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Topic 3: Rivers & Coasts
Transcript of Topic 3: Rivers & Coasts
The rocks chip away at the channel, widening and deepening its bed and banks.
This process of erosion is particularly effective during periods of flood when the river's velocity and force are at their maximum. The river carries a load of eroded materials which contain rock fragments and other materials.
These materials constantly collide with one another as they move.
Overtime, they are broken down into smaller pieces and rough edges may become smoother. As rain falls onto the Earth, it interacts with the carbon dioxide present in the atmosphere, forming weak carbonic acid.
This rainfall increases the acidity of the water in the river, enabling it to dissolve certain minerals. The erosive agent in this process is the force of running water.
In a river which possesses high energy, the water is capable of loosening the rocks and dislodging them from the river bed and banks.
The river channel is eroded by removing rocks which protrude from the river bed and banks. Processes Transportation Traction Suspension Saltation Larger-sized materials, e.g. Boulders & Pebbles, are found on the river bed.
Roll & Slide along the river bed Certain particles such as silt, clay and fine sand are carried along the river without contact with the river bed.
They are held in suspension.
Causing the river's murky appearance. Minerals found in rocks, such as calcium carbonate are dissolved.
Is transported downstream by flowing along with the river. Solution Moves small materials downstream in a bouncing motion.
Such as, sand particles and pebbles are moved along the river's course as they get lifted up and dropped onto the river bed over and over again. Deposition Occurs when...
The river does not have enough energy to either erode or transport material.
Speed or volume of river is reduced.
It occurs along the lower, middle and upper courses of the river. As a river loses its energy, large particles such as rocks and boulders are deposited first, followed by smaller ones such as gravel, sand and silt.
Factors that are likely to cause deposition to occur:
Period of low precipitation..
Loss of speed when a river enters a larger water body, such as a lake or a sea.
Increased friction due to river flowing along the shallower side of a river channel.
Sudden increase in load such as the period after a landslide.
Obstruction created by protruding bed rocks or aquatic plants. Energy Velocity Discharge Channel Management Features Valley Gorges Deltas & Distributaries Artificial Levees Dykes Planting of Vegetation Coasts Features Headlands & Bay Cliff & Shore Platform Beaches Spits & Tombolos Factors Affecting the
Wave Energy Fetch Wind Energy Processes Erosion Transportation Deposition Protection Soft Engineering Beach Nourishment Hard Engineering Seawalls Groynes Breakwater Gabion The Hydrologic Cycle River Course Drainage Basin Oxbow Lakes Revetments Gradient:The steeper the gradient, the faster the flow of the river. Roughness of Channel:
A river with a smooth channel has a higher speed than a river with a rough channel. Wetted Perimeter:
A larger wetted perimeter means that more water comes into contact with the channel. Thus, a lot of friction is generated and the river slows down Drainage Basin:
The bigger the basin, the greater the amount of water. Presence of vegetation:
The more vegetation is present, the more water will infiltrate the ground, thereby reducing the flow of surface runoff. Permeability of rocks:
An area with permeable rocks will allow water to seep into the ground and this would result in lesser surface runoff. Climate:
In areas with distinct wet and dry seasons, the amount of water in channel fluctuates according to the amount of rainfall received during each season. Waterfall Unequal Resistance of Rocks: A river may flow over an area of rocks with different resistance.
The river will erode the less resistance layer more rapidly.
The differential rate of erosion will result in a change of gradient along the less resistant layers.
Overtime, water will plunge from great heights to hit the river bed with great amount of force.
The repeated pounding by the plunging waters will leave a depression. Rocks and boulders will swirl around in the depression, and thus, deepening it. Hence, creating a plunge pool. Faulting: Faulting involves the displacement of rocks.
This displacement may result in one block of land lifted higher than the other.
When a river flows down from the edge of the uplifted block to the lower block, a waterfall and plunge pool are formed. When a river flows through an area of resistant rocks, it carries out vertical erosion, leaving the sides relatively unaffected.
Overtime, a deep, narrow and steep-sided valley known as a gorge is formed. Formed By: Erosion, Deposition & Transportation.
Speed of river differs along its outer & inner banks.
Outer Bank: Higher speed due to less friction. The river has more energy and erosion can occur. Overtime, it gets undercut & a stoop-sided bank (River Cliff) is formed.
Inner Bank: This reduces the speed & cause the river the deposit its load. Overtime, water becomes shallower at the inner bank, causing the river to be asymmetrical. Gentle slope (Slip-Off slope) is formed. Floodplain & Levees When there's a flood, the floodwater from the river will spread out over a larger area.
When the water recedes, the river deposits materials its carrying on the land.
The coarser sediments are deposited closer to the rivers' edge due to it's weight.
Deposited materials or alluvium buils up on both sides of the river as more floods occur. Overtime, flat and wide plains called floodplains are formed.
