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.

Erosion within a river

Erosion Processes

Vertical erosion: This form of erosion deepens channels, aided by weathering mass movement and soil creep. Characteristics of a channel undergoing vertical erosion include large bed load comprising coarse hard particles. Potholes and deep narrow gorges are common.

Lateral erosion: This process increases a river’s width. A large sediment load has to be entrained for this process to work most effectively. It is responsible in conjunction with the processes of slope transport and mass movement for valley widening, meander migration and river cliff formation.

Headward erosion: This increases the length of a river. This process is most active in the source area of a river or where a bed is locally steep. It causes accelerated erosion and is commonly associated with waterfall formation.

Abrasion: Smaller material, carried in suspension, rubs against the riverbanks and wears it away.

Attrition: When bed load is moved downstream, boulders collide with other material and the impact break the rock into smaller pieces. In time, angular rocks become increasingly rounded in appearance.

Corrosion/ Solution: This occurs continuously and is independent of river discharge or velocity. When acids in the river dissolve rocks, which form the river’s bed/ bank. It is related to the chemical composition of the water e.g. the concentration of carbonic acid and humic acid.

Hydraulic action: The sheer force of the turbulent current hits riverbanks, pushes water unto cracks. The air in the cracks is compressed, pressure is increased and over time the back will collapse. Cavitation is a rare form of hydraulic action and the sudden and violent implosion of gas bubbles caused by this process shatters banks extremely rapidly. The resultant shockwaves hit and slowly weaken the banks. This is the slowest and least effective process.

Corrasion: Corrasion occurs when the river picks up material and rubs it along its bed and banks, wearing them away by abrasion. This process is most effective during times of flood and is the major method by which the river erodes both vertically and horizontally. If there are hollows in the riverbed, pebbles are likely to become trapped. Turbulent eddies in the current can swirl pebbles around to form potholes. This form of erosion occurs most often during times of higher river flow, bed load being used as an abrasive agent, scratching and scraping of the solid bedrock. 

Peltier Diagram *AS*

Peltier Diagrams

Physical Weathering

•      Frost shattering is important in a climate where temperatures fluctuate at around 0^oC but if a climate is too cold, or too warm, or too dry, or too wet (covered by vegetation) it will not operate.

Chemical Weathering

•       
This increases as temperature and rainfall totals increase. The rate of chemical weathering (around about) doubles (Increase by about 2 1/2 times) with every 10⁰C temperature increase.

•       Recent theories suggest that in humid tropical areas, direct removal by solution may be a major factor in the lowering of landscape, due to the continuous flow of water through the soil.

•       Chemical weathering is strong in warm moist climates e.g. rainforests.

Weathering Regions

·      Peltier constructed this diagram as an attempt to predict weathering at a place in the world by the mean annual rainfall and mean annual temperature. Physical and chemical weathering operates together at the same time and at the same place, but usually one process is more significant than the other. 

chemical weathering *AS edition*

Chemical Weathering - AND yes you should attempt to know the formulas.

Chemical Weathering: The decomposition of rock caused by a chemical change in the rock. It produces changed substances and soluble, and usually forms clay. It is more likely to occur in areas in warm moist climates where there is associated vegetation on rocks. It tends to attack certain minerals selectively and occur in zones of alternate wetting and drying (where the level of the water table fluctuates). It tends to occur mostly on the base of the slope where there tends to be wetter and warmer. These processes are more likely to occur in conjunction with another.

Hydrolysis: Hydrogen in water reacts with minerals in the rock; there is a combination of H+ and OH- ions in the water and ions of mineral (combines rather than dissolves the mineral).  It affects mostly granite (igneous rock – crystallised magma underground), which is composed of Feldspars (aluminium and potassium silicates). Feldspars (pink-grey rock forming mineral) + water à kaolinites (soft clay that is the residual weathering products) + potassium + silica oxide (Potassium and silica oxide are soluble and are washed away). The kaolinites represents the decomposition of feldspar, and the chemical weathering of granite by hydrolosis produces a chemical change in the rock. It occurs mostly in the tropics. The rate of hydrolysis depends on the amount of H+ ions, which in turn depends on the composition of the air and water in the soil, the activity of organisms, the presences of organic acids and the cat ion exchange.

Carbonation - solution: Rainwater contains carbon dioxide in solution, which produces carbonic acid (H₂CO₃). The weak acid reacts with rocks that are composed of calcium carbonate, such as limestone/ chalk and rocks that have calcareous rock. The limestone dissolves and is removed in solution by running water. Carboniferous limestone is well jointed and bedded, which results in the development of a distinctive group of landforms. Carbonation = CaCo₃ + H₂Co₃ (rainwater) à Ca (HCo₃)₂. The calcium bicarbonate is the weathered product, and is soluble (thus washed away).

