rock coast inheritance an example from galicia northwestern spain 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

fracture mechanisms in soft rock identification and quantification of evolving displacement discontinuities by extended digital image correlation deformation of rocks at shallow depths i e at low confining pressures is primarily driven by the evolution of new fractures or the propagation and coalescence of pre existing ones in this paper fracture evolution from inclined flaws cuts in a soft rock deformed under plane strain uniaxial compression is analyzed using high resolution digital photographs and digital image correlation dic dic provides a full field measurement of the kinematics and thus allows the analysis of non homogeneous deformation however standard dic approaches do not handle correctly fractures as they are displacement discontinuities therefore an extended dic method is developed that allows automatic tracing of discontinuities and their quantification in terms of the displacement jumps along their length the analysis of spatial and temporal fracture evolution using this method enhances the understanding of the phenomena in that it provides experimental quantification of fracture mode opening closing and shearing

modeling the role of weathering in shore platform development a mathematical wave erosional model was modified to study the additional effect of weathering by wetting and drying and salt weathering on the development of shore platforms in macro to mesotidal environments model rates of downwearing by these processes at different tidal elevations were based on data obtained from a series of laboratory experiments on sandstones from eastern canada backwearing by mechanical wave erosion was calculated using basic wave equations there were several types of run which were designed to determine the effect of weathering and the production of fine grained sediment the periodic accumulation of debris on weathering in the upper intertidal zone and weathering in reducing rock resistance and facilitating wave quarrying the results implied that compared to mechanical wave erosion the direct effect of weathering and fine grained sediment production makes only a small contribution to the long term development of shore platforms the relationship between cliff foot debris occurrence and platform development and morphology was inconsistent because of the negative feedback relationship between erosion rates surface gradients and rates of wave attenuation the model suggested that weathering can play an important indirect role in assisting wave quarrying of joint blocks and other rock fragments

shore platform downwearing in eastern canada the mega tidal bay of fundy abstract 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

development and morphodynamics of beaches on horizontal shore platforms on the microtidal southeast coast of brazil abstract the differences between beaches completely composed of sand and beaches underlain or fronted seaward by hard structures have been well addressed in the current literature considering their morphological and morphodynamic particularities trenhaile 2004 modeled the accumulation and dynamics of beaches on shore platforms on mesotidal and macrotidal coast although this model had not been verified or tested in the field it can be useful to understand coastal geomorphology therefore this study aims to apply the trenhaile 2004 model to beaches on horizontal shore platforms located on the microtidal coast of espirito santo brazil and validate it using local data for this the morphological and morphodynamic particularities of the studied beaches were considered when applying and interpreting the model results besides the trenhaile 2004 model variables the role of wave runup was considered in the upward growth of the berm and then in the beach height the model results and field data showed that the gradients and elevations of shore platforms in the study area are suitable to form the beaches while wider shore platforms seem to contribute to the development of wider beaches the platforms in the study area also influence accommodation space cross shore sediment exchanges and beach mobility field data was used to test the applicability of trenhaile s 2004 model in the study area the model was found to be a potentially useful tool in coastal management helping to understand coastal geomorphology

modeling the development of marine terraces on tectonically mobile rock coasts abstract a mathematical model was used to study the effect of glacially induced fluctuations in sea level on the formation of wave cut terraces on tectonically mobile rock coasts two deep water wave sets were used to calculate breaker height and depth which along with surf zone width bottom roughness and the gradient of the submarine slope dictated the force exerted at the shoreline an erosional force threshold was employed to represent the variable strength of the rocks the model also considered tidal range the time that the water level spent at each intertidal elevation and the protective effect of debris accumulation at the cliff foot three hundred runs were made with constant sea level and with different rates of rising and falling relative sea level rates of erosion increased with the rate of rising and falling sea level but eventually decreased in some runs with very rapid changes in relative sea level fifty five longer runs were also made with a quaternary sea level model that consisted of 26 glacial cycles representing the period from 2 million to 0 9 million years ago and nine cycles of approximately twice the amplitude and wavelength in the last 0 9 million years these runs were made on a landmass experiencing slow 0 11 mm yr 1 or fast 0 74 mm yr 1 positive or negative changes in the elevation of the land it was found that on rising landmasses erosional coastal terraces are formed during interglacial stages and on subsiding landmasses during glacial stages the number of terraces increased with the rate of uplift and subsidence and with the slope of the hinterland terrace gradient increased with tidal range and decreased with rock resistance the was an inverse relationship between terrace width and the strength of the rock

hydrodynamic constraints and storm wave characteristics on a sub horizontal shore platform few studies of wave processes on shore platforms have addressed the hydrodynamic thresholds that control wave transformation and energy dissipation especially under storm conditions we present results of a field experiment conducted during a storm on a sub horizontal shore platform on the east coast of auckland new zealand small 2 5 times the gravity wave height 0 05 0 33 hz is necessary for waves to propagate onto the platform without breaking on the platform surface the wave height is a direct function of water depth with limiting maximum wave height to water depth ratios of 0 55 and 0 78 at the centre of the platform and cliff toe respectively a relative platform edge submergence water depth water height ratio threshold of 1 1 is identified below which infragravity 0 05 hz wave energy dominates the platform energy spectra and above which gravity waves are dominant infragravity wave height transformation across the platform is governed by the relative platform edge submergence finally the paper describes the first observations of wave setup on a shore platform during the peak of the storm wave setup on the platform at low tide 0 21 m is consistent with measurements from planar sandy beaches but at higher tidal stages the ratio between incident wave height and maximum setup was lower than expected copyright 2014 john wiley sons ltd

