Collins Cambridge AS and A Level Geography by Collins

Contents

Introduction How to use this book Locations of case studies used in the book

5 6 7

1: Hydrology and ﬂuvial geomorphology The drainage basin system Discharge relationships within drainage basins River channel processes and landforms The human impact

8–37 10–15 15–20 20–29 29–37

2: Atmosphere and weather Diurnal energy budgets The global energy budget Weather processes and phenomena The human impact

38–59 40–41 41–48 48–52 52–59

3: Rocks and weathering Plate tectonics Weathering Slope processes The human impact

60–85 62–68 68–74 74–79 79–85

4: Population Natural increase as a component of population change Demographic transition Population-resource relationships The management of natural increase

86–107 88–96 96–98 98–106 106–107

5: Migration Migration as a component of population change Internal migration (within a country) International migration The management of international migration

108–129 110–119 119–124 124–129 114–115

6: Settlement dynamics Changes in rural settlements Urban trends and issues of urbanisation The changing structure of urban settlements The management of urban settlements

130–155 132–137 137–147 147–153 153–155

7: Tropical environments Tropical climates Landforms of tropical environments Humid tropical ecosystems and seasonally humid tropical ecosystems Sustainable management of tropical environments

156–177 158–162 162–166 166–173 173–177

8: Coastal environments Coastal processes Characteristics and formation of coastal landforms

178–205 180–187 187–197 3

Contents

Coral reefs Sustainable management of coasts

197–201 201–205

9: Hazardous environments Hazards resulting from tectonic processes Hazards resulting from mass movements Hazards resulting from atmospheric disturbances Sustainable management in hazardous environments

206–231 208–215 215–219 219–225 225–231

10: Hot arid and semi-arid environments Hot arid and semi-arid climates Landforms of hot arid and semi-arid environments Soils and vegetation Sustainable management of hot arid and semi-arid environments

232–255 235–242 242–250 250–254 254–255

11: Production, location and change Agricultural systems and food production The management of agricultural change Manufacturing and related service industry The management of change in manufacturing industry

256–281 258–268 268–271 271–278 278–281

12: Environmental management Sustainable energy supplies The management of energy supply Environmental degradation The management of a degraded environment

282–307 284–291 291–295 295–302 302–307

13: Global interdependence Trade ﬂows and trading patterns International debt and international aid The development of international tourism The management of a tourist destination

308–337 310–318 318–324 324–334 334–337

14: Economic transition National development The globalisation of economic activity Regional development within countries The management of regional development

338–367 340–351 351–359 359–362 362–367

15: Geographical skills Diagrams and graphs Maps Satellite images and aerial photographs Data types

368–384 370–373 373–380 380–380 380–384

Glossary Index Acknowldegements Key concepts

385–399 400–416 417–418 419

Introduction

Collins Cambridge A and AS Level Geography Student Book, written by a team of experienced geography teachers, is fully matched to the Cambridge A and AS Level Geography syllabus (9696). The book covers all the core syllabus topics, as well as the physical and human geography options. The aim of the book is to help the student obtain the knowledge, understanding and skills to succeed in their geographical studies. Content is accessible and clearly organised, with a student-friendly layout. Content coverage is suitable for the whole range of abilities. Illustrated throughout, it contains a wealth of maps, photographs, graphs, diagrams and info-graphics to support the geographical content. Case studies and locational examples are included to help provide context and real-life meaning. As well as supporting studies at A Level and helping students to fulfil their potential in the subject, it is to be hoped that they gain an awareness of some of the wider issues related to specific topics. The understanding of current human and environmental problems, the processes at work that create them and their possible solutions form the basis of geographical study. In order to do this effectively, students need to be reading widely and developing their own local case studies to supplement the examples given in the book. Another important aspect of geographical study at this level is learning about the complexity of many of the topics, namely the inter-relationships between human and physical processes, the concepts of space and time and the impact they have on change within both the physical and human landscape. The development of a range of geographical skills also underpins A Level Geography and the value of geography as a subject in today’s world. By undertaking fieldwork, students collect both primary and secondary data to research an issue, then present and interpret the data using a range of illustrative and statistical techniques. Finally, they analyse that data to reach a conclusion about the issue under investigation before critically evaluating the methodology they used. All these techniques are valuable transferable skills to take into higher education and/or the workplace.

