Cambridge International AS and A Level Geography Revision Guide PDF

Cambridge International AS and A Level Geography Garrett Nagle & Paul Guinness To Angela, Rosie, Patrick and Bethany Hodder Education, an Hachette UK company, 338 Euston Road, London NW1 3BH Orders Bookpoint Ltd, 130 Milton Park, Abingdon, Oxfordshire OX14 4SB tel: 01235 827827 fax: 01235 4004...

Cambridge International AS and A Level Geography Garrett Nagle & Paul Guinness To Angela, Rosie, Patrick and Bethany Hodder Education, an Hachette UK company, 338 Euston Road, London NW1 3BH Orders Bookpoint Ltd, 130 Milton Park, Abingdon, Oxfordshire OX14 4SB tel: 01235 827827 fax: 01235 400401 e-mail: [email protected] Lines are open 9.00 a.m.–5.00 p.m., Monday to Saturday, with a 24-hour message answering service. You can also order through the Hodder Education website: www.hoddereducation.co.uk © Garrett Nagle & Paul Guinness 2013 ISBN 978-1-4441-8148-7 First printed 2013 Impression number 5 4 3 2 1 Year 2018 2017 2016 2015 2014 2013 All rights reserved; no part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without either the prior written permission of Hodder Education or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6–10 Kirby Street, London EC1N 8TS. Cover photo by Chris Guinness Typeset by Datapage (India) Pvt. Ltd. Printed by Anthony Rowe Ltd, Chippenham, Wiltshire This text has not been through the Cambridge endorsement process. Hachette UK’s policy is to use papers that are natural, renewable and recyclable products and made from wood grown in sustainable forests. The logging and manufacturing processes are expected to conform to the environmental regulations of the country of origin.  Get the most from this book Everyone has to decide his or her own revision strategy, but it is essential to review your work, learn it and test your My revision planner understanding. This Revision Guide will help you to do Paper 1 Core Geography that in a planned way, topic by topic. Use this book as the Revised Tested Exam cornerstone of your revision and don’t hesitate to write in it Physical Core 1 Hydrology and fluvial geomorphology ready — personalise your notes and check your progress by ticking 7 1.1 The drainage basin system............................................... 9 1.2 Rainfall–discharge relationships within drainage basins........................................................................ off each section as you revise. 11 1.3 River channel processes and landforms........................... 15 1.4 The human impact............................................................................................................................... 2 Atmosphere and weather 19 2.1 Local energy budgets......................................................................................... 23 2.2 The global energy budget................................................................................. 28 2.3 Weather processes and phenomena.................................................................. 33 2.4 The human impact........................................................... Tick to track your progress.................................. 3 Rocks and weathering 2.2 The global energy budget 2 Atmosphere and weather 37 3.1 Elementary plate tectonics............................................... Use the revision planner on pages 4 and 5 to plan your.................................. 40 3.2 Weathering and rocks........................................................................................ 44 3.3 Slope processes and development................................... revision, topic by topic. Tick each box when you have:.................................. Latitudinal 49 variations 3.4 The human impact.in radiation............................................................................................ Revised Human Coreenergy Atmospheric l revised and understood a topic 4The Population atmosphere is an open energy system receiving energy from both Sun and Earth.4.1 54 Although Natural theincrease latter is very assmall, a component it has an important of population local effect, changeas in the................................... l tested yourself case of urban climates. Incoming solar radiation is referred to as insolation. 58 4.2 Demographic transition................................................... There4.3 61 are important variations in the receipt Population–resource relationships of solar radiation...........with...... latitude.......... and............................................................................ season (Figure 2.3). The result is an imbalance: positive budget in the tropics, l practised the exam-style questions 66 4.4 The management of natural increase.............................. negative one at the poles. However, neither region is getting progressively hotter 5orMigration.................................. colder. To achieve this balance the horizontal transfer of energy from the 69 5.1toMigration equator as place the poles takes a component by winds andofocean population currents. This change gives rise.....to........................................ You can also keep track of your revision by ticking off each an important 73 between 77 5.3low secondmigration 5.2 Internal energy budget latitudes and high International....in...the latitudes. to migration.....atmosphere................–.. the..... horizontal compensate for differences in global.................................................................transfer............................................................................... topic heading in the book. You may find it helpful to add insolation. 81 5.4 A case study of international migration............................................................ 6 Settlement dynamics 90S your own notes as you work through each topic. 