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River channel processes.

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Geography revision. RIVER CHANNEL PROCESSES How are things transported in a river? - SUSPENSION, this is where solid particles are suspended in the river - TRACTION. This is where the bedload is rolled along the bottom of river by the force of the water above it. - SOLUTION. This is when chemicals, e.g. co2 dissolve in the water and are transported while dissolved in the river. - SALTATION. This is when rocks bounce along each other, thus being transported done the river. The amount of sediment shifted depends on these factors, - NATURE OF BED AND BANKS. Is the river cutting loose gravel or solid rock. Is it travelling over chalk or limestone? - FLOW OF THE RIVER. The speed at which river flows through its channel. If its flows are constant or do they fluctuate. - HUMAN INTERVENTION. If a dam has been built or not. What erosional processes do rivers go through. - CORRASION (is abrasion). This is when the particles that are suspended within the river rub against the bed or bank, wearing them down. - SOLUTION. This is when the minerals dissolved in the water reaction with the bed or banks. E.g. co2 dissolved in the water can form a weak acid and therefore eat away at limestone etc. - HYDRAULIC ACTION. This is used when referring to the sheer force of the water hitting the bed and banks. This type of erosion is particularly abundant at waterfalls. What controls these processes? VELOCITY. (deposition of particles will only occur when the velocity of the water is not sufficient enough to maintain the particles in the suspended loads. As the particles get bigger so to would the velocity of river have to increase in order to keep these particles in suspension otherwise they will become deposited on the bed. <Hjulstrom curve> As we travel down a river we observe; VELOCITY increases CHANNEL SIZE increases CHANNEL SMOOTHNESS increase SIZE OF PARTICLES decreases PARTICLE ROUNDNESS increases PARTICLE SMOOTHNESS increases DISCHARGE increases FRICTION ...read more.


COASTAL EROSION Erosion is one of the foremost processes that shape a coastline. There are many interacting factors; WEATHERING PROCESSES As we know, salt weathering, frost shattering, wetting and drying, solution and biological weathering are all prominent here. We find that because of the large amounts of salt and water this happens more effectively at the coastline. MASS MOVEMENT Rock falls landslips and rockslides are all common place at the beach. These landforms cause erosion of the beach. MARINE EROSION PROCESSES This is the main way in which a coast is eroded. The waves contribute a lot. They are made by the frictional drag of wind on the surface of the water. The fetch, wind speed and wind direction all effect the type of waves tat is created. There are two main types of waves; - Constructive - the break of these waves is weaker. These waves have a strong swash and a weak backwash. This means that the profile of the beach becomes steeper - Destructive waves have a strong break. They come down on to the beach meaning that they have a weak swash but a strong backwash which takes the beach sediment back with it creating a shallower beach profile. This is an example of negative feedback because when the beach becomes steeper the waves are forced to become destructive as the wave orbit is changed by the shape of the beach. But when the beach becomes shallower then the orbit of the wave then causes the wave to become constructive. Tides also have a big part in the erosional processes of the beach. The tidal range is what affects the amount of erosion that occurs at the beach. Narrow ranges of tide height result in greater amounts of concentrated erosion and deposition. HUMAN ACTIVITY Constant walking can wear down rock etc. CASE STUDY: LULWORTH COVE Lulworth cove is a great example of coastal erosion. We find many features in the stretch of the Dorset coastline commonly associated with erosional features. ...read more.


Extremes of discharge The discharge of a river is hardly ever constant. Often antecedent condition means that the amount of water is high so to will the discharge is high. Often after prolonged times of rainfall a river may breach its banks and cause a flood. 1 - Flooding A river flood is the land inundated with water that is not usually submerged in it; usually the river cannot sustain the amount of water running through it. Either by rapid through flow or overland flows does flooding occur. What causes flooding? > antecedent condition = prolonged rains > intense rainfall event > snow melt > impermeable bedrock > steep valley sides > lack of vegetation > urbanisation > Silting up of river banks e.g. after bankside erosion. Flooding is usually made worse by the activities of man. Most of our activities are usually on a floodplain meaning that they must be constantly monitored. How can we deal with floods? > basin and channel management o Reservoirs and dams o Controlled flooding to prevent intense flooding downstream o By pass channel to divert water o Creating embankment so river can sustain more water. o Channel dredging so river become more efficient and thus can remove water quickly from a basin. o Channel straightening again to speed up the water flow. 2 - Low flows These flows are mainly concerned with the discharge of the water being low. River then tend to dry up and fish die etc. CASE STUDY: River Wylye, Wiltshire. This river experienced extreme low flows. The water in this area is managed by Wessex water. They examined that trout population in the water were falling and less water meant the river dried up. The cause was the boreholes drilled by Wessex water for abstraction. This caused more water to be taken out of the water table (river runs over chalk [aquifer]) so this caused the water table to fall. They were told to then monitor the aquatic environment, increase river augmentation, a constant rate of abstraction as opposed to more abstraction in the summer. ...read more.

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