978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

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VegetationVegetated areas have a greater capacity to intercept precipitation and absorbsoil moisture. The type, nature and extent of vegetation will determine its abilityto retain moisture. Estimates suggest that tropical rainforests intercept up to80 per cent of rainfall (30 per cent of which may later evaporate) whereas arableland may only intercept 10 per cent.In the United Kingdom, large broadleaved deciduous trees have a largerbiomass and expansive canopy in the summer months leading to greaterinterception rates than in winter where intake is greatly reduced due to the lossof leaves in autumn months.Deforestation is an activity widely associated with flooding. The removal ofvegetation whether for the clearance of land for development or harvesting ofa cash crop often has negative consequences and widespread implications on ariver regime. Flows can be considerably faster.In addition, the stability of soil profiles can be compromised by loggingtrails and disturbed ground with further areas vulnerable to erosion by the fastflowing surface flows. The resultant runoff is often heavily silted, which makesrivers thick and dirty with sediment. Areas heavily reliant on rivers for washingand drinking are the first to suffer.Tides and storm surgesThe daily rise and fall of the tides affects the relative base level to which a riverflows. High spring tides may prevent water from discharging into the sea,increasing the potential for flooding. Low pressure systems such as depressionsand tropical storms reduce the amount of air pressure acting on sea levelleading to a slight rise in water level at these times. This coupled with strongwinds create further pressure on low-lying coastal areas. Storm surges occur whenstrong wind conditions affect a coastline, forcing waves landward and inlandthrough estuaries.River channel processes and landformsThe long profileThe long profile is the name given to the gradient of a river from the startof the river (source) to its mouth. Rivers always work under the influence ofgravity, cutting a path downhill through the landscape. The higher up a river’sUPPERCOURSEMIDDLECOURSELOWERCOURSEcross profilescharacteristicsand processesHeight above sea level5004003002001000Vertical erosionwith hydraulicaction, abrasion andattrition dominantprocessesTraction andsaltation at highflowLoad size is largeand angularV-shaped valleysChannel is deeper andwiderVertical erosiondecreasing inimportance, morelateral erosion anddepositionSuspension is themain transportationtypeLoad becomes smallerand less angularChannel is at its widest anddeepest, and may be tidalDeposition more importantthan erosionFine material depositedLarge amount of load but thesize is very small and veryroundedLong profile is thechange in gradient withdistance. It starts offsteep but reduces withdistance from source,and has a concave profilesea orocean–100Source Increasing distance downstream Mouthfigure 1.11 Long and cross profiles on a typical river.20Hydrology and fluvial geomorphology
source is, the higher the gravitational potential. As a result the upper reaches ofa river are often steep with deeply incised valleys: the result of vertical erosion.In the lower reaches however, as the gravitational pull is lessened, rivers tendto expel their energy by eroding laterally across the landscape. A graded profileshows an idealised view of a river’s change in altitude that is in equilibrium,starting steeply and becoming ever more flattened. In reality changes in theunderlying geology and human influences (such as dams) may distort thisidealised view.As water flows downhill under gravity it seeks the path of least resistance. Inthe higher reaches the river has greater potential energy but channels are oftenrough and poorly formed. Further downstream channels become wider, deeperand more efficient as more water joins from tributaries and is able to shape asmoother route.The upper courseThe upper course is a high-energy environment that experiences a highlevel of erosion and turbulent flow. The source of the river can often befound in boggy upland areas with no distinct channel or form. As wateraccumulates it starts to carve out shallow paths in the soil and vegetationbefore descending more rapidly under the influence of gravity. At altitudethe combined processes of weathering and fluvial erosion contribute to thehigh level of bedload (sediments that lie on the riverbed) and large angularmaterial including frost shattered boulders and scree. Partly as a result ofthe large material, traction (the largest stones, boulders and cobbles rolledalong the riverbed by strong turbulent flow) and saltation (a transportationalprocess where smaller bedload such as pebbles bounce along the riverbed) arecommon.The middle courseThe middle course is a longer section of river characterised by a decreasinggradient and greater lateral erosion. As a result the valley sides are less incisedthan the upper reaches and the river starts to become more sinuous (winding).The river itself here becomes more established with a greater number oftributaries bringing additional water. There is a high proportion of suspendedload and bedload is smaller and less angular than upstream.The lower courseThe lower course is the low-lying portion of the river that joins with the sea. Itis characterised by wide flat sweeping floodplains and large meander bends. Itis the depositional zone of the river, featuring small rounded stones that havebeen worked on by fluvial action and erosion. There is a high proportion ofsuspended material in the low profile.FlowA river’s function is to transport water to the lowest point of its catchment.In doing so the water interacts with the landscape, channel and underlyinggeology. The flow of the river is the manner in which the water travels. There arethree types of flow:• Laminar flow is characterised by a smooth horizontal motion often toosimplistic for complex natural river environments that have many changes,steps and gradients. A laminar-style flow may be found in carefully managedchannellised sections on a relatively small scale where there are fewadditional influences.• Turbulent flow is characterised by a series of erratic horizontal andvertical spiral flows (known as eddies) that disturb the smooth appearanceof the water. Turbulent flow is the dominant method of flow in a riverfigure 1.12 Characteristic turbulent flow of the uppercourse, showing large rock debris.figure 1.13 A sweeping curve of the middle course.Rivers become more sinuous as they have more energyto expel downstream.figure 1.14 The lower course where the river joins the seaat the depositional zone.figure 1.15 Turbulent glacial water in Norway.Hydrology and fluvial geomorphology 21
Page 3 and 4: ContentsIntroductionHow to use this
Page 5 and 6: IntroductionCollins Cambridge A and
Page 7 and 8: 1234567891011121314151617181920Loca
Page 9 and 10: figure 1.3 The forested banks of th
Page 11 and 12: transpirationevaporationprecipitati
Page 13 and 14: amount of water available, vegetati
Page 15 and 16: 0.25Group 10.25Group 50.25Group 90.
Page 17: Parent materialThe parent material
Page 21 and 22: ErosionThe power of the water and t
Page 23 and 24: Fluvial features: erosionV-shaped v
Page 25 and 26: • The radius of curvature of the
Page 27 and 28: There are three main types of delta
Page 29 and 30: commercial reasons though there are
Page 31 and 32: GroundwaterHuman activity has serio
Page 33 and 34: Event modifications refer to action
Page 35: • Storm drains: Storm drains have

978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

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