Processes on Cliffs Pages Bird, Eric. Cliff Hazards Pages Bird, Eric. Stabilisation of Cliffs Pages Bird, Eric. Summary Pages Bird, Eric. Show next xx. Recommended for you. An earthquake-induced subsidence occurred in the year , and has been followed by a gradual aseismic uplift. Along the southern half of the Oregon coast and near the Oregon-Washington border in the north, this tectonic uplift has exceeded the global eustatic rise in sea level during the past century, resulting in a drop in relative sea level that has largely eliminated sea-cliff erosion, although evidence remains for the large-scale erosion that followed the abrupt subsidence during the subduction earthquake years ago.
Along the north-central Oregon coast, the present rise in eustatic sea level exceeds the tectonic uplift, and the relative sea-level rise continues to be a factor in sea-cliff erosion, although the erosion rates are highly variable due to site-specific factors. The seasonal occurrence of variations in monthly mean sea levels is due mainly to changes in water temperatures and densities, with additional contributions due to the geostrophic effects of offshore currents.
The most important phenomenon that may accelerate sea-cliff erosion is an abnormal rise in water level caused by strong onshore winds and an abrupt reduction in atmospheric pressure. A remarkable water-level rise may occur on tidal coasts when storm surges are superimposed on high tides. The storm surge on the North Sea caused dramatic erosion of glacial-sediment cliffs at Covehithe, Suffolk, England, 83 as described before. A good correspondence between erosion of bluffs composed of glacial till and surge-induced extreme water level rises superimposed on seasonal variations has been reported from the Ohio shore of Lake Erie.
Beaches front most eroding cliffs. Where the cliff material is so soft that only the hydraulic action of waves is sufficient to cause erosion, critical for its initiation is whether waves running up the beach are able to reach the cliff-beach junction. This is controlled by many factors: input wave characteristics wave height and period , the beach morphology beach width, beach level and slope and the water-level factors discussed above. The relationship between the width of the fronting beach and the rate of erosion of sea cliffs has been examined by Everts, 84 selecting five sites along the km coast of the Oceanside littoral cell, north of San Diego, California, where the beach is backed by cliffs composed of Neogene sandstones.
At each site the erosion rate decreased as the beach became wider; a marked decrease occurred when the beach width exceeded 20 m, and erosion ceased altogether when it attained a width of 60 m.
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- Rocky coast processes: with special reference to the recession of soft rock cliffs.
Dornbusch et al. The reduction or removal of the fronting beach occasionally leads to a dramatic increase in sea-cliff erosion. This was caused by the attack of storm waves with extreme high runup levels, which reached the cliff base with less energy dissipation due to localized lowering and narrowing of the beach in front of the cliff, these corresponding to one of rip-current embayments of a cuspate shoreline formed by storm waves.
The role of the morphology and behavior of a fronting beach is further illustrated by erosion of sea cliffs cut into Pleistocene sandstones along the central Oregon coast.
Coastal Cliffs: Morphology and Management
The coarser-grained reflective beaches are steeper sloped and respond more quickly to winter storms with larger changes in beach-profile levels than do the fine-grained, gently-sloping dissipative beaches. As a result, the reflective beach is a weaker buffer against wave attack, and the sea cliff is more susceptible to erosion compared with areas where the cliff is fronted by a dissipative beach.
A further decrease in buffer protection on the reflective beach is brought about by the more pronounced development of embayments eroded by rip currents, allowing for easy landward penetration of storm waves to the toe of the sea cliff. The height of a fronting beach can be uniquely determined by the volume of beach material if a uniform beach slope is assumed. Along the North Norfolk and Suffolk coasts, England, Lee 88 has examined on the year-by-year basis the recession rate of Pleistocene soft rock cliffs in connection with the volume above High Water Level of the fronting beach during a period of 11 years, and provided for each coast the relationship between recession rate and beach volume, plotted with highly scattered data.
An envelope of the data cluster shows that, as the sediment volume increases, the maximum recession rate for a given sediment volume tends to increase abruptly and decrease after the volume excesses some optimal value. A more marked trend is seen on the Suffolk coast. Let us examine here the effect of a fronting beach on cliff toe erosion considering the wave and cliff strength factors. Bluffs composed of glacial deposits on the US shore of Lake Erie have suffered severe recession. The study has discussed erosion magnitude in connection with storm waves accompanying surges, water level storm surge plus lake level and beach width.
Data acquired at Helen Drive, one of the five sites, will be used here for a quantitative examination of the effect of beach width on the cliff toe erosion by waves. The Helen Drive bluff 7 m high is composed of weak till having a mean compressive strength of 0. In front of the bluff a narrow, sandy beach developed with a width varied from 0 to 9 m with time. Seven of them are selected here, which occurred under almost similar water-level conditions including surge height , 1. The magnitude of storm surges at the seven events was in a range from 0.
Carter and Guy 49 have observed that 1 wave breaking occurred offshore at a storm and broken waves acted on the cliff, 2 mean values for storm wave height and period in the surf zone were 1 m and 6 s, respectively, 3 the largest erosion events occurred when the storm durations exceeded 10 h, and 4 sand-laden waves abraded the cliff toe.
2B.2C Geological Structure and Cliff Profiles - A-LEVEL GEOGRAPHY REVISION: EDEXCEL
No detailed data are provided on the height of the cliff-platform junction. From the surge heights described above, 0.
A further assumption is made that the storm waves always had a breaker height of 1 m, a period of 6 s, and a duration of 10 h. The erosion rate is the mean value obtained by dividing the erosion distance during a single storm event by 10 h, the distance read from the diagram of Carter and Guy , Fig.
