wave erosion

A mathema�cal wave-erosion model is used to simulate postglacial shoreline profiles along the rocky, high energy coast of the north of Ireland. The wave erosion model is driven by a suite of rela�ve sea-level (RSL) curves for the last 16,000 years produced from four glacial rebound models. Mul�ple runs are performed with different ini�al shore profiles and rock resistances to inves�gate shoreline evolu�on and the significance of inherited morphology on the resultant profile shape. The simulated profiles are then compared with mapped profiles from three areas of the north of Ireland with different lithological and hydrographic proper�es. Modelled profiles generally replicate the overall mean shoreline gradients observed across the region when rock resistance is rela�vely high and erosion rates correspondingly low. In these profiles, breaks in mean slope are observed at depths comparable to the RSL minima in several of the RSL scenarios (at c.-10m,-15m and-20m for North Antrim, Derry and Donegal respec�vely). At Portrush and Portballintrae (Derry), profiles may be influenced by structural controls rela�ng to the underlying basalt surface and the removal of overlying glaciogenic sediments. All RSL scenarios replicate the observed eastward increase in cliff-pla�orm junc�on height, reflec�ng the differen�al glacioisosta�c rebound experienced along the coast. However, the precise eleva�on at which the simulated cliff base occurs is sensi�ve to the choice of RSL scenario, sugges�ng that this parameter may prove useful in evalua�ng glacial rebound model performance. Several of the RSL scenarios generate raised shore pla�orms or terraces in North Antrim and Derry at heights comparable to raised shoreline features reported in the literature. However, no single curve or combina�on of parameters is capable of genera�ng the range of pla�orm and terrace features observed in the bathymetric and topographic data. These misfits are consistent with the idea that many rock pla�orm or terrace features are inherited from an earlier phase or phases of RSL.

Divers in the 1980s discovered a large concentration of archaeological material on the seafloor off Kaulonia, a Greek colony on the Ionian coast of southern Italy. It has been propo.sed that a small hook-shaped cape, now submerged, was present off the site during settlement from about 700 to 389 BC. Since 1992, however, most of Kaulonia's seafloor sector has been buried by a thick (to 4 m) sand cover. This investigation resumes study of Kaulonia's margin by highresolution seismic survey, analysis of sediment in cores and excavated sections buried along the coast, and petrologic examination of rock used for construction of a temple, the site's major structure. The sector, located in the tectonically active Calabrian arc, has been subject to considerable elevation rise of land onshore, and an equivalent rate of seafloor subsidence (~1 mm/yr). A distinct Holocene sequence, composed of beachrock-type sandstone and fluvial-marinetransported conglomerate, in sediment sections landward of the beach can be traced seaward to depths of 7 m about 300 m from the present shoreline. Correlation of the sequence helps define changes of coastline position from before Greek settlement to the present. Rather than a generally progressive migration landward of the shoreline in response to relative sea-level rise, the coast has experienced more complex shifts (sea-to-land, land-to-sea, and return to land) due to up-and-down motion in the sector between the alluvial plain behind the beach and innermost shelf. By the mid-Holocene, the shoreline had advanced seaward, resulting in coastal emergence, and it did not return landward (resubmerging the margin) until after the site's abandonment; submergence was complete by late Roman time. We now recognize a broad arcuate headland, rather than a small hooked cape, seaward of the site •2500 years ago. Without a lagoon, sufficiently deep lake, or man-made port available for protected anchorage, vessels sailing to Kaulonia for trade were either beached or anchored close to shore and, perhaps seasonally, within the mouth of the Assi River. Composition of beachrock-type sandstone in Holocene sections on land is comparable to that offshore and also to sandstone used for temple base construction. Extensive sandstone exposures along this coast may have been one reason why Greeks selected the site for settlement.

Progressive erosion of channel-bank sandbars in Grand Canyon has long been thought to be associated with emplacement and operation of Glen Canyon Dam, although the specific physical mechanisms causing local erosion are poorly understood. A shortterm study (order of days) of detailed flow patterns and morphologic adjustments at Stone Creek and Fern Glen sandbars during constant discharge demonstrates that surfacegravity waves and other quasi-periodic flow oscillations are important to the stability characteristics of these alluvial deposits. The primary role of waves is to agitate bottom sediments, entraining them on an intermittent basis. Mean currents associated with recirculating eddies (ordinarily of insufficient strength to entrain sediments) act, in the presence of waves, as a net background drift able to transport sediments away from the bar face and into the main channel. Thus combined wavecurrent interactions provide for sediment transport possibilities that might not occur in the absence of waves. Simple models of beach-foreshore equilibria developed for coastal environments show that the faces of Grand Canyon sandbars behave very much like coastal, wave-dominated features. But the wave-dependent equilibria predicted by coastal models are never attained fully because mean eddy-recirculation currents associated with the river play an important role in the fluid-sediment interactions observed on nearshore terraces. Unlike a coastal system where mean alongshore currents owe their existence to wave motion, eddy-recirculation currents in a fluvial system are completely independent of waves, vis-i-vis their hydrodynamic origin. Thus neither a purely fluvial approach nor a purely coastal approach will be completely successful in describing sandbar stability in Grand Canyon, and a hybrid model should be adopted.

Progressive erosion of channel-bank sandbars in Grand Canyon has long been thought to be associated with emplacement and operation of Glen Canyon Dam, although the specific physical mechanisms causing local erosion are poorly understood. A shortterm study (order of days) of detailed flow patterns and morphologic adjustments at Stone Creek and Fern Glen sandbars during constant discharge demonstrates that surfacegravity waves and other quasi-periodic flow oscillations are important to the stability characteristics of these alluvial deposits. The primary role of waves is to agitate bottom sediments, entraining them on an intermittent basis. Mean currents associated with recirculating eddies (ordinarily of insufficient strength to entrain sediments) act, in the presence of waves, as a net background drift able to transport sediments away from the bar face and into the main channel. Thus combined wavecurrent interactions provide for sediment transport possibilities that might not occur in the absence of waves. Simple models of beach-foreshore equilibria developed for coastal environments show that the faces of Grand Canyon sandbars behave very much like coastal, wave-dominated features. But the wave-dependent equilibria predicted by coastal models are never attained fully because mean eddy-recirculation currents associated with the river play an important role in the fluid-sediment interactions observed on nearshore terraces. Unlike a coastal system where mean alongshore currents owe their existence to wave motion, eddy-recirculation currents in a fluvial system are completely independent of waves, vis-i-vis their hydrodynamic origin. Thus neither a purely fluvial approach nor a purely coastal approach will be completely successful in describing sandbar stability in Grand Canyon, and a hybrid model should be adopted.

Bedrock shore platforms are common features along the Indian Ocean coast of South Africa but their properties and controls have not been well reported. This study presents high-resolution evidence for rock surface properties and hardness values from a sandstone platform at Morgan’s Bay (Eastern Cape, South Africa) where the relative roles of environmental versus lithological factors on bedrock surface hardness can be evaluated. Results show that there is very little correspondence between rock hardness and either absolute platform elevation or distance from low water position of sea level. Two contrasting conceptual models are proposed to explain these results. One model proposes that a uniform geological control dominates, despite variations in environmental forcing. The alternative model considers that environmental controls such as waves and subaerial weathering do not significantly vary over the platform, despite any differences in platform relief and microenvironment. These different models can be used as testable hypotheses to evaluate the relative controls on, and thus interpretations of, measurements of rock surface hardness on shore platforms.