Downwearing Rates Research Articles

Cross-scale erosion on shore platforms using the micro-erosion meter and Structure-from-Motion (SfM) photogrammetry

Investigations comparing surface downwearing rates derived from photogrammetry data and the micro-erosion meters (MEM) are limited due to the former's coarse spatial resolution and the temporal scale of data collection. These space and time limitations were revisited with the possibility of simultaneously using the Structure-from-Motion (SfM) photogrammetric workflow and the MEM. Using a cross-scale nested survey approach over one year to cover micro (10−2 m2) and meso (100 m2)-spatial scales, this paper presents erosion rates on uplifted intertidal shore platforms and marine terraces at Kaikōura Peninsula, New Zealand. The survey represents one of the first investigations combining the MEM and the SfM technique to estimate erosion rates on horizontal coastal bedrock. In a unique way, we applied established methods which allowed comparative accuracy between models via similar point matching to provide a detailed representation and visualisation of the changing rock surfaces at similar spatial and temporal scales. Over one year, across a microscale area (~0.01 m2) of rock, the MEM and SfM point clouds (SfM PCs) measured the lowest and highest erosion rates on a similar rock type and erosion site. The mean annual erosion rate of 2.244 mm/yr from the MEM was lower than the rates of 2.608 mm/yr and 10.299 mm/yr estimated using the SfM micro- and mesoscale PCs, respectively. A lower range of erosion rates (0.131 to 4.750 mm/yr) was measured using the MEM compared to erosion rates estimated from the SfM microscale PCs (0.194 to 10.106 mm/yr). Across the mesoscale (1 m2) areas, higher erosion rates were recorded using the SfM mesoscale PCs on all but one erosion site showing that a significant amount of erosion was undetected by the MEM. Using the SfM-orthomosaics, we link the erosion rates measured across the micro and mesoscale areas to intense granular disintegration, flaking, micro- and polygonal cracking and biological activities. These results provide statistical evidence to argue against extrapolating MEM erosion data obtained over small areas to larger areas on shore platforms affected by recent tectonic uplift. The implications of these findings are considered within the context of techniques for measuring shore platform erosion across spatial and temporal scales.

Tectonics and shore platform development: Rates and patterns of erosion on recently uplifted mudstone and limestone rocks at Kaikōura Peninsula, New Zealand

On tectonically active coasts, there is a dearth of erosion data documenting how rock coasts adjust (either fast or slow) in response to marine and subaerial processes immediately after coseismic uplift. Here we report erosion rates and evidence of reshaping of shore platform morphology at Kaikōura Peninsula, South Island, New Zealand, following the November 2016 Kaikōura 7.8 (Mw) earthquake, that uplifted platforms by ~1 m and extended their widths. High-resolution topographic data obtained from quarterly surveys over four years using a micro-erosion meter (MEM) and Structure-from-Motion Multi-View Stereo (SfM-MVS) surveys have provided rates of erosion and visual representation of surface morphologies. MEM data revealed variations in erosion, weathering and deposition rates across lithology, tidal positions, and platform elevation after the uplift. After four years, surface downwearing rates for all platforms were on average 2.25 mm/yr, a 104% increase from the pre-uplift rate of 1.10 mm/yr. Average lowering rates on limestone, hard, and soft mudstone platforms were 1.19 mm/yr, 2.31 mm/yr, and 3.21 mm/yr, respectively. Results show a change in patterns of erosion rates and faster rates on mudstone than limestone lithologies, but both faster when compared to pre-uplift rates. Previously reported seasonal trends in erosion rates have disappeared because post-uplift erosion rates are similar during the summer and winter. In contrast, statistically significant differences in lowering rates now exist across rock lithology and elevation. On one MEM site on the harder mudstone platform, a large increase from a pre-uplift erosion rate of 0.43 mm/yr to 19.23 mm/yr (1-year after uplift) and subsequent decline to 1.54 mm/yr after four years demonstrates the role of block detachment not previously seen in the MEM data from Kaikōura. For the first time, we complement MEM data with SfM-MVS-derived orthomosaics to provide evidence of changing rock morphology and processes such as granular disintegration, flaking and algal growth. Results from this study demonstrate how tectonism can fundamentally alter the way in which rock shore erosion rates are influenced by waves, tides, and weathering processes.