Rocky coast processes: with special reference to the recession of soft rock cliffs
There is clearly an optimal value for beach width giving rise to the maximum erosion rate. In order to express mathematically the erosion-rate vs. The relationship between the cliff toe erosion rate and the width of a fronting beach at Helen Drive on the Ohio shore of Lake Erie. Based on data of Carter and Guy. The application of Eq. For this purpose, the following relation 93 will be used:. Broken waves with a height of 0. The rate of erosion caused by the waves is assumed to be 0. The line and curve in Fig.
Equation [ 11 ], thus determined Fig. The geometry and dimensions of the fronting platform may affect the wave intensity at the cliff base. Philpott 95 and Kamphuis 60 have considered that downcutting of the platform surface immediately in front of the cliff must have occurred prior to the cliff recession: increasing water depth at the cliff base will reduce energy dissipation of waves arriving at the cliff base to further cliff erosion.
Based on extensive studies on cliffs and shores composed of cohesive materials, Hutchinson 96 has also suggested that the lowering of shore platforms controls the cliff erosion. The platform downwearing has been widely observed along the till shores on the Canadian side of the lower Great Lakes. The height of a cliff-platform junction tends to increase as the strength of coastal rocks increases, as conjectured from the result of laboratory experiments on notch initiation under the action of broken waves.
On the chalk coast of southeast England, shore platforms are of concave upward profiles with some having steeply inclined ramps at the cliff base, and the cliff-platform junction is located at about the high tide level, generally several meters above the mean platform level. The lowering of chalk platforms involves various erosive processes: abrasion by waves armed with beach deposits residing on the platform, plucking of jointed blocks by hydraulic force and their removal, direct erosion of platform substrates due to grazing by marine organisms and deterioration of platform material due to frost and salt weathering especially in harsh winters.
Measurements of chalk platform downwearing have been carried out by use of Micro Erosion Meters — and laser scanners. Foote et al. Dornbusch and Robinson have attempted to investigate long-term chalk platform lowering on the basis of measurements of the block removal and step backwearing on the East Sussex coast, England, using air photographs taken in and recent field studies done in They converted the amount of step erosion into equivalent mean annual platform lowering rates during the period of 28 years.
The offshore bottom topography and its effect on wave refraction or wave-energy attenuation may control the assailing force of waves acting on a sea cliff. Robinson has examined the long-term variation in erosion rates at Dunwich soft cliffs in Suffolk, England. The average rates were 1. This drop can be attributed to the reduction in wave energy reaching the coast. Before the early 20th century, wave refraction over the offshore Sizewell Bank focused the wave energy on the shoreward Dunwich site, but the northward growth of the Bank during the last century produced a divergence in wave refraction and a reduction in the assailing force of the waves.
Lowering of shore platforms located offshore of rapidly receding till cliffs has been extensively studied along the Canadian shore of the lower Great Lakes, 60 , 97 — 99 , where till bottoms are covered with a sparse veneer of sand. These studies have been conducted with a view that nearshore lowering will reduce the dissipation of wave energy reaching the cliff toe, which in turn will expedite cliff erosion.
The presence of talus is a major controlling factor for toe erosion, because talus can protect the foot of the cliff from wave attack until it is removed by waves and currents. Knowledge on the residence time of the talus is necessary for predictive studies of cliff recession. In the Lincoln City littoral cell on the mid-Oregon coast, USA, the life time of talus at the base of a cliff fronted by a dissipative beach is much longer than where a steeper reflective beach is found.
On the chalk coasts in southeast England most cliff failures yield small amounts of debris, less than 1, m 3 in volume, which may be removed in a few weeks or months. Lageat et al. Cliff undercutting by waves results in slope instabilities, eventually leading to intermittent mass movement. Such movement can be occasionally destructive to coastal properties.
For example, the initial movement of the Miocene mudstone cliff in the Jump-Off Joe area on the Oregon coast resulted in the loss of a dozen homes during the s. Such a serious problem threatening coastal communities has been reported from many locations in California. A variety of terms are used for mass movement phenomena. These types depend mainly on lithological factors such as geological structures, stratigraphic features, and geotechnical properties. Hybrid types exist between two or more of these failure modes.
Falls denote movement of a rock mass that travels most of the distance through the air as a freely-falling body, but mass movement occurring along an almost vertical failure plane is also categorized into this mode, also referring to as a vertical failure type. An example of deep-notch development responsible for the vertical failure can be taken from Eocene sandstone cliffs on the southern California coast.
Topples differ from falls in that little free-fall movement takes place because rotation of a block around a fixed hinge dominates the motion.
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Topples are most common on precipitous, sometimes overhanging, cliffs composed of vertically-jointed hard rocks like those found on the Liassic coasts of South Wales. Flows move with increasing velocity towards the surface of the moving body; no block movement is present due to differential shearing within the body. Flows occurring on some coasts cut into soft clayey materials with fluidized potential are called mudflows, and are frequently initiated by mudslides.
The sliding mass may disintegrate during its movement — the flow component increases, resulting in the generation of mudflows in the latter phase of the event. However, the distinction between mudslides and mudflows is not easy in the field.
These two failure modes contribute significantly to the recession of cliffs in soft materials such as clay, mudstone and till. Slides are shearing displacements occurring on a distinct slip surface, and the sliding mass exhibits block movement.
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The former has an almost linear sliding surface, whereas the latter is along a circular plane. Planar sliding with a high-angle failure plane may be found on till cliffs, chalk cliffs , and Pliocene mudstone cliffs. Shallow-seated slides are observed on till cliffs of the Holderness coast in England, 12 , near Kilkeel in Northern Ireland, along the Lake Michigan and along the Canadian shore of the lower Great Lakes.
http://arlivre.net/templates/calls/5193-phone-spyware-no.html Aside from the wave factor, the most common controlling factors for the occurrence of mass movement are rainfall and groundwater.