Gravel Beaches Research Articles

Performance Evaluation of XBeach for Seawater-Aquifer Interaction Simulation in Swash Zone of Gravel Beach: An Attempt to Reduce the Phase Errors

ABSTRACT The swash zone and its processes are very important in a beach system due to their significant effects on beach hydrodynamics, morphodynamics and ecosystems, including beach flows, aquifers, and sediment transport. It is important to develop efficient numerical models to evaluate hydrodynamic-morphodynamic processes, particularly in the swash zone. XBeach is a numerical model for beach simulations. Despite substantial advances, XBeach remains yet to be further developed since it is inefficient in modeling interactions between the aquifer level and seawater. This study aimed to cope with XBeach phase errors in the aquifer (seawater-aquifer interaction) estimations leading to an inaccurate simulation of hydrodynamic-morphodynamic processes in the swash zone. The XBeach process was evaluated by developing a numerical algorithm to tackle this drawback. Six experimental tests were performed using the proposed and original XBeach models. The average morphological beach change errors of the proposed and original models were 5% and 10%, respectively. The groundwater head estimation error of the original model was 8% compare with less than 3% of the proposed model. The proposed XBeach model effectively handled the phase error of the original XBeach and stimulated the hydrodynamic and morphodynamic processes in the swash zone more accurately.

Coastal Sediment Grain Size Estimates on Gravel Beaches Using Satellite Synthetic Aperture Radar (SAR)

Coastal sediment grain size is an important factor in determining coastal morphodynamics. In this study, we explore a novel approach for retrieving the median sediment grain size (D50) of gravel-dominated beaches using Synthetic Aperture Radar (SAR) spaceborne imagery. We assessed this by using thirty-six Sentinel-1 (C-band SAR) satellite images acquired in May and June 2022 and 2023, and three NovaSAR (S-band SAR) satellite images acquired in May and June 2022, for three different training sites and one test site across England (the UK). The results from the Sentinel-1 C-band data show strong positive correlations (R2≥0.75) between the D50 and the backscatter coefficients for 15/18 of the resultant models. The models were subsequently used to derive predictions of D50 for the test site, with the models which exhibited the strongest correlations resulting in Mean Absolute Errors (MAEs) in the range 2.26–5.47 mm. No correlation (R2 = 0.04) was found between the backscatter coefficients from the S-band NovaSAR data and D50. These results highlight the potential to derive near-real time estimates of coastal sediment grain size for gravel beaches to better inform coastal erosion and monitoring programs.

A Study on the Predictions of Wave Breaker Index in a Gravel Beach Using Linear Machine Learning Model

To date, numerous empirical formulas have been proposed through hydraulic model experiments to predict the wave breaker index, including wave height and depth of wave breaking, due to the inherent complexity of generation mechanisms. Unfortunately, research on the characteristics of wave breaking and the prediction of the wave breaker index for gravel beaches has been limited. This study aims to forecast the wave breaker index for gravel beaches using representative linear-based machine learning techniques known for their high predictive performance in regression or classification problems across various research fields. Initially, the applicability of existing empirical formulas for wave breaker indices to gravel seabeds was assessed. Various linear-based machine learning algorithms were then employed to build prediction models, aiming to overcome the limitations of existing empirical formulas in predicting wave breaker indices for gravel seabeds. Among the developed machine learning models, a new calculation formula for easily computable wave breaker indices based on the model was proposed, demonstrating high predictive performance for wave height and depth of wave breaking on gravel beaches. The study validated the predictive capabilities of the proposed wave breaker indices through hydraulic model experiments and compared them with existing empirical formulas. Despite its simplicity as a polynomial, the newly proposed empirical formula for wave breaking indices in this study exhibited exceptional predictive performance for gravel beaches.

