Article

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

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Abstract

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

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... Recent work has examined these mechanisms in the marine environment but not specifically on beachrock (Kline et al., 2014;Herterich et al., 2018). Other work has focussed on geological properties of shore platforms and ways in which geological information can improve geomorphological studies of rocky coastlines, especially in terms of rock strength/hardness and fracture patterns (Thornton and Stephenson, 2006;Dickson et al., 2004;Naylor and Stephenson, 2010;Gómez-Pazo et al., 2021). ...
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Thesis
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Much of the existing research on rocky shore platforms describes results from carefully selected field sites, or comparisons between a relatively small number of selected sites. Here we describe a method to systematically analyse rocky shore morphology over a large area using LiDAR-derived digital elevation models. The method was applied to 700 km of coastline in southwest England; a region where there is considerable variation in wave climate and lithological settings, and a large alongshore variation in tidal range. Across-shore profiles were automatically extracted at 50 m intervals around the coast where information was available from the Coastal Channel Observatory coastal classification. Routines were developed to automatically remove non-platform profiles. The remaining 612 shore platform profiles were then subject to automated morphometric analyses, and correlation analysis in respect to three possible environmental controls: wave height, mean spring tidal range and rock strength. As expected, considerable scatter exists in the correlation analysis because only very coarse estimates of rock strength and wave height were applied, whereas variability in factors such as these can locally be the most important control on shoreline morphology. In view of this, it is somewhat surprising that overall consistency was found between previous published findings and the results from the systematic, automated analysis of LiDAR data: platform gradient increases as rock strength and tidal range increase, but decreases as wave height increases; platform width increases as wave height and tidal range increase, but decreases as rock strength increases. Previous studies have predicted shore platform gradient using tidal range alone. A multi-regression analysis of LiDAR data confirms that tidal range is the strongest predictor, but a new multi-factor empirical model considering tidal range, wave height, and rock strength yields better predictions of shore platform gradient (root mean square error of predictions reduced by 5%). The key finding of this study is that large-scale semi-automated morphometric analyses have the potential to reveal dominant process controls in the face of small-scale local variability.
Article
The subtidal portions of shore platforms are important geomorphic features as they can modify deep-water wave energy before it impacts the intertidal platform edge. In this study an integrated marine and terrestrial aerial LiDAR dataset is used to analyse the morphology of the subtidal portion of shore platforms. Semi-horizontal intertidal platforms on an 85 km along stretch of microtidal, open-ocean, rocky coast in Victoria, Australia are investigated and described quantitatively. Three distinct types of subtidal morphology occur; (i) a steep cliff with a mean slope of 8–18°, (ii) a gently sloping ramp with a mean slope of < 3°, and (iii) a subtidal terrace/reef. It is inferred that the type of subtidal morphology present on a platform will determine the relative impact of marine and subaerial processes in the intertidal and supratidal zones.
