Geography Unit 1 Rivers

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The drainage basin hydrological cycle

The Hydrological cycle is the cycle of water. 

drainage basin is an area of land drained by a river system.It includes water found in the water table and surface run off.

Water shed is an area in which if any precipitation falls then will flow into the river.

Within the catchment area precipitation will reach the stream via three main paths

  • Surface run-off - fastest and characterised by water flowing over the surface
  • Through-flow - a sub-surface flow within the soil layer.
  • Groundwater flow -If the topography of the drainage basin allows water to pool on the surface it is likely to infiltrate
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The water balance

The water budget is calculated through the formula, P =Q +E - ±S.

  • P - Precipitation
  • Q - Surface run-off
  • E - Evapotranspiration
  • ±S - Change in storage. (in soil or bedrock)

(http://blackpoolsixthasgeography.pbworks.com/f/1265793530/hydrologic-cycle_s.jpg)(http://thebritishgeographer.weebly.com/uploads/1/1/8/1/11812015/4948857_orig.gif)

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Hydrographs

River hydrographs show how a river responds to precipitation events within its drainage basin. The hydrograph shows changes in discharge over time.

The hydrograph is important for feeding in data on the potenital magnitude and frequency of floods. They are therefore a vital means to managing rivers and drainage basins.

Lag time is the length of time separating peak rainfall from peak discharge.

Comparing Hydrographs - Response time difference 

Drainage basin responses

  • Urbanisation - increases surface run off
  • Forests - Decrease surface run of and increase through flow
  • Geology - If rock is impermeable will encourage surface run off as well the gradient of landscape can increase or decrease surface run off.
  • Size of a drainage basin - larger the basin slower it is to respond
  • Climate  and time of year
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The long profile

Upper cource - The upper course is typified by v-shaped valleys. The river usually occupies most of the narrow valley floor.

Processes - Mostly vertical erosion ,hydraulic and attritionMainly large boulders are transported  Some suspension and solution

Land Forms- Uneven, steep river bed, rapids, waterfalls.

Middle course - The middle course is typified by the valley becoming wider. This is due to the increase in lateral erosion

Processes- Lateral erosion begins, vertical erosion reduces,mostly attrition with some hydraulic , traction & suspended load increases. Some solution. Deposition on slip off slopes.

Landforms - Meanders , flood plains and rapids

Lower course- The lower course of the river tends to be very wide. Large scale deposition occurs here due to the reduced competence of a river. This part of the river can be tidal. Rivers tend to carry a large amount of sediment at this stage.

Processes-Erosion limited mainly to lateral erosion on the outside of meanders.Small bed load . Most material transported in suspension.Mostly small particles deposited. Levées and slip off slopes formed.

Landforms-Levées, large meanders, floodplain

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Types of erosion

Types of erosion that take place in the river

Hydraulic action - refers to the force of the water's flow against the banks and vertical

Abrasion- Eroded rocks picked up by the river scrape  and rub against the channel wearing it away (sand paper effect)

Attrition- Eroded rocks from the river smash into each other and break into smaller fragments there edges get rounded off as they rub together.

Solution- River water has chemical reactions with some types of rock e.g limestone and chalk they are then dissolved.

Weathering

  • Mechanical weathering - break down of rock without changing its chemical composition
  • Freeze-thaw weathering - Water gets into rocks  , it freezes and expands which puts pressure on the rock , when the water melts the pressure is of the rock this keeps occurring which widens rocks and causes them to break
  • Chemical Weathering-Breakdown of rocks by changing there chemical composition
  • Carbonation weathering - Rainwater had carbon dioxide in , which makes it a weak carbonic acid this reacts with rocks that contain calcium carbonate (limestone) this dissolves the rock.
  • Biological weathering - Breakdown of rocks by living things e.g plants and animals.
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Types of transportation

Types of Transportation - Traction - Saltation - Suspension - Solution

(http://thebritishgeographer.weebly.com/uploads/1/1/8/1/11812015/2631505.jpg?596)

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Bradshaw model

Bradshaw model

  • Suggests that velocity, discharge, cross sectional area and hydraulic radius all increase with distance downstream.
  • As a river travels down its course from the source in the mountains to the low lying floodplains towards its mouth, the nature of the valley, the channel, the flow of water and the sediment transported all change.
  • These changes are reflected in the Bradshaw model.

