AS: Mass Movement

Slope Processes and development

Mass movement

Is the movement of weathered material down a slope under gravity.

The slope is a system with inputs, outputs and flows.  It has exogenetic factors (external factors) and endogenetic factors (internal factors).

The slope as an open system

Humid climate slopes: Convex and concave slope segments (convex, concave, convex, concave formation).

Arid climates slopes:  Resistant jointed vertical rock slope, then a gentler slope with regolith in a concave shape with boulders, mostly rock fall occurs.

Slopes near fault lines: Fault block

AS: A Case study on mass movement MEDC vs. LEDC contrast

Example of a (accelerated by man) Landslide in a MEDC:  Abbotsford Landslide, Dunedin, NZ 1979

Cause:

•              1978 families noticed cracks appearing in their homes.

•              1979 workmen discovered that a leaking water main had been pulled apart. Geologists discovered that water had made layers of clay on the hill soft, and the sandstone above it was sliding on this slippery surface.

•              Construction of the nearby Dunedin southern motorway, an earthquake that occurred in the area in 1974, deforestation (reduced evapotranspiration), increased urbanization (involved cutting into slope and infilling) and quarrying activity on the toe of the slope in the decades before may have further affected the land’s stability.

The landslide:

•              On July 27^th the slide began to accelerate.

•              Early warning system was put in place by Civil defense and a civil emergency was declared on the 6^th of August 1979. This was not thought to have been necessary, as geologists believed the slope would only move slowly.

•              However on the 9^th of August a 7 ha section of East Abbotsford started moving down the slope at a rate of 3m per minute, taking houses with 17 people inside.

•              It was essentially a block of sandstone resting on a bed of weaker clay. Displacement of 50m took place in about 30 minutes, leaving a small rift 30m deep in the head of the slope. In addition the slope was on an angle of 7⁰. Water collected in the impermeable clay, reduced its strength and cohesion, and caused the sandstone to slip along the boundary of the two rocks.

•              The sandstone involved 5.4 million m³ of material. At first the land moved as slow as soil creep, followed by a rapid movement with speeds of 1.7 m per minute.

Impacts:

•              Nobody was killed but 69 homes were destroyed or damaged and 200 people were displaced. The total cost from the destruction of the homes, infrastructure and relief organization amounted to £7 million ($10-13 million NZ today). In total 18 ha was affected.

•              Insurance schemes and government relief to cope with such disasters meant that residents were compensated for any damage.

•              However other impacts such as depressed housing prices, trauma and the cost of a prolonged inquiry were not immediately appreciated.

•              Lessons on landslide preparedness, and the affect human activity has on slopes can be learnt from this.

Case Study on a Physical Landslide in a LEDC: Vargas State, Venezuela  - 1999

Causes:

•              First two weeks of December 1999 saw an unusually high amount of precipitation (40-50% above normal rainfalls).

•              Political corruption – allowing shanty-towns to be built on steep slopes surrounding Caracas. The slopes around the region were changed to accommodate vast squatter settlements.

Landslide:

•              15-16^th December the slopes of the 2000m Mt Avila began to pour forth rock and mud burying 300 km stretch of the central coast.

•              Rains triggered mudslides, landslides and flash floods in between the mountains and the Caribbean Sea.

•              Search and rescue were deployed to search for survivors but very few were found in the first few days.

Impacts:

•              Rains triggered mudslides, landslides and flash floods which claimed the lives of 10,000 -50,000 (unknown accurately as most people were buried under mud or swept to sea) in between the mountains and the Caribbean Sea.

•              150,000 were left homeless by landslides and floods in the states of Vargas and Miranda.

•              Slum dwellings were often buried by mudslides (8-10m deep) or swept out to sea. This is why fatalities are unknown as many went missing and entire families went unreported as missing.

•              Bridges, roads, factories, crops, telecommunications and the tourism industry (in the immediate future) were destroyed. The international airport in Caracas was closed.

•              Containers at the seaport of Maiqueita were damaged. Harzardous material leaked out of these containers. Operations at the port were halted and hampered efforts to bring in emergency supplies. The economic damage was estimated at $3billion.

•              70% of Venezuelan population was living in this small coastal area. The government then made a plan to move some of the population to inland areas.

•              As a result of these landslides a plan to rebuild 40,000 homes was created for Vargas. A $100 million extension was planned for the international airport. The country’s main seaport in Vargas, was also planned to be modernized. Tourist destinations in Macuto and Camuri Chico were also rebuilt. Towns such as Carmen de Uria were not rebuilt, and instead created into parks & bathing resorts.

•              These improvements reduced the number of fatalities to 14 in the next 2005 mudslides in the region.