84 6.1 Changes in rural settlements 60S 30S............................................ 88 6.2 Urban trends and issues of urbanisation............................................................................................. 0 92 6.3 The changing structure of urban settlements................................................... 97 6.4 The management of urban settlements 30N........................................................... 90S 60N 60S 90N Paper 2 30S Advanced Physical Geography Options D N Equinox O 0 S La A tit Solstice ud 30N J Revised Tested Exam e J M 7 Tropical environments 60N Equinox M A ready F 90N Solstice J 104 7.1 Tropical climates.................................................................................................. 107 7.2 Tropical ecosystems.......................................................... The variations of solar radiation with latitude and season for the Expert tip.................................. 110 whole7.3 Tropical globe, assuminglandforms no atmosphere.....This.....assumption............. explains...................................... Make sure................. summer................. when talking about the abnormally high amounts of radiation received at the poles in 114 7.4when summer, Sustainable daylight lastsmanagement for 24 hours each day. of tropical environments........ and winter in the southern................. or northern................. hemisphere that you refer to months – Source: Barry, R. and Chorley, R., Atmosphere, Weather and Climate, Routledge, 1998 it is easy to forget that if it is summer in Figure 2.3 Seasonal and latitudinal variations in insolation one hemisphere it is winter in the other. 4 Cambridge International AS and A Level Geography Revision Guide Now test yourself 11 What does insolation stand for? 12 When does the South Pole receive most insolation (Figure 2.3)? 13 How much insolation does 80°N receive in December and January. Features to help you succeed Answers on p.00 Pressure variations Revised Sea-level pressure conditions show marked differences between the Expert tip hemispheres. In the northern hemisphere there are greater seasonal contrasts Exam-style questions whereas in the southern hemisphere more stable average conditions exist (Figure 2.4). The differences are largely related to unequal distribution of land Throughout the book there are tips from the experts on and sea, because ocean areas are much more equable in terms of temperature and pressure variations. how to maximise your chances. Exam-style questions are provided for each topic. Use Paper 1 Core Geography 23 them to consolidate your revision and practise your exam skills. 181487_02_Cam_19-36.indd 23 24/10/12 7:01 PM Definitions and key words Exam ready Clear, concise definitions of essential key terms are provided on the page where they appear. Key words from the syllabus are highlighted in bold for you throughout the book. Now test yourself These short, knowledge-based questions provide the first step in testing your learning. Answers are at the back of Typical mistake the book. Advice is given on how to avoid the typical mistakes Tested students often make.  My revision planner Paper 1 Core Geography Revised Tested Exam Physical Core ready 1 Hydrology and fluvial geomorphology 7 1.1 The drainage basin system............................................................................... 9 1.2 Rainfall–discharge relationships within drainage basins...................................... 11 1.3 River channel processes and landforms........................................................... 15 1.4 The human impact........................................................................................... 2 Atmosphere and weather 19 2.1 Local energy budgets....................................................................................... 23 2.2 The global energy budget............................................................................... 28 2.3 Weather processes and phenomena............................................................... 33 2.4 The human impact.......................................................................................... 3 Rocks and weathering 37 3.1 Elementary plate tectonics.............................................................................. 40 3.2 Weathering and rocks..................................................................................... 44 3.3 Slope processes and development.................................................................. 49 3.4 The human impact.......................................................................................... Human Core 4 Population 54 4.1 Natural increase as a component of population change...................................... 58 4.2 Demographic transition................................................................................... 62 4.3 Population–resource relationships.................................................................. 67 4.4 The management of natural increase............................................................. 5 Migration 69 5.1 Migration as a component of population change........................................... 73 5.2 Internal migration............................................................................................ 77 5.3 International migration................................................................................... 81 5.4 A case study of international migration.......................................................... 6 Settlement dynamics 84 6.1 Changes in rural settlements........................................................................... 88 6.2 Urban trends and issues of urbanisation......................................................... 92 6.3 The changing structure of urban settlements................................................. 97 6.4 The management of urban settlements........................................................ Paper 2 Advanced Physical Geography Options Revised Tested Exam 7 Tropical environments ready 104 7.1 Tropical climates................................................................................................ 107 7.2 Tropical ecosystems........................................................................................... 110 7.3 Tropical landforms............................................................................................ 114 7.4 Sustainable management of tropical environments......................................... 4 Cambridge International AS and A Level Geography Revision Guide  Revised Tested Exam ready 8 Coastal environments 116 8.1 Waves, marine and sub-aerial processes........................................................... 120 8.2 Coastal landforms of cliffed and constructive coasts....................................... 124 8.3 Coral reefs......................................................................................................... 127 8.4 Sustainable development of coasts.................................................................. 9 Hazardous environments 130 9.1 Hazardous environments resulting from crustal (tectonic) movement......................................................................................... 133 9.2 Hazardous environments resulting from mass movements....................................................................................................... 136 9.3 Hazards resulting from atmospheric disturbances........................................... 139 9.4 Sustainable management in hazardous environments.................................... 10 Arid and semi-arid environments 142 10.1 The distribution and climatic characteristics of hot arid and semi-arid environments.................................................................... 145 10.2 Processes producing desert landforms........................................................... 148 10.3 Soils and vegetation....................................................................................... 151 10.4 Sustainable management of arid and semi-arid environments.................................................................................................. Paper 3 Advanced Human Geography Options Revised Tested Exam 11 Production, location and change ready 154 11.1 Agricultural systems and food production...................................................... 158 11.2 The management of agricultural change: Jamaica......................................... 160 11.3 Manufacturing and related service industry................................................... 164 11.4 The management of industrial change: India................................................. 12 Environmental management 167 12.1 Sustainable energy supplies............................................................................ 172 12.2 The management of energy supply............................................................... 174 12.3 Environmental degradation............................................................................ 179 12.4 The management of a degraded environment.............................................. 13 Global interdependence 182 13.1 Trade flows and trading patterns................................................................... 186 13.2 Debt and aid and their management............................................................. 191 13.3 The development of international tourism.................................................... 196 13.4 The management of a tourist destination: Jamaica....................................... 14 Economic transition 199 14.1 National development.................................................................................... 203 14.2 The globalisation of industrial activity........................................................... 208 14.3 Regional development.................................................................................... 211 14.4 The management of development................................................................. My revision planner 5  Countdown to my exams 6–8 weeks to go 1 week to go l Start by looking at the syllabus — make sure you l Try to fit in at least one more timed practice of know exactly what material you need to revise an entire past paper and seek feedback from your and the style of the examination. Use the revision teacher, comparing your work closely with the planner on pages 4 and 5 to familiarise yourself mark scheme. with the topics. l Check the revision planner to make sure you l Organise your notes, making sure you have haven’t missed out any topics. Brush up on any covered everything on the syllabus. The revision areas of difficulty by talking them over with a planner will help you to group your notes into friend or getting help from your teacher. topics. l Attend any revision classes put on by your l Work out a realistic revision plan that will allow teacher. Remember, he or she is an expert at you time for relaxation. Set aside days and times preparing people for examinations. for all the subjects that you need to study, and Revised stick to your timetable. l Set yourself sensible targets. Break your revision down into focused sessions of around 40 minutes, The day before the examination divided by breaks. This Revision Guide organises the basic facts into short, memorable sections to l Flick through this Revision Guide for useful make revising easier. reminders, for example the expert tips, typical mistakes and key terms. Revised l Check the time and place of your examination. l Make sure you have everything you need — extra pens and pencils, tissues, a watch, bottled water, sweets. l Allow some time to relax and have an early 4–6 weeks to go night to ensure you are fresh and alert for the examinations. l Read through the relevant sections of this book Revised and refer to the expert tips, typical mistakes and key terms. Tick off the topics as you feel confident about