Article

High resolution mapping and analysis of shore platform morphology in Galicia, northwestern Spain

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Abstract

Most research on shore platforms has been at fairly large spatial scales over distances ranging from hundreds to thousands of metres. Other work at much smaller scales, ranging from one to several decimetres, often corresponds to the dimensions of micro-erosion meter stations. Few studies have been concerned with platform morphology in which the basic data are acquired at intermediate or meso-scales ranging up to a few metres. This is due, in part, to terrestrial surveying at meso-scales being time-consuming while aerial surveys using LiDAR are expensive. A meso-scale study was made on three shore platforms in western Galicia, northwestern Spain using data from an Unmanned Aerial Vehicle (UAV) to produce high resolution Digital Surface Models (DSMs) and to calculate local surface elevation, roughness, slope, and joint density at a 0.5 m pixel scale, and joint orientation and length. This was supplemented by Equotip field measurements of rock hardness. A granitic platform was higher and had rougher surfaces and steeper slopes than two platforms in schist. The relationship between platform morphology and rock hardness and joint density was complex, however, reflecting in part the role of schistosity in accounting for the formation of low, regular platforms in hard schists with low joint density. The study suggested that while tidal range, inheritance, and other environmental and evolutionary factors can be dominant in determining platform morphology at the macro- or regional scale, geological, and particularly structural, factors become increasingly important in Galicia as the scale diminishes, and they are generally dominant at the local or meso-scale.

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... Recent work has examined these mechanisms in the marine environment but not specifically on beachrock (Kline et al., 2014;Herterich et al., 2018). Other work has focussed on geological properties of shore platforms and ways in which geological information can improve geomorphological studies of rocky coastlines, especially in terms of rock strength/hardness and fracture patterns (Thornton and Stephenson, 2006;Dickson et al., 2004;Naylor and Stephenson, 2010;Gómez-Pazo et al., 2021). ...
... surface features and architecture of the outcrop, and this is compared to quantitative fracture and structural analysis. For example, the lengths of fractures (persistence) and density (fractures per m 2 ) help determine the extent to which wave quarrying, block dislodgement and weathering can be directed into the rock mass of the platform (Trenhaile, 1972;Buchanan et al., 2020) and thus meso-scale geomorphological aspects (platform elevation, slope, terrain ruggedness) may, if only partly, be geologically controlled (Naylor and Stephenson, 2010;Gómez-Pazo et al., 2021). The quantification of such at Mission Rocks may apply to other parts of the world where beachrock is exposed to subaerial conditions like those of the Mission Rocks coastline. ...
... This result is somewhat counterintuitive, given that one would expect a highly fractured rock mass to have its elevation and slope reduced over time (Trenhaile, 2000). However, a similar result was found recently in a shore platform comprising granites, in which surfaces with the highest elevations and slopes also contained the highest fracture density (Gómez-Pazo et al., 2021). This suggests that the inherent structure of the rock dictates, along with the distribution of erosive forces, the way in which the elevation and slope of the platform evolves. ...
Article
The intertidal zone and raised shore (supratidal) platform at Mission Rocks, northern KwaZulu-Natal coast, South Africa, consists of fracture-bounded Holocene beachrock units, and megagravel (boulder) deposits. This area is microtidal and wave-dominated, with swells approaching from the southeast. The area is also influenced by storm wave events. The seaward edge of the intertidal platform is undercut, leading to the formation of overhangs, which are modelled here as cantilevered beams. This analysis shows that beam volume/weight does not control failure because relatively short, thick beams (l:h < 2) are less susceptible to failure, and the beam length-to-thickness ratio is important. This accords with tensile strength testing which demonstrates that the bedding planes are weak and that saturated beachrock (i.e., at the platform edge) is ~25% weaker than unsaturated rock. Fracture, block and boulder analyses of seven coastal sections, and their geomorphological features (elevation, slope, terrain ruggedness), reveal three key relationships: (1) The number of fracture intersections and fracture density are higher in the raised shore platforms than in the intertidal, which is relatively unfractured, lower in elevation and less rugged than the raised shore platform. These parameters correlate positively with the number of fracture-bounded blocks. (2) Fracture-bounded blocks on the raised shore platform (n = 145) have a mean shape ratio of 2.25 and are 10–20 m² in size, while the disarticulated boulders (n = 172) have a mean shape ratio of 1.3–1.5 and are 2–3 m² in size. It is considered that the reduction in shape and size of the boulders is the result of fracture-bounded blocks being quarried and transported from the raised shore platform, and not from the intertidal platform, during high-energy storm events. (3) Coast-perpendicular fractures in the intertidal platform deepen and widen towards the sea, forming gullies in which wave run-up and retreat was observed. A relatively good negative correlation (r² = 0.7) was found between the number of these gullies and values of maximum terrain ruggedness which suggests that platform lowering occurs when an appropriate fracture network (relatively dense and suitably orientated) receives unimpeded swells and waves. Overall, we find that the evolution of the beachrock platform in terms of ruggedness and downwearing is influenced, at least in part, by the fracture networks present, most notably in terms of their orientation, density, persistence and intersection geometry.
... Asturias and the north-facing coast of Galicia, is broadly E-W oriented and the western part, i.e. the western coast of Galicia and the coast of Portugal, is broadly N-S oriented. The area is very mountainous, and the coast essentially consists of rock-shore platforms and cliffs fronted by beaches of different dimensions (Chao et al., 2003;Gómez-Pazo et al., 2021). The coastline is extremely irregular especially on the western part where numerous "Rias", i.e. long, drowned valleys, are observed while the coast of Portugal is broadly rectilinear and homogenous. ...
... En canto á súa formación, varía enormemente en función da localización, podéndose atopar plataformas formadas sobre xistos ou granitos en distintos estados evolutivos. Isto vén marcado polo tempo que a superficie estivo exposta aos axentes erosivos, así como pola súa estrutura mineralóxica e o patrón de fracturación a distintas escalas (Gómez-Pazo et al., 2021a). ...
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Galicia is a region marked by its relationship with the sea. Along its more than 2,000 km of coastline, many coastal typologies give it incredible biodiversity. In this work, we try to synthesize the existing knowledge about the Galician coasts with a vision at multiple scales, from the general one, where the coast is divided into three categories: sedimentary, rocky, and artificial, to another more specific one, where different types are defined in each area according to its origin and evolution. This analysis allowed us to show the great importance of rocky coasts (65 %) and that within them, in the boulder beaches, we can differentiate up to 5 different categories depending on their design and the disposition of the material. It should also be pointed out how the changes in the occupation of the territory in recent decades are marking its possible evolution, as well as the risks for the population and the infrastructures in the context of global change.
... En las últimas décadas existe un gran número de ejemplos de uso de las nuevas tecnologías para estudios costeros, a partir de imágenes de satélite (Naylor et al., 2010), de datos de dron Boletín de la Asociación de Geógrafos Españoles, (96) (Hoffmeister et al., 2020;Long et al., 2016;Pérez-Alberti & Trenhaile, 2015), con el uso de dispositivos de medición en el terreno (Aoki & Matsukura, 2007;Feal-Pérez & Blanco-Chao, 2013;Viles et al., 2011) o mediante la introducción de sensores (Gómez-Pazo et al., 2021a, 2021bHastewell et al., 2020;Hastewell et al., 2019;Payo et al., 2020;Stephenson & Finlayson, 2009). ...
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Coastal studies have a great relevance historically. This increase in the last with the new technologies and the possible global change impact. In this context, open-source tools represent a key element in these investigations. The present project analyzes the advantages and problems of open-source tools for estimate coastal variations and volumetric changes. For this purpose, a small sector of the Galician coast was used as an example. The open-source approaches have produced similar results to those obtained with proprietary software (ratios ≥ 0.97). In some cases, these methods improve processing times and allowing to users better possibilities for customization and decision-making.