Article
Extreme wave events in coastal zones are principal drivers of geomorphic change. Evidence of boulder entrainment and erosional impact during storms is increasing. However, there is currently poor time coupling between pre- and post-storm measurements of coastal boulder deposits. Importantly there are no data reporting shore platform erosion, boulder entrainment and/or boulder transport during storm events - rock coast dynamics during storm events are currently unexplored. Here, we use high-resolution (daily) field data to measure and characterize coastal boulder transport before, during and after the extreme Northeast Atlantic extra-tropical cyclone Johanna in March 2008. Forty-eight limestone fine-medium boulders (n=46) and coarse cobbles (n=2) were tracked daily over a 0.1km2 intertidal area during this multi-day storm. Boulders were repeatedly entrained, transported and deposited, and in some cases broken down (n=1) or quarried (n=3), during the most intense days of the storm. Eighty-one percent (n=39) of boulders were located at both the start and end of the storm. Of these, 92% were entrained where entrainment patterns were closely aligned to wave parameters. These data firmly demonstrate rock coasts are dynamic and vulnerable under storm conditions. No statistically significant relationship was found between boulder size (mass) and net transport distance. Graphical analyses suggest that boulder size limits the maximum longshore transport distance but that for the majority of boulders lying under this threshold, other factors influence transport distance. Paired analysis of 20 similar sized and shaped boulders in different morphogenic zones demonstrates that geomorphological control affects entrainment and transport distance - where net transport distances were up to 39 times less where geomorphological control was greatest. These results have important implications for understanding and for accurately measuring and modelling boulder entrainment and transport. Coastal managers require these data for assessing erosion risk. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Article
Shore platform widths in the Glamorgan Heritage Coast, Wales, UK (21 sites) and the Kii (14 sites) and Izu Peninsulas (19 sites), Japan, were analysed with respect to rock strength parameters. Specifically, uniaxial compressive strength (from Schmidt hammer readings), joint density and bed thickness, were measured at cliff base and high, mid and low tide locations. Platform width values were derived from telemetric field measurements and in the case of the Glamorgan Heritage Coast, augmented by LIDAR values. Analysis of the data indicated a significant and consistent correlation between platform width and the Schmidt hammer readings derived from the more resistant, dry, unweathered rocks exposed at high tide and cliff base sites, with R(2)= 0.35 at Glamorgan, 0.42 at Kii and 0.41 at Izu. This suggests that a high proportion of the platform width variation could be related to this specific rock resistance parameter (uniaxial compressive strength). No significant relationship was evident between joint density and platform width. This was unexpected and may reflect the difficulty of objective measurement of this parameter in the field. In the case of some of the Japanese sites, no jointing was apparent (within some of the massive volcanic series). Similarly, no consistent relationship was found between platform width and bed thickness.
Article
Variations in the width of shore platforms were investigated within southern Wales, Gaspe in eastern Canada, and southern Japan. The strong, positive linear relationship between platform gradient and tidal range implies that platform width may be largely independent of tidal range, although regional negative correlations between platform gradient and width suggest that platforms become narrower with increasing tidal range. Platform width increases with wave intensity, and the widest platforms are oriented toward the dominant waves. The width of the platforms also decreases with increasing rock dip. A structural classification, based upon combinations of rock dip and strike, relative to the orientation of the cliff face, helped to explain variations in platform width in Canada and Japan. Changes in structural class induced by variations in coastal orientation may partially account for differences in the width of platforms on headlands and in embayments. Platform width is also influenced by the presence and type of cliff-foot deposits.
Article
The Vale of Glamorgan may be described as a macro-tidal, storm-wave environment. Shore platforms occur along most of this coastline, attaining widths commonly greater than 250 m. An investigation of the form of these platforms showed that, although the junction of cliff and platform is related to the level of the Mean High Water Spring tides (r = 0.46) and although the gradient of the platform is related to the magnitude of the tidal range (r = 0.50), much of the variation in the detailed geometry of the platform is related to local lithology. The platforms showed no tendency to develop a parabolic profile, although minor secondary planation has occurred between -0.61 m and +1.22 m O.D. Investigation of the rate of cliff recession indicated that, although present recession is slow, much more rapid rates have occurred in periods since the Neolithic. With reference to work done in adjacent areas, platform origin appears to be a result of the Neolithic modification of a series of interglacial platforms, although further modification and extension of the platforms has taken place since that time.
Article
Differences in slope between Australasian and southern British shore platforms may be explained by morphogenic conditions, the high slopes of platforms in England and Wales being related to high tidal range, storm waves, large and abundant erosional debris, and climatic conditions unsuitable for effective waterlayer levelling. Other typical Australasian features such as the low tide cliff and true platform ramps are, however, generally absent from southern Britain. A classification of contemporary and 'raised' shore platforms and ledges is proposed for England and Wales, in which lithologically controlled storm ledges are distinguished from shore platforms. The relevance of detailed studies of contemporary shore platforms to investigations of 'raised' planation surfaces is emphasized.
Article
This reference text deals with the geomorphology of rock coasts. It brings together the results of research conducted by coastal engineers, biologists, geologists and physical geographers from many parts of the world. The first section of the book discusses changes in sea level and the effects of mechanical wave action, chemical weathering, solution, bio-erosion, frost, and mass movement on rock coasts. The second section is concerned with the major landforms which result from these processes. These include cliffs, shore platforms, coastal karst, bays, headlands, and elevated marine terraces.