Limestone Sea Stacks (Rauks) Record Past Sea Levels and Rocky Coast Evolution in the Baltic Sea (Gotland and Fårö Islands, Sweden)

In this article, we explore the potential for sea-level change and rocky coastal evolution reconstruction hidden in one of the most intriguing rocky coastal landforms in the Baltic Sea region—rauks. Those unique limestone sea stacks are preserved along the coasts of a number of Swedish islands, with Gotland and Fårö as primary locations. We contributed to the long-standing debate on their origin and attempted to investigate the modern geomorphological processes operating on rauks using novel approaches in rock coast studies, including Schmidt hammer rock tests (SHRT)—to characterize zonation in the degree of coastal landforms weathering; traversing micro-erosion meters (TMEM)—to calculate shore platform downwearing rates; and terrestrial laser scanning (TLS) to monitor coastal abrasion and detect sea-level markers (notches) and describe their size and shapes. Our study shows the dominance of mesoscale changes in the shore platform morphology (erosion scars, spalling, and block erosion) over the microscale downwearing carried out by abrasion. We argue that the preservation of rauks along the modern coast of Baltic islands is a result of a favorable sequence of events including the rapid land uplift and their lithological resistance, significantly higher than observed on surrounding platforms and cliffs formed in different types of limestone. Our findings prove that the microrelief of rauks in the form of well-preserved notches records the sea-level changes from at least the late Ancylus Lake period. Finally, we incorporate the results of our observations into the classic notions of rauk formation and highlight the effect of cutting off sea stacks from the operation of littoral processes as a consequence of land uplift and dominance of subaerial weathering over the wave action.

Spatial scaling in geomorphology: Extrapolating micro-erosion measurements to the landform scale on shore platforms using unoccupied aerial vehicles

Downwearing rates of shore platform have been quantified with a number of ground-based techniques (such as the micro-erosion meter (MEM)), providing knowledge of the evolution of coastal landforms. However, it remains problematic to extrapolate the micro erosion results to the landform scale. This is because only a very small proportion of the platform is monitored and usually biased to flat rock surfaces. In this study, we used the shore platforms in south-eastern Australia, which has the world's longest continual erosion records, as an example to address the fundamental question of how to extrapolate across spatial scales. Surveying using an unoccupied aerial vehicle (UAV) – post-processing kinematic (PPK) – structure-from-motion (SfM) workflow, indicates the shore platforms at Marengo and Jump Rock have similar morphology. The generated mm-scale dataset was used to contextualize 4 and 12 MEM sites into the orthomosaics and digital surface models (DSMs) at each location. By using two different methods, we assessed how these monitored surfaces were representative of the morphology of the entire platform. On average, the 4 MEM sites represented 52.2% of platform surfaces at Marengo and 73.2% with the 12 MEM sites at Jump Rock. To improve the representativeness of sampled surfaces, it was found that 20 randomly sampled MEM-size (4.33 × 10−3 m2) surfaces were able to represent (~90%) the landform morphology at the scale of 104 m2. The representativeness was only slightly improved by upscaling the surface size from original cm2, to dm2 (0.16 m2) and m2 (1 m2) scales, implying the importance of sample number over sample size. Using the cm-scale dataset covering a larger geographic region, a bias of MEM installation was found with all 43 MEM sites concentrated at the fast-eroding upper intertidal zone. An elevation-weighted erosion rate of 0.21 mm/yr was calculated, lower than the original mean of 0.25 mm/yr provided by the MEM network. Moreover, a spatial pattern of rock surface morphology was observed across the platform with higher slope and roughness at the seaward end, suggesting more intensive wave processes and varying erosion agents compared to those surfaces further inland.