... This work aims to analyze this volcanic rocky coast setting at the island scale and to assess the constructional and erosive degradational effects on the coast at the scale of one volcanic edifice. The spatial scale ranges from hundreds of meters to kilometers, so it can be considered a macro-scale, according to Gómez-Pazo et al. (2021). We analyze El Hierro Island's rocky coast and the Montaña del Tesoro eruption case study, a Holocene Strombolian basaltic volcanic event that produced a scoria cone, pyroclastic fall deposits, and a lava flow that reached the coastline forming a delta. ...
Article
Marine and subaerial erosion of volcanic ocean islands form coastal cliffs and shore platforms, particularly during stable sea levels. Posterosional lava flows can spill over these coastal cliffs and fill the platforms, leading to the progradation of lava deltas. This work aims to analyze this volcanic rocky coast setting at the island scale and to assess the volcanic constructional and erosive degradational effects on the coast at the scale of one volcanic edifice. El Hierro Island, Canary Islands, exemplifies a rocky coast with an active sea-cliff profile, reflecting its early evolutionary stage as a young ocean volcanic island with no fringing reef. The occurrence of a contemporary insular shelf formed during the Holocene sea-level highstand (<7 ka) allows constraining the ages of those eruptions forming lava deltas affecting this geomorphological landform. A detailed bathymetry around the island allowed us to distinguish 17 eruptions fulfilling this criterion. The Montaña del Tesoro, which occurred about 1050 years BP, is one of these eruptions and was selected as a case study for morphometric modeling integrating subaerial and submarine data at the scale of a volcanic edifice. This eruption was a Strombolian basaltic volcanic event that produced a scoria cone, pyroclastic fall deposits, and lava flows that reached the ocean in the eastern rift zone of El Hierro island. We combine field-based observations with topographic and bathymetric data analysis to reconstruct the pre- and post-eruption Digital Elevation Models (DEMs) and, comparing with present-day DEM, to analyze morphometrically the influence of volcanism on the coastal landscape's development. The resulting landform complexity required the discretization of the lava field according to the coastline evolution and lava front sectors, and the subaerial or submarine lava placement. The pyroclastic materials' total erupted bulk volume (12,829,578 m³) corresponds to a volcanic eruption index (VEI) of 3. This event was primarily effusive. From a dense rock equivalent (DRE) volume of 25,615,424 m³, 87 % flowed as lava, 10 % formed the cinder cone, and 3 % the tephra fall deposits. We quantitatively demonstrate that dominant degradation occurs in the lava field, mainly disturbed by marine erosion. Marine erosion removed 9 % of the erupted volume of lava flows against 1 % by fluvial erosion. This work provides methods and results of great interest with different implications in oceanic volcanic islands, among which we can mention coastal planning (e.g., rock coast evolution) and volcanic risk assessment (e.g., the importance of Holocene sea-level rise on the development of shore platforms facilitating the progradation of lava deltas).
... Parece clara la presencia de dos fases en la acumulación de bloques, una primera vinculada al período Eemiense (Pérez-Alberti et al., 2018;Gómez-Pazo et al., 2021), que se caracteriza por bloques de mayor tamaño y en posiciones superiores, mientras que los que se mantienen activos en la actualidad aparecen a elevaciones menores y tienen un menor tamaño. ...
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Esta investigación analiza la importancia del control estructural en la dinámica de las costas rocosas a partir de la fotointerpretación y la elaboración de campañas de trabajo de campo en las que se midieron megabloques y posibles sectores fuente. Esto permitió diferenciar dos fases de acumulación, una a unos 3 m sobre el nivel del mar actual, y otra más reciente emplazada en el intermareal. En ambos casos el patrón de fracturación dirigió en avance de la meteorización favoreciendo el arranque de los clastos, que presentan tamaños distintos, siendo mayores los más antiguos, que según las dataciones de los depósitos sedimentarios cercanos se podrían situar en el Eemiense.
... En este entorno se ha comprobado la importancia que tienen los movimientos en masa y el acusado retroceso costero registrado, que se ha acelerado en la última década.En el Bloque B se estudiaron las plataformas costeras gallegas y sus características. Para esto se optó por la selección de tres zonas con distintas litologías, dos desarrolladas sobre esquistos y una con mezcla de esquistos y granitos(Gómez-Pazo et al., 2021a). Este análisis se realizó a partir de vuelos de drones, de la generación de los principales parámetros geomorfológicos y de la realización de varias campañas de trabajo de campo en las que se registró la resistencia de las rocas con un durómetro. ...
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Resumen de la tesis doctoral de Alejandro Gómez Pazo, presentada en la Universidad de Santiago de Compostela con el título "Aplicación de novas tecnoloxías no estudo da costa de Galicia dirixidas a unha nova xestión no contexto do cambio global". Publicado en la sección de tesis doctorales del Boletín de la Asociación de Geógrafos Españoles
... En relación con estes sectores, a xistosidade está mellor desenvolvida no caso de Sanxenxo, pero máis importante ca isto son os seus planos de estratificación paralelos á superficie da plataforma. Isto difire do que acontece en Caamaño onde planos son verticais ou forman ángulos moi altos coa superficie(Gómez-Pazo et al., 2021a). A erosión é máis uniforme que a que se produce ao longo dos planos de fracturación, especialmente cando a estratificación é subparalela á superficie. ...
Thesis
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The aim of this research is to deepen the knowledge of rocky coasts in general, and the Galician coasts in particular. This project carries out an analysis in different scales using the new technologies as the base to improve the coastal knowledge. Devices such as the Equotip are used at the microscale, in the mesoscale this project used RFID (Radio Frequency IDentification) sensors to analyze the displacements in boulder beaches, and lastly, UAV flights data are used in the macroscale to understand the behavior of different coastal typologies, as boulder beaches or cliff areas. Based on the obtained results and from the literature review, we proceed to analyze the risks associated with each type of coast, with particular emphasis in their vulnerability in the global change context. Thus, we developed the necessary recommendations for the existing risks, to improve the coastal management, reviewing the coastal management plans evolution and their possible future.
... En las lagunas y playas de bloques que tienen una extensión menor se usó la escala 1:500. Para la cartografía de las plataformas costeras, de pequeña extensión, se han usado ortofotografías y modelos digitales elaborados mediante el uso de UAVs (Pérez Alberti y Trenhaile, 2015;Gómez Pazo et al., 2021). Por último, con el objeto de poder hacer visibles los diferentes tipos de costa cartografiados en las figuras se ha creado una malla con cuadrículas de 10 km de lado. ...