modeling the development of wave cut shore platforms abstract a mathematical model was used to investigate the development of wave cut shore platforms with constant sea level the model considered the effects of deep water wave height spectra period and wavelength breaker height and depth breaker type the width and bottom roughness of the surf zone the gradient of the submarine slope an erosional threshold related to the strength of the rocks the number of hours each year in which the water level is at each intertidal elevation and the amount and persistence of the debris at the cliff foot intertidal erosion rates were calculated according to the force of the surf reaching the shoreline whereas submarine erosion rates were determined using a depth decay variable each of the two hundred and fifty model runs consisted of 300 000 iterations platform width increased and gradient progressively decreased through model runs and states of static equilibrium were attained by the end of one hundred and nine runs when the wave induced force at each point in the intertidal zone had became too weak to continue eroding the rock runs with mesotidal spring tidal range 3 m generally produced subhorizontal platforms with cliff platform junctions at the mean low water neap tidal level a greater variety of profiles was produced in runs with macrotidal spring tidal ranges 9 m although almost 30 of these runs produced gently sloping platforms with average gradients of only 0 3 about two thirds of the simulated profiles had average gradients ranging from about 1 to 3 simulated platform width increased with tidal range and decreased with the rate of submarine erosion rock resistance the roughness or irregularity of the platform surface the amount and persistence of the cliff foot debris and wave period because higher waves break in deeper water than lower waves there was no consistent relationship between simulated platform width and wave height

micro erosion meter measurements of rock wear on the otway coast of southeast australia abstract the use of the micro erosion meter mem has been refined by providing 1 a base plate on which to check the relationship of legs and probe and 2 a micrometer level to check the relationships of the three studs countersunk in the rock at each site along 50 km of the otway coast lines of mem stations were installed across shore platforms normal to the shore and surveyed to mean low water readings were made bimonthly from 1978 to 1980 average site downwear ranged from 0 02 to 1 8 mm yr 1 according to rock type and the amount of exposure to the almost continuous swell waves the results of this study combined with other information available are used to discuss the origin and evolution of otway coastal morphologies

modeling the accumulation and dynamics of beaches on shore platforms a mathematical model was used to study the development and dynamics of beaches on shore platforms the model was based on assumptions that the gradient of the beachface at its seaward edge must be greater than platform gradient beach sediment tends to move landwards and beach occurrence and extent depends upon the amount of sediment available equilibrium beachface gradient was related to breaker height wave period and sediment grain size three grain sizes were used representing fine and coarse sand and pebble two deep water wave sets representing storm wave and west coast swell wave environments and two tidal ranges of 9 1 and 3 3 m runs were made using linear concave and convex platform profiles with gradients ranging between about 0 6 and 8 because platform and beachface gradients are normally quite similar shore platforms are unable to store much sediment for a linear platform with a lower gradient than the beachface sediment first accumulates at the cliff base and then depending on the sediment available extends increasingly seawards beach development first occurs on the gently sloping seaward portions of concave platforms and on the more landward portions of convex profiles the amount of available material partly determines the landward and seaward extent of these beaches respectively although it is also controlled by beachface and platform gradients beaches on shore platforms become thinner and more gently sloping during storms and they may disappear completely depending on such factors as sediment grain size platform gradient the amount of sediment that can be stored during calm conditions wave characteristics and the duration of the storm the response of beaches on rocky coasts to rising sea level will be different from those on sedimentary coasts and will depend in part on the morphology of the rocky foundation

development of shore platforms on kaikoura peninsula south island new zealand ii the role of subaerial weathering the role of subaerial weathering in the development of shore platforms on kaikoura peninsula is investigated evidence for weathering came from a number of distinctive surface morphologies on platforms salt crystal growth water layer weathering and slaking schmidt hammer test data showed that weathering had occurred and that rock strength was reduced through weathering by as much as 50 weathering processes on shore platforms rely on repeated wetting and drying and for this reason the number of wetting and drying cycles was estimated it was found that the number of wetting and drying cycles depend not only on the pattern of tidal water levels but also on algae growth and desiccation the number of cycles ranged from 104 to 379 per year at elevations low in the tide range fewer cycles occurred the greatest number occurring between the peaks of spring and neap tides and where rainfall adds to the number the zone of most cycles was estimated to occur between 0 6 and 0 9 m above mean sea level on the more landward margins of platforms it is at these elevations that the highest rates of erosion occur based on these results it is argued that the development of shore platforms at kaikoura results from subaerial weathering caused by repeated wetting and drying this is contrary to the view that shore platforms result from marine erosion

sea stack formation and the role of abrasion on beach mantled headlands sea stacks are common and striking coastal landforms but few details are known about how how quickly and under what conditions they form we present numerical and analytical models of sea stack formation due to preferential erosion along a pre existing headland to address these basic questions on sediment rich rocky coasts as sea cliffs erode and retreat they produce beach sediment that is distributed by alongshore sediment transport and controls future sea cliff retreat rates depending on their width beaches can encourage or discourage sea cliff erosion by acting either as an abrasive tool or a protective cover that dissipates wave energy seaward of the cliff along the flanks of rocky headlands where pocket beaches are often curved and narrow due to wave field variability abrasion can accelerate alongshore directed sea cliff erosion eventually abrasion induced preferential erosion can cut a channel through a headland separating it from the mainland to become a sea stack under a symmetrical wave climate i e equal influence of waves approaching the coastline from the right and from the left numerical and analytical model results suggest that sea stack formation time and plan view size are proportional to preferential erosion intensity caused by for example abrasion and or local rock weakness from joints faults or fractures and initial headland aspect ratio and that sea stack formation is discouraged when the sediment input from sea cliff retreat is too high i e sea cliffs retreat quickly or are sand rich when initial headland aspect ratio is too small and the headland is rounded much wider in the alongshore direction at its base than at its seaward apex the headland is less conducive to sea stack formation on top of these geomorphic and morphologic controls a highly asymmetrical wave climate decreases sea stack size and discourages stack formation through rock sediment interactions copyright 2014 john wiley sons ltd