Assessment of Shoreline Change from SAR Satellite Imagery in Three Tidally Controlled Coastal Environments

Coasts are continually changing and remote sensing from satellites has the potential to both map and monitor coastal change at multiple scales. Unlike optical technology, synthetic aperture radar (SAR) is uninfluenced by darkness, clouds, and rain, potentially offering a higher revision period to map shoreline position and change, but this can only be feasible if we have a better interpretation of what shorelines as extracted from SAR imagery represent on the ground. This study aims to assess the application of shorelines extracted from SAR from publicly available satellite imagery to map and capture intra-annual to inter-annual shoreline variability. This is assessed in three tidally controlled coastal study areas that represent sand and gravel beaches with different backshore environments: low-lying dunes and marsh; steep, rocky cliff; and urban environments. We have found that SAR shorelines consistently corresponded to positions above the high-water mark across all three sites. We further discuss the influence of the scene geometry, meteorological and oceanographic conditions, and backshore environment and provide a conceptual interpretation of SAR-derived shorelines. In a low-lying coastal setting, the annual change rate derived through SAR presents a high degree of alignment with the known reference values. The present study contributes to our understanding of the poorly known aspect of using shorelines derived from publicly available SAR satellite missions. It outlines a quantitative approach to automatically assess their quality with a new automatic detection method that is transferable to shoreline evolution assessments worldwide.

Phytosociological study of the vascular plant communities of the lower Neretva delta (southeastern Adriatic)

The Neretva delta (estuary) is an internationally important wetland included in the Ramsar list and is one of the Important Plant Areas (IPA) in Croatia. The paper presents data from phytosociological research collected in the lower Neretva delta in the period 2006-2021. Based on 28 phytosociological relevés, 15 associations and one subassociation were identified and classified into 13 alliances, 12 orders and 11 vegetation classes. The most valuable communities belong to halo-nitrophilous short-lived pioneer vegetation on sand and gravel beaches (Cakiletea maritimae) and salt-marsh herblands and scrub (Therosalicornietea, Salicornietea fruticosae). Anthropogenic impacts on the delta are strong, and the diversity of flora and vegetation is threatened by habitat degradation and loss, unsustainable exploitation of natural resources, and pollution.

Multi-phase simulation for understanding morphodynamics of gravel beaches

This paper presents the use of an Eulerian–Eulerian multi-phase model to examine sediment transport over a gravel beach under regular waves. A previously published full-scale experiment is reproduced numerically; the simulated wave deformation, flow velocity, pore pressure, and morphological evolution are consistent with measurements. The multi-phase model can produce berm formation for smaller waves and beach erosion for larger waves as observation in previous field studies. The sensitivity analysis suggests that the nonlinear component of drag has significant effect on the berm formation through infiltration and exfiltration. The two simulated forces applied on the sediment (drag and buoyancy) are examined. The drag is the main force directly for sediment transport; therefore the sediment transport rate has high correlation with depth-averaged flow velocity. The buoyancy that is caused by plunging flow impacting beach face can push the sediment landward in the bore front, where the flow acceleration has its peak. The flow acceleration is considered in some previous sediment transport models empirically for simulating morphological evolution of gravel beach under waves. The buoyancy links the flow acceleration and sediment transport rate in the bore front.

EFFECT OF GRAVEL PARTICLE SIZE ON THE RESHAPING OF DYNAMIC REVETMENTS

Gravel beaches are prevalent natural features on many coastlines and play a vital role in shore protection, often inspiring protection structures (Bayle et al., 2020) and nature-based solutions. Dynamic revetments are constructed features, designed to mimic gravel beaches, dissipating wave energy and preventing or limiting erosion. Compared to sand beaches, gravel revetments dissipate incident wave energy and dampen backwash intensity by percolating through the relatively large pore volume space (Komar, 2007). During storm events, gravel particles move up the beach, accreting at the crest. This predominately onshore-directed transport of gravel material differs from the storm-driven morphodynamic behaviour of sandy beaches, which tend to exhibit higher rates of offshore-directed transport (Komar, 2007). Ahrens’ Theorem continues to be used as a design volume estimator for dynamic revetments (Bayle et al., 2020). This primary objective of this study is to investigate how dynamic revetments with different D50 reshape under various wave conditions.