Article
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There are several coastal classifications. Most of them have been elaborated worldwide using tectonic, climatic, topographic, or oceanographic criteria. Other classifications have been generated on a larger scale and focused on classifying the coastal forms, as cliffs, beaches, estuaries, lagoons, or dune complexes in different places. This project analyzes the types of coastlines, understanding as such each sector that presents certain topographic conditionsmarked by the elevation and slope, and that wasmodeled on a concrete type of rock in a specific climatic and marine environment. This paper describes a methodological approach for a detailed scale classification. This approach based on the delimitation of the different coastal systems, exemplified in cliffs and boulder beaches, sandy beaches, and dunes. In this case the shore platforms, marshes and lagoons have not been considered for the technical problems derived from the LiDAR data source, from which the 2 m spatial resolution digital terrain models (DTM) are derived. The first step in the classification was a manual delimitation combining DTMs and orthophotographs. Subsequently, other typification has been carried out through the automatic creation of Coastal Topographic Units (CTU). This index is the combination of two variables: coastal elevation and slope. The possible integration of others, such as orientation or lithology, is possible, but generate a very high number of units and make it difficult to interpret. For this reason, this study did not consider more variables. In this project 30 CTUs was generated, and then selecting only those that appear in the cliffs, boulder beaches, sandy beaches, and coastal dunes sectors. The possibility of viewing one or several CTUs in any sector of the coast allows to know more accurately the conditions of each sector and these categories could be improve the coastal management plans. PAPER ACCEPTED. PREPRINT VERSION
... Las plataformas costeras son formas típicamente modeladas en un ambiente marino (Fig. 5). Se pueden citar las investigaciones de TRENHAILE et al. (1999), BLANCO CHAO et al. (2003) o GÓMEZ PAZO et al. (2021) en las que se aborda sus principales características lo que ratifica, por una parte, que en las rías existen formas claramente marinas y por otra, como ya decían los ya citados anteriormente Pannekoek , Vanney , García-Gil et al. o Martínez-Carreño et al., que las rías serían mucho más antiguas de lo que suponían los primeros autores. ...
Chapter
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Rias are elongated inlets, arms of the sea, that penetrate in the continental zone. These forms are understood as sea flooded river valleys since the German researcher von Richthofen introduced the term into the scientific literature in 1886. Many authors have dealt with this rocky coast macroform, and most of them highlight their fluvial origin. Nevertheless, a detailed study of the different Galician rías shows the lack of fluvial deposits or forms inside them. This fact force to reconsider the contribution of fluvial and marine factors in shaping the coast, as well as the age of these forms. With regard to this last topic, many studies show the relevance of the falling sea level in rias formation during the last glacial period, but these approaches does not take into account that inside of rías there are Eemian interglacial dated forms and deposits. A careful revision on the rías’literature and the shaping processes and agents (i.e., lithology, joint degree, or the existing of forms and deposits) lead to the conclusion that rias are polygenic forms where the tectonic structure define its morphological pattern. The fluvial dynamics, initiated in the Cenozoic, triggered the valleys incision; and the marine processes were the main agent to shaping the actual character and form of this rock coast macroforms; therefore the fluvial contribution should be understood as a secondary process.
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The inventory and categorization of an extensive coastal boulder assemblage originating from storm wave transport on the coastline of Fenoliga Island (Northern Adriatic Sea, southern Istria, Croatia) are presented and discussed herein. The study adopted the use of a commercial Uncrewed Aerial Vehicle (UAV) and Structure from Motion-MultiView Stereo (SfM-MVS) photogrammetry for the construction of a 3D model of the island. A Digital Elevation Model (DEM) and an orthomosaic were produced and employed for the mapping of the boulder assemblage in a GIS. In total, 592 boulders were identified and mapped. Using SfM-MVS-derived products allowed for the identified boulders to be categorized based on size classification. Amassed data relating to the boulder characteristics was inserted and stored in a GIS, including the results of a comparative assessment with historical Google Earth imagery which enabled the ‘quantification of boulder transport over a 9-year timeframe’. Field evidence indicates that boulders were created in-situ via the quarrying of bedrock strata by breaking waves causing increased water pressure within preexisting surfaces of weakness such as bedding planes and sub-vertical fractures. Once detached, the boulders were transported and deposited during storm wave events. Repeated storm events can further displace previously detached clasts.
Chapter
In the preceding chapter, the lithological, tectonic, orographic and erosive factors determining the general forms of relief and landscape were described, in particular pointing out the morphology of inland Galicia. This chapter refers to the main geoforms of the Galician shoreline. The Galician coast is uniquely sinuous and contrasted. The genesis and main existing coastal landforms are indicated. Continental and marine geological action have been a determinant of its morphological and landscape conformation. Structural, lithological, tectonic and erosive factors shaped the current profile of the coastline. Finally, a global reflection on the valuation and sustainability of geological heritage is added.KeywordsGalician coastRíasCoastal morphologyGeological heritage
Chapter
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The 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 (< 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.
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Rock platforms are dangerous environments commonly subject to high wave energy on the open coast. Platform morphology is central to understanding what makes one stretch of coastline more hazardous than another, and it can be used to create site-specific morphological exposure hazard indices to assess the relative risk of being washed into the sea, assisting coastal managers in an effort to reduce the number of injuries and drowning incidents. This paper describes the use of an unmanned aerial vehicle (UAV) to derive morphological parameters for two data-poor rock platforms along the Illawarra coast of southern New South Wales, to fill the gap using an easily replicable site-specific hazard index, developed previously, that can be applied to other microtidal wave-dominated settings. The approach is based on the subdivision of the terrestrial seaward edge of platforms into segments, classified according to mean elevation, orientation and edge type, to model different weighting scenarios of predominant southeasterly and northeasterly wave direction. UAV-derived results were deemed satisfactory for all study sites, and a comparison of results derived from LiDAR for two platforms suggested that UAV data can be successfully used to guide risk policy on rock coasts, despite differences in the delimitation of the seaward edge due to tidal level during survey acquisition.
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The Iberian microcontinent stands out because of its intense Alpine intraplate deformation. This is reflected in a large number of Cenozoic basins of very different sizes. Most of the contacts between topographic highs and basins are thrust or strike-slip faults. All these basins seem to have undergone a common sedimentary evolution, comprising four stages: initiation of sedimentation, intense syn-tectonic infilling, change from endorheic to exorheic drainage, and accelerated erosion related to fluvial incision. This simple evolutionary model shows a migration from East to West, in which basins are still tectonically active at the Atlantic margin of Iberia. This common evolution is also found in a series of geometrical characteristics, such as the ratio r of length of strike-slip fault and length of thrust fault, that are very similar in both types of basin border settings. Thrust-related basins are mainly associated with segmented pop-downs, whereas the main basins have the characteristics of open-ramp basins. Strike-slip related basins are mostly transpressive structures, although small pull-apart basins are usual along the Vilariça and Messejana faults. For basin areas larger than 100–1000 km2, a constant r value of 0.6 is found (including the Ebro, Duero, Madrid, Lower Tagus and Badajoz basins). Within the Iberian microcontinent, the total amount of Cenozoic contractional deformation was distributed between strike-slip and thrust faults with an r ratio close to 0.6. However, for small basins this parameter seems to depend on the type of fault, range or deformation belt (pure strike-slip, transtension, transpression, and pop-up) independently of its local tectonic development.
Article
Using micro-erosion meters (MEM) and traversing micro-erosion meters (TMEM), surface lowering rates of shore platforms on Kaikōura Peninsula, South Island, New Zealand have been measured over a total of 43 years. This record is the longest monitored network of this type. Since 1973, erosion rates have been calculated over two, two year periods 1973–1975, (n = 31) and 1993–1996, (n = 55) and at decadal scales; 1973–1993 (n = 15), 1973–2004 (n = 12), 1993–2004 (n = 46), 1993–2008 (n = 34), 1993–2016, (n = 18), 1973–2016 (n = 6). After 43 years, surface lowering rates remain similar to previously published rates at an average of 0.525 to 1.181 mm. a⁻¹. Statistical analysis shows that erosion rates over all measurement periods are derived from the same population. Thus, short term rates derived from two years of monitoring remain indicative of decadal rates of erosion. Variations between measurement periods are best explained by the loss of the more rapidly eroding bolt sites. These losses point to the difficulty of maintaining monitoring over longer time scales. A means of statistical manipulation (previously published) allows for these losses to be accounted for in determining long term rates of platform lowering. In November 2016 a Mw 7.8 earthquake raised the Kaikōura Peninsula approximately 0.8–1 m, elevating much of the shore platforms above the tide range. This earthquake has reset the MEM record ending this long running erosion monitoring site of shore platform surface lowering. However the MEM network is now being used to monitor post-earthquake response of the uplift shore platforms and new marine terraces.