How to use this book

Sections of the book This Student Book covers all the content in the Cambridge AS and A Level Geography syllabus. It follows the sequence of the syllabus and is divided into several sections. Section 1 is colour coded blue and matches the ﬁrst three themes of the syllabus – hydrology and ﬂuvial geomorphology; atmosphere and weather; and rocks and weathering. This section covers all topics included in Paper 1 - Core Physical Geography. Section 2 is colour coded red and matches the next three themes of the syllabus – population; migration; and settlement dynamics. This section covers all topics included in Paper 2 - Core Human Geography. Section 3 is colour coded green and matches the next four themes of the syllabus – tropical environments; coastal environments; hazardous environments; and hot arid and semi-arid environments. This section covers all topics included in Paper 3 - Advanced Physical Geography Options. Section 4 is colour coded brown and matches the last four themes of the syllabus – production, location and change; environmental management; global interdependence; and economic transition. This section covers all topics included in Paper 4 - Advanced Human Geography Options. Topics within each section follow the order of content within the syllabus.

Case studies Case studies in every topic focus on particular locations around the world, providing real-life examples and consolidating the themes being discussed. These diﬀerent locations are shown on the world map on the page opposite.

Now investigate Each chapter also has suggestions of further topics for research, to expand your knowledge and understanding.

Geographical skills The last section, colour coded purple is an illustration and explanation of the many diﬀerent types of data that geographers collect, process and analyse. Many examples of how data can be presented visually are illustrated in this section.

Glossary The key terms are highlighted in the text like this, and are explained in the glossary. These are words and phrases which have speciﬁc meanings in Geography – check out the meaning of geographical vocabulary that you come across.

Locations of case studies used in the book 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

32 35–37 55–58 77–78 82 89–90 105 106–107 111 112 114–115 121–122 135 136–137 140–141 144 153–154 155 173–175 175–177

Inter-basin water transfer: The Aral Sea: Kazakhstan/Uzbekistan Harnessing the River Harbourne: UK Urban climate in Chicago: USA Nevado del Ruiz volcano: Colombia Aberfan mudflow: UK Population growth: China Inadequate food supply: Yemen One-child policy: China Seasonal migration to Goa: India Push and pull factors: Turkey Deadly migration routes: Mediterranean Sea Urbanisation: Fiji Rural economy, Hilmarton: UK Mwandama: Rural issues: Malawi Suburbanisation: Los Angeles and Tyson’s Corner: USA Melbourne Docklands: Australia Slum housing, Mtandire: Malawi City transport infrastructure, Bogota: Colombia Tropical rainforest ecosystem: Papua New Guinea Savanna ecosystem, Queensland: Australia

2,5,13,31 22

15,29

1 37

15 40

28 26

4 18 38

6,8,33 34 32

25,39

36 27

24 14,17

30 16

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Coastal erosion at Wamberal Beach: Australia The Columbia River littoral cell: USA Sand dredging at Diani Beach: Kenya Tourism and coral reef management issues: Timor Leste Managing the effects of earthquakes: Japan Sustainable management of volcanic hazards: Montserrat Sustainable management of areas of mass movement: Malaysia Sustainable management of arid and semi-arid environments, Rajasthan: India The Mojave desert: an arid area in a HIC: USA Beef rearing, an extensive pastoral system: Australia Market gardening, an intensive arable system: UK The management of industrial change: Bangladesh Electrical energy strategy: China The Three Gorges Dam: China Darfur: Sudan Fair Trade coffee: Vietnam The Butler Model, Majorca: Spain Ecotourism in the Galapagos Islands: Ecuador A Transnational Corporation - Toyota The management of development: Morocco

185 186–187 203 204–205 227 228–230 230–231 254 254–255 263–264 264–265 278–281 291–293 294–295 302–307 317 332 334–337 356 362–367 7

1 Hydrology and ﬂuvial geomorphology Amongst the hillslopes and valleys of Earth, water has played a clear part in shaping the landscape. This chapter will look at the hydrological cycle and its interactions between the atmosphere, lithosphere (geological world) and biosphere (living world).

The drainage basin system

figure 1.1 A small tributary river of the upper Amazon Basin.

The drainage basin system is a complex system that is governed largely by the impact of hydrological conditions interacting with geology over time. It is an area of land surrounding a principal waterway and its tributaries on a local scale. The boundary of a drainage basin is known as the watershed and is simply the highest contour of land surrounding a river or stream. Factors such as climate, vegetation, soil structure and land use may inﬂuence the character and geomorphological development of a drainage basin resulting in wide and varied spatial diﬀerences. Drainage basins can vary in size from the most extreme example; the Amazon basin, which covers 40 per cent of South America – nearly 7 000 000 sq km – and contains over 1100 tributaries, to the micro-scale that may contain just one river or stream.