(http://image.slidesharecdn.com/ib-geography-drainage-basins-bradshaw-model-12671/95/ib-geography-drainage-basins-bradshaw-model-2-728.jpg?cb=1224482736)

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Hjulstorm curve

  • Shows the relationship between bedload size, velocity and transportation
  • The lower line is the critical deposition line and if velocity falls below it then bed load will be deposited. The upper line is called the critical erosion line. If velocity reaches this line then the corresponding bed load size will be eroded (picked up) 
  • Firstly, it generally requires more velocity to erode bed load than it does to transport it and deposit it. Secondly, the larger the bed load (above 0.1mm) the greater the velocity required to both transport and erode it. Finally, for micro-fine muds, it requires a high velocity; greater than 100 cumecs to actually erode but once picked up by the flow, the river requires very little velocity to continue to transport it
  •                                             (http://dlgb.files.wordpress.com/2008/09/hjulstrom_curve_task.jpg)

 

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Types of deposition

Deposition is the process of eroded material being dropped. This happens when a river loses energy. A river can lose its energy when rainfall reduces, evaporation increases, friction close to river banks and shallow areas which leads to the speed of the river reducing and therefore the energy reduces, when a river has to slow down it reduces its speed (and ability to transport material) and when a river meets the sea.

(http://www.geography.dur.ac.uk/Projects/Portals/72/Photographs/Ti_G25.jpg)

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The long profile and changing cross profile down s

(http://www.coolgeography.co.uk/GCSE/AQA/Water%20on%20the%20Land/Processes/Long%20Profile.bmp)

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Potential and kinetic energy

Kinetic energy

A still body of water at any point above sea level has a certain amount of stored energy, this is called potential energy

Its quantity depends on the height of the water

Kinetic energy is due to the movement of the water and is derived from it's potential energy. The amount of kinetic energy is determined by the discharge, gradient and average velocity.

An increase in velocity and/or discharge will see an increase in kinetic energy

(http://scienceisntscary.files.wordpress.com/2013/06/potential-and-kinetic-energy1.png)

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Changing channel chracteristics Part 1

Wetted perimeter - is defined as the surface of the channel bottom and sides in direct contact with the aqueous body

Cross sectional area - Length multiplied by Width

The hydraulic radius - is the term used to describe the shape of a channel. It is the ratio between the length of the wetted perimeter and and the cross-section area.The higher the hydraulic radius the lower the amount of water in contact with the bed and banks which means there is less friction and water can move at a higher velocity.

Formula - Cross sectional area divide by Wetted perimeter

River roughness decreases as the river progresses downstream because there is little resistance and friction due to the smoother beds and banks, therefore increasing river efficiency. This is also linked to the fact the sediment in the lower courses of the river are normally silts and clays and any rocks are small and rounded compared to the large angular ones in the upper course.

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Changing channel chracteristics Part 2

(http://astronomyonline.org/Science/Images/Velocity1.gif)

River discharge is the volume of water flowing through a river channel. This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs).

Cross sectional area multiplied velocity

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Land forms of fluvial erosion and deposition Part

Pot holes

Potholes are cylindrical holes drilled into the bed of a river that vary in depth & diameter from a few centimetres to several metres. They’re found in the upper course of a river where it has enough potential energy to erode vertically and its flow is turbulent. In the upper course of a river, its load is large and mainly transported by traction along the river bed. When flowing water encounters bed load, it is forced over it and downcuts behind the bed load in swirling Eddie currents. These currents erode the river’s bed and create small depressions in it. As these depressions deepen, pebbles can become trapped in them. As a result of the Eddie currents, the pebbles drill into the depressions making them more circular, wider & deeper.