Article
Historical air photographs, LiDAR, and an unmanned aerial vehicle (UAV) were used to record the movement, from 1956 to 2018, of a clay and clast earthflow in a coastal valley in northwestern Spain. Two procedures were employed. The first tracked changes, in a GIS environment, in the location of eight, easily identified objects on the surface of the deposit (large boulders, topographic lobes, and the foundations of an old hut). The second used DEMs of Difference (DoDs) based on Digital Elevation Models from a 2011 LiDAR flight and two UAV flights in 2016 and 2018 obtained by Structure from Motion-Photogrammetry techniques. While the first procedure provided estimates for earthflow movement over a 62-year period, the second produced more precise data for periods of up to 8-years. The first procedure indicated that the mean rate of movement was 0.48 m•yr-1, increasing from only 0.14 m•yr-1 from 1956 to 1983 to between 0.50 and 0.83 m•yr-1 from 1983 to 2018. Despite some temporal and spatial changes in direction, rates of surface movement were quite uniform on the deposit. The increase in earthflow movement after 1983 may be related to an increase in rainfall, although human activities associated with the removal of a wrecked ship from the nearshore may have been a contributing factor. The role of debuttressing due to the wave-induced removal of lateral support from the toe of the deposit is less clear. While there was no clear relationship between wave erosion and rates of movement, coastal retreat may have triggered changes in the direction and sediment flux in the toe of the deposit. This effect could have been tempered by negative feedback, however, whereby coastal erosion and increased flow activity were countered by the protection afforded by the accumulation of large, dislodged boulders on the beach. Because of this feedback, it is difficult to predict the impact of sea level rise and other elements of climate change along this coast.
Article
An unmanned aerial vehicle (UAV) was flown over a boulder beach (area 20,000 m²) on the southern coast of Galicia (northwestern Spain) in May 2016, continuing earlier work based on flights over the same beach in July 2012, May 2013, and late March 2014. Digital Surface Models (DSMs) with 1.8 cm resolution were constructed from the 2014 and 2016 data to identify changes in beach morphology over the intervening period. Analyses were conducted using a Limit of Detection (LoD) of 0 cm and 3.71 cm. In both cases, the analyses showed that erosion dominated over 19 % of the beach area. Accretion occurred over the rest of the beach, which acquired an additional 1500 m³ of material over the study period. Re‐analysis of the data from earlier flights suggested that erosion dominated on the beach in 2012‐2013 and deposition in 2013‐2014. Without any clear relationship between beach behaviour and storm severity during each winter period, it is proposed that gravitationally induced erosion and storm‐wave induced deposition are the result of perturbations about an equilibrium beach gradient. The UAV data also suggested that an essentially random component modulates regional patterns of movement.
Article
A traversing micro‐erosion meter (TMEM) was used to measure short‐term microtopographic changes on a supratidal rock surface at Marengo, Australia. In order to describe the characteristics of rock surface behaviour at different temporal scales, the TMEM site was monitored at 2‐hourly, daily and multiday (3.5 days) periods. The rock surface was highly dynamic at 2‐hourly scale, repeatedly falling and rising by up to 0.644 mm. Two‐hourly surface change was also characterised by spatial heterogeneity, with contraction and expansion occurring concurrently at centimetre scale across the rock surface. Two‐hourly microtopographic change was linked to the microclimate with significant relationships between surface movement and relative humidity (R² = 0.27) and air temperature (R² = 0.24). Expansion was observed across the rock surface when there was a fluctuation of 18% in relative humidity, and when cloudy the rock surface remained active during the day. Temporal variability in surface movement was also observed over a 24‐hour period with three distinct periods observed: falling (06:00‐12:00), rising (12:00‐20:00) and stable (20:00‐06:00). The daily surface change underwent no net micro‐topographic change (no loss of material). In contrast, an overall contraction was observed at multiday scale. Rock surface behaviour in coastal environments is therefore complex with many cycles of expansion and contraction acting concurrently albeit at different temporal scales from hours to multiple days.
Article
Determining rates and modes of shore platform development and the age of associated elements are among the greatest challenges facing rock coast researchers today. These coasts generally lack dateable deposits but cosmogenic nuclide analysis (CNA) can be applied directly to bare rock surfaces to estimate erosion rates or the time that a surface has been exposed over millennial timescales. Therefore this technique has the potential to revolutionize current theories on rock coast evolution and modes of development. Because environmental and geomorphological factors affect rates of nuclide accumulation and retention, however, the results of CNA analysis are dependent on the validity of the theoretical models that have to be employed according to the particular application. Among the important assumptions that may be made are those concerned with shading, tidal immersion, beach sediment covers, and other factors that influence rates of nuclide accumulation, and with possible spatial and temporal patterns in shore platform erosion, primarily by wave-generated backwearing and weathering-induced downwearing, that reduce nuclide concentrations. Our lack of understanding of the factors that control these variables, and in particular shore platform erosion, which is the subject of this review, has important implications for modelling rock coast evolution, assessing the possible contribution of inheritance, and dating, especially by CNA, the occurrence of important, formative events.
Article
Downwearing rates (erosion in the vertical plane) were measured with a micro-erosion meter (MEM) in eastern Canada, on an argillacious, sub-horizontal shore platform at Mont Louis in eastern Québec, and on two sloping, basaltic and sandstone platforms at, respectively, Scots Bay and Burntcoat Head in the Bay of Fundy, Nova Scotia. The original data covered a period from 2002 to 2009. This dataset was extended by measurements repeated at surviving MEM stations in 2017, producing records ranging over 9–14 years, depending on when each station was installed. Because of rapid surface downwearing, many of the original MEM stations were inoperable in 2017, especially at Burntcoat Head. Nevertheless, data were obtained from 19 stations at Burntcoat (35% of the 2009 original), 25 at Mont Louis (83% of the original), and 38 at Scots Bay (75% of the original). For the stations at Mont Louis and Scots Bay that were still functioning in 2017, there were no significant differences in rates of downwearing over the shorter (from station installation up to 2009) and extended periods (from installation to 2017). Mean rates of downwearing calculated from all the stations in each area declined through time, however, due to the loss of the more rapidly eroding stations. A simple procedure, which was proposed to compensate for this decrease, produced mean downwearing rates that were broadly similar to those reported over the original measurement period. There were significant relationships between downwearing rates and elevation (R² = 0.32) and downwearing rates and rock hardness (R² = 0.41) in the extended record at Scots Bay, and a small but significant relationship between downwearing rates and rock hardness at Mont Louis (R² = 0.17). Differences in downwearing rates across the platforms suggest that salt weathering and wetting and drying are dominant weathering mechanisms at Scots Bay and Mont Louis. Chemical weathering of the sandstone cementing agent and the premature removal of weathered grains by wave-generated bottom currents may, however, be more important at Burntcoat Head.
Article
Rocky shores are complex landforms that result from marine erosion and subaerial weathering. They are time-integrated features where their present day form is the result of instantaneous erosion, often on the millimetre to sub-metre scale, occurring for centuries to millennia. As a result, research on rocky coasts focuses on a range of temporal and spatial scales from granular-scale swelling of a rock surface and instantaneous wave impact to modelling millennial-scale sea level drivers. The challenge for rocky coast researchers is either to upscale or to downscale their results to the human-timescales of greatest interest to managers. The research presented in Earth Surface Processes and Landforms over the past 3 years highlights the range of spatial and temporal approaches to the study of coastal cliffs and shore platforms. We identify a key temporal and spatial gap in current research. Seasonal – annual timeframes over hundreds of metres to kilometre scale studies appear to be lacking and are likely critical in understanding the future evolution of rocky coasts, especially their response to climate change.