The heavier and coarser materials nearer the river formed raised banks known as levees.
Both Floodplains & Levees are found on the lower course of a river. Pronounced meanders that develop into oxbow lakes.
With constant erosion along the outer bends of the meanders, the two meanders edge closer to each other, forming a pronounced loop.
Overtime, as deposition and erosion continues to take place along the inner and outer banks, the two meanders will eventually meet to form a cut-off meander.
As the river flows along its new-found path, deposition will cut off the river from the meander, forming an ox bow lake. When a river enters a lake or a sea, its speed decreases.
Hence, the sediments carried gets deposited at the river mouth.
Overtime, the sediments built at the river mouth will block the flow of the main river.
The main river then branches out into many subsidiary channels known as distributaries.
Overtime, as sediments accumulate at the river mouth, a delta is formed. The widening & deepening of the river channel to allow it to hold more water. Strengths Weakness Ability to hold water.
Surface runoff from surrounding areas into the river also increased.
The river's bed and banks were smoothened using cement & granite, allowing water to flow away quickly. With the river's high speed, more sediment is washed downstream.
The increased sediment in the sea results in less sunlight reaching the sea bed.
This affects the growth of marine life. The straightening of the river channel by removing meanders and reducing the length of the river. Increases the speed of the river, allowing water to flow quickly.
Depth of river channel is maintained as sediment is washed away.
Reduces occurence of floods. The higher speed of the river results in more sediment being washed downstream.
As the volume of the water flowing downstream is also increased, the channel may not be able to contain the increased flow of water and flooding may occur.
Since the river course is shortened, the area of wetland is also decreased. Bank Protection using Gabions Dykes/ Artificial levees, such as sand, stone or concrete walls are built along the river banks to prevent flooding.
Increases the river's capicity to hold water. Sediment is also accumulated behind the structures, causing the river bed to become smaller.
Hence, dykes have to be built higher or sediment has to be dug up and removed regularly. Gabions are free-draining walls constructed by filling large basktes with broken stones. The wire mesh is extremely strong under tension.
The stones provide high permeability, eliminating the need for a drainage system and preventing the build-up of pressure which will displace and crack concrete structures.
Construction costs are low & labour costs are low as unskilled labourers can easily learn to erect the gabion structures. The wire mesh will rust over time and pose as a health hazard to human & animal users of the river.
The rows of wire cages along the river channel destroy the aesthetic appeal assosiated with a natural environment. Rows of rocks placed along the banks of rivers. Stone revetments absorb energy by dissipating the energy of flowing water over their irregular surfaces.
As the water runs over the irregular surfaces, their energy is diverted from the banks of the river towards the cntre of the channel. The stone walls destroy the aesthetic appeal associated to the natural environment.
Unlike dykes or gabions, the rows of rocks are not secured to the river banks. In extreme conditions, the rocks may be dislodged and too repair and re-install them will incur costs. The roots of trees, shrubs & grasses can form a protective barrier against erosion. Roots of trees & plants hold the soil together firmly, reducing the amount of eroded sediments transported to the river.
Vegetation slows down the surface runoff.
Also stabilises other protection measures. Excessive shade from the vegetation prevents sunlight from reaching the river bed.
Hence, killing aquatic plants and affecting the aquatic food chains.
Takes time to stabilise and produce desired effects. Distance over which winds blow across open water. The longer the distance, the bigger the wave. Waves get their energy from winds blowing over the ocean.
Wind is air moving from a region of high pressure to a region of low pressure.
The greater the difference in the pressures, the greater the wind speed.
The speed affects the energy, which determines the size of the waves.
Higher wind speeds will generate higher wind energy, leading to the formation of larger waves. Some coasts are made up of rocks with different level of resistance to erosion.
The less resistant areas of rock curve inwards as they get eroded faster by the waves, forming bays.
The areas made up of more resistat rocks will protrude out from the coastline and are known as headlands. When waves approach the coast in a parallel manner, their energy is concentrated on the headlands because the waves bend as they approach the protruding headlands.
The bending of waves is due to some waves slowing down as they reach the shallower waters in front of the headlands while other waves that are not affected by the headlands continue ahead at the same speed.
Bending of waves= Waves refraction. Wave energy concentrated on the headlands and weakened along the bays, where the material are deposited. 4. As the cliff continues to be eroded, it retreats inland. Which creates a shore platform, which is a surface that slopes gently seaward from a cliff base to the low tide mark. 1. When the waves repeatedly pound against the rock face, they weaken the rocks through erosional processes like hydraulic action and corrasion.
2. Over time, the waves erode the coast to cut a notch along the line of weakness. The notch further erodes into a cave. 3. The overhanging part of the cave collapses. The part left then forms a cliff. Accumulation of sediments on the coast.
