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Clast mobility within boulder beaches over two winters in Galicia, northwestern Spain
... Mega-boulders, those greater than 1 m in length, are frequent sediments on many rock coasts. Although they appear to be motionless, they are in fact able to move during storms (e.g., Fichaut and Suanez, 2011;Paris et al., 2011;Pérez-Alberti and Trenhaile, 2015;Naylor et al., 2016;Erdmann et al., 2018;Kennedy et al., 2019;Hastewell et al., 2020;Oliveira et al., 2020;Scheffers and Kelletat, 2020;Gómez-Pazo et al., 2021) or tsunamis (e.g., Nott, 1997;Marie, 2008;Oetjen et al., 2020). Identifying the type of high energy event that can mobilize boulders on rock coast systems has been a recurring debate among rock coast geomorphologists for about 30 years Trenhaile, 2016). ...
... is unexplained. The low mobility capacity of constrained mega-boulders has been reported by other authors (Dionne, 1988;Dionne and Poitras, 1998;Neumeier, 2011;Boisson and Allard, 2020), including movements that take place in an ice-free environment (Pérez-Alberti and Trenhaile, 2015;Naylor et al., 2016;Kennedy et al., 2019;Gómez-Pazo et al., 2021). The presence of a break in slope can also favour the grounding of ice floes, resulting in the accumulation of boulders. ...
... The basic assumption of this survey was that the mega-boulders studied were only mobilized in winter due to the movement of ice floes. However, the literature has shown that boulders of comparable size and weight can be transported in different environments by wave activity (e.g., Paris et al., 2011;Pérez-Alberti and Trenhaile, 2015;Hastewell et al., 2020), and wave activity can also be more important than floating ice movement for ice-affected coasts (Hansom, 1986). Even though the hydrodynamic conditions at Sainte-Luce make this hypothesis unlikely, it is possible that particularly stormy conditions could displace some mega-boulders. ...
One hundred mega-boulders were tagged to survey their movements (RFID) for five winters (2012–2017) in Sainte-Luce, St. Lawrence Maritime Estuary, where the foreshore is frozen for two to four months per year. In addition, four boulders were fitted with accelerometers to determine the exact periods of motion. It was found that 81% of the boulders moved during at least one of the winters studied. The maximum transport recorded in one winter was 151.87 m for a 3.1 t boulder, but most boulders moved shorter distances (2012–2017 average transport: 9.74 m, standard deviation 23.71). The relationships between length/direction of boulder movements and fast-ice activity as well as weather and oceanic conditions were analyzed. 61% of mobility events took place at the end of the ice period during ice-foot break-up by storm waves or melting, but 37% took place at the beginning of the fast-ice phase during storms and/or high tide, which was unexpected. The length and direction of transport was found to depend on conditions during ice break-up (wind, waves, currents, ice thickness), which have large interannual variabilities. However, a greater proportion of boulders moved landward between 2012 and 2017. Two transport modes occurred: short pushing by ice floes and ice-rafting over longer distances. The first is much more common (70 to 90%) while the second seems to be more frequent during cold winters, when ice is thicker. Moreover, we demonstrated that the geomorphological setting exerts a strong control on boulder transport distance. The longest movements took place near mean sea level, where the ice foot is thicker and the rock platform smooth while cobble and boulder pavements inhibit movements.
... These have been applied to different fields, such as vegetation analysis [41] or wildfire research [42]. In the context of coastal research, these devices have gained importance in the last decade, allowing for new works about coastal areas in general [43,44] and rocky areas in particular [13,[44][45][46], without the need for complicated and expensive field surveys. In this sense, it is worth mentioning the study by Pérez-Alberti et al. [47,48], as the first application of UAVs on rocky coasts, in this case boulder beaches, published in [46]. ...
... This fact was often not possible with classic field surveys, such as topographic profiles, or with aerial images, due to their low spatial and temporal resolution. Even in a novel field such as the use of UAVs on rocky coasts, great technical and methodological advances have taken place, from an early period related to photointerpretation of high resolution images [45,46] to more recent works using photogrammetric techniques such as DSM (digital surface models) generated through SfM (structure from motion) techniques, which enhance the possibilities for quantification [3,[49][50][51]. ...
... The process for image acquisition in the study sites is explained in detail in previous publications [3,45,46] and consisted of the placement of GCPs (ground control points) and their positioning through a GPS device, in this case a Stonex S8 GNSS device. These were used to determine the exact position of the acquired images. ...
Rocky coasts represent three quarters of all coastlines worldwide. These areas are part of ecosystems of great ecological value, but their steep configuration and their elevation make field surveys difficult. This fact, together with their lower variation rates, explains the lower numbers of publications about cliffs and rocky coasts in general compared with those about beach-dune systems. The introduction of UAVs in research, has enormously expanded the possibilities for the study of rocky coasts. Their relative low costs allow for the generation of information with a high level of detail. This information, combined with GIS tools, enables coastal analysis based on Digital Models and high spatial resolution images. This investigation summarizes the main results obtained with the help of UAVs between 2012 and the present day in rocky coastline sections in the northwest of the Iberian Peninsula. These investigations have particularly focused on monitoring the dynamics of boulder beaches, cliffs, and shore platforms, as well as the structure and function of ecosystems. This work demonstrates the importance of unmanned aerial vehicles (UAVs) for coastal studies and their usefulness for improving coastal management. The Galician case was used to explain their importance and the advances in the UAVs' techniques.
... There have been far fewer studies of boulders than of sandy sediments in coastal regions, and most of the former have been concerned with the movement and accumulation of a small number of individual clasts rather than with the response of larger accumulations to changing wave conditions. Exceptions include investigations of spatial changes in the size and shape of clasts in boulder beaches in southeastern China and the work conducted on boulder beaches in Galicia, northwestern Spain (Chen et al., 2011;Perez-Alberti et al., 2012;Trenhaile, 2015a, Pérez-Alberti andTrenhaile, 2015b. Low flying UAVs were used to monitor the movement of thousands of boulders in Galicia (Pérez-Alberti and Trenhaile, 2015a;Pérez-Alberti and Trenhaile, 2015b). ...
... Exceptions include investigations of spatial changes in the size and shape of clasts in boulder beaches in southeastern China and the work conducted on boulder beaches in Galicia, northwestern Spain (Chen et al., 2011;Perez-Alberti et al., 2012;Trenhaile, 2015a, Pérez-Alberti andTrenhaile, 2015b. Low flying UAVs were used to monitor the movement of thousands of boulders in Galicia (Pérez-Alberti and Trenhaile, 2015a;Pérez-Alberti and Trenhaile, 2015b). Flights were made over two beaches at Laxe Brava and Oia in July 2012, May 2013, and late March 2014, with the data being used to produce high resolution digital terrain models and orthophotographs. ...
... The study differs from previous work on boulder beaches and megaclasts in Galicia and elsewhere that has been concerned with the effect of catastrophic events and boulder movement during one or more high intensity storms. It also uses a different methodology from the previous work by Perez-Alberti and Trenhaile (Pérez-Alberti and Trenhaile, 2015a, Pérez-Alberti andTrenhaile, 2015b), being based on photogrammetric analysis of two high resolution (1.8 cm) digital surface models (DSMs) of the entire study area. This procedure, which has previously been applied to sandy beaches (Ierodiaconou et al., 2016), allows the data to be processed semi-automatically, thereby reducing the processing time and facilitating repetition of the analytical procedures and their application to other areas of study. ...
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.
... Drones, or unmanned aerial vehicles (UAVs), now offer a fairly inexpensive way to acquire high resolution datasets over extensive areas and hence the possibility of collecting sufficient, representative data from large accumulations [12][13][14][15]. Repeated 3D imagery data can be used to identify and map changes in the location of hundreds of mobile and immobile boulders along the coasts and relate the changes to boulder size and elevation [16][17][18][19]. ...
Boulder-sized clasts on rocky coasts are considered as erosional signatures of extreme wave events and boulder attributes are often used in numerical models to estimate wave characteristics. The use of unmanned aerial vehicle (UAV) technology and related software has facilitated the monitoring of coastal areas, by generating models from which 2D and 3D measurements can be derived. However, the reliability and preciseness of such measurements is still to be determined. This study seeks to analyse the accuracy of boulder measurements by comparing the dimension data obtained through in-situ measurements with ex situ data generated from digital models, based on UAV images. The study area is a bouldered sloping coast located on the southeast coast of Malta (Central Mediterranean) that has developed into multiple limestone dipped strata with a fractured and heavily jointed morphology. The dimensions of c. 200 boulders in different morphological settings, such as clusters or ridges, have been statistically compared. The results show a very strong correlation between the two datasets, both in 2D and 3D; however some notable differences were observed at the individual boulder level. For the majority of boulders analysed, the A and B axes dimensions varied by ±10% to 20%. The C axis proved to be harder to measure accurately and showed a wider range of difference. Boulder volume results in the majority of cases varied from 0% to ±40%. Some tested methods of volume calculation may be more accurate and realistic than others depending on the boulder position in relation to other clasts and shore morphology. An automated digital analysis of the terrain surface to identify the boulder extents may offer possibilities for a more accurate estimation of boulder attributes.
... 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. ...
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
... and Trenhaile, 2015aand Trenhaile, , 2015bGómez-Pazo et al., 2019;Horacio et al., 2019;. For the present study images were obtained during low tide using an UAV (Microdrones GmbH, Model MD4-200) flying about 30 m above the shore platforms at Caamaño(March 2, 2015), Sanxenxo (April 30, 2015), and Laxe Brava(August 8, 2016) ...
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.
... The data, albeit from only a limited number of tracers, suggest that, despite temporal and spatial variations in boulder movement at Oia over the study period, there was no dominant vector but rather a diversity of local displacements, varying in distance and direction, between boulders that were often very close to each other. Boulder displacement appears to have been essentially chaotic, a conclusion which is consistent with the results of J o u r n a l P r e -p r o o f previous work using UAVs (Pérez-Alberti and Trenhaile, 2015a). It also supports the contention that this behaviour, involving the landward movement and deposition of boulders by high waves, followed by seaward movement, is a manifestation of perturbations that occur about a sub-equilibrium gradient (Gómez-Pazo et al., 2019). ...