them. Highlight those topics you find My exams difficult and look at them again in detail. l Test your understanding of each topic by working Paper 1 through the ‘Now test yourself’ questions in the Date:................................................. book. Look up the answers at the back of the book. Time:................................................. l Make a note of any problem areas as you revise, Location:............................................. and ask your teacher to go over these in class. Paper 2 l Look at past papers. They are one of the best ways to revise and practise your exam skills. Write Date:................................................. or prepare planned answers to the exam-style Time:................................................. questions provided in this book. Check your Location:............................................. answers with your teacher. l Try different revision methods. For example, you Paper 3 can make notes using mind maps, spider diagrams Date:................................................. or flash cards. Time:................................................. l Track your progress using the revision planner and give yourself a reward when you have achieved Location:............................................. your target. Revised 6 Cambridge International AS and A Level Geography Revision Guide 1 Hydrology and fluvial geomorphology 1.1 The drainage basin system The drainage basin system Revised A drainage basin is a natural system with inputs, flows and stores of water and sediment. Every drainage basin is unique, on account of its climate, geology, A drainage basin refers to the area drained by a river and its tributaries. vegetation, soil types, size, shape and human activities. The drainage basin system is an open system as it allows the movement of energy and matter Hydrology is the study of water as it across its boundaries. moves on, and under and through the Earth’s surface. The hydrological cycle refers to the cycle of water between atmosphere, The water cycle or hydrological lithosphere and biosphere. Water can be stored at a number of stages or levels cycle is the movement of water within the cycle (Figure 1.1). These stores include vegetation, surfaces, soil between air, land and sea. moisture, groundwater and water channels. Human modifications to these can be made at every scale. Evapotranspiration, e Precipitation, p ac ai Interception storage, D er Stemflow and drip Surface storage, R Overland flow, qo em Infiltration, f Soil moisture storage, M Throughflow, m el Seepage, s Aeration zone storage, L Interflow, i Groundwater recharge, d Channel Groundwater storage, G Baseflow, g storage, S Channel runoff, q Figure 1.1 The hydrological cycle l Precipitation is the main input into the drainage basin system. The main characteristics that affect local hydrology include: the total amount of precipitation; intensity (mm/hour); type of precipitation (snow, rain etc.); geographical distribution; and temporal variability (i.e. seasonality). Precipitation is the conversion and transfer of moisture in the atmosphere l Interception has three main components: interception loss – water that to the land. It includes rainfall, snow, is retained by plant surfaces and which is later evaporated away or absorbed frost, hail and dew. by the plant; throughfall – water that either falls through gaps in the Interception is the precipitation that is vegetation or which drops from leaves, twigs or stems; stemflow – water collected and stored by vegetation. that trickles along twigs and branches and finally down the main trunk. Paper 1 Core Geography 7  Evaporation and transpiration increase under warm, dry conditions and 1 Hydrology and fluvial geomorphology l Evaporation refers to the decreases under cold, calm conditions. Meteorological factors affecting transformation of liquid water from evapotranspiration (EVT) include temperature, humidity and windspeed. the Earth’s surface into a gas (water Of these, temperature is the most important. Other factors include the vapour). amount of water available, vegetation cover and colour of the surface Transpiration is water loss from (albedo or reflectivity of the surface). vegetation to the atmosphere. l EVT represents the most important aspect of water loss, accounting for Evapotranspiration is the combined the loss of nearly 100% of the annual precipitation in arid areas and 75% in loss of water to the atmosphere humid areas. through transpiration and evaporation. l Potential evapotranspiration (PEVT) is the water loss that would occur if Infiltration is the process by which there was an unlimited supply of water in the soil for use by the vegetation. water soaks into, or is absorbed by, For example, the actual evapotranspiration rate in Egypt is less than 250 mm, the soil. because there is less than 250 mm of rain annually. However, given the high Runoff is water that flows over the temperatures experienced in Egypt, if the rainfall was as high as 2000 mm, land’s surface. there would be sufficient heat to evaporate that water. Hence the potential evapotranspiration rate there is 2000 mm. l The infiltration capacity is the maximum rate at which rain can be absorbed by a soil in a given condition. Infiltration is inversely related to runoff and is influenced by a variety of factors such as duration of rainfall, Typical mistake antecedent soil moisture (pre-existing levels of soil moisture), soil porosity, Drainage basin hydrology is very vegetation cover, raindrop size and slope angle. variable from year to year. This can be Table 1.1 Influence of ground cover on infiltration rate due to natural changes or, increasingly, human-related activities. Ground cover Infiltration (mm/hour) Old permanent pasture 57 Permanent pasture: moderately grazed 19 Expert tip Permanent pasture: heavily grazed 13 You may be asked to draw a diagram of a drainage basin hydrological