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CAPE Geography Cape '06 u1 p2- #5


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Attempt to answer a pastpaper

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CAPE Geography Cape '06 u1 p2- #5

  1. 1. CAPE 2006 UNIT ONE #5 13-Feb-11b) explain how the differences in the composition of the beach material influence theslope of the beach [5marks]c) discuss the role of subaerial and marine processess in the formation of cliffs[20marks]b) Reference Waugh (page 145-146)Particle size complicates the influence of wave steepness on the morphology of the beach. The fact that shingle beaches have a steeper gradient than sandy beaches is due mainly todifferences in percolation rates resulting from differences in particle size. This means that waterwill pass through coarse grained shingle more rapidly than through fine grained sand. In shinglebeaches, shingle may make up the whole or just the upper part of the beach and like sand; it willhave been sorted out by wave action. The larger the size of the shingle, the steeper the gradientof the beach.Shingle beaches are typically steep, because the waves easily flow through thecoarse, porous surface of the beach, decreasing the effect of backwash erosion and increasing theformation of sediment into a steeply sloping beach. Regardless of whether the waves on shinglebeaches are constructive or destructive, most of the swash percolates rapidly downwards leavinglimited surface backwash. On the other hand, sand beaches produce beaches with a gentlegradient. This is because the small particle size allows the sand to become compact when wet.This severely impacts the rate of percolation. The percolation is also hindered by the storage ofwater in pore spaces which enables most of the swash from constructive waves and destructivewaves to return as backwash.c) Reference Waugh (page 149) , Cook (page 464) A cliff is a vertical or near vertical rock face which typically comprises exposed situ rockand capped with present day/ recent soils. The formation of cliffs is the result of sub aerial andmarine processes which include, wave pounding, hydraulic action and weathering.Coastalprocesses act on timescales that range from the few seconds of a wave breaking to the manymillennia of sea-level change. Cliffs are formed by a number of processes, in particular the explosive energy releasedfrom waves crashing into their base, the result which leads to undercutting and eventual collapseof the overlying rock. Such a process may be wave pounding. Wave pounding is an erosionalprocess by which steep waves which have considerable energy break the foot of cliffs andgenerate shock waves. These shock waves may be as strong as 30 tonnes per m2. 1
  2. 2. In addition to this, hydraulic pressure is another marine process which leads to theformation of cliffs. Hydraulic action occurs when waves strike the shore. In storm conditions,the force of the waves can have a heavy impact- equivalent to 100 kg per m2. Where there arejoints or weaknesses in the cliff face, air might become trapped and compressed by the incomingwaves. The resulting pressure expands and weakens joints. This can cause large blocks as well assmaller fragments to be loosened and then removed by the waves. This is seen in the diagrambelow where the waves break off pieces of the cliff causing it to recede causing an eventualcollapse and recession of the cliff. STEEP WAVES BREAKING OFF CLIFF CLIFF SEAFurthermore, sub aerial processes aid in the formation of cliffs. According to J. Pethick, “cliffrecession is primarily the result of mass failure.” Mass failure may be caused by such non marineprocesses such as rain falling directly onto the cliff face, surface runoff of water from the land;and the effects of weathering by the wind and frost. Precipitation (rain, snow and hail) penetratesthe natural cracks and fissures throughout the cliff, washing away and dissolving less resistantrock and weakening the stability of the cliff. The rate of erosion is accelerated by freeze thawaction during the winter months, during which time the expanding ice cracks the rock further.This is most notable on the exposed outer surface. Erosion also occurs during the summermonths, at which the time the cliff is warmed and the evaporating water causes the sediment tocontract and crack (porous rock only).From the aforementioned points, cliffs are constantly changing and on the retreat; inevitably rockfalls and large scale collapse occur unpredictably throughout the year. 2