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Land forms of fluvial erosion and deposition Part

Rapids-

Are most commonly found in the upper course of the river and form as a result of the river cutting down rapidly in a localised section of the river.

The main characteristics of rapids are distinctly steeper gradients marked by steps in the channel and high turbulence, which is the result of large bed load in the channel or steps. It is common that rapids form over different bands of geology that vary in resistance or where the river is transporting large resistent bed load. The velocity of the river is noticeably faster at rapids but not efficient in its flow.

(http://www.coolgeography.co.uk/A-level/AQA/Year%2012/Rivers_Floods/Landforms/River%20Rapids%20diagram.jpg)

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Land forms of fluvial erosion and deposition Part

  • Waterfalls like rapids form mainly in the upper course as a result of differentiated erosion. If the river channel flows across different bands of geology with varying resistence, then the softer less resistent rock will be eroded faster creating a step in the river.
  • The hydraulic action of the water enlarges this step over time creating the waterfall
  • Due to the force of the water a deep plunge pool forms at the base of the waterfall
  • Turbulent flow with Eddie currents and splash back undercuts the cap of resistent rock headwardly, untill the undercut is significant enough for the rock above to collapse under its own weight.
  •  This leaves angular resistent bedload in the plunge pool, which will combine with hydraulic action to further deepen and undercut through the process of abrasion
  • Over time the waterfall becomes higher and retreats upstream. As the waterfall retreats a deep gorge with steep U-faced sides forms down the length of the river and marks the length of retreat

(http://www.bbc.co.uk/staticarchive/588f5bb3bb05d79acd04af3f0c2d456ea545a69e.gif)

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Land forms of fluvial erosion and deposition Part

Over time the waterfall becomes higher and retreats upstream. As the waterfall retreats a deep gorge with steep U-faced sides forms down the length of the river and marks the length of retreat.

(http://image.slidesharecdn.com/formationofwaterfall-100813111153-phpapp02/95/the-formation-of-waterfall-8-728.jpg?cb=1281716093)

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Land forms of fluvial erosion and deposition Part

Meanders

  • The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
  • The force of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy due to decreased friction.
  • On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction.

(http://www.bbc.co.uk/staticarchive/59ff82c9a42269062a53ac9ff24ef92ccc54d1e1.gif)

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Land forms of fluvial erosion and deposition Part

Ox bow lakes

(http://www.bbc.co.uk/staticarchive/a8dcd06cd8cd3559b399ecc64af3a2032812ccf0.gif)

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Land forms of fluvial erosion and deposition Part

BraidingThis unique channel characteristic develops due to changes in velocity and discharge. This occurs as part of its typical river regime. During periods of increased discharge, the capacity of the river to transport sediment increases and eyots become eroded. The competence of the river to transport larger bedload also increases. The braids widen and merge.Avulsion takes different forms, sometimes expanding into a larger areas of the channel and sometimes sweeping across the channel. In doing so sediment banks shift. During periods of lower discharge, eyots build up due to reduced capacity in the channel

(http://thebritishgeographer.weebly.com/uploads/1/1/8/1/11812015/1674185_orig.png)

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Land forms of fluvial erosion and deposition Part

Levees - Natural levées form as the river floods its banks and inundates the floodplain. As soon as the river overflows the bank, it comes in contact with greater surface area and friction. This increased friction in turn, reduces the velocity and energy of the river and its ability to transport bedload. As a result, the river has declining competence with distance away from the banks. Consequently, the river deposits its larger load first but continues to transport finer sediments further across the floodplain. The sediment profile therefore falls in size from the bank to the bluff. Due to larger bedload being deposited closer to the bank the bank becomes raised, forming gentle sloping banks towards the floodplain.