Article
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 700 km 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 50 m 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.
Article
The subtidal portions of shore platforms are important geomorphic features as they can modify deep-water wave energy before it impacts the intertidal platform edge. In this study an integrated marine and terrestrial aerial LiDAR dataset is used to analyse the morphology of the subtidal portion of shore platforms. Semi-horizontal intertidal platforms on an 85 km along stretch of microtidal, open-ocean, rocky coast in Victoria, Australia are investigated and described quantitatively. Three distinct types of subtidal morphology occur; (i) a steep cliff with a mean slope of 8–18°, (ii) a gently sloping ramp with a mean slope of < 3°, and (iii) a subtidal terrace/reef. It is inferred that the type of subtidal morphology present on a platform will determine the relative impact of marine and subaerial processes in the intertidal and supratidal zones.
Article
Extreme wave events in coastal zones are principal drivers of geomorphic change. Evidence of boulder entrainment and erosional impact during storms is increasing. However, there is currently poor time coupling between pre- and post-storm measurements of coastal boulder deposits. Importantly there are no data reporting shore platform erosion, boulder entrainment and/or boulder transport during storm events - rock coast dynamics during storm events are currently unexplored. Here, we use high-resolution (daily) field data to measure and characterize coastal boulder transport before, during and after the extreme Northeast Atlantic extra-tropical cyclone Johanna in March 2008. Forty-eight limestone fine-medium boulders (n=46) and coarse cobbles (n=2) were tracked daily over a 0.1km2 intertidal area during this multi-day storm. Boulders were repeatedly entrained, transported and deposited, and in some cases broken down (n=1) or quarried (n=3), during the most intense days of the storm. Eighty-one percent (n=39) of boulders were located at both the start and end of the storm. Of these, 92% were entrained where entrainment patterns were closely aligned to wave parameters. These data firmly demonstrate rock coasts are dynamic and vulnerable under storm conditions. No statistically significant relationship was found between boulder size (mass) and net transport distance. Graphical analyses suggest that boulder size limits the maximum longshore transport distance but that for the majority of boulders lying under this threshold, other factors influence transport distance. Paired analysis of 20 similar sized and shaped boulders in different morphogenic zones demonstrates that geomorphological control affects entrainment and transport distance - where net transport distances were up to 39 times less where geomorphological control was greatest. These results have important implications for understanding and for accurately measuring and modelling boulder entrainment and transport. Coastal managers require these data for assessing erosion risk. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Article
Shore platform widths in the Glamorgan Heritage Coast, Wales, UK (21 sites) and the Kii (14 sites) and Izu Peninsulas (19 sites), Japan, were analysed with respect to rock strength parameters. Specifically, uniaxial compressive strength (from Schmidt hammer readings), joint density and bed thickness, were measured at cliff base and high, mid and low tide locations. Platform width values were derived from telemetric field measurements and in the case of the Glamorgan Heritage Coast, augmented by LIDAR values. Analysis of the data indicated a significant and consistent correlation between platform width and the Schmidt hammer readings derived from the more resistant, dry, unweathered rocks exposed at high tide and cliff base sites, with R(2)= 0.35 at Glamorgan, 0.42 at Kii and 0.41 at Izu. This suggests that a high proportion of the platform width variation could be related to this specific rock resistance parameter (uniaxial compressive strength). No significant relationship was evident between joint density and platform width. This was unexpected and may reflect the difficulty of objective measurement of this parameter in the field. In the case of some of the Japanese sites, no jointing was apparent (within some of the massive volcanic series). Similarly, no consistent relationship was found between platform width and bed thickness.
Article
Variations in the width of shore platforms were investigated within southern Wales, Gaspe in eastern Canada, and southern Japan. The strong, positive linear relationship between platform gradient and tidal range implies that platform width may be largely independent of tidal range, although regional negative correlations between platform gradient and width suggest that platforms become narrower with increasing tidal range. Platform width increases with wave intensity, and the widest platforms are oriented toward the dominant waves. The width of the platforms also decreases with increasing rock dip. A structural classification, based upon combinations of rock dip and strike, relative to the orientation of the cliff face, helped to explain variations in platform width in Canada and Japan. Changes in structural class induced by variations in coastal orientation may partially account for differences in the width of platforms on headlands and in embayments. Platform width is also influenced by the presence and type of cliff-foot deposits.
Article
The Vale of Glamorgan may be described as a macro-tidal, storm-wave environment. Shore platforms occur along most of this coastline, attaining widths commonly greater than 250 m. An investigation of the form of these platforms showed that, although the junction of cliff and platform is related to the level of the Mean High Water Spring tides (r = 0.46) and although the gradient of the platform is related to the magnitude of the tidal range (r = 0.50), much of the variation in the detailed geometry of the platform is related to local lithology. The platforms showed no tendency to develop a parabolic profile, although minor secondary planation has occurred between -0.61 m and +1.22 m O.D. Investigation of the rate of cliff recession indicated that, although present recession is slow, much more rapid rates have occurred in periods since the Neolithic. With reference to work done in adjacent areas, platform origin appears to be a result of the Neolithic modification of a series of interglacial platforms, although further modification and extension of the platforms has taken place since that time.
Article
Differences in slope between Australasian and southern British shore platforms may be explained by morphogenic conditions, the high slopes of platforms in England and Wales being related to high tidal range, storm waves, large and abundant erosional debris, and climatic conditions unsuitable for effective waterlayer levelling. Other typical Australasian features such as the low tide cliff and true platform ramps are, however, generally absent from southern Britain. A classification of contemporary and 'raised' shore platforms and ledges is proposed for England and Wales, in which lithologically controlled storm ledges are distinguished from shore platforms. The relevance of detailed studies of contemporary shore platforms to investigations of 'raised' planation surfaces is emphasized.
Article
This reference text deals with the geomorphology of rock coasts. It brings together the results of research conducted by coastal engineers, biologists, geologists and physical geographers from many parts of the world. The first section of the book discusses changes in sea level and the effects of mechanical wave action, chemical weathering, solution, bio-erosion, frost, and mass movement on rock coasts. The second section is concerned with the major landforms which result from these processes. These include cliffs, shore platforms, coastal karst, bays, headlands, and elevated marine terraces.
Article
Low altitude flights by a micro-drone were made in 2012 and 2013 over two boulder beaches in northwestern Spain. Geographical information system software was used to map the data. Boulder outlines from the first flight were recorded on 4796 clasts at Laxe Brava and 2508 clasts at Oia. Changes in location were identified by overlaying these outlines on the 2013 images. About 17.5 % of the boulders (mean surface area 0.32 m2) moved at Laxe Brava and about 53 % (mean surface area 0.23 m2) at Oia. Most movement on both beaches was between the mid-tide to about 2 m above the high tidal level. The location and elevation of the highest points were also recorded on the 2012 images on 4093 boulders at Laxe Brava and 3324 boulders at Oia. These elevations were compared with the elevations at the same locations in 2013. The occurrence and scale of the elevational changes were generally consistent with changes in the boulder outlines. The study confirmed that boulder beaches can be cheaply and effectively monitored using high resolution, micro-drone technology. This article is protected by copyright. All rights reserved.