IGCSE: Case Studies: Trans National Corporations – Wal-mart

•        Wal-mart was first opened in 1962 in Arkansas in the USA
•        Stores opened around across the USA soon after and also opened stores in China, Japan, Canada, Brazil, Argentina, Mexico, India and in the UK (where it’s called ASDA).
•        It owns over 8000 stores and employs over 2 million people

Positive effects:

•        Wal-Mart creates lots of jobs for example, in Argentina three new shops opened in 2008 creating 450 more jobs.
•        Wal-Mart invests money in sustainable development for example; in Puerto Rico 23 stores have solar panels fitted onto their roofs to generate electricity.
•        Wal-Mart donates hundreds of millions of dollars to improve health in countries it is based in. E.g. In 2008 in Argentina the company donated $77,000 to local projects and gave food to help feed 12000 people.
•        Wal-Mart offers more skilled jobs to local people in LEDC’s
•        Local Companies and farmers supply to Wal-Mart, which in Canada created about $11 billion of business per year.

Negative effects:

•        Wal-Mart can cause smaller shops in the area to go out of business
•        Not all Wal-Mart workers are payed the same wages e.g. in the USA workers earn about $6 but in China they earn $1 per hour
•        Some companies that supply Wal-Mart work long hours e.g. In Bangladesh the max a worker can work is 60 hours per week, but the company that supplies Wal-Mart with Clothing work workers around 80 hours per week.
•        These stores take up lot’s of land that would have been used for farming e.g. A Wal-Mart in Hawaii is roughly 21000m².

IGCSE Case Study: Maldives tourism

•        Population: 314,000
•        Area: 298 KM squared
•        Started to develop in 1972
•        11% of the Maldives workforce is in tourism
•        It accounts for 29% of the Maldives GDP
•        More than 700,000 tourists visit the Maldives every year
•        Tourism brings in $600 million per year
•        Most tourists are from Europe

     Attractions in the Maldives:

•        Diving
•         Snorkelling
•         Water-skiing
•         Swimming
•         Fishing
•         Windsurfing
•         Mulee Aage Palace
•         Looking around local towns
•         Fish Markets
•         Touring beaches

Positive Impacts:

•         Improvement in Infrastructure e.g. roads, internet, airports
•         Foreign currency brought to the region by tourists can be invested improving local education, health and other services.
•         Jobs for local people are created from tourism giving people the chance to learn new skills in tourism services.
•         National Parks have been created – encouraging people to protect the environment

Negative Impacts:

•         Effects of construction of coastal structures. For development of a resort, it’s vital to have easy access to the island. To make this possible the following has occurred: the movement of sand around the island is obstructed from the construction of coastal structures such as jetties which erodes the island, the destruction of the habitat of many marine creatures
•        Profits usually go to foreign tourism companies
•        Land prices rise
•        Severe effects to reefs and islands occur from construction of coastal structures, including seawalls, submerged breakwaters and groins. Most of these structures protect part of the island though severely affect other parts.
•        Coastal vegetation is removed during the construction of tourist facilities.
•        Sewage and liquid waste disposed on the island seep into lagoons.
•        Pesticides & fertilizers used in resort gardens contaminate reefs causing eutrophication.
•        Conflicts between fisheries & tourism in resource exploitation are another major issue

Fluvial Landforms produced from deposition *AS and some IG courses*

Fluvial Landforms  and resultant effects produced from deposition

Thalweg: (this is not a landform but has to do with erosion and deposition)This is the line of fastest flow in a stream and is usually exaggerated variation of the stream channel shape that crosses to the outside of each meander at the point of inflection. Because erosion is greatest where the stream flow is fastest, the thalweg is also the deepest channel in the stream. It is found in the top middle of a straight channel because this is where the water is the deepest and is where there is the least friction.

Riffle and pool sequence: River channels have irregularities in the bed, which cause the thalweg to shift from the middle. These are known as ‘pools’ and ‘riffles’. In a flowing stream, a riffle-pool sequence (also known as a pool-riffle sequence) develops as a stream's hydrological flow structure alternates from areas of relatively shallow to deeper water. This sequence is present only in streams carrying gravel or coarser sediments. Riffles are formed in shallow areas (the shallow points of inflection) by coarser materials such as gravel deposits on river with a turbulent flow with a lower velocity. Pools are deeper and calmer areas of laminar flows with higher velocities, whose bed load (in general) is made up of finer material such as silt. Streams with only sand or silt-laden beds do not develop the feature. The sequence within a streambed commonly occurs at intervals of from 5 to 7 stream widths. Meandering streams with relatively coarse bed load tend to develop a riffle-pool sequence with pools in the outsides of the bends and riffles in the crossovers between one meander to the next on the opposite side of the stream. The pools are areas of greater erosion where the available energy in the river builds up due to a reduction in friction. The material eroded tends to be deposited in the riffle area between pools as energy is dissipated across the riffle area. Pools and riffles are responsible for the initiation of a meander. The pools are areas of high velocity and the thalweg is fast in a pool. Its energy is reduced and diffused (spread out) as it crosses the riffles. This is because the water is shallower; the bed is covered with bed load, is rough and creates turbulent flow. Therefore in order to overcome, these obstacles the river uses up more energy become slower.