cycle. A Strip-cropped 10 systems diagram – with inputs, stores, Weeds or grain 9 flows and outputs – such as that in Clean tilled 7 Figure 1.1, is much better than one that tries to show trees, clouds, rainfall, Bare, crusted ground 6 glaciers, rivers, lakes and oceans, for example. l Soil moisture refers to the subsurface water in the soil. Field capacity refers to the amount of water held in the soil after excess water drains away, i.e. saturation or near saturation. Wilting points refer to the range of moisture content in which permanent wilting of different plants occurs. Now test yourself They define the approximate limits to plant growth. Throughflow refers 1 Define the following hydrological to water flowing through the soil in natural pipes and percolines (lines of characteristics: concentrated water flow between soil horizons). (a) interception l Groundwater refers to subsurface water. Groundwater accounts for 96.5% (b) evaporation of all freshwater on the Earth. The permanently saturated zone within solid (c) infiltration rocks and sediments is known as the phreatic zone. The upper layer of this is 2 Study Figure 1.1. known as the water table. Baseflow refers to the part of a river’s discharge Outline the differences between that is provided by groundwater seeping into the bed of a river. It is a overland flow, throughflow and relatively constant flow, although it increases slightly following a wet period. baseflow. l Recharge refers to the refilling of water in pores where the water has dried 3 Suggest what is meant by up or been extracted by human activity. Hence, in some places, where interception storage in Figure 1.1. recharge is not taking place, groundwater is considered a non-renewable 4 Outline what may happen to water resource. that is stored on the surface on the ground. l Aquifers are rocks that contain significant quantities of water. A spring is a natural flow of water from the Earth’s surface. It occurs when the water table 5 Comment on the influence of ground cover on infiltration rates (the upper surface of saturation within permeable rocks) occurs at the (Table 1.1). surface. Answers on p.213 Tested 8 Cambridge International AS and A Level Geography Revision Guide 1.2 Rainfall–discharge relationships 1 Hydrology and fluvial geomorphology within drainage basins Annual hydrograph Revised A hydrograph is a line graph showing how water level in a river changes over time. There are two main types of hydrograph – annual hydrographs A flood hydrograph shows how the (also known as river regimes) and storm hydrographs (also known as flood discharge of a river varies over a short time – normally it refers to an individual hydrographs). Annual hydrographs show variations in the flow of a river over storm or group of storms of not more the course of a year, whereas a storm hydrograph shows the variation in the than a few days in duration. flow of a river for a period of between 1 and 7 days. A river regime is the annual variation Stream flow occurs as a result of runoff, groundwater springs and input from in the flow of a river. lakes and from meltwater in mountainous or sub-polar environments. The character or regime of the resulting stream or river is influenced by several 40 Guadalquivir at variable factors: Alcala del Ri Discharge (l/s/km2) l the amount and nature of precipitation 20 l the local rocks, especially porosity and permeability l the shape or morphology of the drainage basin, its area and slope 0 J F MAM J J A S O N D l the amount and type of vegetation cover Figure 1.2 Guadalquivir regime l the amount and type of soil cover 40 Shannon at Killaloe In Figures 1.2–1.4 discharge is shown in litres per second per km2. On an annual Discharge basis the most important factor determining stream regime is climate. Figure 1.2 (l/s/km2) 20 shows a river regime for the Guadalquivir river at Alcala Del Ri in Spain. Its peak flow is about 20 litres/second/km2 in March. It is generally in higher flow during 0 J F MAM J J A S O N D winter whereas in summer it has low flow. In fact, in August there appears to be Figure 1.3 Shannon regime no flow. Discharge in July and September is less than about 5 litres/second/km2. 40 Gloma at This is due to the high-pressure system that characterises Mediterranean regions Lamgnes Discharge in summer, producing a summer drought. In contrast, winters are associated (l/s/km2) 20 with low-pressure systems and the resulting rain they bring, hence higher discharges. 0 J F MAM J J A S O N D Now test yourself Tested Figure 1.4 Gloma regime 6 Study Figures 1.3 and 1.4. Compare the river regimes of the Gloma (Norway) Expert tip and Shannon (Ireland). Make sure that you use units – it 7 Suggest reasons for their differences. would be easy here to just refer to high Answers on p.213 discharge and low discharge (or high flow and low flow). A scale is provided – please make sure that you make use of it. Flood hydrograph Revised A flood hydrograph (Figure 1.5) normally refers to an individual storm or group of storms of not more than a few days in length. Before the storm starts the main supply of water to the stream is through groundwater flow or baseflow. This is the main supplier of water to rivers. During the storm some water infiltrates into the soil while some flows over the surface as overland flow or runoff. This reaches the river quickly as quickflow. This causes the rapid rise in the level of the river. The rising limb shows us how quickly the floodwaters begin to rise, whereas the recessional limb is the speed with which the water level in the river declines after the peak. The peak flow is the maximum discharge of the river as a result of the storm and the lag time is the time Paper 1 Core Geography 9  between the height of the storm (not the start or the end) and the maximum Typical mistake 1 Hydrology and fluvial geomorphology flow in the river. Some students work out the lag time 1.25 from