Oxidation: This occurs when rocks are exposed to oxygen in the air or water. An example of this is when iron rusts. The rock or soil, which may have been blue or grey, is discoloured into a reddish-brown colour – in a process called rusting. Oxidation causes rocks to crumble more easily and occurs in iron rich rocks. In water logged areas oxidation operates in the reverse and the amount of oxygen in the soil is reduced in a process called reduction. Ferrous oxide + water à Ferric oxide. FeO + H₂O à Fe₂O₃. Sandstone is most affected by oxidation.

Hydration: Certain rocks, especially those containing salt minerals, are capable of absorbing water into their structure, causing them to swell (about 0.5%) and to become vulnerable to future breakdown. This process is most active following successive periods of wet and dry weather and is important in forming clay particles. Anhydrite + water à Gypsum. CaSo₄ + H₂O à (CaSo₄ 2H₂O) powder form. Hydration is in fact a physio-chemical process as the rocks may exert pressure as well as changing their chemical structure.

Solution: Some minerals are soluble in water and simply dissolve in situ. The rate of solution can be affected by acidity since many minerals can become more suitable as the pH of the solvent increases.

Organic Weathering/ Chelation: It requires a bio agent e.g. plants (chelates/ organic acid) and animal excretion. The decomposition of minerals in the rock leads to the crumbling of rock. Humic acid, derived from the decomposition of vegetation (humus), contains important elements such as calcium, magnesium and iron. The action of bacteria and the respiration of plant roots tend to increase carbon dioxide levels which helps accelerate solution processes, especially carbonation. Lichen can also extract iron from certain rocks through the process of reduction. High lichen and algae help in the development of the lithosphere. 

Physical Weathering *AS edition*

Physical Weathering

Freeze thaw shattering: Occurs in rocks that contain crevices and joints (e.g. joints formed in granite as it cooled, bedding planes found in sedimentary rocks, and pore spaces in porous rocks), where there is limited vegetation cover and where temperature fluctuates around 0 ⁰C. In the daytime, when it is warmer water enters the joints, but during cold nights it freezes. The process of shattering of rock is due to frost cycles i.e. fluctuating above and below 0⁰C. The process occurs with climates with rapid frost cycles, rocks with joints and rainfall.  Frost leads to mechanical breakdown in two ways:

1.     As ice occupies 9% more volume than water, it exerts pressure within the joints.

2.     When water freezes within the rock it attracts small particles of water, creating increasingly large ice crystals.

In either case the process slowly widens the joints and, in time, causes process of rock to shatter (or disintegrate) from the main rock. Where the block disintegration occurs on steep slopes large angular rocks collect at the foot of the slope as scree; if the slopes are gentle large blockfields tend to develop.

Salt crystallisation: If water entering the pore spaces or joints in rocks is slightly saline then, as it evaporates, salt crystals are likely to form. As the crystals become larger, they exert stresses upon the rock, causing it to disintegrate. This process occurs in deserts and coastal areas (areas contains sodium sulphates, magnesium sulphates and calcium chloride) where capillary action draws water to the surface and where rock is sandstone. Individual grains of sand are broken off by granular disintegration. This process also occurs on the coast with a constant supply of salt. During the day water enters the rock and is heated, water evaporates leaving salt crystals. These are large in volume and put pressure on rocks by expansion and eventually will disintegrate.

Spheroidal Weathering: In jointed rock, the weathering and heating/cooling takes place along all joints so this temperature change produces rounded boulders.

Exfoliation: Occurs in hot arid and desert climates where diurnal ranges can range up to 50⁰C (below zero to 40). It also occurs in places of high altitudes in low latitudes. These rocks are usually heated via conduction. Because the outer layers of the rock warm up faster (and expand) and cool more rapidly (and contracts) than the inner ones, stresses were set up that would cause the outer thickness to peel off (or flake off) like layers – the process of exfoliation. Changes in temperature will also cause different minerals within a rock to expand and contract at different rates. It is also theorised that water is needed for the process to be stimulated or accelerated.

Pressure Release: Many rocks have developed under considerable pressure. The confining pressure increases the strength of the rocks. If these rocks are exposed to the atmosphere, then there will be a substantial release of pressure.  The release of pressure weakens the rock allowing other agents to enter it and other processes to develop. When cracks develop parallel to the surface, a process called sheeting causes the outer layers of the rock to peel away. This process is responsible for the formation of large round rocks called exfoliation domes.

Wetting and drying: Affects less resistant rocks such as clays. The clay is porous and has the ability to absorb. When these rocks are wet they expand and when dry is contracts. Over time they disintegrate the rocks. 

Biological weathering: When tree roots penetrate and widen weaknesses in the rock until blocks of rocks become separated. 