measuring erosion with the micro erosion meter contributions to understanding landform evolution abstract we review the contribution made to understanding landform evolution through the use of the micro erosion meter mem and the variant the traversing micro erosion meter tmem the mem has allowed the direct measurement of bedrock erosion in terrestrial and coastal settings as well as of building stone as recorded in mem measurements world wide the average rate of surface lowering on shore platforms is 1 486 mm a 1 and on terrestrial rock surfaces measured lowering rates are on average 0 903 mm a 1 reported means obscure significant variation in rates from a range of environmental settings and lithologies the mem has in a number of examples allowed the identification of the contribution of individual processes in polygenetic settings particularly in shore platform studies the dynamic nature of rock weathering has been highlighted where the tmem has revealed rock surfaces swelling and contracting at time scales ranging from hours to years the mem will continue to provide useful data on erosion rates and processes but the most fruitful investigations will be those that combine the t mem with other techniques that transcend time and space scale issues in erosion and process measurement

modeling cyclic recession of cohesive clay coasts effects of wave erosion and bluff stability abstract a mathematical model combining the effects of wave erosion beach sediment and bluff slope stability has been developed to study the episodic cyclical recession of cohesive clay coasts the model was applied to the southern great lakes in north america which although non tidal experience seasonal fluctuations in lake level of about 0 5 m and long term fluctuations of up to 2 m model runs were designed to explore the interaction between various components of the system including profile shape bluff recession rates lake level variations beach morphodynamics the geomechanical characteristics of the material and the type slump topple and size of the slope failures model runs were made for the equivalent of 100 years quasi equilibrium conditions developed in the runs which had similar erosion rates to the field with parallel slope retreat in the upper portion of the profiles and slope decline in the lower portion temporal variations in the rate of recession of the crest of the bluff closely matched changes in lake level the model confirmed previous work that suggested that rates of bluff recession are strongly controlled by the rate of erosion below the water surface but it also provided support for a more recent proposal that rates of bluff recession also influence rates of erosion in shallow water it was found that positive and negative changes in the size of the beach can either promote or hinder bluff and profile erosion depending on local circumstances the model demonstrated that bluff height debris mobility wave undercutting and groundwater levels are key factors in determining the stability of coastal bluffs the model represents a promising new tool to forecast and thereby mitigate the effects of climate change on marine and lacustrine coasts

the role of beach morphology on coastal cliff erosion under extreme waves erosion of hard rock coastal cliffs is understood to be caused by a combination of both marine and sub aerial processes beach morphology tidal elevation and significant wave heights especially under extreme storm conditions can lead to variability in wave energy flux to the cliff toe wave and water level measurements in the nearshore under energetic conditions are difficult to obtain and in situ observations are rare here we use monthly cliff face volume changes detected using terrestrial laser scanning alongside beach morphological changes and modelled nearshore hydrodynamics to examine how exposed cliffs respond to changes in extreme wave conditions and beach morphology the measurements cover the north atlantic storms of 2013 2014 and consider two exposed stretches of coastline porthleven and godrevy uk with contrasting beach morphology fronting the cliffs a flat dissipative sandy beach at godrevy and a steep reflective gravel beach at porthleven beach slope and the elevation of the beach cliff junction were found to influence the frequency of cliff inundation and the power of wave cliff impacts numerical modelling xbeach g showed that under highly energetic wave conditions i e those that occurred in the north atlantic during winter 2013 2014 with hs 5 5 m dissipative site and 8 m reflective site the combination of greater wave height and steeper beach at the reflective site led to amplified wave run up subjecting these cliffs to waves over 4 times as powerful as those impacting the cliffs at the dissipative site 39 kwm 1 compared with 9 kwm 1 this study highlighted the sensitivity of cliff erosion to extreme wave conditions where the majority over 90 of the annual value of cliff face erosion ensued during the winter the significance of these short term erosion rates in the context of long term retreat illustrates the importance of incorporating short term beach and wave dynamics into geomorphological studies of coastal cliff change

modelling the effect of waves weathering and beach development on shore platform development a mathematical model was used to study shore platform development mechanical wave erosion was dependent on such variables as tidal range wave height and period breaker height and depth breaker type surf zone width and bottom roughness submarine gradient rock resistance and the elevational frequency of wave action within the intertidal zone also included were the effects of sand and pebble accumulation cliff height and debris mobility and downwearing associated with tidal wetting and drying the occurrence location and thickness of beaches often depended on initially quite minor variations in platform morphology but owing to their abrasive or protective effect on underlying rock surfaces they were able to produce marked differences in platform morphology generalizations are difficult but the model suggests that platform gradient increases with tidal range platform width also increases with tidal range with slow downwearing but it decreases with fast downwearing platform gradient decreases and width increases with wave energy and decreasing rock resistance and platform roughness with low tidal range platform gradient is generally lower and platform width greater with beaches of fine sand than with gravel but the relationship is more variable with a high tidal range platform width increases and platform gradient decreases with the rate of downwearing on bare surfaces particularly in low tidal range environments but the pattern is less clear on beach covered platforms platforms with large amounts of beach sediment tend to be narrower and steeper than bare platform surfaces platform gradient increases and platform width decreases with increasing cliff height and with decreasing cliff debris mobility copyright 2005 john wiley sons ltd