Article
Low altitude flights by a micro-drone were made in 2012 and 2013 over two boulder beaches in northwestern Spain. Geographical information system software was used to map the data. Boulder outlines from the first flight were recorded on 4796 clasts at Laxe Brava and 2508 clasts at Oia. Changes in location were identified by overlaying these outlines on the 2013 images. About 17.5 % of the boulders (mean surface area 0.32 m2) moved at Laxe Brava and about 53 % (mean surface area 0.23 m2) at Oia. Most movement on both beaches was between the mid-tide to about 2 m above the high tidal level. The location and elevation of the highest points were also recorded on the 2012 images on 4093 boulders at Laxe Brava and 3324 boulders at Oia. These elevations were compared with the elevations at the same locations in 2013. The occurrence and scale of the elevational changes were generally consistent with changes in the boulder outlines. The study confirmed that boulder beaches can be cheaply and effectively monitored using high resolution, micro-drone technology. This article is protected by copyright. All rights reserved.
Article
In 1978 E.D. Gill and J.G. Lang installed 53 micro-erosion meter (MEM) sites on shore platforms along the Otway coast in south eastern Australia. Surface lowering rates were originally determined from 2 yr of measurements (1979 and 1980) and showed that the mean annual rate of lowering from all sites was 0.37 mm/yr− 1. We re-measured these MEM sites 32 yr after the first set of readings were taken in February 1979. We relocated 49 of the original 53 bolt sites and were able to measure 45 of them. From the sites we measured, the mean annual rate of shore platform lowering was 0.31 mm/yr− 1. Rates of surface lowering on shore platforms are commonly reported from studies of only two or three years and only one other record exceeds 30 yr (Kaikoura Peninsula, NZ; 1.09 mm/yr− 1), in that case fewer than half of the original bolt sites were functioning because erosion had removed bolts. Along the Otway coast rates of erosion are much slower hence the greater number of still readable sites after 32 yr. Compared to other micro-erosion meter studies the rates reported here are at the lower end of the range of values from around the world but nevertheless consistent with rates from the Otway Coast and others in similar lithology. We found no statistical difference between erosion rates calculated over 2 yr (1979 to 1980) and 32 (1979 to 2011) years supporting the view that shore platform lowering rates measured over 2 yr are representative of decadal scales.
Article
Repeated measurements of erosion rates on seashore platforms on Kaikoura Peninsula, South Island, New Zealand were gathered over a 28-month period using a micro-erosion meter (MEM). The resultant hierarchical dataset was analysed using multilevel regression modeling. Results indicate that there is persistent variation of rates across measurement occasions within individual points and significant variation is also found across sites (or local ‘contexts’). There is little evidence for significant variation across individual points. The explanatory variables of platform/rock type, distance along platform and seasonality all have a significant effect on the erosion rates and aid the explanation of some of the variation in erosion rates across sites.
Article
Knowledge of the long-term rates of erosion of shore platforms are needed to assist modeling platform development, understand rates and modes of development and decipher issues of inheritance from former sea level. Few studies report rates from more than 2 years of monitoring. We report decadal scale erosion rates using both a micro-erosion meter and a traversing micro-erosion meter with measurements from 30 years and 10 years, respectively. Gross mean surface lowering rates measured over 10 years with a traversing micro-erosion meter were 0.901 (S.E.=0.116) mm/yr compared with 1.130 (S.E.=0.217) mm/yr over two years. Over thirty years (1973–2003) 12 micro-erosion meter sites provide a mean surface lowering rate of 1.09 (S.E.=0.126) mm/yr compared to 1.43 (S.E. 0.128) mm/yr calculated over twenty years (1973–1993) from 15 bolt sites and 1.53 (S.D.=1.45) mm/yr from 31 bolt sites. Rates calculated over longer time periods are reduced compared to short-term rates as a result of faster eroding bolt sites being removed from the data set. Comparison of means between short and long time periods for existing bolts show no significant difference. That is, short-term measurements over two years are representative of decadal scale erosion rates. In fact decadal scale rates may under represent rates of platform development because of the loss of faster eroding bolt sites. The loss of such bolts means that obtaining long-term erosion rates is problematic. Future modeling of shore platform development can use MEM rates with more certainty, but we recommend using higher rates from shorter term studies, unless longer records are complete.