Article
In 1978 E.D. Gill and J.G. Lang installed 53 micro-erosion meter (MEM) sites on shore platforms along the Otway coast in south eastern Australia. Surface lowering rates were originally determined from 2 yr of measurements (1979 and 1980) and showed that the mean annual rate of lowering from all sites was 0.37 mm/yr− 1. We re-measured these MEM sites 32 yr after the first set of readings were taken in February 1979. We relocated 49 of the original 53 bolt sites and were able to measure 45 of them. From the sites we measured, the mean annual rate of shore platform lowering was 0.31 mm/yr− 1. Rates of surface lowering on shore platforms are commonly reported from studies of only two or three years and only one other record exceeds 30 yr (Kaikoura Peninsula, NZ; 1.09 mm/yr− 1), in that case fewer than half of the original bolt sites were functioning because erosion had removed bolts. Along the Otway coast rates of erosion are much slower hence the greater number of still readable sites after 32 yr. Compared to other micro-erosion meter studies the rates reported here are at the lower end of the range of values from around the world but nevertheless consistent with rates from the Otway Coast and others in similar lithology. We found no statistical difference between erosion rates calculated over 2 yr (1979 to 1980) and 32 (1979 to 2011) years supporting the view that shore platform lowering rates measured over 2 yr are representative of decadal scales.
Article
Repeated measurements of erosion rates on seashore platforms on Kaikoura Peninsula, South Island, New Zealand were gathered over a 28-month period using a micro-erosion meter (MEM). The resultant hierarchical dataset was analysed using multilevel regression modeling. Results indicate that there is persistent variation of rates across measurement occasions within individual points and significant variation is also found across sites (or local ‘contexts’). There is little evidence for significant variation across individual points. The explanatory variables of platform/rock type, distance along platform and seasonality all have a significant effect on the erosion rates and aid the explanation of some of the variation in erosion rates across sites.
Article
Knowledge of the long-term rates of erosion of shore platforms are needed to assist modeling platform development, understand rates and modes of development and decipher issues of inheritance from former sea level. Few studies report rates from more than 2 years of monitoring. We report decadal scale erosion rates using both a micro-erosion meter and a traversing micro-erosion meter with measurements from 30 years and 10 years, respectively. Gross mean surface lowering rates measured over 10 years with a traversing micro-erosion meter were 0.901 (S.E.=0.116) mm/yr compared with 1.130 (S.E.=0.217) mm/yr over two years. Over thirty years (1973–2003) 12 micro-erosion meter sites provide a mean surface lowering rate of 1.09 (S.E.=0.126) mm/yr compared to 1.43 (S.E. 0.128) mm/yr calculated over twenty years (1973–1993) from 15 bolt sites and 1.53 (S.D.=1.45) mm/yr from 31 bolt sites. Rates calculated over longer time periods are reduced compared to short-term rates as a result of faster eroding bolt sites being removed from the data set. Comparison of means between short and long time periods for existing bolts show no significant difference. That is, short-term measurements over two years are representative of decadal scale erosion rates. In fact decadal scale rates may under represent rates of platform development because of the loss of faster eroding bolt sites. The loss of such bolts means that obtaining long-term erosion rates is problematic. Future modeling of shore platform development can use MEM rates with more certainty, but we recommend using higher rates from shorter term studies, unless longer records are complete.
Article
This paper presents a case study comparing shore platforms of the same lithology and different rock structures (limestone) with those of different lithologies but similar rock structures (limestone, dolerite) in Wales and Sweden. Results of this study illustrate that different erosion mechanisms (i.e. wave action and ice scour) produce similar erosion products and meso-scale (cm–m) landforms (i.e. stepped shore platforms) in structurally similar lithologies. In contrast, lithologically similar rocks (i.e. limestones) generate different meso-scale landforms and shore platform morphologies. Thus, structural controls appear to be more critical than lithological controls in generating meso-scale erosion products, such as rock blocks, in this comparative study. The similarities in the meso-scale geomorphology appear to be caused by uniform rock structure (i.e. rock hardness, joint density, spacing of bedding/horizontal jointing and the slope and direction of bedding/horizontal jointing in relation to the sea). Blocks produced by the jointing have been eroded by high energy waves in the case of Wales and drifting sea ice in the case of one of the Swedish sites. Meanwhile, different rock structures in a similar lithology (i.e. limestone) resulted in very different landforms, under the same environmental conditions as the previous example. As such, this research also provides observational evidence supporting the concept of equifinality.
Article
Erosion rates are 15–18.5 times higher in places where there is a beach at the cliff foot compared with those where there is no beach. The zone of intense erosion associated with a beach of sand and pebbles was below a height of about 10 cm above the surface, maximum erosion being at 14.5 cm below the surface. Erosion was by corrasion and wedging, a previously unrecognised process which is the quarrying of rock aided by fine-grained particles from the beach. Wedging, perhaps with some corrasion, had a median erosion rate of 11.05 · 10−3 cm tide−1 in Upper Lias shale while the rate for corrasion was 5.79 · 10−3 cm tide−1. A smooth, notched cliff foot may be produced by these processes. Where there was no beach, erosion of the cliff foot was by the sporadic quarrying of blocks of rock and the more continuous micro-quarrying of shale fragments. The cliff foot was dominated by structural planes and had a rough surface. The horizontal erosion rate due to quarrying at one cliff foot was 2.3 cm year−1. Quarrying acts on the cliff foot above the intense zone of erosion at a beach surface. The median erosion rate for this quarrying was 0.91 · 10−3 cm tide−1.
Article
Processes of platform development include mechanical wave erosion, weathering and bio-erosion. Form is strongly influenced by lithology and structure. Contrasts between British and Australian forms are noted. These seem to be opposite ends of a tidally regulated wave erosional spectrum of forms. Rates of platform development raise questions of inheritance. -Jennifer Clayton
Article
Work has been conducted, in the laboratory and field, on a micro-tidal, subhorizontal shore platform in argillaceous rocks at Mont Louis, in Gaspé, Québec. Surface downwearing rates for 450 rock samples were measured over 3 years in the laboratory at the neap low, mid-, and high tidal levels, under semi-diurnal tidal conditions. De-ionized water was used to determine the role of wetting and drying, and artificial sea water for the additional effect of salt and possibly chemical weathering. Another 300 samples were immersed in de-ionized water or artificial sea water for 90 min every 1, 2, or 3 weeks, representing conditions between the neap and spring high tidal levels, and exposed in air for 90 min every 1, 2, or 3 weeks to represent conditions between the neap and spring low tidal levels. Surface downwearing was recorded in the field at 34 transverse micro-erosion meter (TMEM) stations, most of which were installed along 4 shore-normal profiles in 2004 and 2005. Mean downwearing rates in the laboratory ranged from 1.25 mm yr− 1 at the neap high tidal level to 0.63 mm yr− 1 at the neap low tidal level. Downwearing was much slower between the neap and spring tidal levels and it was uniformly slow below the neap low tidal level. The presence of salts inhibited downwearing within the neap tidal range, where wetting and drying was dominant, and promoted it between the neap and spring high and low tidal levels. Surface swelling was fairly common in the field, and the mean downwearing rate (0.242 mm yr− 1) was much lower than in the laboratory, possibly because the TMEM stations were installed in harder rocks. Modeling supports the contention that the Mont Louis platform was cut initially by waves at a higher elevation, and then lowered by weathering in the last few thousand years as relative sea level fell to its present elevation.