Shore platform downwearing in eastern Canada; A 9–14 year micro-erosion meter record

Downwearing rates (erosion in the vertical plane) were measured with a micro-erosion meter (MEM) in eastern Canada, on an argillacious, sub-horizontal shore platform at Mont Louis in eastern Québec, and on two sloping, basaltic and sandstone platforms at, respectively, Scots Bay and Burntcoat Head in the Bay of Fundy, Nova Scotia. The original data covered a period from 2002 to 2009. This dataset was extended by measurements repeated at surviving MEM stations in 2017, producing records ranging over 9–14 years, depending on when each station was installed. Because of rapid surface downwearing, many of the original MEM stations were inoperable in 2017, especially at Burntcoat Head. Nevertheless, data were obtained from 19 stations at Burntcoat (35% of the 2009 original), 25 at Mont Louis (83% of the original), and 38 at Scots Bay (75% of the original). For the stations at Mont Louis and Scots Bay that were still functioning in 2017, there were no significant differences in rates of downwearing over the shorter (from station installation up to 2009) and extended periods (from installation to 2017). Mean rates of downwearing calculated from all the stations in each area declined through time, however, due to the loss of the more rapidly eroding stations. A simple procedure, which was proposed to compensate for this decrease, produced mean downwearing rates that were broadly similar to those reported over the original measurement period. There were significant relationships between downwearing rates and elevation (R2 = 0.32) and downwearing rates and rock hardness (R2 = 0.41) in the extended record at Scots Bay, and a small but significant relationship between downwearing rates and rock hardness at Mont Louis (R2 = 0.17). Differences in downwearing rates across the platforms suggest that salt weathering and wetting and drying are dominant weathering mechanisms at Scots Bay and Mont Louis. Chemical weathering of the sandstone cementing agent and the premature removal of weathered grains by wave-generated bottom currents may, however, be more important at Burntcoat Head.

Cirques have growth spurts during deglacial and interglacial periods: Evidence from 10Be and 26Al nuclide inventories in the central and eastern Pyrenees

Cirques are emblematic landforms of alpine landscapes. The statistical distribution of cirque-floor elevations is used to infer glacial equilibrium-line altitude, and the age of their frontal moraines for reconstructing glacial chronologies. Very few studies, however, have sought to measure cirque-floor and supraglacial ridgetop bedrock downwearing rates in order to confront these denudation estimates with theoretical models of Quaternary mountain landscape evolution. Here we use 10Be nuclide samples (n=36) from moraines, bedrock steps, and supraglacial ridgetops among a population of cirques in the east-central Pyrenees in order to quantify denudation in the landscape and detect whether the mountain topography bears any relevance to the glacial buzzsaw hypothesis. Minimum exposure ages (MEAs) obtained for a succession of moraines spanning the Oldest Dryas to the Holocene produced a deglaciation chronology for three different Pyrenean ranges: Maladeta, Bassiès, and Carlit. Based on a series of corrections, calibrations, and chronostratigraphic tuning procedures, MEAs on ice-polished bedrock exposures were further used to model denudation depths at nested timescales during the Würm, the Younger Dryas, and the Holocene. Results show that subglacial cirque-floor denudation was lower during glacial periods (Würm: ~10mm/ka) than during deglacial and interglacial periods (tens to hundreds of mm/ka). The relative inefficiency of glacial denudation in the cirque zone during the Würm would have resulted from (i) cold-based and/or (ii) low-gradient glaciers situated in the upper reaches of the icefield; and/or from (iii) glacier-load starvation because of arrested clast supply from supraglacial rockslopes situated in the permafrost zone. Denudation peaked during the Younger Dryas and Holocene glacial advances, a time when cirque glaciers became steeper, warmer-based, and when frost cracking weakened supraglacial ridgetops, thus enhancing subglacial erosion by providing debris to the sliding glacier base. Cirques, therefore, grow faster during more temperate periods of cirque glaciation than under full glacial conditions. Another key finding was the very low rates of ridgetop lowering averaged over the Würm and Holocene (10–25mm/ka). A comparison of the denudation rates obtained from the cirque zone with regional estimates of crustal uplift indicates that the alpine topography is not in a steady state. The low intensity of glacial denudation failed to bring the topography to a buzzsaw equilibrium state.