Article
This paper presents a case study comparing shore platforms of the same lithology and different rock structures (limestone) with those of different lithologies but similar rock structures (limestone, dolerite) in Wales and Sweden. Results of this study illustrate that different erosion mechanisms (i.e. wave action and ice scour) produce similar erosion products and meso-scale (cm–m) landforms (i.e. stepped shore platforms) in structurally similar lithologies. In contrast, lithologically similar rocks (i.e. limestones) generate different meso-scale landforms and shore platform morphologies. Thus, structural controls appear to be more critical than lithological controls in generating meso-scale erosion products, such as rock blocks, in this comparative study. The similarities in the meso-scale geomorphology appear to be caused by uniform rock structure (i.e. rock hardness, joint density, spacing of bedding/horizontal jointing and the slope and direction of bedding/horizontal jointing in relation to the sea). Blocks produced by the jointing have been eroded by high energy waves in the case of Wales and drifting sea ice in the case of one of the Swedish sites. Meanwhile, different rock structures in a similar lithology (i.e. limestone) resulted in very different landforms, under the same environmental conditions as the previous example. As such, this research also provides observational evidence supporting the concept of equifinality.
Article
Erosion rates are 15–18.5 times higher in places where there is a beach at the cliff foot compared with those where there is no beach. The zone of intense erosion associated with a beach of sand and pebbles was below a height of about 10 cm above the surface, maximum erosion being at 14.5 cm below the surface. Erosion was by corrasion and wedging, a previously unrecognised process which is the quarrying of rock aided by fine-grained particles from the beach. Wedging, perhaps with some corrasion, had a median erosion rate of 11.05 · 10−3 cm tide−1 in Upper Lias shale while the rate for corrasion was 5.79 · 10−3 cm tide−1. A smooth, notched cliff foot may be produced by these processes. Where there was no beach, erosion of the cliff foot was by the sporadic quarrying of blocks of rock and the more continuous micro-quarrying of shale fragments. The cliff foot was dominated by structural planes and had a rough surface. The horizontal erosion rate due to quarrying at one cliff foot was 2.3 cm year−1. Quarrying acts on the cliff foot above the intense zone of erosion at a beach surface. The median erosion rate for this quarrying was 0.91 · 10−3 cm tide−1.
Article
Processes of platform development include mechanical wave erosion, weathering and bio-erosion. Form is strongly influenced by lithology and structure. Contrasts between British and Australian forms are noted. These seem to be opposite ends of a tidally regulated wave erosional spectrum of forms. Rates of platform development raise questions of inheritance. -Jennifer Clayton
Article
Work has been conducted, in the laboratory and field, on a micro-tidal, subhorizontal shore platform in argillaceous rocks at Mont Louis, in Gaspé, Québec. Surface downwearing rates for 450 rock samples were measured over 3 years in the laboratory at the neap low, mid-, and high tidal levels, under semi-diurnal tidal conditions. De-ionized water was used to determine the role of wetting and drying, and artificial sea water for the additional effect of salt and possibly chemical weathering. Another 300 samples were immersed in de-ionized water or artificial sea water for 90 min every 1, 2, or 3 weeks, representing conditions between the neap and spring high tidal levels, and exposed in air for 90 min every 1, 2, or 3 weeks to represent conditions between the neap and spring low tidal levels. Surface downwearing was recorded in the field at 34 transverse micro-erosion meter (TMEM) stations, most of which were installed along 4 shore-normal profiles in 2004 and 2005. Mean downwearing rates in the laboratory ranged from 1.25 mm yr− 1 at the neap high tidal level to 0.63 mm yr− 1 at the neap low tidal level. Downwearing was much slower between the neap and spring tidal levels and it was uniformly slow below the neap low tidal level. The presence of salts inhibited downwearing within the neap tidal range, where wetting and drying was dominant, and promoted it between the neap and spring high and low tidal levels. Surface swelling was fairly common in the field, and the mean downwearing rate (0.242 mm yr− 1) was much lower than in the laboratory, possibly because the TMEM stations were installed in harder rocks. Modeling supports the contention that the Mont Louis platform was cut initially by waves at a higher elevation, and then lowered by weathering in the last few thousand years as relative sea level fell to its present elevation.