Article
The Schmidt Rock Test Hammer was used to study the effect of abrasion on shore platforms in Galicia, northwestern Spain. On platforms where tidally-induced weathering (salt, wetting and drying, etc.) is dominant, rock strength is significantly lower than in areas where abrasion is, or has been active in the recent past. This suggests that abrasion removes weathered surface material, exposing the stronger, less weathered rock below. Abrasion downwearing, measured with a transverse micro-erosion meter, ranged between 0.13 and 1.8 mm yr− 1 over the last year. Most active abrasion occurs in the upper part of the intertidal zone, but weathering is slowly destroying formerly abraded surfaces at lower elevations. These abandoned surfaces were abraded by materials supplied by erosion of fluvio-nival and periglacial slope deposits that covered, or partially covered, parts of the Galician coast during the middle and late Weichselian. During the Holocene, rising sea level and erosion of the slope deposits caused the abrasion zone to gradually migrate up to its present position near the high tidal level. The spatial and temporal role of abrasion on this coast is, therefore, closely associated with the exhumation and inheritance of ancient platform surfaces from beneath Weichselian deposits.
Article
This paper describes a new method to distinguish river terrace levels based on clast hardness and degree of weathering measured with an Equotip hardness tester. The technique was applied to a series of terraces on the Miño River in the northwestern Iberian Peninsula, where the lack of suitable material, high degree of weathering, and intense iron cementation precluded routine dating. Detailed mapping demonstrated that terrace sediments occupy a range of altitudes that make assignment to a specific terrace, and/or correlation between levels, difficult. Statistical analysis of the Equotip hardness data from quartzite clasts using k-means clustering allowed four probable terrace levels to be identified; a series of t-tests generally supported these groupings. A fifth, lowermost terrace level, was not included in the analysis because of limited exposure above a reservoir. Clast hardness and degree of terrace weathering were generally consistent with progressive river downcutting. The occurrence of faulted sediments, however, suggested that terrace elevations were modified locally by post-depositional tectonic movements, which may explain why probable younger terraces in some sectors of the Miño River are at higher elevations than older terraces in adjacent sectors. The Equotip tester helped to resolve stratigraphic uncertainties and to assign deposits to specific terrace levels and was found to be a useful tool to distinguish and correlate river terraces.
Article
This paper is concerned with the effect of sediment accumulation on shore platform development. Boulder accumulations are common on the granitic shore platforms of Galicia, northwestern Spain. Boulders are produced by erosion of shore platforms and of cliffs consisting of cold-climate deposits from the last glacial period. Measurements were made of the long axis length of more than 800 boulders, and additionally of the short and intermediate axes of 340 of these boulders, as well as of their orientation and gradient. There were two study areas. The boulders on the Barbanza Peninsula are generally a little smaller than those in southern Galicia with, respectively; mean long axis lengths of 0.98 and 1.14, and masses of 1.06 and 1.59 t. There are also some isolated, very coarse boulders and megaclasts in southern Galicia. The distribution and extent of the deposits and boulder imbrication and orientation testify to the high levels of wave energy produced by northwesterly and westerly storms in this region. Although the boulders, as well as the underlying shore platforms, were inherited, in part, from previous interglacial stages, some boulder detachment and movement is occurring today during storms, when significant deep water wave heights exceed 8 to 10 m. Despite some abrasion of the shore platforms, the primary effect of large boulder accumulations is protective. The role of sediment on shore platforms has been neglected, but this study suggests that because of arrested development under thick accumulations, platform gradient in areas with abundant sediment increases with the grain size of the material. The occurrence and type of sediment on shore platforms may therefore help to explain the distribution of sloping and subhorizontal platforms under different morphogenic and geological conditions.
Article
Examination of rocky coastal profiles in Australia and Europe suggests that two major and genetically distinct types of erosional surface should be recognized, one of these being the shore platform, whether sloping or horizontal, smooth or rough, which is best developed on promontories and terminates in a low-tide cliff, the other being the wave ramp, often exposed in bays, which slopes gradually down from about high water level to merge with the submarine slope near-shore. A modified terminology for erosional surfaces on rocky coasts is proposed.
Article
Downwearing rates were measured on shore platforms at about 200 transverse micro-erosion meter (TMEM) stations, over periods ranging from 2 to 6 years. There were seven study areas in eastern Canada. The platforms were surveyed and a Schmidt Rock Test Hammer was used to measure rock hardness. More than 1200 rock samples from three of the study areas were also subjected each day, over a 3 year period, to two tidal cycles of immersion and exposure, which simulated the central intertidal zone. A further 840 samples were subjected to longer periods of exposure and immersion, over a 1 year period, which represented different elevations within the upper and lower intertidal zone, respectively. These experiments suggested that tidally generated weathering and debris removal is an effective erosional mechanism, particularly at the elevation of the lowest high tides. In the field, mean rates of downwearing for each study area ranged from 0·24 mm yr−1 to more than 1·5 mm yr−1. Rates tended to increase with elevation in the field, with maxima in the upper intertidal zone. This trend in the field cannot be attributed entirely to the tidally induced weathering processes that were simulated in the laboratory, and must reflect, in part, the effect of waves, frost, ice, and other mechanisms. It is concluded that there are no strong spatial downwearing patterns on shore platforms, and that downwearing rates in the intertidal zone are the result of a number of erosional mechanisms with different elevation-efficacy characteristics. Furthermore, even if only one or two mechanisms were dominant in an area, any resulting relationship between downwearing rates and elevation would be obscured or eliminated by the effect of variations in the chemical and physical characteristics of the rocks. Copyright © 2010 John Wiley & Sons, Ltd.
Article
The shore platforms on Shag Point, southern New Zealand, are quasi-horizontal surfaces and are developed between supratidal and low water spring levels. A range of morphologies occur, with more exposed platforms having a distinct low-tide cliff, in contrast to low-tide surfaces where the seaward edge is buried beneath rubble and macro-algal growth. The platforms range in width from 20 to 80 m and are eroded into Late Cretaceous/Early Tertiary fine marine sandstones and mudstones. Shore platforms have formed in two principal lithological units: a homogeneous unit that is characterized by few discontinuities, and a fractured unit with joints spaced about 0·5 m apart. Rock hardness is low in both units (L-type Schmidt hammer rebound values of 31 ± 4), and there is little systematic variation in values between the two units in which platforms have developed. Case-hardened concretions within the sandstone are significantly harder than surrounding rock and cause local relief of metre scale as the spherical diagenetic features are eroded from the bedrock. They do not, however, appear to affect broad-scale platform geometry. Joints within the bedrock are a primary control on platform elevation. Platforms formed in jointed rock occur at the lower portion of the intertidal zone, in contrast to platforms formed in unjointed bedrock, in which horizontal surfaces occur at or above mean high water spring tide level. Rock structure, therefore, appears to be the primary determinant factor of platform geometry at Shag Point. Copyright © 2006 John Wiley & Sons, Ltd.
Article
We report a series of short-term (diurnal) rock surface monitoring studies on inter- and supra-tidal shore platforms using a traversing micro-erosion meter at two sites, Kaikoura Peninsula, New Zealand, and Apollo Bay, Victoria, Australia. Statistically significant day-to-day changes were measured. Surface rise and lowering occurred at rates above instrument error, with a maximum range of 3·378 mm between 1·697 mm (lowering) and -1·681 mm (rise). Individual measurements showed rises greater than 2 mm. These daily variations reveal that surface lowering and rise occur at a much shorter time scale than previously reported from other studies. The patterns observed suggest wetting and drying is the most likely process causing surface changes at these temporal scales. We argue that traversing micro-erosion meter studies operating at a short-term time scale of day-to-day provide meaningful results that open new opportunities for studying rock weathering and erosion in a coastal environment. Copyright © 2004 John Wiley & Sons, Ltd.