(http://www.bbc.co.uk/staticarchive/69f8d741e09f98538a506a344f5bd385e42bb5a9.jpg)

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Land forms of fluvial erosion and deposition Part

Development of flood plain

  • When rivers flood in the middle valley the cover an area of land known as the flood plain.
  • When they flood velocity is slowed and deposition of any rocks being transported is encouraged.
  • This deposition leaves a layer of sediment across the whole floodplain.
  • After several floods there are several layers of sediment (rocks) deep on the flood plain.

(http://www.coolgeography.co.uk/GCSE/AQA/Water%20on%20the%20Land/Meanders/Floodplains%20and%20Levees.jpg)

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Land forms of fluvial erosion and deposition Part

Deltas are formed when the river deposits its material faster than the sea can remove it. There are three main types of delta, named after the shape they create:

(http://www.coolgeography.co.uk/A-level/AQA/Year%2012/Rivers_Floods/Landforms/Delta%20Formation.jpg)

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Process and impact of rejuvenation

From time to time something can occur that changes and de-stabilises this equilibrium.  This causes the river to vertically erode its channel to re-establish the its long, smooth concave profile. This renewed period is known as rejuvenation.

Knick points -Is a sharp break of slope in the smooth, concave long profile of a river. It is usually marked by the presence of a waterfall (or a series of rapids). At this point vertical erosion associated with rejuvenation is at its greatest.  The knick point retreats upstream over time.

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Process and impact of rejuvenation Part 2

River terraces are old floodplains left perched above the current floodplain.  Following rejuvenation the river will cut down into its channel and will gradually form a new floodplain. The old one is left high and dry.

Settlements are frequently built on river terraces because they represented land safe from flooding. Much of London is constructed on a river terraces

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Process and impact of rejuvenation Part 3

Incised meandersMeanders are sweeping bends in rivers. They are usually found along the lower course of a river where lateral erosion is greater than vertical. However, when rejuvenation occurs vertical erosion begins to dominate the lateral erosion that usually occurs in a meander (with erosion on the outside bend). This results in a steep sided meander that is cut into the floodplain. It may become entrenched (symmetrical steep sides on both sides of the river) and form a winding gorge like that of the Grand Canyon.

(http://www.alevelgeography.com/wp-content/uploads/2013/11/incised.jpg)

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Magnitude - frequency analysis of flood risk

The actual figures vary for each drainage basin and are important information for planners and inhabitants at risk, as well as engineers planning flood alleviation schemes. Recurrence intervals are calculated using the formula

Recurrence Interval =  (n+1)                            (return period)               / r

Where n is the number of discharge levels in the record and r is the rank of that discharge. The data record shows that the flood hazard has increased over the last few decades. Using the return period information, the statistical probability that a magnitude will be equalled or exceeded can be calculated.

Human habitation of floodplain locations, including higher risk areas increased in the 20th century. This was due to urban development, population pressure, and more road and rail routeways. Thus people have increasingly become exposed to the flood risk and the hazard has increased in relative importance without an actual increase in flood events. 

(http://www.tulane.edu/~sanelson/Natural_Disasters/images/FloodFreqRedRiver.gif)

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Flood management strategies

Multi-Purpose Dams are often built across the channel in order to store water and regulate the discharge of the river. Water is stored in a resevoir upstream of the dam and released in a controlled way. This protects the catchment downstream from potential flooding. Large dam projects are used for clean hydro-electric power, for large scale irrigation schemes and for strategies that open up the interior of countries for transport and trade.

Multi-purpose dams therefore offer a country a means of regulating rivers at a vast scale and in doing so they protect the lives of many many people

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Flood management strategies Part 2

River straightening

This technique kind of speaks for itself. By blocking off meanders and constructing alternate, straighter routes across meanders, the river starts to flow faster. Like with wing dykes, this moves water through the river faster preventing it from pooling and so reducing the risk of a flood. A straightened channel is faster to navigate too, a nice benefit of channel straightening.

