has affected the coast of Salento peninsula during the last four millennia probably in response to the recent doming of the region.

Coastal boulders in Martigues, French Mediterranean: evidence for extreme storm waves during the Little Ice Age

Summary. Boulder accumulations occur along a stretch of rocky coast of about 1.5 km near the French Mediterranean city of Martigues. The boulders occur up to 100 m inland from the present shoreline and some contain marine bio-constructions that are proof of residence in a subtidal or intertidal setting. The setting, spatial distribution and morphologic characteristics of these boulders indicate that they were detached from the rocky shore platform and transported landward by high-energy waves. The size, position and distance from the shoreline of 1475 boulders were measured in order to determine their volume and mass, as well as the conditions under which they were transported landward to their present positions. The results were then statistically analyzed and confronted with hydrodynamic models commonly used to evaluate the charactestics of the transporting waves. The wave characteristics thus obtained were compared to recorded and modeled extreme waves in the region. Dating of the boulders shows age ranges that correspond to the Little Ice Age (LIA), thus suggesting a relationship between their deposition and the high storm frequency that characterized the LIA. The results also indicate little likelihood of a tsunami origin for these boulders, although there is historical evidence of tsunamis in this region. The study insists on the potential for storm