National capital Major town Main town Dam Boundary of Amazon Basin rainforest figure 1.2 The major waterways of the Amazon Basin stretching across the northern part of South America.

10 Hydrology and ﬂuvial geomorphology

figure 1.3 The forested banks of the Amazon River.

Drainage patterns The pattern of streams and rivers within a catchment can vary greatly. Often there are similar characteristics based on the underlying geology and structure of the drainage basin. Here are four common types: • • • •

Dendritic – a tree-like pattern where water may converge (meet) from a variety of directions before joining a main river channel. Rectangular – where the streams and channels follow geological weaknesses and gaps in blocky bedrock. Radial – where water drains away from a central high point, hill or mountain into separate channels. Trellised – where streams follow slopes downhill and converge along areas of eroded rock.

Endorheic drainage basins Endorheic drainage basins are inland basins that do not drain to an ocean. Instead their base level is an inland lake or sea. Around 18 per cent of all land drains to endorheic lakes or seas or sinks. The largest of these consists of much of the interior of Asia, which drains into the Caspian Sea, the Aral Sea and numerous smaller lakes. The drainage basin is known as an open system as water is not confined to a specific location and can move from one state to the next at any given time. The different stages are explored in Figure 1.5 in a simplified systems diagram.

Dendritic

fractures Rectangular

Radial ridge valley

Trellised figure 1.4 Drainage basin morphology

Hydrology and ﬂuvial geomorphology 11

storage in ice and snow

moisture over land condensation

precipitation on land surface runoff (overland flow)

precipitation on ocean

evaporation from land evapotranspiration

freshwater storage

soil layer permeable rock layer

percolation

evaporation

lake thro

ugh

flow

impermeable rock layer

evaporation from ocean lake surface outflow

groundwater outflow ocean water table zone of saturation figure 1.5 The hydrological cycle

Inputs Drainage basins principally have one main input – precipitation (ppt), which includes all forms of rainfall, snow, frost, hail and dew. Water is then stored or transferred in the system for an indeterminate amount of time before its eventual output in the form of evaporation (EVP), evapotranspiration (EVT) and runoﬀ. Precipitation refers to the conversion and transfer of moisture from the atmosphere to the land. Precipitation can be very variable and several factors may impact the hydrology of an area: amount and extent of precipitation, intensity, type, duration and geographical distribution. Storage Storage refers to the parts of the system that hold or retain water for periods of time. They can be open stores on the surface of the land, within vegetation or hidden deep within the rock structure. The amount of time that water is stored for is dependent on the processes acting on it. Interception refers to water that is caught and stored by vegetation. It is aﬀected largely by the size and coverage of plants, with large broadleaved trees catching the most water (in summer). Intercepted water may still transfer through the system using three main mechanisms: • • •

interception loss – water retained by plants and later lost as evaporation throughfall and leaf drip – water that is slowed by running oﬀ and dropping from leaves, twigs and stems stemﬂow – water that runs down branches and trunk to the ground.

Urban areas and areas that have been cleared for cultivation have much lower rates of interception.

12 Hydrology and ﬂuvial geomorphology

input transpiration

evaporation

precipitation output interception

transfer

stemflow/ leaf drip

store

surface storage

surface runoff (overland flow)