Article
One of the longest standing debates in rocky coast geomorphology is whether subaerial weathering or wave processes dominate shore platform evolution. The origins of this debate date to the mid-nineteenth century when the first descriptions of Old Hat Islands were provided from northern New Zealand. Old Hat Islands are surrounded by a broad near-horizontal shore platform. Their formation was inferred to relate to subaerial weathering of bedrock to a level of permanent saturation with wave processes acting only to remove the weathered debris. To date, no detailed topographic surveys have been conducted on the Old Hat Islands in New Zealand that initiated this debate; in this study we provide the first quantitative data on the original field sites. Topographical surveying and Schmidt Hammer hardness testing were conducted on >25 profiles of varying wave exposure. In contrast to the classic descriptions, platform elevation varies from just above mean high water spring (MHWS) at the most exposed sites, to mean high water neap (MHWN) at the most protected sites or mean sea level (MSL) where beaches occur on the platform surface. There is no significant difference between the hardness of the cliffs and the platforms fronting them and no clear relationship between wave exposure and platform width. Rather than being exclusively dominated by subaerial process, the formation of microtidal, sheltered, Old Hat Island platforms is considered to be a function of (i) the rate of weathering, (ii) exposure to wave energy, (iii) nearshore water depth and (iv) rock resistance. Shore platform elevation in the study area is thought to be a function of the level at which waves erode cliff rock, and the action of weathering which lowers platform surfaces. The width of platforms is strongly influenced by the ability of waves to dissect the platform edge along vertical joint lines. Copyright © 2010 John Wiley & Sons, Ltd.
Article
Shore platforms frequently exhibit steps or risers facing seaward, landwards or obliquely across-shore. A combination of soft copy photogrammetry, ortho-rectification, geo referencing and field measurement of step height are linked in a GIS environment to measure step retreat on chalk shore platforms at sample sites in the south of England over two periods, 1973–2001, 2001–2007. The methods used allow for the identification, delineation and measurement of historic change at high spatial resolution. The results suggest that while erosion of chalk shore platforms by step backwearing is highly variable, it appears to be of similar magnitude to surface downwearing of the same platforms measured by micro-erosion meters (MEMs) and laser scanning, in a range equivalent to 0·0006 – 0·0050 m y−1 of surface downwearing. This equates to annual chalk volume loss from the platforms, by the two erosion processes combined, of between 0·0012 m3 m−2 and 0·0100 m3 m−2. Results from the more recent years' data suggests that step retreat has variability in both space and time which does not relate solely to climatic variability. The results must be viewed with caution until much larger numbers of measurements have been made of both downwearing and step erosion at higher spatial and temporal resolution. Copyright © 2010 John Wiley & Sons, Ltd.
Article
The coast of Wellington, New Zealand, is tectonically active and contains a series of uplifted and contemporary shore platforms that are developed in Triassic Greywacke. The platform profiles are rugged with relief of metre scale common. The surveyed platforms were formed at, and at two distinct levels 1–1·5 and 2–2·5 m above, mean sea level. They range in width up to 70 m and are highly fractured with fracture densities in excess of 20[sol ]m2 common. The rate of development of these platforms is rapid, with lateral erosion rates of up to 0·15 m[sol ]yr calculated, allowing platform development to occur over centennial scales. Even given this rapid development, continued instantaneous uplift of the coast has meant they are unable to reach an equilibrium state, whereby the effectiveness of wave processes in removing material is reduced by platform extension. The co-seismic uplift means that the rear of the platforms is raised beyond the limits of marine process and has become an area of deposition. Although no direct process measurements were made the highly fractured nature of the bedrock appears to play a major role in platform evolution, with wave processes being easily able to pluck blocks as evidenced by fresh erosion scars and active gravel beaches at the rear of many platforms. This coast therefore represents an extremely dynamic youthful shore platform environment, where the processes of marine abrasion can be observed over historical timescales. Copyright © 2005 John Wiley & Sons, Ltd.
Article
Transverse micro-erosion meter (TMEM) stations were installed in rock slabs from shore platforms in eastern Canada. The slabs were put into artificial sea water for 1, 6 or 11 hours, representing high, mid- and low tidal areas, respectively. The TMEMs were used to record changes in surface elevation as the rocks dried during the remainder of the 12 h of a semi-diurnal tidal cycle. A similar technique was used on the same rock types at intertidal TMEM stations in the field, as the rocks dried during low tide. Argillite and basalt surface contraction was from 0 to 0·04 mm: there was little surface expansion. Sandstones contracted by up to 0·03 mm in the field, but there was almost no contraction in the laboratory. Argillite and basalt contraction tended to be greatest in the upper intertidal zone, and to increase with rates of longer-term surface downwearing, but there was little relationship with rock hardness or air temperature and humidity. Changes in elevation at the same points at TMEM stations in the laboratory and field were quite consistent from one tidal cycle to the next, but there were considerable variations within single tidal cycles between different points within each station. The data suggest that contraction within the elevational zone that is normally submerged twice a day by the tides is by alternate wetting and drying. Short-term changes in elevation are generally low compared with annual rates of downwearing owing to erosion, but they may generate stresses that contribute to rock breakdown. Copyright © 2007 John Wiley & Sons, Ltd.
Article
The Schmidt hammer is a useful tool applied by geomorphologists to measure rock strength in field conditions. The essence of field application is to obtain a sufficiently large dataset of individual rebound values, which yields a meaningful numerical value of mean strength. Although there is general agreement that a certain minimum sample size is required to proceed with the statistics, the choice of size (i.e. number of individual impacts) was usually intuitive and arbitrary. In this paper we show a simple statistical method, based on the two-sample Student's t-test, to objectively estimate the minimum number of rebound measurements. We present the results as (1) the ‘mean’ and ‘median’ solutions, each providing a single estimate value, and (2) the empirical probability distribution of such estimates based on many field samples. Schmidt hammer data for 14 lithologies, 13–81 samples for each, with each sample consisting of 40 individual readings, have been evaluated, assuming different significance levels. The principal recommendations are: (1) the recommended minimum sample size for weak and moderately strong rock is 25; (2) a sample size of 15 is sufficient for sandstones and shales; (3) strong and coarse rocks require 30 readings at a site; (4) the minimum sample size may be reduced by one-third if the context of research allows for higher significance level for test statistics. Interpretations based on less than 10 readings from a site should definitely be avoided. Copyright © 2009 John Wiley & Sons, Ltd.
Article
There is increasing evidence that shore platforms and other elements of rock coasts may be inherited, at least in part, from interglacial stages when sea level was similar to today's. Most of this evidence, which includes ancient beaches and datable terrestrial deposits, has been obtained from areas of resistant, slowly eroding rock, where the platforms often appear to be much too wide to have developed since the sea reached its present level. It is much more difficult to demonstrate that inheritance has occurred in areas of weaker rock, which generally lack any datable material. The coast of western Galicia in northwestern Spain has shore platforms in igneous and metamorphic rocks that were deeply weathered during the Tertiary. These platforms are closely associated with ancient beaches from the last interglacial stage, and associated periglacial and fluvio-nival deposits that covered and fossilized most of the Eemian platforms and cliffs during the late middle and late Weichselian glacial stage. The sedimentary processes and the thickness and facies of the sediments were determined by the height, aspect and gradient of the coastal mountains, and their distance from the coast. Radiocarbon dating, sedimentary analysis and platform morphology indicate that the shore platforms of Galicia have been inherited from at least the last interglacial stage. They were fossilized in places beneath thick Weichselian deposits and then exhumed during the Holocene transgression. The abundant evidence for inheritance in Galicia has important implications for other coasts in fairly weak rocks where such evidence is generally lacking. Copyright © 2003 John Wiley & Sons, Ltd.