Shore platform and cliff notch transitions along the La Paz Peninsula, southern Baja, Mexico

Increasing exposure to wave action produces a northerly transition from high tidal notches to shore platforms inthe andesitic lahar deposits of the La Paz Peninsula, southern Baja, Mexico. Twenty-four notches were surveyedand wear pins were cemented into the apex of each notch. Thirty-six transverse micro-erosion meter (TMEM)stations were installed on three surveyed platforms. Field measurements were made over a 2.5 year period.The wear pins suggested notch backwearing (horizontal erosion) is <2mm yr -1 . The shore platforms were fairlynarrow (a few tens of metres) and steeper (1o) than most platforms in similar microtidal environments, reflectinga weak wave environment and resistant rocks. Mean TMEM downwearing (vertical erosion) rates for each of thethree platforms ranged from 0.14mm yr -1 to 0.42mm yr -1 . There was a good relationship between notch height(difference in elevation between the floor and the roof at the front of the notch) and exposure to wave action, butnotch depth is time-dependent and the relationship with exposure was not statistically significant. Notch heightwas also related to the orientation and wave fetch of the site. Field evidence suggested that the notches were notproduced by bioerosion or chemical weathering but by alternate wetting and drying or salt weathering from hightidal immersion and wave-generated splash and spray. Coastal morphology is fairly well adjusted to present sealevel although notch occurrence in the upper portion of the high tidal zone suggests that there is slow tectonic uplift in this region.

Downwearing rates of vertical limestone surfaces in the intertidal zone (Gulf of Trieste, Italy)

Abstract In 2007, we installed seven micro-erosion meter stations on a vertical limestone slab located in the intertidal zone in the Gulf of Trieste (Italy) in order to observe rock surface changes at elevations ranging between − 0.75 m and + 0.75 m m.s.l. ( Furlani et al., 2010 ). In this paper, we present the micro-erosion metre (MEM) data for most months between February 2008 and May 2013 and the traversing micro-erosion metre (TMEM) data collected twice a year since 2007. The MEM and TMEM data were consistent and indicated that the mean lowering rate ranged between 0.007 mm/yr and 0.205 mm/yr and − 0.001 mm/yr and 0.260 mm/yr, according to the elevation of the stations. Maximum downwearing rates were observed in the mid-lower intertidal zone and decreased upward and downward. The natural trend of erosion started in 2010, after 2 years of exposure, when the slab, formerly smoothed and not colonized by marine organisms, started to be acclimated to the local environment. After 2010, the erosion rate increased to a maximum of 0.31 mm/yr. This value can be more representative for long-term studies and is similar to the downwearing rates locally collected on natural limestone surfaces. This dataset represents the first collection of downwearing rates mainly measured using MEMs, but also TMEMs on vertical limestone surfaces in an intertidal zone and provides interesting new information for addressing the debate on notch genesis and development. The shape of a tidal notch has been found to be consistent with the distribution of erosion rates along the slab, even if the present-day notch was not surveyed in the Gulf of Trieste due to the local tectonic subsidence.

Integrated assessment of bioerosion, biocover and downwearing rates of carbonate rock shore platforms in southern Portugal

Bioerosion on rocky shores has been frequently reported as an important mechanism in coastal evolution, with less attention focussed on determining the bioprotective role organisms may have in mediating coastal erosion. This work aims, for the first time, to provide an integrated assessment of both traversing microerosion meter (TMEM) downwearing rates and activity of intertidal organisms on two carbonate shore platforms in southern Portugal. Paired substations positioned on the same substrate but differing in biological cover (one with bare rock and the other with algal cover colonised between the first and final readings) were monitored for eighteen months using a TMEM. At each station, the volume of burrows produced by macro borers was measured. Downwearing rates were lower in the surfaces protected by algal turf except in the station that experienced the longest time of exposure to subaerial conditions. In contrast, TMEM downwearing rates were higher in the areas containing the higher volume of burrows. Both downwearing rates and burrow volumes were negatively correlated with the mechanical strength of the substrate as measured by Schmidt Hammer rebound.