the start of the storm to the peak Discharge (m3/sec) Lag time discharge. This is wrong – it should be 20 Peak discharge (flow) from the peak of the storm to the peak 1.00 mm/hr of the flood. Channel precipitation 0 and overland flow Rainfall 0.75 Rising limb Recessional limb 0.50 Throughflow Groundwater flow 0.25 0 0 5 10 15 20 Time (hours) Figure 1.5 A simple hydrograph The effect of urban development on hydrographs is to increase peak flow and Typical mistake decrease lag time (Figure 1.6). This is due to an increase in the proportion of You may be expected to draw a labelled impermeable ground in the drainage basin as well as an increase in the drainage hydrograph – or hydrographs – to density. Storm hydrographs also vary with a number of other factors (Table 1.2) show the impact of urbanisation. Pay particular attention to the command such as basin shape, drainage density and gradient. words in the question – are you asked to describe the differences or are you Discharge 1 Completely sewered basin being asked to explain the curves? Not with highly impermeable surface reading the question (and therefore 20 answering it) is one of the most mm/hr common mistakes in exams. 0 2 Completely sewered Rainfall basin with natural surface Expert tip 3 Natural channels and basin surface Practise drawing annotated (labelled) diagrams of simple rural and urban Time hydrographs for the same storm. Label Source: Advanced Geography: Concepts & Cases the rising and recessional limbs, peak by P. Guinness & G. Nagle (Hodder Education, 1999), p.255 flow and lag time. Figure 1.6 The effects of urban development on flood hydrographs Table 1.2 Factors affecting storm hydrographs Factor Influence on flood hydrograph Precipitation type and Highly intensive rainfall is likely to produce overland flow and a steep rising limb and high peak intensity flow. Low-intensity rainfall is likely to infiltrate into the soil and percolate slowly into the rock, thereby increasing the lag time and reducing the peak flow. Precipitation that falls as snow sits on the ground until it melts. Sudden, rapid melting can cause flooding and lead to high rates of overland flow, and high peak flows. Temperature and Not only does temperature affect the type of precipitation, it also affects the evaporation rate evapotranspiration (higher temperatures lead to more evaporation and so less water getting into rivers). On the other hand, warm air can hold more water, so the potential for high peak flows in hot areas is raised. Antecedent moisture If it has been raining previously and the ground is saturated or near saturated, rainfall will quickly produce overland flow and a high peak flow and short time lag. Drainage basin size and Smaller drainage basins respond more quickly to rainfall conditions. For example, the Boscastle shape (UK) floods of 2004 drained an area of less than 15 km2. This meant that the peak of the flood occurred soon after the peak of the storm. In contrast, the Mississippi River is over 3700 km long – it takes much longer for the lower part of the river to respond to an event that occurs in the upper course of the river. Circular basins respond more quickly than linear basins, where the response is more drawn out. Drainage density Basins with a high drainage density – such as urban basins with a network of sewers and drains – respond very quickly. Networks with a low drainage density have a very long lag time. 10 Cambridge International AS and A Level Geography Revision Guide  1 Hydrology and fluvial geomorphology Table 1.2 (continued) Factor Influence on flood hydrograph Porosity and impermeability Impermeable surfaces cause more water to flow overland. This causes greater peak flows. Urban of rocks and soils areas contain large areas of impermeable surfaces. In contrast, rocks such as chalk and gravel are permeable and allow water to infiltrate and percolate. This reduces the peak flow and increases the time lag. Sandy soils allow water to infiltrate, whereas clay is much more impermeable and causes water to pass overland. Slopes Steeper slopes create more overland flow, shorter lag times and higher peak flows. Vegetation type Broad-leafed vegetation intercepts more rainfall, especially in summer, and so reduces the amount of overland flow and peak flow, and increases lag time. In winter, deciduous trees lose their leaves and so intercept less. Land use Land uses that create impermeable surfaces or reduce vegetation cover reduce interception and increase overland flow. If more drainage channels are built (sewers, ditches, drains) the water is carried to rivers very quickly. This means that peak flows are increased and lag times reduced. Now test yourself Tested 8 Define the terms river regime and flood hydrograph. 9 Study Figure 1.6 which shows the impact of urbanisation on flood hydrographs. Describe the differences in the relationship between discharge and time. Answers on p.213 1.3 River channel processes and landforms River processes Revised Transport The load is transported downstream in a number of ways: l The smallest particles (silts and clays) are carried in suspension as the Capacity of a stream refers to the suspended load. largest amount of debris that a stream can carry. l Larger particles (sands, gravels, very small stones) are transported in a series of ‘hops’ as the saltated load. Competence refers to the diameter of the largest particle that can be carried. l Pebbles are shunted along the bed as the bed or traction load. l In areas of calcareous rock, material is carried in solution as the dissolved load. The load of a river varies with discharge and velocity. Deposition and sedimentation: Hjulstrom curves There are a number of causes of deposition, such as: l a reduction in gradient, which decreases velocity and energy l a decrease in the volume of water in the channel l an increase in the friction between water and channel Paper 1 Core Geography 11 