Previous studies using unmanned aerial vehicles and remote sensing techniques have provided data on group sediment dynamics and overall mobility of a boulder beach at Oia in Galicia, northwestern Spain. They recorded changes in the location and disposition of hundreds of clasts but were unable to trace the intermittent movement of individual boulders. Radio Frequency IDentification (RFID) sensors were used in the present study to identify and record the long-term trajectories of boulders in this area. Sensors were installed in 80 boulders (mean intermediate axes approximately 55 cm) in September 2016 and were used to locate and record their displacement on five occasions, terminating in December 2019. Although burial and other factors prevented data being collected from every tagged boulder during these periods, recovery rates ranged from approximately 50%–75% of the original population. There was considerable variability in the displacement of the boulders, with maxima of >20 m and mean values >5 m. The distance of travel tended to decrease during the course of this study, despite a corresponding increase in storm frequency and intensity. Patterns of movement, including transport distance and direction were essentially chaotic in nature, reflecting the complex interaction and compound effect of a multitude of controls related to such factors as the location of boulders on the beach, their relationship to surrounding boulders, including the effects of sheltering, interlocking, and burial, and their size, shape, and other physical characteristics. The unpredictable behaviour and high transport thresholds of boulder beaches are in contrast to sand and gravel beaches that are characterized by more uniform and predictable patterns of sediment transport and morphological change.
... Due to the high frequency of clast activation in boulder beaches, they cannot be used to interpret the age and magnitude of individual (pre-)historical events. Nonetheless, clast movements during storm events and sedimentological trends in boulder beaches have been monitored and analyzed to understand how these beaches respond to storm events, for predicting beach-morphology changes due to rising sea level and climate change Pérez-Alberti and Trenhaile, 2015;Gómez-Pazo et al., 2019). ...
High-energy waves such as tsunamis and storm waves can dislodge and transport enormous clasts. With a growing body of literature that uses subaerial coarse clasts (individual clast sized between tens of centimetres and several metres in the intermediate axis) to interpret paleotsunamis or storms on the one hand, and characterizes coarse-clast deposits formed in recent events on the other, this chapter provides an introduction to wave-deposited coarse-clast research. It (1) addresses how to correctly identify wave-transported coarse-clast deposits; (2) provides an overview of different patterns of coarse-clast deposits, including isolated megaclasts, boulder fields, boulder ridges, coral-rubble ridges, and other gravel-sized deposits; and (3) discusses the potentials for inferring extreme-wave characteristics and deposition mechanisms (tsunami vs. storm) from coarse-clast deposits.
... A gradación, a anchura e a máxima evolución dos coídos nas áreas de estudo reflicten en parte a topografía das plataformas costeiras subxacentes, o que supón a presenza de formas máis abertas alí onde a costa o é e máis arqueadas onde o litoral debuxa numerosos entrantes e saíntes. Con todo, investigacións anteriores amosaron que os coídos son depósitos dinámicos que responden aos cambios na enerxía das ondas (Oak, 1984;Etienne & París, 2010;Chen et al., 2011;Paris et al., 2011;Pérez-Alberti et al., 2012, Pérez Alberti & Trenhaile 2015a2015b). A súa mobilidade responde dun xeito especial á intensidade dos temporais, cun aumento significativo na proporción de clastos que se moven, o seu maior tamaño e a elevación dos móbiles, desde o inverno de 2012 a 2013 ata o de 2013-2014. ...
Coídos are beaches composed of cobbles and boulders that are usually found in association with cliffs and rock platforms, particularly in granite-dominated areas. They consist of accumulations of heterometric, predominantly boulder-sized clasts, often between 1 and 2 m and even with their main axis exceeding 2 m in some areas. The best examples of coídos are found in the area known as Costa da Morte, located north of Cape Vilan (Camariñas, A Coruña); in the Corrubedo area (Ribeira, A Coruña), and along the stretch of coastline located between Cape Silleiro and the Miño river estuary (Baiona, Oia, O Rosal, and A Guarda, in Pontevedra).
According to their shape from top view, five broad types have been defined: longitudinal, double-pointed, arched, corridor-like, and single-pointed. Genetically, four types can also be distinguished: those derived from granite exhumation processes; those generated by the dislodgement of rock fragments from shore platforms and sea cliffs; those produced by rockslides, and those originated by glacial deposition. Their genesis is often of multiple origin.
In order to study boulder mobility, a micro-drone was flown at low heights over two coídos, one in Laxe Brava (Ribeira, A Coruña) and the other in Oia (Pontevedra). Flights were carried out in July 2012, May 2013, and April 2014. These flights yielded ortophotographs and high-resolution digital terrain models (DTM), which were integrated in a GIS and used to monitor thousands of cobbles and boulders for changes in their position. Maximum storm wave height was greater during winter 2013-2014 than during winter 2012-2013, resulting in an increase in the proportion of moved boulders from 17 % to almost 48 % in Laxe Brava and from 53 % to almost 88 % in Oia.
... It is necessary to highlight the great evolution of remote images in the last decades as well as the use of unmanned aerial vehicles (UAVs) for coastal analysis. This technology allows more detailed studies with centimetric accuracy [23][24][25]. ...
Sedimentary coastal areas change rapidly and are economically and environmentally important. This research focuses on determining the extent to which natural dynamics and human activity have contributed to visible changes on Rodas, Cíes Islands in southwestern Galicia (NW Spain). The number of visitors to the islands has increased in recent years, and the port infrastructure has therefore been expanded. Previously, this zone experimented with important sand extraction phases. These changes have influenced the ecosystem directly by modifying the sedimentary behavior and indirectly by promoting even greater numbers of visitors to the area. Aerial images and orthophotographs of the study zone were examined to identify changes that have taken place over the last sixty-one years (1956-2017). Changes in the position of the shoreline, defined as the boundary of the dune vegetation, were mapped at different times between 1956 and 2017. Changes in the shoreline were quantified using GIS (Geographic Information System) technology and Digital Shoreline Analysis System (DSAS) software. The findings revealed that the system regressed by more than 30 m between 1956 and 1981, in part as a result of sand extraction. We also identified different erosion/accretion phases that occurred before the reformation of the Rodas dock in 2010. The system is currently undergoing important changes, especially in the northern area, with a regression of 14.14 m in the last seven years. In this context, LiDAR analysis from 2010 and 2015 using Geomorphic Change Detection (GCD) tools allowed variations in the dune system to be verified. The elevation in the study zone increased in 83% of the area, mainly in the frontal dune and close to the winter inlet (north sector). However, the variations were very small.
... The methodology can also be utilised with emerging technologies, such as UAV monitoring (Pérez-Alberti and Trenhaile, 2015aTrenhaile, , 2015bBiolchi et al., 2016) to provide greater spatial resolution on the mechanisms that facilitate the transport of intertidal boulders. Furthermore, deployment in coastal revetment blocks could help mitigate against damage or loss to coastal defence engineering works. ...
Boulder transport is an area of growing interest to coastal scientists as a means of improving our understanding of the complex interactions between extreme wave activity and the evolution of rocky coasts. However, our knowledge of the response of intertidal boulder deposits to contemporary storm events remains limited due to a lack of quantifiable field‐based evidence.
We address this by presenting a methodology incorporating Radio Frequency Identification (RFID) tagging and Differential Global Positioning Navigation Satellite System (DGNSS) technology to monitor and accurately quantify the displacement of RFID tagged boulders resulting from storm wave activity. Based on preliminary findings we highlight the suitability of the technology and methodology to better understand the spatial and temporal response of intertidal boulders to contemporary storm events.
We inserted RFID tags in 104 limestone boulders (intermediate axes from 0.27 to 2.85m) across a range of morphogenic settings at two sites on the intertidal shore platforms at Bembridge, Isle of Wight (UK). Fifteen topographic surveys were conducted between July 2015 and May 2017 to relocate and record tagged boulder locations (tag recovery rate: 91%). The relocated boulder coordinate data from both sites identified 164 individual transport events in 63% of the tagged boulder array amounting to 184.6m of transport, including the displacement of a boulder exceeding 10 tonnes.
Incidents of boulder quarrying and overturning during transport were also recorded, demonstrating that despite the relatively sheltered location intertidal boulders are created and regularly transported under moderate storm conditions. This suggests that contemporary storm events have a greater propensity to mobilise boulders in the intertidal range than has previously been realised.
Consequently, by documenting our methodology we provide guidance to others and promote further use of RFID technology to enable new hypotheses on boulder transport to be tested in a range of field settings and wave regimes.
... Very recent studies reveal the potential of drones in intertidal reefs to accurately monitor dominant algal communities (Murfitt et al., 2017) and the distribution patterns and patchiness of seagrass (Konar and Iken, 2017). Also, drones have been successfully used for monitoring rocky intertidal boulder position over time to investigate disturbance in Northern Spain (Pérez-Alberti and Trenhaile, 2015). ...
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.
Extreme storm events in coastal zones play significant roles in shaping the morphology of boulder beaches. However, boulder displacement and the geomorphological evolution of boulder beaches driven by different extreme storm events, especially typhoon events, remain poorly understood. Thus, boulder displacement and the geomorphic response on a boulder beach in Fujian, southeastern China, were explored before, during and after a cold wave event (Dec. 1–7, 2020) and before and after Typhoon In-Fa (Jul. 19–27, 2021), a large tropical storm. This was achieved by tracking 42 tagged boulders distributed in the intertidal and supratidal zones using Radio Frequency Identification (RFID) and topographic surveys using real-time kinematic techniques, respectively. The results showed obvious disparities in boulder displacement in different geomorphic zones due to cold wave and typhoon events that were mainly characterized by migration magnitude, range, direction, and mode of transport. The typhoon event led to rapid and substantial changes in the overall morphology of the boulder beach, while the cold wave event impacted the intertidal morphology of the boulder beach to only a small extent. The surrounding structure of boulders, beach slope and beach elevation had a combined dominant effect on boulder displacement under the same extreme event. Hydrodynamic factors (effective wave energy fluxes, incident wave direction, storm surge and water level) had dominant effects on boulder displacement during different extreme events. In terms of a single event, the magnitude of the boulder displacement driven by the typhoon was much greater than that driven by the cold wave. However, considering the frequency and duration of cold waves in winter, the impact of multiple consecutive cold waves on the geomorphology of the boulder beach cannot be ignored in this study area. Alternating and repeated interactions between these two processes constitute the complete geomorphic evolution of the boulder beach. This study contributes to improved predictions of the morphodynamic response of boulder beaches to future storms, especially large tropical storms, and facilitates better coastal management.
Nowadays, a traditional image of Galicia still survives showing the country as a uniform territory, but in fact there is a very different reality that materializes in a wide set of spaces with numerous contrasting landscapes. This is due to several factors: (a) firstly, the existence of an extensive coastal strip and a wide continental territory; (b) secondly, the existence of altitudinal contrasts, linked from the coast to the interior; (c) thirdly, different climatic and biogeographic environments that can be found both on the coast and inland; (d) and, fourthly, a long geomorphological and anthropic evolution affecting to this geographic area.