Article
The Schmidt Rock Test Hammer was used to study the effect of abrasion on shore platforms in Galicia, northwestern Spain. On platforms where tidally-induced weathering (salt, wetting and drying, etc.) is dominant, rock strength is significantly lower than in areas where abrasion is, or has been active in the recent past. This suggests that abrasion removes weathered surface material, exposing the stronger, less weathered rock below. Abrasion downwearing, measured with a transverse micro-erosion meter, ranged between 0.13 and 1.8 mm yr− 1 over the last year. Most active abrasion occurs in the upper part of the intertidal zone, but weathering is slowly destroying formerly abraded surfaces at lower elevations. These abandoned surfaces were abraded by materials supplied by erosion of fluvio-nival and periglacial slope deposits that covered, or partially covered, parts of the Galician coast during the middle and late Weichselian. During the Holocene, rising sea level and erosion of the slope deposits caused the abrasion zone to gradually migrate up to its present position near the high tidal level. The spatial and temporal role of abrasion on this coast is, therefore, closely associated with the exhumation and inheritance of ancient platform surfaces from beneath Weichselian deposits.
Article
This paper describes a new method to distinguish river terrace levels based on clast hardness and degree of weathering measured with an Equotip hardness tester. The technique was applied to a series of terraces on the Miño River in the northwestern Iberian Peninsula, where the lack of suitable material, high degree of weathering, and intense iron cementation precluded routine dating. Detailed mapping demonstrated that terrace sediments occupy a range of altitudes that make assignment to a specific terrace, and/or correlation between levels, difficult. Statistical analysis of the Equotip hardness data from quartzite clasts using k-means clustering allowed four probable terrace levels to be identified; a series of t-tests generally supported these groupings. A fifth, lowermost terrace level, was not included in the analysis because of limited exposure above a reservoir. Clast hardness and degree of terrace weathering were generally consistent with progressive river downcutting. The occurrence of faulted sediments, however, suggested that terrace elevations were modified locally by post-depositional tectonic movements, which may explain why probable younger terraces in some sectors of the Miño River are at higher elevations than older terraces in adjacent sectors. The Equotip tester helped to resolve stratigraphic uncertainties and to assign deposits to specific terrace levels and was found to be a useful tool to distinguish and correlate river terraces.
Article
This paper is concerned with the effect of sediment accumulation on shore platform development. Boulder accumulations are common on the granitic shore platforms of Galicia, northwestern Spain. Boulders are produced by erosion of shore platforms and of cliffs consisting of cold-climate deposits from the last glacial period. Measurements were made of the long axis length of more than 800 boulders, and additionally of the short and intermediate axes of 340 of these boulders, as well as of their orientation and gradient. There were two study areas. The boulders on the Barbanza Peninsula are generally a little smaller than those in southern Galicia with, respectively; mean long axis lengths of 0.98 and 1.14, and masses of 1.06 and 1.59 t. There are also some isolated, very coarse boulders and megaclasts in southern Galicia. The distribution and extent of the deposits and boulder imbrication and orientation testify to the high levels of wave energy produced by northwesterly and westerly storms in this region. Although the boulders, as well as the underlying shore platforms, were inherited, in part, from previous interglacial stages, some boulder detachment and movement is occurring today during storms, when significant deep water wave heights exceed 8 to 10 m. Despite some abrasion of the shore platforms, the primary effect of large boulder accumulations is protective. The role of sediment on shore platforms has been neglected, but this study suggests that because of arrested development under thick accumulations, platform gradient in areas with abundant sediment increases with the grain size of the material. The occurrence and type of sediment on shore platforms may therefore help to explain the distribution of sloping and subhorizontal platforms under different morphogenic and geological conditions.