INVESTIGATION INTO THE MECHANISMS OF CREST GROWTH ON GRAVEL BERMS

The continual sea-level rise predicted over the next century poses a significant threat to coastal regions. Preserving the coastline will require innovative coastal protection techniques and structures. A potential defence is to introduce artificial gravel beaches (Loman et al., 2010), or dynamic revetments (Bayle et al., 2020), a gravel berm placed at the high tide berm(Bayle et al., 2020). A key feature of these types of defences is their ability to maintain the berm elevation relative to the water level. As the water level rises, the structure is overtopped by waves which drives sediment up and over the berm crest peak. This leads to elevation gain for the berm and limits the landward excursion of waves. Berm crest behaviour has only been recorded at frequencies that do not allow wave-by-wave analysis of the process and the resultant ‘snapshot’ profiles may not be representative of the true variability of gravel berms. Through application of continuous 2-D Lidar monitoring on several active gravel berms this paper addresses this gap. It presents new findings related to sediment transport at the berm crest, the morphodynamic behavior under wave attack and a conceptual model for berm state.

THE EFFECT OF ANCHORED LARGE WOODY DEBRIS ON BEACH MORPHOLOGY: A PHYSICAL MODEL STUDY

As the demand for nature-based coastal protection methods increases globally, there remains a stringent need to develop evidence-based design guidance for many of these methods and techniques. Anchored Large Woody Debris (LWD) has been used as an economical method of coastal protection for several decades and has, more recently, gained notoriety as a nature-based approach. Existing design guidance, however, is both limited and not significantly rooted in academic research. This paper presents results from the first experimental study related to coastal protection using LWD. Gravel beach response to various LWD configurations were tested at a large scale based on site characteristics and LWD design characteristics made by the authors during the previous field investigation phase of this research project. Tests were also conducted to assess experiment repeatability, sensitivity to test duration, sensitivity to wave height, wave period, and relative water level, and influence of log roughness. The LWD placement elevation relative to the still water level was found to be strongly linked to the beach morphological response and a theoretical relationship was developed between LWD elevation and sediment volume change. LWD design configurations which included LWD below the still water level, such as the Benched configuration, were found to be most effective at stabilizing the beach profile. To realize potential benefits of coastal protection using LWD, significant additional research is needed on the topic, including studies focused on how to best anchor LWD structures and a wider variety of parameters (hydrodynamic conditions and placement techniques).

Morphosedimentary characteristics and formation mechanisms of new beaches generated after the Tajogaite volcano eruption of 2021 (La Palma, Spain)

Only in very special circumstances can new beaches develop due to natural processes in areas where they did not exist before. One such circumstance is related to volcanic eruptions, when a lava flow reaches the ocean. The sudden formation of new beaches is examined in this paper. Following the Tajogaite volcano eruption in September 2021, two lava deltas formed after the arrival of several lava flows to the coast. Attached to the flanks and front of these lava deltas, thirteen gravel beaches formed in just a few months. Detailed topographic and sedimentologic information was collected both on the beaches and adjacent submarine areas. The volume of sediments accumulated on these beaches was obtained after comparison of the actual topography with a previous one, yielding a total net accumulation of about 79,000 m3 of volcanoclastic pebbles and cobbles. This material comes from two major source areas: extensive offshore deposits of volcanic clasts within the depth of closure which are moved onshore by low steepness swell waves, and rock fragments derived from erosion of the lava delta front by high energy waves. Two types of beaches were identified. Beaches that already existed prior to the eruption showed significant increases in cross-shore length and height, with a net accumulation of 67,000 m3. In contrast, the dismantling of the lava delta front led to the formation of totally new beaches in certain locations, accounting for about 12,000 m3 of sediments. Several features, such as a steep foreshore slope, a well-developed storm berm and the presence of wood debris in many of these beaches at several meters height and tens of meters inland, are examined conjointly with the sediment characteristics, showing how textural maturity can change in response to the forcing agents acting in the different parts of the beach profile. The future evolution of these beaches is also considered. This study can enrich the knowledge regarding how new gravel beaches can form in volcanic settings.

Microhabitat partitioning between sympatric intertidal fish species highlights the importance of sediment composition in gravel beach conservation.