The contrast between the coast and the interior is, without a doubt, a first element that explains the diversity of Galicia’s landscapes. The length of its coasts-more than 2000 km-and its sinuous profile, with numerous inlets and outlets, due to the chaining of estuaries, bays, inlets, and capes, favored the genesis of contrasting spaces, with linked rocky coasts, beaches, lagoons, estuaries and rías and dune systems. In addi-tion, from the coast inland, a clear staggering of terrain levels is observed. On the same seashore stands a set of mountains, which reach 500/600 m of altitude. This is the case of A Capelada and Montes da Candieira, between the Ortigueira and Cedeira estuaries; O Barbanza, between those of Muros/Noia and Arousa; the Montes do Castrove, between those of Arousa and Pontevedra, or the Montes da Groba and O Galiñeiro, situated in the south of the Vigo estuary.
The above commented staggering becomes more relevant towards the interior. If a west–east cut is made, it is seen that the terrain rises to 1100 m in the western sierras, descends to 400/500 m in the interior of Lugo and Ourense, and reaches 2000 m again in the eastern sierras, in the limits with Asturias and Castilla y León. Also to add the existence of a wide and diverse fluvial network that is intensely embedded, with a large number of narrow and deep valleys that give rise to hills with a diversity of slopes. If lithological diversity is also added to this, it is easier to understand the reasons for the visible differences in the landscapes. From west to east, igneous and metamorphic rocks and, to a lesser extent, sedimentary rocks, are situated in a linking chain. On the Atlantic façade, granites and granodiorites are dominant. In the central regions, mafic and ultramafic rocks are present (as peridotites, eclogites, or gabbro). In the north, schists. In the center and in the south, again granites. In the eastern regions, there are slate, quartzite, limestone and dolomite bands, with small granitic intrusions.
This diversity of lithology, topography and endogenous or exogenous processes, that took place in Galicia over millions of years, has given rise to the presence of landscapes marked by geological factors and that are visible in different places in the Iberian northwest.
Keywords: Geomorphology Landscape, Galicia, Northwestern Iberian Peninsula
Galicia is a complex territory from the environmental (lithology, relief, climate) and socio-economic (population, distribution of productive activities) perspectives. Its geographical position has determined its paleoenvironmental dynamics throughout the last glacial cycle (Würm), in whose final phase a large part of the territory was under glacio-nival conditions, and has favored the emergence of favorable conditions for the domain of deciduous forests throughout the last two thirds of the Holocene. However, as a result of human colonization of most of this territory over the last 5,000 years, its vegetation cover has been greatly homogenized, first favoring the expansion of agricultural crops and scrublands (particularly heaths) and, already in recent times, forest plantations with introduced species (pines and Eucalyptus). The best examples of the characteristic vegetation of this territory take refuge in the steepest mountains, certain slightly altered coastal stretches, and some wetlands. Nonetheless, Galicia still harbors important elements (species, habitats) for the maintenance of biodiversity on a European scale. Its long-term persistence requires territorial planning and management of land use consistent with this singularity and in line with the evolving scientific knowledge and environmental regulations.KeywordsCurrent vegetationResidual forestsHeathsCoastal complexesWetlandsBiodiversity threats
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.
Understanding the interactions between dune systems and beaches is critical to determining the short‐term shoreline response and the long‐term resilience. In this study, almost 15 years of monthly beach/dune measurements were analyzed for three different profiles at Vougot Beach, France to understand and predict shoreline changes from intra‐annual to multi‐annual time‐scales. Four migration modes: advance/retreat (translation modes) and steepening/flattening (rotation modes) were identified through a centroid analysis. The analysis showed that translation and rotation can occur simultaneously, with long‐term trends of beach retreat and profile steepening (lower beach retreating and upper beach advancing), which was interrupted by two energetic wave events causing profile flattening (lower beach advancing and upper beach retreating). These two observations are evidence of how the sediment contribution resulting from the dune erosion events temporarily caused a large advance in the shoreline position. A recent modeling approach that accounts for different time‐scales is applied to predict the shoreline changes, showing significant improvements in comparison to a traditional shoreline equilibrium model when time‐scales related with the dune erosion and recovery are considered. The results showed that the dune system affects the beach profile evolution both spatially, with different impacts at different elevations along the cross‐shore profile, and temporally, by periodically redistributing the sediment in the system.
The coastal zone is extremely active, where multiple geodynamic agents converge, contributing to its constant morphological evolution. One of the most common coastal forms are cliffs, where it occurs mass movements that contribute to the change of its profile, representing a high risk for people and goods, both at its top and base. The evolution of coastal cliffs is spatially different and it is the result of a combination of various factors – such as morphology, rock strength, exposure of slopes and the action of erosive agents – which determine their stability.
In the context of coastal risk prevention and spatial planning, the monitoring of cliffs plays a key role in monitoring their dynamics, trends, and geomorphological changes. This work aims to present a low-cost experimental methodological approach, applied at a municipal level which, combining the use of an unmanned aircraft (UAS) with the application of the Structure-from-Motion with Multi-View Stereo (SfM-MVS) photogrammetry technique, allows to safely check for signs of instability and anticipate dangerous events; obtain high resolution and accurate cartographic data often; determine and quantify the changes occurred in a certain period; and, in a medium/long term, assess the cliff retreat rate and contribute to the identification of patterns that promote coastal erosion.
The results accomplished demonstrate the potential of this methodology. It contributes to the management of the coast and support the decision making. Through the products generated, the results prove the effectiveness and decimetric precision obtained, highlighting the dynamics of the coastline of the municipality of Torres Vedras.
Several occurrences of mass movements were recorded in a short period time, revealing signs of instability that during the bathing season can result in a high risk for exposed vacationers, demonstrating the importance of implementing a regular coastal monitoring plan.
The catalog of tsunami deposits in the western Mediterranean, with the exception of the Balearic Islands, is not very large. In total there are 82 localities, of which 65 correspond to the Balearic archipelago. From chose two sites have tsunamites while the rest correspond to large blocks on cliffs. This work is the result of a bibliographic scan of all the articles related to this type of event with the aim of facilitating an adequate weighing of the risks in this half of the Mediterranean. Tsunami frequency is highest in the central Mediterranean and high in the eastern Mediterranean where the vast majority of research work is found. The work describes the general characteristics of the western Mediterranean deposits and their relation with the mathematical models of propagation towards the coast. Resumen: El catálogo de depósitos de tsunami en el Mediterráneo occidental, a excepción de las Baleares, no es muy numeroso. En total son 82 localidades, de las que 65 corresponden al archipiélago Balear. De este conjunto tan solo dos localidades presentan tsunamitas, mientras que el resto corresponde a bloques de gran tamaño sobre acantilados. Este trabajo es el resultado de un revisión bibliográfica de artículos relacionados con este tipo de depósitos sedimentarios asociados a eventos de tsunami, con el objetivo de facilitar una adecuada ponderación de los riesgos en este tercio del Mediterráneo. La frecuencia de tsunamis es más alta en el Mediterráneo central y sobretodo en el oriental donde se encuentra la gran mayoría de los trabajos de investigación. El trabajo describe las características generales de los depósitos del Mediterráneo occidental y su relación con los modelos matemáticos de propagación hacia las costas. Palabras clave: Mediterráneo occidental, tsunami históricos, bloques, tsunamitas. REVISIÓN DE LOS DEPÓSITOS DE TSUNAMIS, BLOQUES Y TSUNAMITAS, EN LAS COSTAS DEL MEDITERRÁNEO OCCIDENTAL Review of tsunami deposits, blocks and tsunamites on the coasts of the western Mediterranean
Extreme storm events are known to produce, entrain, transport and deposit sizable boulders along rocky coastlines. However, the extent to which these processes occur under moderate, fetch‐limited wave conditions is seldom considered. In this study we quantify boulder transport at a relatively sheltered location subject to high‐frequency, low‐magnitude storm activity. This was achieved by deploying radio frequency identification (RFID) tags within 104 intertidal limestone boulders ranging in size from fine to very coarse (intermediate axis: 0.27–2.85 m). The study was conducted over 3 years (July 2015–July 2018) and encompassed numerous storm events. Tagged boulders were relocated during 17 field surveys and their positions recorded using a differential global positioning navigation satellite system (DGNSS).
On completion, we identified boulder displacement in 69% of the tagged array. The accrued boulder transport distance amounted to 233.0 m from 195 incidents of displacement, including the movement of a boulder weighing an estimated 11.9 t. Transport was not confined to autumn and winter storms alone, as displacement was also recorded during summer months (April–September), despite the seasonally reduced wave magnitude.
Boulder production by wave quarrying was documented in three tagged clasts, confirming observations that the shore platform is actively eroding. Incidents of overturning during transport were also recorded, including multiple overturning of clasts weighing up to 5 t. We further identify a statistically significant difference (maximum p‐value ≤ 0.03) between the transport distances attributed to constrained and unconstrained boulders, suggesting that the pre‐transport morphological setting exerts considerable control over boulder transport potential.
The findings establish low to moderate storm waves as a key component in the evolution of the study site. More broadly, we claim that high‐frequency, low‐magnitude storms regularly modify these overlooked rocky coastal locations, suggesting that the hydrodynamic capability at such sites may previously have been underestimated. © 2020 John Wiley & Sons, Ltd.
La costa de Galicia tiene más de 2.100 km de longitud. Se caracteriza por el encadenamiento de sectores de costa rocosa y arenosa y se halla situada en un contexto de alta energía, sometida al paso de borrascas y temporales marinos. Por ello, en el contexto del cambio global en el que nos encontramos, es fundamental conocer las dinámicas que interactúan en ella para identificar los sectores más vulnerables en el presente y prevenir posibles consecuencias negativas en el futuro. Este estudio analiza la su vulnerabilidad centrándose en las variables físicas que afectan a los primeros 100 m de la fachada marítima: pendiente, altitud, orientación, tipo de costa, litología, cambio en el nivel del mar, altura media de ola y distancia a la línea batimétrica de los 20 m.El uso de herramientas SIG ha permitido generar un índice de vulnerabilidad costera (CVI) con valores entre 1 y 5, de menor a mayor vulnerabilidad que muestra que el 4,52% de la fachada costera presenta valores de alta o muy alta vulnerabilidad, asociada principalmente a los arenales, mientras que el resto de la costa 57,96%, mayoritariamente costa rocosa, presenta unos valores de baja vulnerabilidad.