storm swash deposition on an embayed rock coastline facies formative mechanisms and preservation abstract storm swash terraces mark the most distal landward deposition of storm wave events on high energy rock coasts they form a potential but as yet unexploited sedimentary archive of past storminess this paper describes storm swash terraces at morgan bay eastern cape south africa they form as steeply seaward dipping 30 60 partially vegetated terraces comprising alternating beds of organic rich sand mud and gravel the terraces are up to 5 m thick and the basal elevations range from 0 5 m amsl in the embayment to 6 5 m amsl on the headland margins regardless of location they are always fronted by a rocky shore platform on which most storm wave energy is attenuated six sedimentary facies are delineated on the basis of their lithological and paleontological characteristics and the nature of each bounding surface deposits in the most exposed headland locations are thicker and coarser grained than those formed in the embayment composition of the depositional terrace reflects the fronting shore platform indicating a short source to sink transport distance the thickest storm deposits form where the backing topography is high and accommodation is limited storm swash terraces have a high preservation potential with successive storms depositing material in an aggradational manner reworked material is incorporated into subsequent depositional units the spatial controls on facies partitioning in each terrace appear to be a function of position in the embayment the availability of material to seaward and the presence of accommodation space areas of particularly limited accommodation space produce aggradational storm sequences that form especially good archives of marine storminess

shore platform abrasion in a para periglacial environment galicia northwestern spain abstract 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

rates and patterns of erosion on inter tidal shore platforms kaikoura peninsula south island new zealand this paper presents measured rates of erosion on shore platforms at kaikoura peninsula south island new zealand surface lowering rates were measured with a micro erosion meter and traversing micro erosion meter the mean lowering rate for all shore platforms was 1 130 mm a 1 differences in lowering rates were found between different platform types and lithologies the rate of surface lowering on type a sloping mudstone platforms was 1 983 mm a 1 and 0 733 mm a 1 on type b mudstone platforms subhorizontal on limestone platforms the lowering rate was 0 875 mm a 1 a previously reported cross shore pattern of surface lowering rates from kaikoura was not found rates were generally higher on the landward margins and decreased in a seaward direction season is shown statistically to influence erosion rates with higher rates during summer than winter the interpretation given to this is that the erosive process is subaerial weathering in the form of wetting and drying and salt weathering this is contrary to views of shore platform development that have favoured marine processes over subaerial weathering copyright 1998 john wiley sons ltd

modelling the development of dynamic equilibrium on shore platforms abstract a numerical model was used to determine whether the landward migration of shore platforms at the low tidal level due to downwearing and possibly other mechanisms can match the recession caused by wave erosion at the high tidal level the hybrid model calculated the time required to undercut a cliff face and to remove the debris and the amount of recession accomplished at the low tidal level during that time these iterations were repeated over the equivalent of a 40 000 year period the rates of high tidal erosion calculated during model runs were representative of cliff recession rates recorded in the field and downwearing rates specified for each run were based on data recorded on shore platforms with micro erosion meters almost all runs demonstrated that platform gradients decline asymptotically to a state of dynamic equilibrium that is strongly related to tidal range and rock resistance although its relationship to wave height varies according to attenuation rates in the surf zone periodic changes in rock resistance and continuous changes in cliff height can introduce perturbations that delay equilibrium or depending on their severity prevent its occurrence the conclusion that platforms trend towards dynamic as opposed to static equilibrium is contrary to other models which have considered only the effect of wave erosion the results of this work have important implications for coastal modelling and the estimation of platform lowering for cosmogenic dating

the morphodynamics of shore platforms in a micro tidal setting interactions between waves and morphology abstract the interactions between waves and morphology on shore platforms were investigated at four platforms located at kaikoura peninsula new zealand and the otway coast victoria australia waves were measured over six tidal peaks of a spring tidal sequence at each site during a six month period february 2007 july 2007 this paper reports the results of an investigation into the research question how are waves modified by platform morphology in a micro tidal setting through the investigation of wave interactions on shore platforms it was found that complex hydrodynamic patterns formed on the shore platforms studied through direct measurement of waves on the platforms it was found that wave attenuation was not dependent on platform width but rather influenced by platform gradient and depth of water on the platform spectral analysis was used to assess how wave energy was transformed across the platforms at three of the four platforms investigated energy gradients formed across the platform with wave energy recorded within the infragravity range

evolution and inheritance of a rock coast western galicia northwestern spain 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 dif cult 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 uvio 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

development of shore platforms on kaikoura peninsula south island new zealand abstract the role of waves in shore platform development has been investigated on kaikoura peninsula by direct measurement of waves in deep water and on platforms at high tide this showed that the deepwater wave environment off the kaikoura peninsula is very energetic but the amount of energy delivered to platforms is very low in deep water wave power ranged from 5900 to 106 700 n m s 1 per m of crest when calculated from mean significant wave height and when calculated from maximum wave height it ranged from 31 400 to 5 370 000 n m s 1 per m of crest a comparison of deep water wave energy flux with wave energy flux at the high tide cliff of platforms showed that there was a reduction by as much as five orders of magnitude during storm conditions an analysis of the role of breaking waves revealed that these are ineffective as an erosional agent because the depth of water offshore causes breaking before waves arrive on platform surfaces depth of water controlling wave breaking causes larger storm waves to break further from shore preventing direct attack at the cliff base shear stresses and dynamic forces under waves were calculated from waves measured on shore platforms this showed that these forces never exceeded the compressive strength of the platform rocks at kaikoura it is concluded that wave forces are not directly capable of causing erosion on shore platforms at kaikoura