1 Hydrology and fluvial geomorphology 10.0 Velocity (m/s) 5.0 Erosion 1.0 rt po y 0.5 cit ns y Ent lo cit Tra r ain ve elo men i o n v t a nd e r o s ng 0.10 i ttl nt Se ime 0.05 -sed Transport Deposition d ende 0.010 0.005 Susp 0.001 0.01 0.050.1 0.5 1.0 5 10 50 100 1000 Grain diameter (mm) Clay Silt Sand Gravel Source: Advanced Geography: Concepts & Cases by P. Guinness & G. Nagle (Hodder Education, 1999), p.263 Figure 1.7 Hjulstrom curves Hjulstrom curves show what work a river will do (erosion, transport, deposition) depending on its velocity and the size of material present. There are three important features of Hjulstrom curves: l The smallest and largest particles require high velocities to lift them – for example, particles between 0.1 mm and 1 mm require velocities of around 100 mm/s to be entrained, compared with values of over 500 mm/s to lift clay (less than 0.01 mm) and gravel (over 2 mm). Clay resists entrainment due to its cohesion; gravel due to its weight. l Higher velocities are required for entrainment than for transport. l When velocity falls below a certain level those particles with a particular settling or fall velocity are deposited. Erosion l Abrasion (or corrasion) is the wearing away of the bed and bank by the load carried by a river. Abrasion increases as velocity increases. l Attrition is the wearing away of the load carried by a river. It creates smaller, rounder particles. l Hydraulic action is the force of air and water on the sides of rivers and in cracks. l Corrosion or solution is the removal of chemical ions, especially calcium. Velocity and discharge l Velocity refers to the speed of the river. Velocities increase in rivers that are deeper and when rivers are in flood. l Discharge refers to rate of flow of a river at a particular time. It is generally found by multiplying cross-sectional area by velocity, and is usually expressed in cusecs (cubic feet per second) or cumecs (cubic metres per second). l The hydraulic radius is a measure of a stream’s efficiency – it is calculated by dividing the cross-sectional area by the wetted perimeter (the length of bed and bank in contact with water). The higher the hydraulic radius, the more efficient the river. Patterns of flow There are three main types of flow: laminar, turbulent and helicoidal. l For laminar flow a smooth, straight channel with a low velocity is required. This allows water to flow in sheets or laminae parallel to the channel bed. l Turbulent flow occurs where there are higher velocities and an increase in bed roughness. Turbulence is associated with hydraulic action (cavitation). Vertical turbulence creates hollows in the channel bed. l Horizontal turbulence often takes the form of helicoidal flow – a ‘corkscrewing’ motion. This is associated with the presence of alternating 12 Cambridge International AS and A Level Geography Revision Guide  pools and riffles in the channel bed, and where the river is carrying large 1 Hydrology and fluvial geomorphology amounts of material. The erosion and deposition by helicoidal flow creates meanders. Channel landforms Revised Sinuosity is the length of a stream channel expressed as a ratio of the valley length. A low sinuosity has a value of 1.0 (i.e. it is straight), whereas a high sinuosity is above 4.4. The main groupings are straight channels (1.5). Straight channels are rare. Even when they do occur the thalweg (line of maximum velocity) moves from side to side. These channels generally have a central ridge of deposited material, due to the water flow pattern. Meandering is a natural process and creates rivers with an asymmetric cross-section (Figure 1.8). a Pool Riffle Erosion Point bar River cliff Slip-off slope X Y X Y Erosion on the outside bank Deposition Movement of water: on the on the surface inside bank on the river bed fast slow fast slow Plan erosion accretion Long profile erosion accretion initial bed Cross-section final bed initial bed Erosion of initial channel Accretion of initial bed b Initial uniform channel pool to pool Pool Riffle Figure 1.8 Characteristics of meanders Paper 1 Core Geography 13  Braiding occurs when the channel is divided by islands or bars. Islands are 1 Hydrology and fluvial geomorphology vegetated and long-lived, whereas bars are unvegetated, less stable and often short-term features. Braided channels are formed by many factors, including: l a large proportion of coarse material l easily erodable bank material l highly variable discharge Pools are the deep parts of a meander, whereas riffles are the ridges of sediment found in the straighter sections of a meander. Riffles are generally formed of coarse gravel, whereas in a pool, erosion is the main process. Helicoidal flow in the river causes erosion on the outer bank of a meander, and the formation of river cliffs. In contrast, on the inner bank, helicoidal flow leads to the formation of point bars (also known as slip-off slopes). A bluff is another term for a river cliff, but may also be used to describe the edge of a river terrace (a former floodplain). A floodplain is the part of a river valley, adjacent to the river channel, over which the river flows in times of flood. It is a zone of low relief and gentle gradients. The floodplain may contain oxbow lakes and is generally formed of alluvium. Waterfalls occur where the river spills over a sudden change in gradient, undercutting rocks by hydraulic impact and abrasion, thereby creating a waterfall. The change in gradient is usually related to a band of resistant rock. A gorge is a deep, narrow valley with very steep sides. It is currently occupied by a river or had a river in the past. Gorges may be formed as a result of: Typical mistake l retreat of waterfalls (e.g. Niagara Falls) Many students think that most l glacial overflow channelling erosion occurs in upland areas and deposition in lowland areas. Both occur l collapse of underground caverns in carboniferous