Point bars: On a meander, material deposited on the convex inside of the bend may take the form of a curving point bar. Material is deposited here where velocity is at its lowest round a bend.

 Ox Bow lakes: Continual erosion on the outside bends, results in the neck of the meander getting narrower until the river undercuts through the neck and shortens the coarse. The current will take the path of least resistance, giving it renewed energy. The faster current will now be flowing in the centre of the channel and deposition is more likely next to the banks. The original meander will now be blocked off to leave a crescent shaped ox bow lake with a meander core in the centre. The lake will slowly dry up except during heavy rain. This lake can also be become filled with alluvium over time (marshland).

Flood Plains:

It’s a flat wide expanse of alluvium covering the valley floor formed due to deposition when the river is in overbankful. As the river floods, the river slows down, loses energy and consequently deposits its large (capacity) load of small material (competence) usually silt (alluvium). Rivers have the most energy at their bank full stage. Should the river continue to rise, and then the water will cover any adjacent flat land. The land susceptible to flooding in this way is known as the floodplain. As the river spreads over its floodplain, there will be a sudden increase in both the wetted perimeter and the hydraulic radius. These results in an increase in friction, a corresponding decrease in velocity and the deposition of material (alluvium) previously held in suspension. The thin veneer of silt, deposited each flood, increases the richness of the soil, while each successive flood causes the floodplain to increase in height. The floodplain may also be made up of material deposited as point bars on the inside of meanders and can be widened by the lateral erosion of the meanders. Prominent slopes known as bluff lines often mark the edge of the flood plain. These bluff lines can change as the flood plains become wider and more sinuous as they migrate downstream – which in turn widens the valley. 

Levees: When a river overflows its banks, the increase in friction produced by the contact with the floodplain causes material to be deposited. The coarsest material is dropped first to form a small, natural embankment (levee) alongside the channel. During subsequent periods of low discharge, further deposition will occur within the main channel causing the bed of the river to rise and the risk of flooding to increase. To try to contain the river, the embankments are sometimes artificially strengthened and heightened. Some rivers flow above their floodplains so if levees increase the river can cause serious damage to properties.

River Terraces: They are the remnants of former floodplains which, following vertical erosion caused by rejuvenation, have been high and dry above the maximum level of present day flood plains. If a river cuts rapidly into its floodplain, a pair of terraces of equal height may be seen flanking the river and creating a valley-in-valley feature. However, more often than not, the river cuts down relatively slowly, enabling it to meander at the same time. The result is that the terrace to one side of the river may be removed as the meanders migrate downstream. If the uplift of land continues, the river may cut downwards to form incised meanders. There are two types of incised meanders. Entrenched meanders have a symmetrical cross-section and a result from either a very rapid incision by the river, or valley sides being resistant to erosion. Ingrown meanders occur when the uplift of the land, or incision by the river, is less rapid, allowing the river time to shift laterally and to produce an asymmetrical cross-valley shape. As with meanders in the lower course, incised meanders can also change their channels to leave an abandoned meander with a central meander core.

Deltas: Deltas are usually composed of fine sediment, which is deposited when a river loses energy and competence as it flows into an area of slow-moving water such as a lake or the sea. When the river meets the sea the meeting produces an electric charge, which causes clay particles to coagulate and to settle on the seabed, a process called flocculation (larger coagulated particles carried out into the shallow water offshore and deposited, and the river loses energy on meeting the sea water). The water flows into a delta via distributaries. They are usually highly populated, not very navigable and have a great risk of flooding. Crops are usually grown on these deltas and are usually staple crops e.g. Rice. Deltas are named after the fourth letter in the Greek alphabet (∆). Yet Deltas range in geomorphology into three main types:

·      Arcuate: (Wave dominant) Having rounded, convex outer margins. They also have smooth coastlines and have well developed beaches/ sand dunes. Lagoons form near coastal areas e.g. the Nile Delta.

·      Cuspate: (Tide dominant) Where material brought down by a river is spread out evenly on either side of the channel. It is tide dominant and is covered by the high tide and left dry at low tide e.g. the Bangladesh Delta.

·      Bird’s foot: (River Dominant) Where the river has many distributaries bounded by sediment and which extend out to sea like the claws of a bird’s foot. The river has a large load from a huge drainage basin, a low energy river into the Gulf of Mexico, and a small tidal range e.g. Mississippi Delta.

Alluvial Fans: In order to form, alluvial fans require a flat or a gently sloping plain near the foot of a hill or plateau, where a stream carrying sediment emerges abruptly from a mountain front and spreads out. As the stream reaches the flat plan, known as the piedmont, its velocity slows and it loses competence to carry sediment load. The coarse sediment is therefore deposited at the junction of the hill and the piedmont, and a fan-shaped deposit builds up. Arid environments are well suited to alluvial fan development because they are prone to flash flooding. Furthermore, they have hill slopes that erode easily and therefore provide alluvial material suitable for deposition. Alluvial channels are considered disconnected from the channel.