modelling the effect of pliocene quaternary changes in sea level on stable and tectonically active land masses a mathematical model was used to examine the effect of pliocene and quaternary changes in sea level on the development of tectonically active and inactive rock coasts the model calculated rates of mechanical wave erosion according to such factors as the deep water wave regime bottom topography and surface roughness and the resistance of the rocks subaerial terraces were truncated or eliminated by subsequent terrace formation at lower elevations especially on steeply sloping landmasses experiencing slow rates of uplift submarine terraces formed during glacial stillstands were best preserved when rapid subsidence quickly carried them below the level of wave action on slowly subsiding landmasses submarine terraces formed during interglacials and glacial periods experienced repeated erosional modification during subsequent periods of rising and falling sea level and were generally less distinctive on rapidly rising or subsiding 5 mm yr 1 landmasses terraces that formed during interglacial stages alternated above and below present sea level with terraces formed during glacial stages despite some differences in terrace occurrence and elevational distribution it may be difficult to distinguish profiles cut during accelerating or decelerating uplift the amount of erosion during sea level oscillations increases with oscillation amplitude and the larger oscillations in the middle to late quaternary were therefore more conducive to erosion than the smaller oscillations of the pliocene and early quaternary the effect of oscillation amplitude may have been countered during the earlier stages of profile development however by steeper submarine gradients and reduced rates of wave attenuation copyright 2014 john wiley sons ltd

boulder coastal deposits at favignana island rocky coast sicily italy litho structural and hydrodynamic control abstract boulders are frequently dislodged from rock platforms transported and deposited along coastal zones by high magnitude storm waves or tsunamis their size and shape are often controlled by the thickness of bedding planes as well as by high angle to bedding fracture network we investigate these processes along two coastal areas of favignana island by integrating geological data for 81 boulders 49 rupture surfaces called sockets and fracture orientation and spacing with four radiocarbon dates numerical hydrodynamic analysis and hindcast numerical simulation data boulders are scattered along the carbonate platform as isolated blocks or in small groups which form as a whole a discontinuous berm underwater surveys also highlight free boulders with sharp edges and sockets carved out in the rock platform boulders are composed of ruditic to arenitic size clastic carbonates their size ranges from 0 6 to 3 7 m 0 55 to 2 4 m and 0 2 to 1 m on the major a medium b and minor c axes respectively the highest value of mass estimation is 12 5 t almost all of boulders and sockets are characterized by a tabular or bladed shape the comparisons between a the fractures spacing and the length of a and b axes and b the frequency peaks of c axis with the recurrent thickness of beds measured along the coastal zone demonstrate the litho structural control in the size and shape of joint bounded boulders these comparisons together with the similarity between the shapes of the boulders and those of the sockets as well as between the lithology of boulders and the areas surrounding the sockets suggest that blocks originate by detachment from the platform edge thus the most common pre transport setting is the joint bounded scenario hydrodynamic equations estimate that the storm wave heights necessary to initiate the transport of blocks diverge from 2 m to 8 m for joint bounded boulders and from few tens of centimeters up to 11 m for submerged boulders the comparison between the wave heights at the breaking point of the coastal zones with the results of hydrodynamic equations shows that waves approaching the coastline are able to transport all surveyed boulders our data suggest that boulders have been transported by several storm events even in very recent times

sedimentology and morphodynamics of a macrotidal beach pendine sands sw wales abstract the foreshore of pendine sands forms the seaward part of an extensive sandy coastal barrier in a shallow carmarthen bay sw wales the sedimentological features of the macrotidal foreshore reflect a tide induced modification of nearshore wave characteristics as the tide ebbs the breaker height may decrease the surf zone widens and becomes increasingly dissipative and swash backwash velocities diminish a concomitant change from plunging to spilling breakers and increasingly symmetrical swash zone flows are associated with a decreasing beach gradient a zero net transport model demonstrates that the beach profile is self stabilising in the short term and periodic levelling has shown that the beach is in long term equilibrium with prevailing conditions though this does not preclude a significant dynamic response to changing tides and waves the flow regimes of wave generated currents decline as the tide ebbs and normal beach processes do not usually affect the lower foreshore accordingly there is an overall seaward fining of the primary framework component of the sands in more detail this framework component displays a slight seaward coarsening across an upper foreshore dominated by high water swash and surf a rapid seaward fining across the mid foreshore in response to the ebb attenuating swash zone flow velocities and a slight seaward fining across the lower foreshore under the action of nearshore shoaling waves bedforms vary from a swash backwash emplaced flat bed across the upper foreshore to the small ripples of nearshore asymmetric oscillatory flows across the lower foreshore the surface sediment veneer is not representative of the subsurface sediments which form in response partly to fairweather conditions partly to storms the upper foreshore is characterised by swash backwash emplaced plane bedding in fine sands frequently disrupted by bubble cavities the mid foreshore is composed of coarser grained shelly traction clogs arranged as landward and seaward dipping large scale cross bedding and or plane bedding these are probably storm breaker surf deposits the lower foreshore though partially and sometimes totally bioturbated shows landward dipping small scale cross bedding in very fine sands sorted by nearshore shoaling waves tide and storm induced modification of the nearshore flow regimes therefore produces a distinctive shore normal array of sedimentary facies each facies is characterised by diagnostic textural and structural signatures a prograding sequence of such macrotidal deposits would be similar to but more extensive than a comparable microtidal sequence