Gravel beaches in the Mediterranean ecoregion represent an economically important and unique habitat type. Yet, burgeoning tourism, intensive coastal development and artificial nourishment of beaches may jeopardize their ecological communities. To date, species that reside on gravel beaches and the consequences of beach alterations are poorly understood, which hampers the development of a sustainable coastal tourism industry along the region's shorelines. Using a simple collection method based on dredging buckets through the intertidal section of beaches, we quantified the microhabitat association of two sympatric clingfish species in the genus Gouania at seven natural and an artificial gravel beach based on sediment characteristics. We hypothesized that slender (G. pigra) and stout (G. adriatica) morphotypes would partition interstitial niche space based on sediment size, which may affect the vulnerability of the species to changes in gravel beach composition due to coastal development. We detected substantial differences in gravel composition within and among the sampled beaches which suggests scope for microhabitat partitioning in Gouania. Indeed, we found significant relationships between species identity and the presence/absence and abundance of individuals in hauls based on their positioning on PC1. Our results suggest that modifications of gravel beaches through coastal development, including beach nourishment, intensifying coastal erosion, or artificial beach creation, may have detrimental consequences for the two species if sediment types or sizes are altered. We posit that, given the simplicity and efficacy of our sampling method and the sensitivity of Gouania species to prevailing gravel composition, the genus could serve as an important indicator for gravel beach management in the Mediterranean ecoregion.

Beach-gravel armouring response to sea level rise: Case studies from Patagonia and Tierra del Fuego

High-latitude coasts are composed dominantly by coarse-grained sediments derived from Pleistocene glaciations. These inherited composition led to gravel armouring either of beaches or spits. The expectance for the anthropogenic-induced sea-level rise will be therefore attenuated due to this resistance of gravel beaches to migrate onshore. Several examples from the Patagonian and Fuegian coasts are described and analysed. Armouring, migration and cannibalisation of gravel spits should be considered in order to forecast the evolution of these coasts in relation to sea-level rise.

GraSSAMS: A new instrument designed for the determination of grain size and shape of sand-gravel-sized sediment

Natural coasts define the interface between land and ocean and constitute a natural barrier to protect humans and infrastructure from waves and storm surges. In the context of global sea level rise and coastal erosion, beaches, especially sand-gravel beaches, can achieve better marine energy reduction at the expense of a smaller spatial buffer zone (i.e., the lateral range of the coastal zone), meaning that nature-based coastal reconstruction and protection are becoming increasingly important. The need for beach replenishment and coastal protection necessitates fast, accurate, portable, and wide-range gravel grain size measurement technology to provide technical support and data for the designing and monitoring of artificial gravel beaches. However, existing grain size analysis methods for gravel-sized sediment are either inefficient (such as sieve analysis) or have a limited range of sizes (such as dynamic image analyzers, up to −3 φ), or have difficulty in eliminating errors caused by gravel overlap (e.g., remote-sensing in situ image analysis). In this study, a Gravel Grain Size and Shape Automatic Measurement System (GraSSAMS) was designed to analyze grain size and shape parameters of gravel-sized sediments. The mean diameters of three types of coins and some Zirconia balls measured using this method were compared with those measured by a vernier caliper. The results show that the GraSSAMS method has good accuracy and yields an acceptable relative error. The grain sizes of 67 field gravel samples were analyzed using both the GraSSAMS method and sieve analysis. The irreducible random error between the two methods is 1.41 φ. In addition, a Camsizer XT and GraSSAMS were used to measure samples with a grain size range of 1∼8 mm (0 φ ∼ -3 φ), respectively. The results of the two methods show good correspondence, with an irreducible random error of 0.31 φ. GraSSAMS was also tested in the field on several gravel beaches. Results revealed that GraSSAMS could complete the surface gravel measurement of a 0.5 m × 0.5 m quadrat within 8 min, and the analysis results avoided the fabric error involved in remote-sensing (photographic) measurement. Based on these various results, we propose that GraSSAMS can be used to analyze the grain size and shape of sand-gravel sediments in the size range of 1∼330 mm (0 φ ∼ -8.37 φ) in a laboratory as well as in the field. Compared with traditional methods, GraSSAM has the advantages of accuracy, high efficiency, and portability and can be used as an alternative for grain size analysis of gravel sediments, with broad application prospects to gravel beaches and fluvial deposits worldwide.