Are formed when waves, tides or currents bring and deposit materials onto the shore.
A beach which is made up of coarse materials is steeper than one made up of less coarse materials.
Beaches are constantly changing as their shapes and sizes affected by waves, tides and currents. Longshore currents transport sediments parallel to the coast.
When the longshore currents encounter a bay or a bend in the coast with shallow sheltered water, the materials they carry will be deposited in the direction of the longshore drift. Over time, the accumulation of materials will result in a long, narrow ridge of sand, with one end attached to the mainland, and the other end extending seawards. This coastal feature is known as a spit.
Sometimes. the spit extends seawards and joins an offshore island nearby to form a tombolo.
A tombolo is an extension of the coastline that is made up of mainly sand and other materials that join an offshore island to the mainland. Refers to physical structures to defend against the erosive power of waves. Also known as structural approach. Made of concrete and are built in front of a cliff or parallel to the coast.
Absorbs the wave energy and protects the coast against strong waves. Made of granite & built parallel off the coast.
Waves will break against breakwaters before reaching the coast. Reduce impact of erosion.
Low walls built at right angles with one attached to the coast and the other extending into the sea. wire cage containing small rocks that protect the coast against erosion.
Form a wall along the coast. The use of natural materials & natural processes to stabilise the coast and reduce erosion. Involves the adding of large amounts of sand to a beach that is eroded. Planting of Vegetation Planting mangroves is a natural way to protect the coast.
Mangroves have prop or kneed roots that anchor them firmly into the muddy soil. Hence, they protect the coast against erosion by strong waves and winds. A coastal dune is a ridge of sand piled up on the coast by the wind.
Coastal dunes can be stabilised by planting vegetation.
The roots of vegetation helps to trap & bind the sand together. Stabilising Dunes Coraql reefs are mineral structures produced by organisms known as corals which line in warm and shallow waters.
The corals secrete calcium carbonate which forms the hard outer skeletons of the reefs. Coral Reefs Waves as agents of deposition Deposition occurs when waves lay down sediments along the coast.
Small or low-energy waves are responsible for deposition.
The lack of energy could be due to a decrease in wind speed or an increase in the size of sediments carried.
These waves do not have the energy to carry the load. Hence, deposition occurs. Currents as agents of deposition When longshore currents meet a bend in the coast with shallow and sheltered water, the materials carried may be deposited. Waves & Currents as agents of transportation Longshore current influences the development of landforms along the coast.
Longshore current flows parallel to the coast, & is generated by winds, tides & waves approaching at an angle.
Due to the direction of the flow, the longshore current plays an important role in moving or transporting the materials along the coast.
Waves usually approach the coast at an angle, depending on the wind direction. Carries the sediments up onto the coast at that angle Carries the sediments back to the sea at right angles to the coast due to the influence of the gravity. This motion is repeated and causes a zig-zag movement of sediments along the coast.
This zig-zag movement is one way in which sediments are moved along the coast.
The overall movement of sediments parallel to the coast is known as the longshore drift. Waves as agents of erosion A wave is a rising and falling movement of the water surface.
It is usually produced by winds blowing across the sea or ocean.
Erosion takes place when waves wear away the rocks along the coast and carry away the eroded materials.
Whether waves can erode a coast depends on the amount of energy they possess. The amount off energy a wave possesses corresponds to its size. The following factors affect the size of waves:
the greater the wind speed, the greater the wind energy which forms larger waves
Fetch is the distance over which wind blows across oopen water. The longer the distance, the biggger the wave.
To understand how waves erode the coast , we have to understand the movement of waves on the coast
As a wave appproaches the coast, it experiences friction with an increasingly shallow seabed
This causes the wave to lose energy and slow down
However, the waves behind it keep pushing it forward, forcing the wave to rise
When the wave becomes too steep, it collapses and breaks onto the beach
The forward movement of a wave onto the beach is known as a swash
The backward moment of the wave into the sea is known as a backward.
Large or high-energy waves have a backwash that is stronger than the swash. the stronger backwash causes more materials to be removed from the beach instead of being deposited. Hence, high-energy waves are also called destructive waves.
Small or low-energy waves have a swash that is stronger than the backwash. The backwash is weak because the wave has spent most of its energy overcoming friction with the shallow seabed. As a result, when the backwashh returns to the sea some sediments are left on the coast. Hence, low-energy waves are also called constructive waves. Currents as agents of erosion Currents are flows of water that move either horizontally or vertically in a cetain direction.
As the currents approach the coast, they are able to cause the movement off sediments along the coast.
Together with waves, currents become agents of transportation and deposition.
Currents are produced by a combination of winds, the Coriolis effect, water density and temperature differences in the ocean.
The coriolis effect is the deflection of wind as they move across the Earth. due to the Earth's rotation.