It is a long-standing maxim that coarser sediment sizes are associated with steeper beach-face slopes; but because most work has focused on sandy beaches, few data are available for the pebble, cobble, or boulder size ranges. Little is known, therefore, about how beach morphology and grain size relate at the coarser size grades. We compiled data from the literature—2144 measurements of beach-face slope with associated grain sizes—covering the range from very fine sand to boulders. This meta-analysis shows that beach faces do tend to steepen as average grain size increases, at least up to the cobble size range; but the trend is not simple. Although previous studies suggested a simple power-law relationship between grain size and beach-face slope, in fact the data distribution is best fit by a curve that is steep at the finer grain sizes but is much more gentle as grain size coarsens (and may trend downward for boulders).
Fine and medium sands have the greatest range of reported slopes and the data support the importance of effective-weight modification and boundary layer dynamics as primary controls on beach-face steepness at these fine size grades. Around the very coarse sand size grade, a plateau marks a shift in beach dynamics: as hydraulic conductivity increases, beaches switch to being infiltration-dominated. The trend toward steeper slopes continues, but at a lower and steady rate, from granules through cobbles. There appears to be a falloff in slope for the coarsest boulder deposits (but we note that this is based on very few locations, so should be considered preliminary).The overall data distribution is described by a power-law function: tan β = a (D50 − 0.125)^b + c . The broadness of each transition zone also shows that a range of factors, other than grain size, governs where that tipping point occurs.
In the cobble and boulder size ranges there are two deposit categories: boulder beaches, and supratidal boulder ridges. Although not commonly thought of as beaches, boulder ridges record long-term storm deposition and reworking on the highest energy coasts. Supratidal ridges and ‘regular’ boulder beaches seem to show different slope/grain-size relationships, with the steepest slopes occurring in boulder ridges, and with boulder beaches showing more gentle gradients than other coarse clastic beaches; but our dataset reveals how little quantitative information exists about the coarsest end of the beach spectrum, and invites further work to investigate the sedimentology of cobble- and boulder-dominated systems.
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.
Coastal boulder fields provide clues to long‐term frequency‐magnitude patterns of coastal flooding events and have the potential to play an important role in coastal hazard assessment. Mapping boulders in the field is time and labour‐intensive, and work on intertidal reef platforms, as in the present study, is physically challenging. By addressing coastal scientists not specialized in remote sensing, this contribution reports on the possibilities and limitations of digital applications in boulder mapping in Eastern Samar, Philippines, where recent supertyphoons Haiyan and Hagupit induced high waves, coastal flooding and boulder transport. It is demonstrated how satellite imagery of sub‐metre resolution (Pléiades, WorldView‐3) enables efficient analysis of transport vectors and distances of larger boulders, reflecting variation in latitudes of both typhoon tracks and approaching angles of typhoon‐generated waves. During the investigated events, boulders with a‐axes of up to 8 m were clearly identified to have been shifted for up to 32 m, mostly along the seaward margin of the boulder field. It is, however, hard to keep track of smaller boulders, and the length of a‐axes and b‐axes including their orientation is often impossible to map with sufficient accuracy. Orthophotographs and digital surface models created through the application of an unmanned aerial vehicle and the ‘Structure from Motion’ technique provide ultrahigh‐resolution data, and have the potential to not only improve the results of satellite image analysis, but also from field mapping and may significantly reduce overall time in the field. Orthophotographs permit unequivocal mapping of a‐ and b‐axes including their orientation, while precise values for c‐axes can be derived from the respective digital surface models. Volume of boulders is best inferred from boulder‐specific Structure from Motion‐based three‐dimensional models. Battery power, flight speed, and altitude determine the limits of the area covered, while patches shielded by the boulders are difficult to resolve. For some tasks field mapping remains mandatory and cannot be replaced by currently available remote sensing tools: for example, sampling for rock type, density and age dating, recording of lithological separation of boulders from the underlying geological unit and of geomorphic features on a millimetre to decimetre‐scale, or documentation of fine‐grained sediment transport in between the boulders in supratidal settings. In terms of future events, the digital products presented here will provide a valuable reference to track boulder transport on a centimetre to decimetre‐scale and to better understand the hydrodynamics of extreme‐wave events on a fringing reef coastline. This article is protected by copyright. All rights reserved.
The coast of Galicia corresponds to the NW end of the Iberian Penninsula and extends from the estuary of the Eo River, in the north, to the mouth of the Miño River in the border with Portugal. This coast has distinctive characteristics compared with the rest of the Atlantic Iberian coasts given the importance of geological and structural controls that define the deeply indented coastline. Although is a coast exposed to a energetic storm and swell wave regime, at different scales there are great variations in the orientation of the coastline that causes a high variability in the exposition and the energetic regime.
The coastline of Galicia is more than 2100 km long (POL Galicia 2010) (Fig. 2.1). Two broad types of coast can be differentiated in the region: zones with rías and zones without rías. Marine inlets dominate in the former, whereas rectilinear stretches dominate in the latter and only small coves or estuaries occur. The megaforms of coastal relief in northwest Spain are clearly determined by the tectonic structure, whereas lithological differentiation has played a predominant role in the genesis of meso and microforms (Pérez-Alberti and Blanco-Chao 2005). In general, different factors are involved in shaping the coastline: the overall structure is determined by tectonic processes; the lithology causes differential erosional processes that define the broad features of the coastal front; and, finally, the succession of geomorphological processes that have taken place over time have determined the specific forms and distribution of the different environments. In addition, human activity has affected many areas, particularly the low-lying coastline.
Las investigaciones que se llevan a cabo para alcanzar la tesis doctoral que lleva por título “Territorio, paisaje y planificación en el litoral mediterráneo andaluz. El área articulada por el sistema urbano de pequeñas ciudades: Motril, Vélez -Málaga y Adra” pretenden desvelar los factores y elementos que se están desarrollando sobre el territorio y que están provocando, desde hace algunas décadas, procesos de rearticulación que han terminado por configurar una nueva realidad territorial que ha roto con la tradición histórica de áreas desorganizadas y sin relación en torno a los escasos y pequeños valles costeros.
Hoy el eje costero está completamente desarrollado y tanto en el litoral como en el antepaís se están produciendo una serie de procesos de rearticulación, configurando así una gran área funcional a las que las grandes aglomeraciones de Málaga y Almería parecen ajenas. Estos procesos, de diversa índole, pero sobre todo atractivos para su análisis desde el punto de vista socioeconómico, se han visto reforzados por la reciente llegada de las infraestructuras. Por tanto, la tesis se ocupará de analizar estos procesos, los factores y elementos que están interviniendo en ellos y las repercusiones territoriales de los mismos con el objetivo de establecer, de cara a una mejor planificación y gestión territorial, los límites del área funcional que se está configurando y que queda totalmente ajena a las divisiones administrativas actuales.
Esta investigación busca profundizar en el conocimiento de las costas rocosas en general, y de las gallegas en particular. En el estudio se realizarán análisis a tres escalas: en la microescala se emplearán sensores TMEM para comprobar la microerosion en la costa sur gallega. En la mesoescala el estudio se realizará con sensores RFID para conocer los desplazamientos que se producen en las playas de bloques. Por último, en la macroescala se trabajará con información obtenida con el vuelo de drones para analizar el comportamiento de un sector acantilado. A partir de los resultados obtenidos y de la revisión bibliográfica, se analizarán los riesgos asociados a cada tipología costera.
A partir de los análisis anteriores, se elaborarán las recomendaciones necesarias sobre los riesgos existentes, para mejorar la gestión costera, promoviendo la conservación y restauración en las zonas en las que pudiese ser pertinente.
The coast of Galicia has more than 2.100 km for length. It is characterized by the chaining of rocky and sandy coast and it's located in a high energy context, subject to the passage of depressions and marine storms. In the context of global change, it is essential to know the dynamics that interact in the Galician coast to identify the most vulnerable sectors in the present and prevent potential negative consequences in the future. This study analyzes the vulnerability of coastal focusing on physical variables that affect at the first 100 meters of the coastal zone: slope, altitude, orientation, type of coast, lithology, change in the sea level, average wave height and the distance from de 20 meters bathymetric line. A coastal vulnerability index (CVI) is generated from the Geographical Information Systems (GIS) tools with values between 1 and 5, from lower to higher vulnerability.
This index shows that 4.52% of the Galician coast territory presents values of high or very high vulnerability, mainly associated with sedimentary zones, while a majority of the coast (57,96%), frequently rocky coasts presented values of low vulnerability.
Recent technological advances have made it possible to improve the accuracy of coastal studies through devices such as RFID (Radio Frequency IDentification). These sensors allow the monitoring of elements of the
environment at any time since its placement and offer a great level of detail. This research is developed in a boulder beach in Oia (Pontevedra), where 80 RFID sensors were installed in the natural boulders, without changing their position. The sensors were placed in lines of 5 elements perpendicular to the coastline. After the first winter, there is a general movement of the boulders. Of the 48 recovered, 81% have displaced more than 50 cm, minimum value to consider that there was movement. The greater displacements have occurred in boulders whose axes didn’t exceed 100 cm, with a maximum distance of 20,47 meters. As for the areas of greater mobility, it is clearly perceived that the central sector of the study area present greater variations in the position of the boulders, while the north and the lower part of the beach, where it is located coastal platform, are the most stable areas.
Coastal monitoring is fundamental to studying dune and beach behaviour related to natural and
anthropogenic factors as well as coastal management programs. Various tools have been used in recent years
for such investigations, including LIDAR, satellite images, terrestrial laser scanning, and photogrammetry,
allowing for both qualitative and quantitative analyses. These tools are applied based on the spatial and
temporal scales of the coastal zone being studied. In this study, seasonal coastal changes are monitored using
high-accuracy and high-quality photogrammetry frames acquired using unmanned aerial vehicle (UAV). The
UAV surveys were performed over two sessions during summer 2014 (September) and winter 2015 (March).
Additional data were also acquired during the strongest winter storms of the 2014-2015 season. The results
of these two UAVs surveys are compared to identify the changes that occurred on beaches and dunes due to:
i) anthropogenic changes such as tourist facilities and winter storm defence construction; and ii) the winter
storms of 2014-2015 from dune erosion, investigating the magnitude of impact across all coastal zones under
study. Moreover, this study demonstrates the efficacy and applicability of photogrammetry from UAVs for
coastal work, analyses whether it is a useful technique for scientific studies and authorities to use due to its
greater cost/benefit, and also whether it contributes to the application of local Integrated Coastal Zone
Management.
La preocupación por las regiones costeras ha sido muy importante a lo largo de la historia. En los últimos años, con la evolución experimentada por los SIG ha surgido una gran oportunidad de profundizar en su mejor conocimiento. La presente investigación se ha centrado en buscar una aproximación metodológica semiautomática a las costas rocosas que ayude a poder caracterizar mejor las costas rocosas. Para ello se han seleccionado las Illas Cíes. Se han usado ortofotografías y datos LIDAR, a partir de los cuales se ha generado información del terreno de 0,5 m de resolución. Su combinación ha permitido diferenciar 10 tipos de costa singularizados por su asociación a plataformas litorales, acumulaciones de bloques en su base o distintos perfiles topográficos, así como por enmarcarse en sectores acantilados o de sistemas playa-duna.