Article
Examination of rocky coastal profiles in Australia and Europe suggests that two major and genetically distinct types of erosional surface should be recognized, one of these being the shore platform, whether sloping or horizontal, smooth or rough, which is best developed on promontories and terminates in a low-tide cliff, the other being the wave ramp, often exposed in bays, which slopes gradually down from about high water level to merge with the submarine slope near-shore. A modified terminology for erosional surfaces on rocky coasts is proposed.
Article
Downwearing rates were measured on shore platforms at about 200 transverse micro-erosion meter (TMEM) stations, over periods ranging from 2 to 6 years. There were seven study areas in eastern Canada. The platforms were surveyed and a Schmidt Rock Test Hammer was used to measure rock hardness. More than 1200 rock samples from three of the study areas were also subjected each day, over a 3 year period, to two tidal cycles of immersion and exposure, which simulated the central intertidal zone. A further 840 samples were subjected to longer periods of exposure and immersion, over a 1 year period, which represented different elevations within the upper and lower intertidal zone, respectively. These experiments suggested that tidally generated weathering and debris removal is an effective erosional mechanism, particularly at the elevation of the lowest high tides. In the field, mean rates of downwearing for each study area ranged from 0·24 mm yr−1 to more than 1·5 mm yr−1. Rates tended to increase with elevation in the field, with maxima in the upper intertidal zone. This trend in the field cannot be attributed entirely to the tidally induced weathering processes that were simulated in the laboratory, and must reflect, in part, the effect of waves, frost, ice, and other mechanisms. It is concluded that there are no strong spatial downwearing patterns on shore platforms, and that downwearing rates in the intertidal zone are the result of a number of erosional mechanisms with different elevation-efficacy characteristics. Furthermore, even if only one or two mechanisms were dominant in an area, any resulting relationship between downwearing rates and elevation would be obscured or eliminated by the effect of variations in the chemical and physical characteristics of the rocks. Copyright © 2010 John Wiley & Sons, Ltd.
Article
The shore platforms on Shag Point, southern New Zealand, are quasi-horizontal surfaces and are developed between supratidal and low water spring levels. A range of morphologies occur, with more exposed platforms having a distinct low-tide cliff, in contrast to low-tide surfaces where the seaward edge is buried beneath rubble and macro-algal growth. The platforms range in width from 20 to 80 m and are eroded into Late Cretaceous/Early Tertiary fine marine sandstones and mudstones. Shore platforms have formed in two principal lithological units: a homogeneous unit that is characterized by few discontinuities, and a fractured unit with joints spaced about 0·5 m apart. Rock hardness is low in both units (L-type Schmidt hammer rebound values of 31 ± 4), and there is little systematic variation in values between the two units in which platforms have developed. Case-hardened concretions within the sandstone are significantly harder than surrounding rock and cause local relief of metre scale as the spherical diagenetic features are eroded from the bedrock. They do not, however, appear to affect broad-scale platform geometry. Joints within the bedrock are a primary control on platform elevation. Platforms formed in jointed rock occur at the lower portion of the intertidal zone, in contrast to platforms formed in unjointed bedrock, in which horizontal surfaces occur at or above mean high water spring tide level. Rock structure, therefore, appears to be the primary determinant factor of platform geometry at Shag Point. Copyright © 2006 John Wiley & Sons, Ltd.
Article
We report a series of short-term (diurnal) rock surface monitoring studies on inter- and supra-tidal shore platforms using a traversing micro-erosion meter at two sites, Kaikoura Peninsula, New Zealand, and Apollo Bay, Victoria, Australia. Statistically significant day-to-day changes were measured. Surface rise and lowering occurred at rates above instrument error, with a maximum range of 3·378 mm between 1·697 mm (lowering) and -1·681 mm (rise). Individual measurements showed rises greater than 2 mm. These daily variations reveal that surface lowering and rise occur at a much shorter time scale than previously reported from other studies. The patterns observed suggest wetting and drying is the most likely process causing surface changes at these temporal scales. We argue that traversing micro-erosion meter studies operating at a short-term time scale of day-to-day provide meaningful results that open new opportunities for studying rock weathering and erosion in a coastal environment. Copyright © 2004 John Wiley & Sons, Ltd.