where is the seaward edge a review and definition of shore platform morphology abstract shore platforms are erosional coastal landforms that have attracted scientific attention since the mid 19th century the defining element of a platform is width that is used in many calculations such as determining a platform s evolutionary state or inferring how wave energy is distributed along the shore although a critical variable there are no uniform criteria for defining the seaward edge quantification of platform width has been driven by site specific variables with the seaward edge defined on the basis of tides morphology biology processes and sediment coverage the lack of a uniform definition has meant that comparative studies are difficult and results are possibly spurious as widths derived from very different criteria can vary by an order of magnitude just on the basis of which criteria is used to determine its edge in this review a combination of morphologic and process elements is used to define the seaward edge of a shore platform the development of strict criteria is especially needed in an environment of rising sea levels if measurements of landscape change are to be made in addition the advent of seamless datasets that cross the land sea boundary means that the delineation of platform morphology is no longer limited by physical access this review concludes that the seaward edge of a shore platform will occur at or landward of wave base and should be defined as the point where active erosion of the bedrock ceases characterised by erosional features such as notches and block plucking scars or the deposition of sediment of such a thickness that the underlying bedrock is not exposed during storm events

the unsteady nature of sea cliff retreat due to mechanical abrasion failure and comminution feedbacks sea cliff retreat is often linked to large waves heavy precipitation and seismic events but the specific operative mechanics have not been well constrained in particular what is the role of mechanical abrasion by beach sediments in cliff platform evolution and how does it relate to the episodic nature of cliff retreat observed at certain locations here we present a simple numerical model of sea cliff retreat that incorporates mechanical abrasion of a basal notch threshold controlled failure of the cantilevered block and a feedback mechanism wherein retreat is dependent on the rate of sediment comminution within the surf zone using shore platform and cliff characteristics found in two coastal settings the central california coast and the english north sea coast the model produces retreat rates comparable to those observed via field measurements the highest retreat rates coincide with the steepest shore platforms and increasing wave height steeper platforms promote wave access to the cliff toe and correspondingly the receding cliff face produces additional accommodation space for the platform beach preserving the erosive efficacy of the beach sediments when exposed to energetic wave forcing the slope of the inner platform segment controls retreat rates for concave platforms whereas the slope of the outer platform segment exerts greater control for convex platforms platform beaches approached a long term dynamic equilibrium on the concave profiles leading to more consistent and steady retreat platform beaches were ephemeral on convex profiles mirroring observed sand wave ord migration on the holderness coast uk these findings agree with previous field observations and support mechanical abrasion as a viable cause of temporal heterogeneity in cliff retreat rate for both coastlines

morphological features and processes in the central algarve rocky coast south portugal morphological features along the algarve rocky coast south portugal are identified and described with an emphasis on shore platforms and notches the contributions of processes such as wave attack chemical weathering and bioerosion to sculpting the shore platforms are discussed the preferential localization of shore platforms on sites exposed to waves and the lack of significant chemical weathering point to wave erosion as the first order factor in platform formation whilst bioerosion bioprotection lithology and geological structure determine platform morphological variations in addition platforms above the present intertidal zone appear to have a polygenetic evolution being inherited from former sea level highstands and currently undergoing chemical weathering the occurrence of notch features is independent of the degree of exposure to waves but they mostly occur where the substratum is sand hydrostatic pressure appears to be an important factor in the formation of marine caves in the more sheltered sites

wave planation surfaces in the mid western east sea sea of japan indicators of subsidence history and paleogeographic evolution of back arc basin abstract this study focuses on revealing the origin of deep marine wave planation surfaces and its geomorphic and tectonic implications for the back arc evolution based on multi channel seismic reflection profiles acquired from the mid western east sea sea of japan on seismic reflection profiles the wave planation surfaces are recognized as continuous and high amplitude single reflectors outlining the flattened top of submarine terraces or isolated topographic highs the wave planation surfaces are classified into four types depending on the geomorphic characteristics and origins of substratal landmasses i e continental margin terrace type 1 rifted continental fragment type 2 volcanic edifices type 3 and uplifted sedimentary successions type 4 which have common origin of wave erosion near the sea level but different subsidence history the mean subsidence rate of wave planated substratum is calculated by dividing the maximum depth of a wave planation surface below modified datum sea level d e by the time elapsed since the onset of subsidence t information of local subsidence history enables reconstruction of paleogeographic evolution of mid western east sea during the neogene quaternary time 1 incipient continental rifting and onset of marine incursion ca 23 18 ma 2 progressive continental rifting and onset of wave planation ca 18 11 ma 3 final eruption of post rift volcanoes ca 11 ma 4 uplifting of shelf margin banks and onset of the pliocene holocene volcanic activities ca 11 2 5 ma 5 change in the locus of volcanic eruption and cessation of contractile uplifting in the basin margin ca 2 5 0 ma

onshore offshore sediment transport on beaches abstract the onshore offshore transport of sediment on beaches in analyzed by considering the time averaged behaviour of a control volume bounded by the wave breaking zone and by the beach face interactions are determined between the wave radiation stress the wave set up the shear stress on the beach face and the beach permeability simple theoretical relationships are derived which specify beach slope in terms of incident wave attack and beach material properties these relationships indicate that coarser sediment beaches are steeper because of the higher beach permeability it is also shown that any particular beach will decrease its slope as the wave attack increases according to an inverse function of the wave height not only the beach permeability but also the sediment grading curve is shown to be a key factor in determining beach slope the theory is used to discuss and explain some observed behaviour on real beaches and the possible implications for beach failure under storm attack