Wind–Wave Conditions and Change in Coastal Landforms at the Beach–Dune Barrier of Cesine Lagoon (South Italy)

Several coastal barriers experienced significant erosion and change in shape throughout the Mediterranean coasts over the past decades, and the issue has become of increasing concern for scientists and policymakers. With reference to a case study and by meteorological and geomorphological investigations, this note aims to define the wind–wave conditions, infer the net longshore transport, and detect the geomorphological processes that shape the landforms of the Cesine Lagoon barrier (South Italy). Despite the importance of the site in coastal defense and environmental conservation, there are still no specific studies. A challenge for this research was to obtain significant results from publicly available sources and simple and inexpensive methods. Geomorphological changes, such as the retreat of dune toes, accretion of washover fans, and formation of gravel beaches, are related to the analyzed wind–wave conditions. The net longshore transport is found in accordance with the direction of the more intense winds. The role of extreme events in the shaping of coastal landforms is yet to be established, even if they greatly increase the vulnerability to flooding of the study area. The results achieved so far are starting points for further data collection and analysis in the perspective of assessing the impact of climate changes and the threatening hazards on the lagoon barrier.

Early Pleistocene River-Fed Paleocoast in Western Umbria (Central Italy): Facies Analysis and Coastal Models

Pliocene (?)—early Pleistocene shallow marine deposits, varying from gravel to sand to clay, characterize the southernmost sector of the Valdichiana Basin, between Orte and Città della Pieve, across Tuscany, Umbria and Latium (Central Italy). Facies associations, referring to the evolution of a river-fed coast, with a sensible facies heteropy, and a sub-environment articulation, both across and alongshore, have been recently described. Although the main part of the territory responds to a wave-dominated coastal model, a clear fluvial sediment origin and the presence of localized river mouths have also been documented. Nearshore is mainly represented by interbedded sand and gravel beachface to upper shoreface deposits, in which both a mouth bar organization and a lateral distribution of gravel beaches are recognizable. Sediment origins largely depend ondebris flow processes, related to small alluvial fans/fan deltas. In constrained areas, debris flow and current continental deposits occur, referring to coalescent alluvial fans, organized as a smoothly seaward-dipping piedmont band, drained by shallow braided channels. This roughly organized fluvial system feeds a coastal area, with a fandeltabuild-up. The as-described fan delta and beach systems are characterized by a smooth seaward morphology, according to models resembling, on a coast-transverse profile, the shelf-type fan delta. Although the proposed models differ from each other’s, with respect tothe shelf-type one, this is mainly on a lateral facies distribution.

Formation of crevasse‐squeeze ridges at Trygghamna, Svalbard

AbstractCrevasse‐squeeze ridges (CSRs) are landforms that have been unequivocally linked to surge‐type glaciers. The formation of CSRs has been discussed since they were first defined in the mid‐1980s. Here, we describe geometric CSR networks from the terrestrial glacier forefields of two glaciers in Trygghamna, Western Svalbard. No glacier surges have been observed in Trygghamna; however, the presence of the CSRs signifies past surge activity. Detailed geomorphological maps were constructed, and the spatial context of these landforms described. Cross‐sections of several CSRs highlight ridge architecture, structure and relationships to surrounding landforms and sediments. Most CSRs are symmetrical in cross‐profile, orientated perpendicular or oblique to the ice‐flow direction. Like previous investigations, we observe these ridge networks on top of till and flutes. Additionally, we, for the first time, document CSRs deposited directly on non‐glaciogenic subsurfaces, namely, beach gravels and bedrock. Our findings confirm previous CSR formation theories; basal sediments are squeezed into bottom‐up crevasses during surges, which are subsequently transported englacially until surge termination and are finally released by melt out from stagnant ice. Consequently, a network of CSRs is the product of a significant reorganisation and down‐glacier transport of basal sediment, exemplifying how single surges are agents of glacial sediment redistribution. These formation processes are illustrated in a refined schematic model. The results further contemporary understanding of CSRs in terrestrial surge‐type glacier settings and may also apply to landforms and sediments in certain marine settings and palaeoglacial environments.

