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Case Studies - Slope processes - Coggle Diagram
Case Studies - Slope processes
St Oswald's Bay
CAUSES
:
Rock type
(chalk is a very porous rock, capable of holding lots of water, within its numerous voids or air spaces. During long periods of wet weather, it can become saturated which causes an increase in pore pressure which reduces the rock's internal shear strength.)
Rock structure
(The presence of water also increased the bulk density of the rock causing overloading and an increase in sheer stress. This together with overloading were the major causes of the cliff collapse.)
29/30th April 2013 a major cliff fall occurred in St Oswald's Bay, to the East of Durdle Door. Coastal wardens discovered that a 90m stretch of chalk cliff has collapsed, removing with it part of the coastal path.
Weathering processes: The folded nature of the rocks and the steepness of the cliff also contributed to the fall. Chalk is composed of at least 95% calcium carbonate, therefore prone to carbonation and solution. These processes widened small fissures and enlarged pore spaces. When solution occurs, the rock goes red so it's easy to see where this has occurred.
Folded rock: Intense folding had overturned the chalk so it dipped at an angle of 115 towards the north. This meant it was dipped seaward which would have encouraged a sliding motion.
The steepness of the cliff would have created internal stresses within the rock, increasing the risk of failure.
With clay being a soft rock and having a permeable nature in that rainwater passes through it, rather than running across it increasing the risk of erosion.
IMPACTS
: Fortunately the cliff fall occurred at night, but it had happened during the day the results could have been fatal. Some people such as local boat owners got good business as tourists wanted to see the cliff fall which he could offer from his boat trips.
Aberfan
9:15am, October 21, 1966.
500,000 tons of coal waste slid down, hitting the town of Aberfan.
CAUSES
:
Key terms
Mass movement = The downhill transfer of material, acting under the influence of gravity. 3 types of rapid moement are falls, slides, and flows.
Falls = Occur on steep slopes where well-joined and/or bedded rock fractures and material moves rapidly downhill. the rock often breaks up at the base of the slope into smaller fragments, crating a talus/debris cone.
Slides = Occur when large, coherent blocks of material break away and move down slope on a slip plane.
Flows = Involve the rapid movement of unconsolidated material that's saturated with water. Common on cliffs made of mudstone, clay or glacial deposits.
Burton Bradstock
24 July 2012, 2 cliff falls occurred 20 mins apart.
The cliff has horizontal layers of silts and fine-grined sandstones which went soft, hard, soft, hard in the layers.
CAUSES
: Wet weather in June 2012, would have saturated the normally permeable sandstone and contributed to an increase in positive pore pressure. But a drier period immediatly before the failure may have caused some shkrinage, further weakening in the rock.
IMPACT
: Beach and coastal path was closed to the public. Locals expressed concern about the negative publicity, especially at the height of tourist season. Today, the coastal path has reopened. Money has been spent for new fencing and stiles.