The role of rocky coasts as depositional environments has been generally neglected by rock- and depositional-coast workers. Although rock coasts can be hostile sedimentary environments, they frequently host a variety of deposits consisting of mixes of boulders and megaclasts to sand and cobbles. These deposits include pockets of sand and pebble trapped in topographic depressions or at the foot of scarps and cliffs, isolated boulders, pocket beaches trapped between headlands, and sand to boulder beaches in shallow bays exposed to strong wave action. The tectonic setting, which is reflected in the degree of geological heterogeneity alongshore, together with the wave regime and other elements of the morphogenic environment, help to determine the amount and mobility of the sediment, and consequently whether it plays an essentially abrasional or protectional role. Many beaches have a resistant rock foundation which will modulate their response to rising sea level and possibly increased storminess during this century. Wave refraction and temporal and spatial changes in sediment abrasional and protectional efficacy influence the development of crenulated planforms on coasts with a high degree of geological heterogeneity, and possibly on more geologically homogeneous coasts with longshore variations in cliff height or in the amount of sediment. Theory suggests that these coasts may develop equilibrium planforms with uniform rates of erosion in bays and on headlands, or they may undergo a series of cyclical transformations involving alternating increases and decreases in the depth of the bays relative to the headlands.
Baseline Surveys Ltd is a company which specialises in the supply of accurate geospatial data, such as cadastral, topographic and
engineering survey data to commercial and government bodies. Baseline Surveys Ltd invested in aerial drone photogrammetric
technology and had a requirement to establish the spatial accuracy of the geographic data derived from our unmanned aerial vehicle
(UAV) photogrammetry before marketing our new aerial mapping service. Having supplied the construction industry with survey
data for over 20 years, we felt that is was crucial for our clients to clearly understand the accuracy of our photogrammetry so they
can safely make informed spatial decisions, within the known accuracy limitations of our data. This information would also inform
us on how and where UAV photogrammetry can be utilised.
What we wanted to find out was the actual accuracy that can be reliably achieved using a UAV to collect data under field conditions
throughout a 2 Ha site. We flew a UAV over the test area in a "lawnmower track" pattern with an 80% front and 80% side overlap;
we placed 45 ground markers as check points and surveyed them in using network Real Time Kinematic Global Positioning System
(RTK GPS). We specifically designed the ground markers to meet our accuracy needs. We established 10 separate ground markers
as control points and inputted these into our photo modelling software, Agisoft PhotoScan. The remaining GPS coordinated check
point data were added later in ArcMap to the completed orthomosaic and digital elevation model so we could accurately compare the
UAV photogrammetry XYZ data with the RTK GPS XYZ data at highly reliable common points. The accuracy we achieved
throughout the 45 check points was 95% reliably within 41 mm horizontally and 68 mm vertically and with an 11.7 mm ground
sample distance taken from a flight altitude above ground level of 90 m.The area covered by one image was 70.2 m × 46.4 m, which
equals 0.325 Ha. This finding has shown that XYZ data derived from UAV photogrammetry has a similar practical accuracy to RTK
GPS, which is commonly used for cadastral, topographic and engineering survey work. This means that UAV photogrammetry can,
for the most part, replace GPS surveying as the main method of data capture for engineering projects, boundary mapping and
topographical surveying. Aerial Photogrammetry, in conjunction with RTK GPS, can now be used for projects with a 1:200 map
scale accuracy requirement.
The Udden-Wentworth grain-size scale is widely used as the standard for objective description of sediment, but it inadequately covers gravel, the dominant fraction in many environments such as alluvial fans. The scale is most detailed in the sand and mud fractions, where grades such as "fine sand" are defined by particle intermediate axial length (</,). We propose similar detailed grades for gravel with dl boundaries consistently determined by extending the Udden-Wentworth scheme of multiples of 2 (whole increments). The 2 to 4 mm granule class (- 1 to -2 (f>) in this system consists of just one grade, but the pebble class comprises four: fine pebbles with </, from 4 to 8 mm (-2 to -3 <£), medium pebbles from 8 to 16 mm (-3 to -4 <£>), coarse pebbles from 16 to 32 mm (-4 to -5 ), and very coarse pebbles from 32 to 64 mm (-5 to -6 cb). Coarser grades are/we cobbles with di from 6.4 to 12.8 cm (-6 to -7 tb), coarse cobbles from 12.8 to 25.6 cm (-7 to -8 tb),fine boulders from 25.6 to 51.2 cm (-8 to -9 <£), medium boulders from 51.2 to 102.4 cm (-9 to -10 </)> coarse boulders from 102.4 to 204.8 cm (-10 to -11 </>), and very coarse boulders from 204.8 to 409.6 cm (-11 to -12 tb). These terms can be used in Folk's texture classification to derive detailed descriptions such as "angular, poorly sorted, fine to coarse boulder conglomerate". This grain-size scheme is further extended to account for particles coarser than boulders (rf, > 4.1 m), which we collectively call megaclasts, and the sediment they comprise megagravel or, if lithified, megaconglomerate, Megagravel is divided into four classes based on </ including blocks from 4.1 to 65.5 m (-12 to -16 (b), slabs from 65.5 to 1049 m (-16 to -20 ), medium blocks from 8.2 to 16.4 m (-13 to -14 <b), coarse blocks from 16.4 to 32.8 m (-14 to -15 tb), very coarse blocks from 32.8 to 65.5 m (-15 to -16 (}>),fine slabs from 65.5 to 131 m (-16 to -17 ), medium slabs from 131 to 262 m (-17 to -18 tf>), coarse slabs from 262 to 524 m (-18 to -19 tb), very coarse slabs from 524 to 1049 m (-19 to -20 ), very fine monoliths from 1.0 to 2.1 km (-20 to -21 4>),fine monoliths from 2.1 to 4.2 km (-21 to -22 tb), medium monoliths from 4.2 to 8.4 km (-22 to -23 tb), coarse monoliths from 8.4 to 16.8 km (-23 to -24 tb), and very coarse monoliths from 16.8 to 33.6 km (-24 to -25 tb). These grades also can be used in Folk's texture classification for objective sediment description. We reserve the megalith class and five attendant grades for even coarser megaclasts, with dl spanning from 33.6 to 1075.2 km (-25 to -30 tb).
A boulder-strewn rock platform on an Atlantic-facing coastline in Gweebarra Bay, NW Ireland, is examined using an integrated geomorphological, ecological and geotechnical methodology. Here, a steep granite shore platform is cut by intersecting subvertical and subhorizontal fractures and shows a clear biological and geomorphological zonation associated with level within the tidal frame. Minimum surface strength values of the bedrock platform (as derived from Schmidt hammer tests) corresponds to the supralittoral zone, which is dominated by Ramalina siliquosa. Bedrock surface strength increases into the littoral zone, which is characterized by a succession through Verrucaria maura, Semibalanus balanoides and Mytilus edulis communities. Granite boulders in the upper intertidal zone are clustered and stacked into ridges that have a consistent spacing and northeast-southwest alignment. Boulders within the ridges are imbricated, stacked, and have west-facing dips. Boulder surface strength is higher on northwest-facing ridge sides than on southeast-facing sides, and is higher at the landward than seaward ridge end. Variations in surface weathering across the platform show where blocks have been recently removed by storms. Pulverized bedrock and boulder surfaces show where boulders have knocked into each other and been dragged across the platform by backwash. It is likely that such boulder features are formed and destroyed on decadal or shorter time scales.
Estimating the magnitude of a past tsunami from its deposits is one of the major topics to be developed in
future studies on tsunami hazard assessment. Main limitations are (1) the great variability of tsunami sandy
sheets deposited on land (the sediment source and the topography controlling many aspects of the
deposition), (2) the preservation of these soft sediments, and (3) the controversial interpretations of coastal
boulder accumulations. In this paper, we investigate sediment transport and deposition during the December
26, 2004 tsunami inflow and outflow in the Lhok Nga Bay, located 10 km west of the city of Banda Aceh
(northwest Sumatra, Indonesia). Side-scan sonar data of the near shore area are used to study the
morphometry and distribution of boulders offshore. Entrainment of finer sediments offshore is inferred by
estimating the movable grain sizes based upon the simulated current velocities of the tsunami waves. Results
demonstrate that the tsunami waves raised the bed shear velocities to levels above critical values for the
entrainment of coarse sands on the continental shelf. Most of the sediments deposited on land came from
offshore, from fine sands to coral boulders. With very high values of u⁎ (>30 cm/s), the outflow (backwash)
reworked and re-deposited large volume of sediments offshore. All rocky outcrops offshore were affected by
the tsunami (down to 25 m deep). The fraction of boulders transported from offshore and deposited inland
represents only 7% of the total number of boulders moved during the tsunami. Characteristics of the boulders
can help to estimate flow velocities required for detaching them, and their imbrication (if any) to infer flow
directions. However, calculations of flow depth and transport distance do not provide convincing results.
Future studies coupling offshore–onshore mapping of boulder accumulations with reconstitutions of the
morphological history (sea-level variations, coastal sediment discharge and landform evolution) may allow
distinguishing storms and tsunami deposits.
Experiments have been conducted on the settling rates of cylindrical-shaped grains in a fluid, having application to the settling of certain heavy minerals and fecal pellets in water. The settling objects were fabricated from circular glass rods with diameters 3-8 mm, giving length to diameter ratios ranging 2-12. Glycerine was used as the fluid in the experiments, yielding Reynolds numbers equivalent to quartz-density silt and fine sand grains of similar shape settling in water. Analysis of the data shows that the settling velocity can be calculated with a modified Stokes relationship which is applicable only at low Reynolds numbers, analogous to the Stokes region for the settling of spheres. - from Author
Boulder accumulations occur along a stretch of rocky coast of about 1.5 km near the French Mediterranean city of Martigues. The boulders occur up to 100 m inland from the present shoreline and some contain marine bio-constructions that are proof of residence in a subtidal or intertidal setting. The setting, spatial distribution and morphologic characteristics of these boulders indicate that they were detached from the rocky shore platform and transported landward by high-energy waves. The size, position and distance from the shoreline of 1475 boulders were measured in order to determine their volume and mass, as well as the conditions under which they were transported landward to their present positions. The results were then statistically analyzed and confronted with hydrodynamic models commonly used to evaluate the charactestics of the transporting waves. The wave characteristics thus obtained were compared to recorded and modeled extreme waves in the region. Dating of the boulders shows age ranges that correspond to the Little Ice Age (LIA), thus suggesting a relationship between their deposition and the high storm frequency that characterized the LIA. The results also indicate little likelihood of a tsunami origin for these boulders, although there is historical evidence of tsunamis in this region. The study insists on the potential for stormrelated hazards on this heavily populated and industrialized part of the Mediterranean coast of France.