Coastal survey data for Perranporth Beach and Start Bay in southwest England (2006–2021)

Records of beach morphologic change and concurrent hydrodynamic forcing are needed to understand how coastlines in different environments change over time. This submission contains data for the period 2006 to 2021, for two contrasting macrotidal environments in southwest England: (i) cross-shore dominated, dissipative, sandy Perranporth Beach, Cornwall; and (ii) longshore-dominated, reflective gravel beaches within Start Bay, Devon. Data comprise monthly to annual beach profile surveys, annual merged topo-bathymetries, in addition to observed and numerically modelled wave and water levels. These data provide a valuable resource for modelling the behaviour of coastal types not covered by other currently available datasets.

Analiza dohranjivanja i nasipavanja plaža u Hrvatskoj

Beaches on the eastern Adriatic coast are the basis of Croatia’s tourism offering while also being under pressure from climate change. Data is necessary to manage beaches effectively, but data is lacking as well as the long awaited national strategy for coastal management. This paper collected data about beaches from regional documents, nourishment data was obtained by survey from local municipalities while beach construction data was obtained from aerial photogrammetry. Croatia has more than 1904 small gravel beaches and nourishment is performed in small increments beinnaly, while more than 27 % of existing beach area has been constructed all primarily for the needs of beaches, marines and tourism.

Grain-scale investigation of swash zone sediment transport on a gravel beach using DEM-MPS coupled scheme

ABSTRACT Sediment transport in the swash zone directly affects beach changes such as shoreline recession; thus, detailed understandings of sediment transport mechanisms are necessary to accurately estimate the short-time scales sediment transport rate. However, these detailed mechanisms under runup waves have not been elucidated because of the complex solid-gas-liquid multiphase turbulence flow. In this study, we attempt to numerically investigate the sediment grain-scale mechanism to overcome the shortcomings of experimental measurements and the free surface treatment in many numerical simulations. The gravel transport process on a sloped beach under regular waves was simulated using a 2D coupled model of the discrete element method (DEM) and a modified moving particle semi-implicit (MPS) method; a sub-model was built into the DEM-MPS model to improve fluid volume conservation. After validating the simulated performance by comparing it to a previous experiment, the gravel motions were investigated for turbulence and inner beach structure. The Shields number, estimated using the drag force distribution, revealed that significant turbulence contributed to onshore gravel transport near the rundown limit. The inter-gravel contact structure inside the beach explained the decrease in offshore sediment transport during backwash as increased resistance to gravel motions resulting from beach compaction.

Sedimentary Coastal Cliff Erosion in Greenland

AbstractClimate change will increase the duration of annual sea‐ice‐free periods and shift precipitation patterns across the Arctic. Those factors are likely to increase erosion rates along its coasts. Large parts of the Arctic coast consist of hard rock. However, glacial, deltaic, and coastal sedimentary deposits occur in deglaciated areas and isostatic uplift following glaciations has created beach ridge plains and pocket beaches with coarse soft‐sediment cliffs. Hitherto, very little was known about the spatial distribution, erosion rates, and morphodynamics of soft sediment cliffs along the coast of Greenland. Here, we investigate a 3‐km sedimentary cliff section on the south coast of Qeqertarsuaq (Disko Island). We measured 2D cliff top erosion over 50 years between 1964 and 2014 as well as 3D cliff profile change over 2 years between 2019 and 2021. Morphometric indices of the gravel beach and cliff were calculated based on a series of cross‐shore elevation profiles. Wave run‐up at the beach fronting the cliff was modeled with XBeach‐G for a series of storm events under present day sea‐ice conditions and for a reduced sea‐ice scenario. Cliff top erosion rates varied along the cliff with maximum rates of 0.3 m y−1. The investigated coastal cliff erodes by two coupled processes: (a) precipitation‐driven surface runoff downslope the cliff and (b) wave‐driven erosion at the cliff toe. In a continuously warming climate, this study shows that erosion of soft coastal cliffs in Greenland thus can accelerate due to increased storminess and prolonging open water periods.