(http://www.coolgeography.co.uk/GCSE/AQA/Water%20on%20the%20Land/Managing%20Floods/River%20straightening.jpg)

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Flood management strategies Part 3

Building up of levees

They act as embankments, essentially extending the channel’s height and increasing its bankfull discharge. Unlike natural levées, artificial levées are significantly larger and are generally constructed out of a material like concrete that is resistant to erosion. The main advantage of an artificial levée is that it allows the floodplain to be built on. This is their downfall though as they encourage the development of the floodplain which can increase the risk of flooding.

(http://www.lakeforest.edu/livewhale/content/images/138/14439_building-up-the-levee_f5c94f9c32077c779ff08835f82fdde8.jpg)

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Flood management strategies Part 4

  • Diversion spillways are artificial channels that a river can flow into when its discharge rises. These channels move water around an area at risk of flooding and send it either back into the river (but further downstream) or into another river. Spillways generally have floodgates on them that can be used to control the volume of water in the spillway.
  • Spillways pose a threat to areas near the confluence between the spillway and whichever river it flows into as the discharge here will be increased and so too will the risk of flooding. In addition, the path that spillways take can take water around areas not usually used to flooding. If the spillway was to fail for some reason, this could cause widespread damage.

(http://images.publicradio.org/content/2010/07/08/20100708_river-diversion1_33.jpg)

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Flood management strategies Part 5

Flood Warning systems - Gives people time to prepare / evacuate.

Floodplain zoning- Involves placing restrictions on land usage in the areas surrounding a river. Essentially, construction is unlimited outside of the floodplain but the floodplain itself is limited to the construction of public outdoor facilities like playing fields and parks. This has several effects in reducing the risk and impact of flooding. Obviously if no one is allowed to build on a floodplain then the damage caused by the river flooding will be greatly reduced because there isn’t much to damage. In addition, floodplain zoning ensures that land on the floodplain isn’t urbanised so infiltration can occur and surface run off is reduced. This reduces the likelihood of a river flooding.

The problem with floodplain zoning is that it limits development to certain areas. In addition, if a floodplain has already been developed on, there isn’t anything that can be done to un-develop it without forcing people off of the floodplain.

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Flood management strategies Part 6

Wetland and Riverbank conservation:

Wetlands are areas that are deliberately allowed to flood at times of high discharge. They can reduce flooding to settlements by allowing the water to overflow into them instead of a town. River conservation is the removing of hard management therefore allowing the river to follow its natural course and processes.

  • Advantages: it is not costly to allow land to be left for wetland, and the conservation of rivers can be good for the natural environment and animal habitats. Also, wetlands and river conservation prevent flood damage to settlements.
  • Disadvantages: there are no hard structures that can prevent flooding in severe conditions. The most fertile land - the wetlands - cannot be used, furthermore wetlands can often take up a lot of land. Finally, the cost of removing hard engineering can sometimes be a large sum.
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Flood management strategies Part 6

River restoration involves restoring a river that has undergone hard engineering back to its original course. This can involve un-straightening a channel, removing artificial levees, diversion spillways & wing dykes. This may seem silly as it is going to reintroduce flooding back in the area where these structures were located but, if this land is no longer valuable, river restoration can help reduce the risk of flooding downstream.

  • River restoration is a good thing if it’s done properly as allowing the river to take its natural course prevents and reverts any environmental and ecological damage introduced by hard engineering projects. River restoration also has as good as 0 maintenance costs making it very cheap.
  • The issues arise when river restoration takes place in areas that are still being used by people. I said previously that river restoration can be used where land is no longer valuable. Well, what defines valuable? Farmland? A small village? A town? This decision comes down to the local environmental agency. If they make the wrong call, the restoration project can cause a lot of damage.
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