Article
One of the longest standing debates in rocky coast geomorphology is whether subaerial weathering or wave processes dominate shore platform evolution. The origins of this debate date to the mid-nineteenth century when the first descriptions of Old Hat Islands were provided from northern New Zealand. Old Hat Islands are surrounded by a broad near-horizontal shore platform. Their formation was inferred to relate to subaerial weathering of bedrock to a level of permanent saturation with wave processes acting only to remove the weathered debris. To date, no detailed topographic surveys have been conducted on the Old Hat Islands in New Zealand that initiated this debate; in this study we provide the first quantitative data on the original field sites. Topographical surveying and Schmidt Hammer hardness testing were conducted on >25 profiles of varying wave exposure. In contrast to the classic descriptions, platform elevation varies from just above mean high water spring (MHWS) at the most exposed sites, to mean high water neap (MHWN) at the most protected sites or mean sea level (MSL) where beaches occur on the platform surface. There is no significant difference between the hardness of the cliffs and the platforms fronting them and no clear relationship between wave exposure and platform width. Rather than being exclusively dominated by subaerial process, the formation of microtidal, sheltered, Old Hat Island platforms is considered to be a function of (i) the rate of weathering, (ii) exposure to wave energy, (iii) nearshore water depth and (iv) rock resistance. Shore platform elevation in the study area is thought to be a function of the level at which waves erode cliff rock, and the action of weathering which lowers platform surfaces. The width of platforms is strongly influenced by the ability of waves to dissect the platform edge along vertical joint lines. Copyright © 2010 John Wiley & Sons, Ltd.
Article
Shore platforms frequently exhibit steps or risers facing seaward, landwards or obliquely across-shore. A combination of soft copy photogrammetry, ortho-rectification, geo referencing and field measurement of step height are linked in a GIS environment to measure step retreat on chalk shore platforms at sample sites in the south of England over two periods, 1973–2001, 2001–2007. The methods used allow for the identification, delineation and measurement of historic change at high spatial resolution. The results suggest that while erosion of chalk shore platforms by step backwearing is highly variable, it appears to be of similar magnitude to surface downwearing of the same platforms measured by micro-erosion meters (MEMs) and laser scanning, in a range equivalent to 0·0006 – 0·0050 m y−1 of surface downwearing. This equates to annual chalk volume loss from the platforms, by the two erosion processes combined, of between 0·0012 m3 m−2 and 0·0100 m3 m−2. Results from the more recent years' data suggests that step retreat has variability in both space and time which does not relate solely to climatic variability. The results must be viewed with caution until much larger numbers of measurements have been made of both downwearing and step erosion at higher spatial and temporal resolution. Copyright © 2010 John Wiley & Sons, Ltd.
Article
The coast of Wellington, New Zealand, is tectonically active and contains a series of uplifted and contemporary shore platforms that are developed in Triassic Greywacke. The platform profiles are rugged with relief of metre scale common. The surveyed platforms were formed at, and at two distinct levels 1–1·5 and 2–2·5 m above, mean sea level. They range in width up to 70 m and are highly fractured with fracture densities in excess of 20[sol ]m2 common. The rate of development of these platforms is rapid, with lateral erosion rates of up to 0·15 m[sol ]yr calculated, allowing platform development to occur over centennial scales. Even given this rapid development, continued instantaneous uplift of the coast has meant they are unable to reach an equilibrium state, whereby the effectiveness of wave processes in removing material is reduced by platform extension. The co-seismic uplift means that the rear of the platforms is raised beyond the limits of marine process and has become an area of deposition. Although no direct process measurements were made the highly fractured nature of the bedrock appears to play a major role in platform evolution, with wave processes being easily able to pluck blocks as evidenced by fresh erosion scars and active gravel beaches at the rear of many platforms. This coast therefore represents an extremely dynamic youthful shore platform environment, where the processes of marine abrasion can be observed over historical timescales. Copyright © 2005 John Wiley & Sons, Ltd.