Deposits formed by extreme waves can be useful in elucidating the type and characteristics of the depositional event. The study area on the southeast coast of the island of Hawaiʻi is characterized by the presence of geologically young basalts of known age that are mantled by recent wave-derived sedimentary deposits. The area has been impacted by large swells, storms and tsunamis over the last century, and in combination with known substrate ages makes this an ideal location to study recent deposits produced by such events.Three distinct coarse-clast deposit assemblages can be recognized based on clast size, composition, angularity, orientation, packing, elevation and inland distance of the deposit. These deposits are characterized as one of three types. 1) Gravel fields of isolated clasts, primarily boulder-size material, and scattered pockets of concentrated sand and gravel in topographic lows. 2) Shore-parallel and cuspate ridges composed mostly of rounded basalt gravel and sand with small amounts of carbonate detritus. The ridges range in height from about 1 to 3 m and are 10s of m wide. 3) Cliff-top deposits of scattered angular and sub-angular clasts along sea cliffs that are generally greater than 5 m elevation. The gravel fields are primarily of tsunami origin from either the 1975 Kalapana event, or a combination of the 1975 tsunami, and 1868 tsunami or earlier events. The ridge deposits are presently active and sediment continues to be added during high wave events. The cliff-top deposits contain evidence of deposition by both tsunami and storm processes and require further investigation.
At exposed sites on the deep water coasts of the British Isles, cliff-top storm deposits (CTSDs) occur on cliffs at a variety of elevations above sea level and may occasionally reach up to 50 m above sea level. Time-series field mapping of CTSDs has demonstrated their formation over the historical period and their continued modification during major past storms. This paper seeks to clarify the morphogenetic context of CTSDs, model the wave conditions and forces encountered at the cliff-face and cliff-top platform and propose mechanisms to link wave processes to cliff-face quarrying and landwards cliff-top transport of quarried blocks to deposition zones at the rear of the cliff-top platform. We report on wave-tank experiments using scaled cliff and wave conditions from a cliff in the Shetland Islands to focus on three situations: incident waves lower than the cliff edge height; at the same height as the cliff edge height; and higher than the cliff edge height. The modelling suggests that steep waves of 10 m and above impacting on a 15 m high cliff will result in impact pressures sufficient to promote crack propagation, block detachment and lifting of large blocks. Large, but not necessarily steep, waves of the same height as the cliff edge produce sufficient impact pressures and water flow over the cliff edge and platform to entrain blocks, transport and deposit them on the cliff-top. Where cliff-top height is below wave crest elevation “green water” bore flow occurs, sufficient to force rotation or lifting of blocks out of cliff-top and cliff-top platform ‘sockets’. High flow velocities rapidly accelerate and transport blocks inland until the flow attenuation results in deposition of blocks at the limit of run-up. The model results provide a good explanatory framework to account for the quarrying of the upper part of the cliff-face and cliff-top platform under storm wave conditions and provide an insight into the exceptional velocities experienced over the cliff-top platform under bore flow conditions. The modelling results show that extreme storm waves are capable of quarrying, transporting and depositing large blocks at altitude and significant distances inland and so present serious questions about the use of such deposits as diagnostic of palaeo-tsunami.
This paper reviews the common theories for determining the magnitude and distribution of wave forces on vertical walls and sloping, rubble-mound breakwaters and compares these theories, selecting the best available for use. The results of the better theories are compared with experimental data, and an outline is given of experimental investigations considered necessary to obtain data sufficient for the design of economical and safe breakwaters.
Using a critical value for ϒb = Hb/hb as a wave breaking criterion, where Hb and hb are respectively the wave breaker height and depth, applying Airy wave theory, and assuming conservation of the wave energy flux, one obtains Hb=kg1/5(TH∞2)2/5 relating Hb to the wave period T and to the deep-water wave height H∞. Three sets of laboratory data and one set of field data yield k = 0.39 for the dimensionless coefficient.
The relationship, based on Airy wave theory and empirically fitted to the data, is much more successful in predicting wave breaker heights than is the commonly used equation of Munk, based on solitary wave theory. In addition, the relationship is applicable over the entire practical range of wave steepness values.
The critical threshold mass for boulders composing a beach is the mass of the largest stone entrained by the hydraulic forces associated with wave breaking and swash run-up. For any given storm event there is a maximum boulder mass that can be moved and another slightly larger boulder that has the minimum mass necessary to remain stable. Two equations are derived: one to estimate critical threshold mass and another to estimate minimum stable mass for boulders on a beach. The equations incorporate: stone density, beach slope, breaking wave height, water depth, wave period, run-up height, maximum swash velocity and average swash velocity. In both equations the wave force applied to the beach face is scaled relative to the elevation that wave energy raises the water surface. Scaling the wave force relative to the run-up elevation results in a critical threshold formula. This is given as equation (45). Its predictions accurately match field data giving the largest boulder transported on a beach during storm events. Scaling the wave force relative to the breaking wave height at the toe of the beach results in a stability formula. This is given as equation (46). It predicts stable mass in the range defined by the Hudson formula. Equation (46) has an advantage over the Hudson formula by incorporating the physically important parameters of wave period and swash velocity. Both equations could be useful in the initial evaluation and design of dynamic revetments constructed with quarry stone.
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.
A combined field and laboratory study in northern Lake Erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non-magnetic sediment. Work was conducted on the cross-shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement, in the field, of samples of magnetic (magnetite) and non-magnetic (quartz and calcite) grains, respectively. Laboratory experiments examined how the burial of small, dense magnetic minerals is affected by the grain size of the non-magnetic host material, and how grain burial affects magnetic susceptibility measurements at the surface. The field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser, non-magnetic grains under low to moderate wave conditions, and subsequently were unable to move in the longshore or cross-shore directions. The laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals, and that magnetic grain burial was most effective beneath coarser rather than finer non-magnetic sand and, for the latter sediments, under less rather than more energetic conditions. The results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions, and subsequent settling, exposure and concentration in the upper swash zone during more energetic periods, when the non-magnetic grains are eroded. It is probably during these erosional periods, when the magnetic minerals are exposed in fairly homogeneous deposits, that longshore and cross-shore transport takes place.
We have conducted hydraulic experiments in an open channel with cubic
and rectangular shaped solid blocks on the slope for investigating the
boulder transport process by tsunami. In our experiments, the block was
mainly seen to be transported by a bore due to rolling or saltation
rather than by sliding. Previous models for the boulder transport by
tsunamis assumed sliding as a mode of transport for the boulder.
Therefore, these models underestimated the distance of the boulder moved
by the tsunami when it was transported due to rolling or saltation. In
this study, we have developed a practical model for the transport of a
boulder by tsunami, which takes into account the various transport
modes. We introduce an empirical variable coefficient of friction by
assuming that the coefficient decreases with decrease in ground contact
time when the block was transported by rolling or saltation. With the
aid of this parameter, the model can explain various modes of transport,
i.e., sliding, rolling, and saltation, and reproduces the experimental
results well. We further applied this improved model to a tsunami
boulder at Inoda area in Ishigaki Island, Japan, which was transported
by the 1771 Meiwa tsunami. The calculated distance of transport of the
boulder was approximately 650 m, which is consistent with the
description in the historical document. Based on our calculations, we
estimated hydraulic values of the tsunamis. Estimation of such hydraulic
values is important for understanding the behavior and power of the
historical tsunamis, besides aiding future disaster mitigation efforts.
This paper is the first review of coastal boulder studies; it serves as a peer-reviewed introduction to a special issue on the topic of ‘Boulders as a signature of storms on rock coasts’. Since 2004 and the Indian Ocean tsunami, most coastal boulder research has been focused on using boulders as sedimentary signatures of palaeo-tsunami events and the most commonly used transport equations typically suggest that large boulder deposits are products of tsunami rather than storms. There is growing empirical evidence demonstrating (on inter-annual to century timescales) that storm waves are capable of transporting and depositing boulders of significant size at elevations well above sea level. Current process studies of how boulders are eroded, transported and deposited have not received wide attention by rock coast geomorphologists or palaeoenvironmental reconstruction scientists. This special issue is framed around the goal of highlighting our current understanding of the role of storm waves as a driving mechanism for boulder erosion, transport and deposition. Here, we review the current state of knowledge about boulder dynamics, boulder responses to storms and boulders as sedimentary signatures of storm events. We identify gaps in our conceptual and quantitative understanding. Lastly, we propose means of addressing research gaps, improving consistency between researchers and collecting multi-purpose data. This review and the papers contained within the special issue provide an improved understanding of coastal dynamics; particularly the role storms can play in influencing erosion rates and depositional patterns of coarse material (i.e. boulders or larger), which has hitherto been under represented in the literature.
On 10 March 2008, a particularly high energy storm hit the French Atlantic coast and the western part of the English Channel. This storm did not generate exceptionally high waves: significant and maximum wave heights recorded in the Iroise Sea (Brittany) reached 10.85 and 18.17m respectively, whereas during the 1989–90 winter storms, they had reached 12 and 20m, respectively. The exceptional character of the March 2008 storm event arises from the fact that it occurred during an exceptional spring tide. From a morphogenetic point of view, the effects of this storm in terms of block quarrying, transport and deposition on Banneg Island (Brittany) were significant. This study shows that the weight of the blocks displaced during the event was between 0.07 and 42.64t, with a median value of 0.72t. More than 60% of the blocks were quarried from the wave-scoured cliff-top platform, demonstrating that the favoured zone for supplying material was located inland of the cliff. Two dominant modes of transport were involved depending on the relationship between extreme water levels and cliff height. At the centre of the island, the height of the waves breaking over the top of the cliffs on the western coast resulted in a torrential surge that flowed out towards the eastern coast of the island over a landward-sloping platform. On the western coast of the island, blocks weighing between 0.3 and 1.4 t were displaced between 50 and 90m from the cliff edge by this flow. At the flow outlet on the eastern coast (90m from the western cliff edge), a pit 1.6m deep was excavated at Porz ar Bagou cove and some of the mobilised blocks were deposited in two parallel lobes about 40m seaward of the eastern shoreline. Elsewhere on the island, block transport occurred by airborne projection although wave heights were lower than the altitude of the cliffs. The pressure exerted by breaking waves on the bedrock was sufficient to quarry and displace blocks. A temporal comparison of the changes recorded in the double and triple boulder ridges showed that the most seaward ridges were practically untouched with the most important changes occurring in the second and third ridges. These observations allow us to propose a spatio-temporal model for the accretion of cliff-top storm deposits (CTSDs), with the various stages of CTSD formation being directly related to the morphological evolution of the cliff.