Late Holocene seacliff retreat recorded by 10Be profiles across a coastal platform: Theory and example from the English Channel

We explore a new method for documenting the long-term retreat rate of seacliffs based on measurements and modeling of 10Be concentration transects across present-day shore platforms. The proposed forward numerical model relies on a scenario of sea-level rise since the last deglaciation, and predicts the shape of 10Be concentration transects as a function of prescribed cliff recession and vertical coastal platform downwearing rates. Two independent transect features allow fitting the long-term recession rate model to field observations: a sharp 10Be concentration drop predicted at the former stationary location of the cliff during the last glacial period ∼100ka ago, and a characteristic dome shape whose magnitude is directly related to the recession rate of the cliff. A retreating chalk cliff site from the English Channel coast of France, at Mesnil-Val, where the 7m tidal range broadly exposes the shore platform, was selected to test the proposed method. Although retreat rates were too high to pinpoint the predicted 10Be concentration drop at the last glacial cliff position, the 10Be concentration of the flints sampled across the shore platform is consistent with the expected dome shape. When modeled using the proper tidal range and proposed Holocene sea-level rise history, the 10Be data yield a cliff retreat rate since the mid-Holocene of 11–13cm/yr. This is consistent with a 30-year-long measurement record, strongly supporting the utility of the 10Be method.

Downwearing rates on shore platforms of different calcareous lithotypes

Vertical lowering (downwearing) of shore platform surfaces is a very important mechanism in their morphological evolution albeit much remains incompletely understood. The efficacy of mechanical and chemical weathering acting on a given substrate, together with erosional processes, influences downwearing rates. In order to determine the relationship between lithotypes and downwearing rates, data collected from a Transverse Micro-erosion Meter were obtained for shore platforms of three different calcareous lithotypes (biocalcarenite, calcarenite and carbonated siltstone) along the central Algarve coast (Southern Portugal). Downwearing rates ranged between 0.096 mm year − 1 and 1.676 mm year − 1 in biocalcarenite and weakly cemented calcarenite, respectively. In addition, physical properties of the rocks comprising the platforms were measured, including uniaxial compressive strength (as determined by the Point Load Test), porosity, and calcium carbonate content. The results show that downwearing depends primarily on the intrinsic properties of the substrate. Porosity, in particular, acts in two ways: (i) it tends to weaken the substrate; and, (ii) it controls the downward extent of the water percolation and therefore the depth of the weathering mantle subject to erosion by waves and currents.

Patterns of surface downwearing on shore platforms in eastern Canada

AbstractDownwearing 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−1to 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 &amp; Sons, Ltd.

Shore platform downwearing in eastern Canada: The mega-tidal Bay of Fundy

Laboratory and field work was conducted to determine rates of surface downwearing on shore platforms at two sites in the Bay of Fundy, Canada; these data are needed to assess and model the effect of downwearing mechanisms on platform evolution. Sandstone and basalt samples (900) were exposed to semidiurnal tidal cycles over 3 years, using de-ionized water or artificial sea water. They were immersed for 1, 6, or 11 h over each 12 h tidal cycle, representing the lowest high (LHT), mid-, and highest low (HLT) tidal levels, respectively. A further 600 samples were immersed in de-ionized water or artificial sea water for 90 min every 1, 2, or 3 weeks, representing increasingly higher elevations above the LHT level, or exposed for 90 min every 1, 2, or 3 weeks, representing increasingly lower elevations below the HLT level; these experiments ran for 12 months. In the field, surface downwearing was measured at 108 transverse micro-erosion meter (TMEM) stations over 2 to 6 year periods. In the laboratory, mean downwearing rates between the LHT and HLT levels were 0.61–1.80 mm yr − 1 in sandstones and 0.01–0.18 mm yr − 1 in basalts; rates were highest at the LHT level. Rates decreased with elevation above the LHT level and were uniformly low below the HLT level. Salt weathering was dominant above the LHT level. Salt weathering and wetting and drying were important in sandstones between the LHT and HLT levels, but salts inhibited rock breakdown in basalts. In the field, the mean downwearing rate was 1.254 mm yr − 1 in sandstones, which was similar to the experimental data, and 0.722 mm yr − 1 in basalts, which was much higher than in the experiments with artificial sea water but similar to the experiments with de-ionized water. There was no relationship in the field between downwearing rate and rock hardness or TMEM station elevation.