modeling the effect of late quaternary interglacial sea levels on wave cut shore platforms abstract a mathematical wave erosional model was used to study the effect of high sea levels during the penultimate oxygen isotopic stage 7 and last interglacials substage 5e and in the late holocene stage 1 on the present morphology of wave cut shore platforms sea level was considered to have been either the same as today or 2 25 m lower during the penultimate interglacial and 2 25 4 5 or 6 75 m higher than today during the last interglacial stage the model suggested that inherited gently sloping shore platforms in resistant rocks may be essentially protected today from erosion by high storm waves the lowest platform gradients were in runs with mesotidal 3 m range and usually with low wave periods low surf attenuation rates and weak rocks modern platform gradients increased with the difference in elevation between sea levels during successive interglacial stages shore platforms were widest in runs in which sea level was the same as today in the penultimate interglacial and 4 5 m higher than today during the last interglacial constant sea level and high last interglacial sea levels with considerable overlapping between the zones of high duration values between the mean neap high and low tidal levels in stages 7 5e and 1 were conducive to the development of wide shore platforms whereas sea levels lower than today s in stage 7 tended to produce narrower platforms in general higher sea levels during the last interglacial tended to produce higher cliff platform junctions than constant sea level unless the sea was lower than today during the penultimate interglacial stage there was a lack of supratidal ledges in macrotidal 9 m model runs with high initial gradients this suggests that gently sloping inherited shore platforms are essential for the subsequent development of supratidal nonstructural ledges in high tidal environments during periods of higher sea level intertidal ledges developed in the upper portion of the modern intertidal zone under a variety of tidal and sea level conditions these ledges can develop independently of lithological or structural influences and without any change in sea level a single high sea level may also simultaneously produce two ledges at different supratidal elevations in mesotidal environments

the influence of rock resistance on coastal morphology around lord howe island southwest pacific an erratum has been published for this article in earth surface processes and landforms 27 7 2004 931 lord howe island in the northern tasman sea is a remnant of a much larger late miocene basaltic shield volcano much of the island s coastline is exposed to waves that have unlimited fetch but a marked contrast is provided by a fringing coral reef and lagoon that very effectively attenuate wave energy along a portion of the western coastline the geology of the island is varied with hard and resistant basalt lavas breccias and tuffs of intermediate resistance and highly erodible eolianites this variability provides an excellent opportunity to examine the in uence of rock resistance on the development of the spectacular rock coast landforms that occur around the island the hardness of rocks and the extent of weathering around the coastline were assessed using a schmidt hammer and statistical analysis was undertaken to remove outlying values on all but one occasion higher mean rebound values were returned from fresh surfaces than weathered surfaces but only half of these differences were statistically signi cant shore platforms with two distinct levels are juxtaposed along two stretches of coastline and schmidt hammer results lend support to hypotheses that the raised surfaces may be inherited features relative rock resistance was assessed through a combination of schmidt hammer data and measurements of joint density and constrained on the basis of morphological data this approach formed a basis for examining threshold conditions for sea cliff erosion at lord howe island in the context of the distribution of resistant plunging cliffs and erosional shore platforms copyright 2004 john wiley sons ltd

rates and patterns of bedrock denudation by coastal salt spray weathering a seven year record rapid surface lowering of bedrock is taking place in the supratidal zone by salt spray weathering a seven year run of data demonstrates a mean rate of lowering of 0 625 mm a 1 considerable variation exists in annual point lowering values within measurement sites although between site variation is not significant aggregate year to year variations in surface lowering are not significant spatial variation in individual point values may be compensated by temporal variation over an 11 year period there is a marked summer maximum in surface lowering rate and this is strongly correlated with monthly air temperature spatially and temporally episodic swelling of the rock surface is demonstrated this does not correlate statistically with any available climatic variable and is deemed to be a real and largely stochastic phenomenon it is interpreted as rock bursting at the granular scale due to haloclasty the processes most likely to be responsible for the observed rapid denudation are crystallization and thermal expansion of halite both of which are enhanced by high summer temperatures

geomorphologic equifinality a comparison between shore platforms in hoga kusten and faro sweden and the vale of glamorgan south wales uk 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

equilibrium responses of cliffed coasts to changes in the rate of sea level rise abstract basic formulae have long been used to predict the effects of sea level rise on coastal recession for instance the geometric bruun rule and its modifications has often been applied to sandy coasts both low lying and steep however the behavior of rocky coasts whether strongly or poorly lithified should be significantly different than that of sandy coasts given that rocky coast evolution depends upon the irreversible breakdown of rock whereas sandy and depositional systems are controlled by the transport and related transport gradients of mobile sediment here we investigate the basis of a modeled relationship which suggests with a number of caveats that the equilibrium soft rock cliff recession rate can be estimated by the square root of the relative change in sea level rise rate although this relationship was derived using the numerical model scape soft cliff and platform erosion which simulates a broad soft rock cliffed coastal system driven by stochastic environmental forces here we show that a simplified modeling approach also reproduces the relationship we then extend this approach to develop a general theoretical framework within which it is possible to consider the potential responses of the different types of cliffed coasts to changes in the rate of sea level rise although a wide variety of processes affect different coastal settings this framework demonstrates how the strength and the nature of feedbacks within cliffed system control their response to sea level rise this suggests that cliffed environments controlled by different processes can still respond in similar ways to changes in the rate of sea level rise most rocky coasts would be expected to respond as a damped or negative feedback system between the extremes of a no feedback system that is unresponsive to sea level rise rate and an instant response system characterized by a linear response similar to the bruun rule this framework suggests that a potential inverse feedback case could also exist in which increased rates of sea level rise reduce the rate of coastal recession in almost all cases it is apparent that cliffed coast response to sea level rise depends not only upon the total elevation change of sea level but on the rate of the sea level rise these theoretical investigations and the classifications presented provide a framework to understand the behavior of systems affected by a wide array of processes and provide expectations that can be tested using more complex models of cliffed coast evolution in a variety of environments whether sandy or rocky hard or soft