Size and mass of boulders and megaclast deposited from intense wave events of the past coupled with measures of depositional elevation, and transport distance are useful metrics to distinguish between tsunami or storm wave period. This outcome provides for a wave-competence scaling approach to better help distinguish between large storms and tsunamis because wave period for tsunamis and wind driven storm waves differ by an order of magnitude. However, wave-competence equations are simple linear approximations of a complex non-linear processes, hence interpretations of the results need to be taken carefully. At best they may help approximate wave forcing based on size of material transported coupled with other pieces of information gained from stratigraphy, imbrications and morphology of boulder and megaclast deposits, as well as aging techniques for temporal correlation with known storms and/or tsunamis. The focus here is to present a conceptual example of how to apply a wave-competence approach to a beach exposed to both tsunamis and major storm waves and how variance in backshore topography (hills and valleys) and source material (sea cliff erosion and scree slope development) may play into interpreting the mathematical results.
In this paper we examine the sedimentological properties and morphodynamic evolution of a boulder beach at Huayanpeng Cape on Putuo Island off the southeast China coast. Focus is given to the formation of the boulder beach (256–2000mm diameter) and resulting morphology in relation to high-energy storms and seasonal typhoon. Holocene sea level rise to the present position at ca. 7000–6000yrs BP provides a long term window for the formation of the beach. The beach is primarily characterized from east to west by: 1) the convex to concave beach morphology, 2) reduced longest axis of the skeleton boulders, 3) decreased flatness of the skeleton boulders, and 4) greater sphericity of the skeleton boulders. Infilling cobbles/pebbles in the beach demonstrate a bi-modal distribution, corresponding to dominant sizes of 100–250mm and 50–100mm in diameter, on the eastern beach and western beach, respectively. This reflects changed intensity of storm waves alongshore. Multiple-peaks of the distributions of infilling clasts for the western beach (supratidal) probably reflect storm and calm weather sedimentation. Statistically, both the skeleton boulders and infilling clasts follow a β-distribution, reflecting the high-energy control of beach formation and the dominance of crushing in boulder size reduction. A boulder transport model applied in the present study further reveals that boulders with an a-axis length of 500–1000mm (the dominant component on the beach) move under a wide range of wave conditions depending on wave height and wave period with a cluster of critical wave heights in the range 2–6m and wave periods of 5–15s.
Field observations from previous studies of boulder movements were compiled to augment our own work on boulders that have been displaced by storm waves during recent typhoon events in the Okinawa Islands, Japan to elucidate the ability of storm waves to move boulders. Our observations reveal that recent storm waves displaced by sliding and overturning 100-ton boulders emplacing them on the reef or high cliff tops and that storm waves might also be capable of displacing 200ton boulders on the reef. The weights of storm wave boulders at the Okinawa Islands are of comparable order to those boulders displaced by historical tsunami origins. Consequently, boulder weight alone is an inappropriate parameter to discriminate between tsunami or storm wave processes. However, these heavy storm wave boulders are close to the reef and cliff edges, while tsunami boulders can be deposited much further inland. Hence, horizontal displacement distance of boulders could be a useful parameter to discriminate boulders deposited by the tsunami and storm waves on the wide fringing reef. The storm wave boulders were characteristically concentrated on the southeastern (Pacific Ocean) sides of each island but large boulders are rarely found on the northwestern (East China Sea) side. This is probably because the storm wave intensities are generally stronger at the southeastern side than at the northwestern side, although differences of reef strength and initial condition of boulders should also be taken into account. Consideration of the high frequency of typhoons at the area suggests that effects of the storm waves and the consequent displacement of boulders on the reef might have contributed to the formation of the reef-moat framework that typifies the Okinawa Islands especially, if the moat is located within the transport limit of the storm wave boulders (approximately 300m from the reef edge at the islands).
Coastal boulders are good evidence of high-energy events, but the distinction between storm and tsunami boulders remains difficult to identify and mathematical models are still in their preliminary stages. In a pioneering contribution, Nott (1997, 2003) developed hydrodynamic equations to assess the minimum wave height required to initiate transport of a coastal boulder by tsunamis or storm surges. These equations are widely cited and used, but they can be improved. In this study, Nott's equations have been revised: (1) the equation for the submerged boulder scenario has been revised by rearranging the lift area of the lift force, (2) the subaerial boulder scenario has been reconsidered by rearranging lift area and omitting inappropriate use of inertia force, and (3) the joint bounded scenario was revised by balancing force components in the lifting direction, and the effect of slope at the pre-transport location is tested. Calculations are performed for four case studies: boulders in Western and Eastern Australia (data after Nott, 1997, 2003), boulders in southeastern Italy (data after Scicchitano et al., 2007), storm boulders in Iceland (data after Etienne and Paris, 2010), and 2004 tsunami boulders in Sumatra (data after Paris et al., 2009).
The rock coast of northwest Ireland comprises steep cliffed headlands and more open coastal sections where bedrock shore platforms are developed. Many shore platforms are overlain by boulders; the locations on the platform from which boulders are derived are marked by ‘holes’ of fresh and unweathered rock surfaces that are not, or are poorly, covered by lichen. These areas of boulder detachment are termed sockets. This paper examines the mapped distributions and physical properties of boulders, sockets and shore platform context of an Atlantic-facing granite shore in County Donegal, northwest Ireland. Results from Schmidt hammer rebound tests show statistically-significant differences in rebound values between areas inside and outside of sockets and between sockets and boulders. Based on their distributions and physical properties, relationships between sockets and boulders are explored. We calculate that sockets are formed rapidly by winter storms but are also rapidly weathered over c. 5years, becoming indistinguishable from the surrounding bedrock platform. We argue that, in contrast to some studies, boulders here were formed during recent winter storms (episodically during the last 150–200years) rather than by more ancient waves or by tsunamis. However, a significant proportion of boulders (c. 20%) are morphometrically dissimilar to sockets; we argue that these were formed by infrequent and unusually-powerful waves that stripped whole bedrock sheets off the platform surface and which detached boulders from the lower shoreface.
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.
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.
Abstract Five boulder beaches located along the coast of New South Wales, Australia, were examined in order to determine if beaches composed of boulders differ in basic sedimentological structure and behavior from beaches composed of cobbles or pebbles. Each beach is aligned obliquely to the approaching waves and is composed of locally derived sediment. Investigation of beach-particle size, morphology, and roundness, along with foreshore slope, reveals consistent up-beach particle fining, positive size skewness, absence of shape zoning, absence of sphericity grading, and low foreshore slope. These characteristics of the five studied boulder beaches contrast markedly with the known characteristics of beaches composed of finer sediment, which suggests that boulders appear to form fundamentally distinct coastal sedimentary assemblages.
Important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain. A series of experiments has been undertaken to determine how this angle depends on grain shape (rollability and angularity), on the ratio of the size of the pivoting grain to those beneath, and on factors such as imbrication. The experiments involved gravel‐sized spheres (ball‐bearings and marbles), natural pebbles selected for their approximately triaxial ellipsoid shapes, and angular crushed basalt pebbles. The pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined, the angle of the board being equal to the pivoting angle at the instant of grain movement.
The pivoting angles of spheres showed reasonable agreement with a theoretically derived equation, showing much better agreement than in previous studies which utilized sand‐sized spheres. A series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size. Our results are therefore consistent with the earlier studies limited to sand‐size spheres. The cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out. Similar size dependencies are also found for the ellipsoidal pebbles and angular gravel.
The experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle, being a function of the ratio of the pebble's smallest to intermediate axial diameters. This ratio controls the grain's ability to roll and pivot; with small ratios of these diameters the pebbles tended to slide out of position, whereas with ratios closer to unity (circular cross‐section) true pivoting took place and the angles were smaller. Experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position, the pivoting angle being increased approximately by the imbrication angle. The angular crushed gravel also required high pivoting angles, apparently due to interlocking of the grains resulting from their angularity.
Other factors being equal, the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres, ellipsoidal grains, angular grains, and imbricated grains. The results obtained here make possible the quantitative evaluation of these shape effects on grain threshold, as well as evaluation of the selective entrainment of grains from a bed of mixed sizes.
Observations of the threshold of movement of loosely packed gravel in a tidal current are described. For gravel with equivalent ‘spherical’ diameters D in the range 0.2 ≲D≲ 5.0cm the critical friction velocity u*c, corresponding to the initiation of sediment transport, is given by u*c=7.0 D0.2. At large values of D within the quoted range, the value u*c is significantly lower than would be obtained by a Shields experiment (u*c∞D0.5). By comparing our values of u*c with those obtained under well-controlled laboratory conditions, the discrepancy with Shields is shown to be due to the open spacing between, and exposure of, individual pebbles on the seabed. By comparing our results with those from upland gravel streams and flume experiments, it is suggested that Shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0c is valid.
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.
Understanding the distribution of wave energy on shore platforms is important for models of the long term development of both shore platforms and backing cliffs. Little is known, however, of the magnitude of the wave currents generated on shore platforms in storms. This study uses the distribution of boulders of different sizes on a shore platform on the North Sea coast of Scotland to investigate patterns of storm wave current velocities. In storms over the last 40–240 yr, blocks as large as 9 m 3 have been quarried from the rock step at LWMS and boulders of N 0.5 m 3 have been moved landward over extensive areas of the platform. In earlier storms, boulders N1 m 3 have been moved widely. Wave current velocities in storms have likely reached 3–4 m/s on many parts of the platform, especially in the slightly deeper water along smooth-floored strike canals developed in marl beds. Attenuation of velocities across the platform is limited. Storm wave action is a key process in erosion across large parts of this shore platform. Wave impacts quarry large blocks at or near LWMS and wave current velocities in storms are sufficient to transport landwards large clasts and smaller debris. Weathering of soft marl beds also contributes directly and indirectly to development of the shore platform. Lowering of marl beds at long term rates of ~ 1 mm/yr in response to recent sea-level fall leads to the undercutting and eventual collapse of overlying, more competent rock beds, releasing blocks for wave transport. Deepening of strike canals, by both water-layer weathering and wave action, allows faster wave currents to reach and erode cliffs at the rear of the platform. On this exposed, macro-tidal coast, shore platforms are eroded by both wave action and weathering processes operating over various spatial and temporal scales.