Shore platform downwearing in eastern Canada: Micro-tidal Gaspé, Québec

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.

A device for measuring downwearing rates on cohesive shore platforms

AbstractThe device described in this paper, known as the traversing erosion beam, is believed to be the first to be designed specifically for measuring rates of downwearing of cohesive foreshores, such as clay shore platforms, under high‐energy wave conditions. It is primarily constructed of Flexlink Aluminium Structural System components and consists of a horizontal beam with a main leg and two further support legs. A sliding ‘dolly’ module is moved along the beam at precise intervals using a longitudinal scale as a reference. The topographic profile of the measurement surface is transferred to the dolly by a vertical sliding steel alloy rod, from which the relative height differences are measured by a separate engineer's digital height gauge placed on the dolly. The main leg of the device fits into a marine grade stainless steel box, which is deeply embedded into the foreshore or shore platform to act a permanent datum. The box is robust and can be expected to have a lifetime of at least a year, and possibly even five years or more.Sample data are provided from a clay shore platform at Warden Point in the Isle of Sheppey, Kent (UK), which demonstrate that the TEB can provide information on seasonal as well as annual rates of downwearing. The device has also been tested successfully on a peat and clay foreshore at Pett Level in Sussex and on a shore platform cut in glacial till at Easington, Yorkshire. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Can shore platforms be produced solely by weathering processes?

Field measurements and laboratory experiments provided data on surface downwearing rates on horizontal, mesotidal and sloping, macrotidal shore platforms in eastern Canada. The experimental data were used in a simple mathematical model to determine whether weathering, acting alone, can produce shore platforms. The model failed to produce realistic platform profiles: it did not account for the relationship in the field between platform gradient and tidal range, and downwearing rates were much too low to produce wide platforms in the few thousand years that the sea has been at its present level. Although weathering undoubtedly plays an important role in the development of shore platforms it cannot, by itself, account for characteristic differences in platform morphology in different morphogenic environments. It seems more likely that platform morphology is the result of mechanical wave erosion controlled by the tidally controlled expenditure of wave energy, and some related or independent mechanism that controls weathering efficacy at different tidal elevations according to the tidal range.

Tidal wetting and drying on shore platforms: An experimental assessment

Rocks were subjected to wetting and drying cycles with de-ionized water under real time conditions. Using tidal simulators, one-third of the samples experienced 11 h of exposure and 1 h of inundation over a 12-h tidal cycle (representing high tidal conditions), one-third experienced 6 h of exposure and 6 h of inundation (representing mid-tidal conditions), and one-third experienced 1 h of exposure and 11 h of inundation (representing low tidal conditions). We obtained 324 cores from igneous, metamorphic, and sedimentary rocks from Ontario, which were exposed to more than 930 wetting and drying cycles over 16 months. We obtained 675 cores and cubes from basalts, sandstones, and argillites from shore platforms in eastern Canada, which were exposed to about 700 cycles over 12 months. Many of the cores and cubes experienced very little to no breakdown, particularly those from the igneous and metamorphic rocks of Ontario. Sandstones from a sloping platform in the macrotidal Bay of Fundy and argillites from a horizontal, mesotidal platform in Gaspé, Québec, were the most susceptible rock types, with equivalent surface downwearing rates ranging from 0 to 4 mm year − 1. The results suggest that downwearing rates decrease with elevation within the intertidal zone, probably because rocks require much longer to desorb than to absorb water. The experiments demonstrated that wetting and drying can be an important process on shore platforms in some types of rock, helping to lower the height and reduce the gradient of sloping macrotidal platforms and to reduce the height of horizontal micro- and mesotidal platforms.