limestone areas throughout the course of a river. Most l surface runoff over limestone during a periglacial period erosion only takes place when the river is in flood. Rapids occur when there is a series of small bands of hard rock outcropping in a river. This causes turbulence within the river and a series of ‘mini-waterfalls’. The Nile Cataracts are a good example of rapids. Now test yourself Levées are formed when a river bursts its banks repeatedly over a long period 10 Define the terms hydraulic action, of time. The floodwater quickly loses velocity, leading to the rapid deposition of attrition and abrasion. coarse material (heavy and difficult to move a great distance) near the channel 11 Outline the ways in which a river edge. These coarse deposits build up to form embankments, or levées. The finer transports its load. material is carried further away to be deposited on the floodplain. 12 State the approximate values needed to (a) deposit, (b) transport, Alluvial fans are found in semi-arid areas where swiftly flowing mountain (c) entrain and (d) erode a particle streams enter a main valley or plain at the foot of the mountains. There is a of 1.0 mm. sudden decrease in velocity, causing deposition. Fine material is spread out as an alluvial fan, with a slope angle of less than 1°. By contrast, coarse material forms a Answers on p.213 Tested relatively small, steep-sided alluvial cone, with a slope angle of up to 15°. Deltas are river sediments deposited when a river enters a standing body of Expert tip water such as a lake, a lagoon, a sea or an ocean. For a delta to form there must For most landforms (of erosion be a heavily laden river and a standing body of water with negligible currents, and deposition) you should learn such as the Mediterranean or the Gulf of Mexico. Deposition is enhanced if the an annotated diagram, and ensure water is saline, because salty water causes small clay particles to flocculate or that you explain how the landform is formed. For many features, e.g. adhere together. waterfalls and levées, you may need to learn a sequence of diagrams. 14 Cambridge International AS and A Level Geography Revision Guide 1.4 The human impact 1 Hydrology and fluvial geomorphology Human impacts on hydrology Revised Urbanisation is a major cause of hydrological changes. These are summarised in Table 1.3. Table 1.3 Potential hydrological effects of urbanisation Urbanising influence Potential hydrological response Removal of trees and vegetation Decreased evapotranspiration and interception; increased stream sedimentation Initial construction of houses, streets and culverts Decreased infiltration and lowered groundwater table; increased storm flows and decreased base flows during dry periods Complete development of residential, commercial and Decreased porosity, reducing time of runoff concentration, industrial areas thereby increasing peak discharges and compressing the time distribution of the flow; greatly increased volume of runoff and flood damage potential Construction of storm drains and channel improvements Local relief from flooding; concentration of floodwaters may aggravate flood problems downstream Human impact on precipitation There are a number of ways in which human activity affects precipitation. Cloud seeding has probably been one of the more successful. Rain requires particles, such as dust and ice, on which to form. Seeding introduces silver iodide, solid CO2 (dry ice) or ammonium nitrate to attract water droplets. Human impact on evaporation and transpiration The human impact on evaporation and transpiration is relatively small in relation to the rest of the hydrological cycle but is nevertheless important. Dams – there has been an increase in evaporation due to the construction of large dams. For example, Lake Nasser behind the Aswan Dam loses up to a third of its water due to evaporation. Water loss can be reduced by using chemical sprays on the surface, by building sand-fill dams and by covering the dams with plastic. Urbanisation leads to a huge reduction in evapotranspiration due to the lack of vegetation. There may also be a slight increase in evaporation because of higher temperatures and increased surface storage. Human impact on infiltration and soil water Human activity has a great impact on infiltration and soil water. Land use changes are important. Urbanisation creates an impermeable surface, with compacted soil. This reduces infiltration and increases overland runoff and flood peaks. Infiltration is up to five times greater under forests compared with grassland. Human impact on interception Interception is determined by the density and type of vegetation. Most vegetation is not natural but represents some disturbance by human activity. Deforestation leads to: l a reduction in evapotranspiration l an increase in surface runoff l a decline of surface storage l a decline in lag time Abstraction and water storage In the High Plains of Texas, groundwater is now being used at a rapid rate to supply centre-pivot irrigation schemes. In under 50 years, the water level has declined by 30–50 m over a large area. The extent of the aquifer has reduced by Paper 1 Core Geography 15  more than 50% in large parts of certain counties. By contrast, in some industrial 1 Hydrology and fluvial geomorphology areas, recent reductions in industrial activity have led to less groundwater being taken out of the ground. As a result, groundwater levels in such areas have begun to rise, adding to the problem caused by leakage from ancient, deteriorating pipe and sewerage systems, and resulting in: l surface water flooding l pollution of surface waters and spread of underground pollution l flooding of basements l increased leakage into tunnels There are various methods of recharging groundwater resources, providing that Recharge is the topping up of sufficient surface water is available. Where the materials containing the aquifer