modelling the quaternary evolution of shore platforms and erosional continental shelves a mathematical model was used to investigate the effect of glacially induced fluctuations in sea level on the evolution of wave cut shore platforms and erosional continental shelves during the quaternary the model used two deep water wave sets which were used to calculate breaker height and depth and the force of the waves at the waterline according to the width and bottom roughness of the surf zone and the gradient of the submarine slope the model also incorporated an erosional threshold related to the strength of the rocks the number of hours each year in which the water level is at each intertidal elevation and the amount and persistence of the debris at the cliff foot most runs were made using a sea level model that consisted of 26 glacial cycles from 2 million to 0 9 million years ago and nine of approximately twice the amplitude and wavelength in the last 0 9 million years the model emphasized the dynamic association between the contemporary intertidal platform and the continental shelf both surfaces trend towards a state of static equilibrium under oscillating sea level conditions when attenuated waves are unable to continue eroding the rock if there has not been enough time to reduce the gradient of the shallower portions of the continental shelf however intertidal shore platforms can be in a temporary though possibly long lasting state of dynamic equilibrium the model suggests that most platforms are at least in part inherited from one or in many cases more interglacial stages when sea level was similar to today s copyright 2001 john wiley sons ltd

frost weathering and the saturation of coastal rocks measurements were made of the water content in coastal rocks by simulating tidal oscillations in the laboratory and by field measurement in eastern canada if rapid freezing takes place upon exposure to the air saturation levels may be high enough to permit frost weathering in fine grained rocks in the lower portions of the intertidal zone near the high tidal level however it may be dependent upon a supply of water from the ice foot and from melting snow if freezing is slow frost action may be inhibited by desorption of the rocks while they are exposed by the ebb tide there was no evidence of a level of permanent sea water saturation within the intertidal zone ambient temperature and humidity may affect the rate of rock desorption

temporal and spatial variations of chalk cliff retreat in east sussex 1873 to 2001 spatial and temporal variations of chalk cliff retreat rates over a 130 yr period are examined for 22 km of the east sussex chalk coastline between brighton and belle toutetout lighthouse england data are derived from maps surveyed between 1873 and 1925 28 and air photographs dating from 1973 and 2001 the long term average cliff retreat rate over the period 1873 to 1973 is 0 36 m y 1 and from 1873 to 2001 is 0 35 m y 1 over the whole 23 km study area rates calculated over shorter time slices indicate a decline in cliff retreat 0 37 m y 1 between 1873 and 1925 28 0 35 m y 1 between 1925 28 and 1973 0 27 m y 1between 1973 and 2001 temporal variations are unlikely to be due to variations in precipitation and frost days though the relationship remains uncertain due to the resolution of available cliff retreat and climatic data changes in wave climate are small and cannot be responsible for the decline in cliff retreat the study suggests that the most important factors causing the measured decline in cliff retreat rates are firstly the loss of cliff toe beaches over time and secondly widening of the rocky shore platforms that front the cliffs some of the spatial variability in retreat rates can be explained by local variations in lithology

the formation of beaches on shore platforms in microtidal environments beaches are common features of many rocky shorelines and can be considered to be constrained by the underlying geology in mesotidal to macrotidal areas the slope of the substrate and sediment supply are the primary factors in constraining the size and development of beaches on shore platforms in microtidal settings it is not known if these factors are wholly responsible for determining the presence of beaches on shore platforms nor the contribution of other factors such as hydrodynamics the microtidal coast of victoria australia is surveyed in this study in order to quantify the morphological boundary conditions that constrain beach development on semi horizontal shore platforms an ample sediment supply indicates that the underlying geology is controlling the presence and absence of beaches where beaches occur they always overlie a rock ramp which is the steepest part of the platform the two most important morphological constraints were platform width and height both of which significantly correlated with beach volume an elevational threshold exists at just over 1 0 m where beaches cannot accumulate below this threshold platform width appears to be the principle constraining factor in beach accumulation an evolutionary model is inferred which suggests that dissipation of wave energy associated with platform widening plays an important role in allowing beaches to accumulate the model suggests beaches on platforms will be particularly sensitive to sea level rise copyright 2014 john wiley sons ltd

modeling cohesive clay coast evolution and response to climate change a model was used to study the evolution of cohesive clay coasts under different environmental and geological conditions and the effect of future climate change the model considered wave impact on the bluff and in the intertidal zone and bottom generated shear stresses and the protective and abrasive action of beach sediment in the intertidal and subtidal zones quasi equilibrium conditions developed in model runs with parallel slope retreat in the upper portion of the profiles and slope decline in the lower portions profile gradient below the low tidal level increased with the threshold shear stress for bottom erosion therefore rates of wave attenuation were lower in resistant than in weak materials and the intertidal zones were wider and more gently sloping the widest and gentlest intertidal zones developed in runs that had no sediment the largest waves played little role in platform development in part because of low frequency but mainly because they expend most of their energy in crossing wide turbulent surf zones the model suggested that rising sea level will usually result in accelerated rates of bluff recession especially if the rise is rapid simultaneous increases in the frequency of moderately large waves as opposed to the largest waves may also generate marked increases in erosion on bluffs and in intertidal and supratidal zones although erosion may be reduced at some elevations where there is beach sediment positive or negative changes in beach volume can increase or decrease rates of bluff erosion

rocky coasts their role as depositional environments abstract 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