Weathering 101 - basic introduction

Weathering: The disintegration and decomposition of rock in situ (in their place of origin). There are two types of weathering: Mechanical (Physical) or Chemical

Physical Weathering: The disintegration of rocks into smaller pieces caused by physical processes without any change to the chemical compound of the rock. It occurs on bare rock that lacks vegetation. Physical weathering usually produces sand.

Chemical Weathering: The decomposition of rock caused by a chemical change in the rock. It produces changed substances and soluble, and usually forms clay. It is more likely to occur in areas in warm moist climates where there is associated vegetation on rocks.

River Processes: Transportation

This is for all levels:

Rivers can either deposit, erode or deposit material.

Transportation

Load is either transported through suspension, solution or bed load (traction & saltation). For sediment to move resisting forces have to overcome, competent velocity has to be achieved (this is the lowest velocity at which particles of a particular size are set in motion), and critical tractive force must be achieved (This is when drag and embedded particle inertia is overcome and the particle begins to move).

Traction: Traction occurs when the largest cobbles (100-1000mm) and boulders (bed load) roll or slide along the bed. The largest of these may only be moved during times of extreme flood (high discharge).

Saltation: Bed load is either moved through saltation or traction. Saltation occurs when pebbles (1-100mm), sand (0.1-1mm) and gravel are temporarily lifted by the current and bounced along the bed in a hopping motion.

Solution: If the bedrock of the river is readily soluble, it is constantly dissolved in flowing water and removed in solution. Except in limestone areas, the material in solution forms only a relatively small proportion of total load.

Suspension: Very fine particles of clay and silt (0.001-0.1mm) are dislodged and carried by turbulence in a fast-flowing river. The greater the turbulence and velocity, the larger the quantity and size of particles which can be picked up. The material held in suspension usually forms the greatest part of the total load; it increases in amount towards the river’s mouth, giving the water its brown or black colour. 

How did mid years go?

Hope you are all pleased with your results of all your exams from the may/june exams. Hope you did extra well in geography. I hope to get more AS Geography notes and posts up by the end of year exams. Will also try to do more physical geography posts. Hope this helps ^_^.

IGCSE Physical Geography - Google Docs

The formatting may be strange from converting on google docs

https://docs.google.com/open?id=0B0RgfrfwviNwbE5QQmdDTU9mMVE

IGCSE Case studies

Again the formatting may be a bit strange form converting it on google docs

https://docs.google.com/open?id=0B0RgfrfwviNwNV8tcmQ3MHE2MnM

Economic development - Google Docs

Here is a booklet though some of the formatting is strange because I had to convert it into this format

https://docs.google.com/open?id=0B0RgfrfwviNwLWpOMkZkTjFfUE0

Competition for land *AS*

(Sorry I can't put pics up of the bid rent theory diagram, but it's simple to find)

·      There are numerous theory’s related to locational rent. Though the main assumption is the highest bidder will obtain the land. It is also assumed the highest bidder is the one that can obtain the most profit from the site.

·      Land competition is highest at the CBD, mainly because it’s accessibility and the shortage of space.

·      Businesses such as shops conduct business using a relatively small amount of ground space and due to the high rates of sales and turnover they can bid a high price for land (they use the land intensively). Usually offices are built in high rises above ground level shops.

·      The Peak Land Value Intersection (PLVI): The most valuable site in the CBD.

·      Retailers compete with offices – which rely upon good transport systems and proximity to other commercial buildings

Urban trends and the issue of urbanisation

Urban trends and issues of urbanisation

Urbanisation: The process by which an increasing proportion (%) of the total population of a country lives in towns and cities.

Counter-urbanisation: The process of moving from an urban environment to a rural environment.

Gentrification: Where old substandard housing is bought, modernised and occupied by wealthy families.

Counter-Urbanisation in Sydney Australia

·      Many leave due to congestion, crumbling infrastructure, expensive real estate prices and soaring rent prices.

·      25% of Sydney residents are looking to leave

·      In New South Wales government created a $3.75 million campaign to lure Sydney residents to ‘Evocities’ such as Bathurst and Orange. The campaign included billboards along congested city roads.

·      The current population of Sydney is 4.5 million and is expected to reach 5.7 million by 2030.

·      A June 2020 survey said 50% of people needed an annual income $100,000 to live comfortably in Sydney. Only 15% of those surveyed had such an income.

·      In 2010 Sydney was one of the least affordable cities to live in the world.

·      Sydney’s annual house price in June 2010 was $700,000 compared to Evocity prices of $300,000.

·      60% of those that pay rent in Sydney pay over $350 per month – in Evocities only 20% pay this.

·      Evocities are said to have good education, healthcare, transport and opportunities for new businesses.

·      On average a Sydney resident only has 10% of their income left to spend on leisure – In Evocities it’s 25%.

·       A Large Evocity (Wagga Wagga) has around 58,000 people