The pre-transport environment of a coastal boulder along with its shape, size and density determines the height of wave required for it to be transported. Different forces act on sub-aerial boulders as opposed to submerged boulders when struck by a wave. Boulders derived from joint bounded blocks on shore platforms predominantly experience lift force and require a wave of greater height to be transported than boulders in other environments. No one equation is applicable to determine the height of palaeo-waves responsible for depositing a field or ridge of imbricated coastal boulders. A range of equations and their derivation is presented here which can be applied to the respective pre-transport environment of a boulder. Such an approach is necessary when attempting to reconstruct the frequency and magnitude of past coastal wave hazards and for differentiating between tsunami and storm wave deposited boulder fields. ß 2003 Elsevier Science B.V. All rights reserved.
A shore platform on the western coast of Galicia in northwestern Spain has been inherited from interglacial stages when sea level was similar to today. The wide, gently sloping intertidal platform is backed in places by supratidal rock ledges, and in other places by a steeper and narrower supratidal ramp. The gradient of the intertidal platform is consistent with the relationship between platform gradient and tidal range, but the slope of the ramp is much too high. The abandoned and degraded sea cliff is grass-covered along most of this coast, and the ledges and the ramp, which extend up to several metres above the highest tides, are covered by lichen and, in places, by salt-tolerant plants. Radiocarbon-dated sediments in the cliff, which range up to 36 000 years in age, lie on top of an ancient beach deposit. The former beach, remnants of which are found in situ on the ramp and rock ledges, as well as two caves that are filled with the dated sediments, are probably last interglacial in age. The morphological and sedimentary evidence suggests that the supratidal ramp and ledges were also formed during the last interglacial stage, whereas the wider intertidal platform is probably the product of several older interglacials, when sea level was generally similar to today. A general model is proposed for the inheritance of shore platforms in macro- and microtidal environments. Copyright © 1999 John Wiley & Sons, Ltd.
Hypotheses advanced concerning the origin of the Pleistocene Hulopoe Gravel on Lanai include mega-tsunami, abandoned beach, ‘multiple event,’ rocky shoreline, and for parts of the deposit, Native Hawaiian constructions and degraded lava flow fronts. Uplift of Lanai shorelines has been suggested for deposits occurring up to at least 190 m. These conflicting hypotheses highlight problems with the interpretation of coarse gravel deposits containing marine biotic remains. The geological records of the processes implied by these hypotheses should look very different. Discrimination among these or any other hypotheses for the origins of the Hulopoe Gravel will require careful study of vertical and lateral variations in litho- and biofacies, facies architecture, contact relationships and stratal geometries of this deposit.
This study investigates the size, position and the long axis orientation of 210 boulders at Kudaka Island, Japan. These boulders were deposited from the reef crest to the slope of the back reef moat, distributed within 275 m from the reef edge. Most boulders were rectangular to ellipsoidal, without sharp broken edges. They are reef rock fragments estimated as < 63 m3 (< 127 t). The second largest boulder (54 t) was not observed in aerial photographs taken in 1977 and 1993, although it appears in photographs taken in 2005 and 2007. Considering that no large tsunami event occurred during 1993–2005, the second largest boulder is expected to have been emplaced by typhoon-generated storm waves. Moreover, the positions of many boulders were found to have shifted after 1977. These boulders were highly likely to have been repositioned by the storm waves. Results showed that boulders' motion follows an exponential fining trend shoreward. This trend fits well with the distribution of the height of the storm wave after breaking on the reef flat. The largest storm waves after 1977 (typhoon 0704 in 2007) were probably responsible for the current boulder distribution. Using the relation between the distributions of boulders and the significant wave height of typhoon 0704, the approximate transport distance of boulders by an arbitrary storm wave at the island can be estimated. The storm wave boulders' distribution is also useful to estimate the storm wave properties: we estimated the maximum current velocity distribution of waves generated by typhoon 0704 on the reef flat as up to 6.5 m/s using the boulder distribution.
Coastal boulder accumulations are often mentioned in the literature, even though their interpretation remains difficult, especially along rock coasts affected both by storms and tsunamis. Studies on the geomorphic impact of such high-energy events are actually of great interest, since their intensity and frequency are key issues for the future evolution of coasts in the framework of the global change. The southwest coast of Iceland faces the powerful storms of the North Atlantic Ocean, with wave heights of more than 15 m. The probability for past and present tsunamis to hit this coast is very low. In this paper, we describe boulder accumulations along the volcanic rock coast of Reykjanes (southwest Iceland). They consist of cliff-top boulders, clusters and ridges, beaches, and boulder fields. Large boulders, up to 70 t in weight, have been transported and deposited up to 65 m inland (6 masl). The maximum limit of boulder deposition and driftwood was found respectively 210 m and 550 m inland. Storms appear to be a predominant factor in the geomorphic evolution of Reykjanes coasts. Our observations also give new insight for the interpretation of coastal boulder accumulations. Processes of erosion and deposition by tsunamis are a rising topic in the literature, and the effects of recurrent and powerful storms are neglected.
This study is based on the morphosedimentary analysis of the cliff-top storm deposits accumulated on the coast of Banneg Island located in the archipelago of Molène (Brittany, France). These CTSDs comprise large, tabular clasts quarried from the upper part of the cliff and the backing scoured platform by giant oceanic storm waves impacting directly the western coast of the island. An analysis of the distribution and the geomorphology of these accumulations were carried out using the DGPS topographic surveys. Most of the clasts are organised into imbricate boulder clusters or ridges deposited between 7.5 and 14.5 masl. 52 accumulations were inventoried from the north to the south of the island, representing a global volume of 1000 m3. The median size of the clasts calculated is equivalent to 0.8 × 0.6 × 0.4 m and a weight of 0.6 t. The largest one measuring 5.3 × 3.9 × 0.5 m (≈ 32 t) is located in the centre of the island (ridge #28). It has been deposited 14 m inland from the edge of the cliff at the elevation of 9 m. Sediment analysis shows that clast sizes become smaller with increasing distance from the shoreline, but there is no relationship between the sorting and the distance inland. A study of the hydrodynamic conditions inducing clast transport was undertaken by an analysis of the wave data from the 1989 to the 1990 winter storms. Models of wave runup indicate that their highest water levels may have reached up to 19 masl, 5 to 10 m higher than the top of the cliff. Submersion by giant storm waves has been more important and frequent in the centre and the south of the island. Wave data over the 1979 to the 2007 period shows that no events as powerful as those of the 1989 to the 1990 winter were recorded during the last 30 years. Yet, it appears that the 1979–1990 decade was characterized by important morphogeneous events while the following period (1990–2007) has experienced a sharp decrease in storm events. These variations could be attributed to the inversion from a negative towards a positive phase in the North Atlantic Oscillation index.
In the frame of its research concerning real-time positioning and control of road construction equipment, the Laboratoire Central des Ponts et Chaussées, has carried out in 1996 a study to know more about the actual vertical accuracy that a real-time kinematic (RTK) global positioning system (GPS) sensor could reach, under work site conditions.This study has widely used the dedicated testing facility called SESSYL, built to perform high-accuracy and real-time evaluation tests on positioning systems. It has been performed in collaboration with the French road contractor COLAS and the Ecole Supérieure des Géomètres et Topographes (ESGT).First, there is the proposed adapted geodetic transformation procedure, compatible with the high accuracy requirements. Then, the main results of a special SESSYL tests program are presented, where the impacts of several influencing parameters on the vertical accuracy have been carefully examined.The core part of the paper is the analysis of the typical RTK GPS set of data, from which we have tried to extract two different components: a high-frequency noise, rather easy to filter, and a low-frequency bias. This bias, given its good repeatability, can be modelled and used in prediction to improve in real-time the raw accuracy of the data.As a full-scale validation of our study, a site experiment is finally described, carried out this time on a real piece of equipment (an asphalt paver) during real roadwork.
Past occurrences of major tsunami in the global record have often been inferred from the presence of megaclast accumulations at significant heights above sea level, which sometimes exhibit a pronounced seaward-directed imbricate fabric. The Aran Islands, off the west coast of Ireland, exist in a marine environment which is totally unprotected from the most extreme sea states encountered in the North Atlantic. Spectacular megaclast accumulations are found along the tops of vertical cliffs on these islands. The elevation of these deposits varies from 0 m to 50 m above mean sea level and they include clasts weighing 250 tonnes at sea level, over 117 tonnes at 12 m and 2.9 tonnes at 50 m above sea level. Imbrication measurements on these megaclasts indicate a well defined and consistent mean direction from the southwest for the emplacing waves, one that is consistent with the prevalent storm wind direction for the area. The megaclast accumulations are subject to episodic and on-going erosion by cliff retreat but the existing accumulations are estimated to record events since at least 1839. Recent emplacement and reworking is evidenced by trapped plastic detritus within the deposits and eyewitness accounts of large storms during which clasts have been emplaced. Similar deposits are found on the exposed coasts of the Shetland and Orkney islands off Scotland. There is at present no evidence of any tsunami in recorded history affecting the western Irish and Scottish coasts other than those generated by the Lisbon earthquake of 1755, where the wave effects were slight. Thus, the presence of megaclast accumulations at significant heights above sea level, on exposed coasts, and exhibiting imbrication should not be used as a definitive criterion for the past occurrence of tsunami. These deposits attest to extreme wave conditions occasionally encountered in the North Atlantic and act as a record of such events extending back at least 160 years. Such data on extreme wave conditions are essential for the prediction of the effects of deepwater waves on structures such as offshore drilling platforms.
The Grind of the Navir is an ignimbrite headland on the exposed Atlantic coast of the Shetland Islands, Scotland. During storms, offshore wave heights exceed 20 m and deep water close inshore allows high-energy waves to impact on these cliffs. The cliff top at ∼ 15 m above sea level is awash with wave water when wave heights exceed 8 m, a condition met in storms in most years. Detailed mapping, ground photography and patterns of lichen colonisation and growth allow the processes, patterns and rates of erosion to be assessed on different parts of the headland over the past 100 years.
Una formula para el calculo de los disques de escollera. Revista de Obras Publicas, Madrid. (or see: A formula for the calculation of rock-filled dykes
- R Iribarren
Iribarren, R., 1938. Una formula para el calculo de los disques de escollera. Revista de
Obras Publicas, Madrid. (or see: A formula for the calculation of rock-filled dykes,
translated by D, Heinrick, University of California, Dept of Engineering T.R.-He-116-295 Berkley, 1948).
Shore Protection Manual (SPM)
USACE, 1984. Shore Protection Manual (SPM). 4th edition. U.S. Army Corps of Engineers,
Coastal Engineering Center, Waterways Experiment Station, Vicksburg, Miss, Govt.
Printing Office, Washington, D.C (2 vols).