Article
Transverse micro-erosion meter (TMEM) stations were installed in rock slabs from shore platforms in eastern Canada. The slabs were put into artificial sea water for 1, 6 or 11 hours, representing high, mid- and low tidal areas, respectively. The TMEMs were used to record changes in surface elevation as the rocks dried during the remainder of the 12 h of a semi-diurnal tidal cycle. A similar technique was used on the same rock types at intertidal TMEM stations in the field, as the rocks dried during low tide. Argillite and basalt surface contraction was from 0 to 0·04 mm: there was little surface expansion. Sandstones contracted by up to 0·03 mm in the field, but there was almost no contraction in the laboratory. Argillite and basalt contraction tended to be greatest in the upper intertidal zone, and to increase with rates of longer-term surface downwearing, but there was little relationship with rock hardness or air temperature and humidity. Changes in elevation at the same points at TMEM stations in the laboratory and field were quite consistent from one tidal cycle to the next, but there were considerable variations within single tidal cycles between different points within each station. The data suggest that contraction within the elevational zone that is normally submerged twice a day by the tides is by alternate wetting and drying. Short-term changes in elevation are generally low compared with annual rates of downwearing owing to erosion, but they may generate stresses that contribute to rock breakdown. Copyright © 2007 John Wiley & Sons, Ltd.
Article
The Schmidt hammer is a useful tool applied by geomorphologists to measure rock strength in field conditions. The essence of field application is to obtain a sufficiently large dataset of individual rebound values, which yields a meaningful numerical value of mean strength. Although there is general agreement that a certain minimum sample size is required to proceed with the statistics, the choice of size (i.e. number of individual impacts) was usually intuitive and arbitrary. In this paper we show a simple statistical method, based on the two-sample Student's t-test, to objectively estimate the minimum number of rebound measurements. We present the results as (1) the ‘mean’ and ‘median’ solutions, each providing a single estimate value, and (2) the empirical probability distribution of such estimates based on many field samples. Schmidt hammer data for 14 lithologies, 13–81 samples for each, with each sample consisting of 40 individual readings, have been evaluated, assuming different significance levels. The principal recommendations are: (1) the recommended minimum sample size for weak and moderately strong rock is 25; (2) a sample size of 15 is sufficient for sandstones and shales; (3) strong and coarse rocks require 30 readings at a site; (4) the minimum sample size may be reduced by one-third if the context of research allows for higher significance level for test statistics. Interpretations based on less than 10 readings from a site should definitely be avoided. Copyright © 2009 John Wiley & Sons, Ltd.
Article
There is increasing evidence that shore platforms and other elements of rock coasts may be inherited, at least in part, from interglacial stages when sea level was similar to today's. Most of this evidence, which includes ancient beaches and datable terrestrial deposits, has been obtained from areas of resistant, slowly eroding rock, where the platforms often appear to be much too wide to have developed since the sea reached its present level. It is much more difficult to demonstrate that inheritance has occurred in areas of weaker rock, which generally lack any datable material. The coast of western Galicia in northwestern Spain has shore platforms in igneous and metamorphic rocks that were deeply weathered during the Tertiary. These platforms are closely associated with ancient beaches from the last interglacial stage, and associated periglacial and fluvio-nival deposits that covered and fossilized most of the Eemian platforms and cliffs during the late middle and late Weichselian glacial stage. The sedimentary processes and the thickness and facies of the sediments were determined by the height, aspect and gradient of the coastal mountains, and their distance from the coast. Radiocarbon dating, sedimentary analysis and platform morphology indicate that the shore platforms of Galicia have been inherited from at least the last interglacial stage. They were fossilized in places beneath thick Weichselian deposits and then exhumed during the Holocene transgression. The abundant evidence for inheritance in Galicia has important implications for other coasts in fairly weak rocks where such evidence is generally lacking. Copyright © 2003 John Wiley & Sons, Ltd.