infiltration vegetation storage variable level water table

soil moisture storage

throughflow

channel storage

channel flow

percolation groundwater storage

groundwater/ base flow

river discharge

figure 1.6 Systems diagram – inputs, transfers, stores and outputs

When vegetation absorbs moisture directly through its root system it becomes stored within the organism/plant and is called vegetation storage. The amount of water stored relates to the size and variety of plant and the local conditions at any given time. A large leafy and ‘thirsty’ plant will require more than a well-watered shrub. Surface storage is the name given to any parts of the system where water lies above the ground on the Earth’s surface. Within a drainage basin water may naturally accumulate in lakes, ponds and puddles or through human intervention whereby engineering creates structures to contain water such as reservoirs and swimming pools. Surface stores have a high potential evapotranspiration rate as there is a large amount of moisture available with limited cover. Channel storage refers to water that is contained within a river channel or stream at any given time. Groundwater storage refers to water that has become stored in the pores and spaces of underlying rocks. Despite being hidden, this water is fundamentally important to the hydrological system accounting for almost 97 per cent of all freshwater on Earth. Although a significant part of the hydrological cycle, water contained here may be stored for 20 000 years. Any large quantities of water are contained in aquifers. An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or silt) that can be found at any depth. Those nearest the surface are often used for water supply and irrigation. Areas that suffer from a large extraction of groundwater through wells and pumps require good recharge rates (where water stores naturally fill back up). Those areas with little recharge consider groundwater to be a non-renewable resource. Many groundwater reserves are being used at an unsustainable rate too. Groundwater recharge occurs as a result of percolation, infiltration from precipitation, leakage and seepage from the banks and beds of water bodies as well as artificial recharge through from reservoirs and irrigation. In 2013 large freshwater aquifers were discovered under continental shelves off Australia, China, North America and South Africa. They contain an estimated half a million cubic kilometres of low salinity water that could be economically processed into potable (drinkable) water.

Hydrology and ﬂuvial geomorphology 13

Transfers Overland flow is the movement of water over the land, downslope to a body of water. It has two main mechanisms. Where precipitation exceeds the infiltration capacity accumulated water will flow downslope due to the effects of gravity. An alternative mechanism occurs when the soil saturation exceeds its maximum capacity due to groundwater uplifting, base flow, and lateral subsurface water discharges, resulting in the appearance of saturation excess overland flow. Channel flow is the movement of water within a defined channel such as a stream or river. The speed and flow of the water will depend on a variety of factors such as gradient and efficiency; these are considered in more detail in river channel processes and landforms (pages 20–29). Base flow is considered to be the lowest flow within a channel, often occurring due to a lack of precipitation leaving only the influence of water trapped in rocks and soil. It is maintained by groundwater seeping into the bed of a river. The channel is topped up by precipitation events and the arrival of water through other mechanisms such as throughflow, overland flow etc. It is relatively constant but increases following wet conditions. Throughflow refers to the movement of water through the soil substrata. As the soil type of an area is closely linked to the underlying bedrock flow rates through different soil profiles can be varied. Clay-rich soils are known for their water retention whereas sandy loams are characteristically free draining. The influence of land use also plays a part as it can influence soil density and aeration (page 19). Groundwater flow is subsurface water (lies under the surface of the ground) that travels downwards from the soil and into the bedrock through cracks and pores. This process is called percolation. Differing rock types and structures will affect the flow of water into underlying layers, with porous sedimentary/carboniferous rocks such as chalk and limestone being the most effective carriers of water. The layers of rock that become saturated form the phreatic zone (Figure 1.7 (a)) in which the uppermost layer is known as the water table. Where there is a small area of underlying impermeable substrata (aquiclude), water may be held higher up the basin profile as a perched water table (Figure 1.7 (b)). Water that cannot pass through the rock layers will emerge as a spring. Outputs Evaporation is the process by which water is converted to water vapour in the atmosphere. This is most significant where there are large bodies of water such as the oceans and seas and on a local scale – rivers and lakes. Rates of evaporation are dependent on climatic variables such as temperature, humidity and wind speed. Other factors include the

spring

perched water table

aquiclude

river (dry in summer) zone of inte

n rmittent saturatio

te win

able te r t r wa

table r water umme

figure 1.7 (a) Seasonal variation in the level of the water table.

14 Hydrology and ﬂuvial geomorphology

wa ter

unsaturated zone

ta b

le river

figure 1.7 (b) Perched water table

aquifer

amount of water available, vegetation cover, and albedo (reflectivity of the surface). Evaporation rates change throughout the day and with seasonality. Transpiration is the process of evaporation of water from plants through pores (stomata) in their leaves. Broadleaved trees, such as beech, can hold more water and so have greater potential for high transpiration rates. Some species of plant, such as the saguaro cacti, are specially adapted to retain moisture by reducing their rates of transpiration. Evapotranspiration is the combined effect of evaporation and transpiration and represents the major output from the drainage basin system. In humid areas 75 per cent of moisture may be lost in this way and up to 100 per cent in arid areas. River discharge is a measure of the volume of water moving in a river. It can also be used to describe the output of river water from a drainage basin. At its lowest point a river will discharge into an ocean. Although a river cannot change catchments its drainage basin may be part of a larger complex system that links a number of drainage basins. In some cases water may escape from the system by other means not highlighted by Figure 1.6. Some examples may include when geology at lower levels may cause leakage allowing water to seep from one drainage basin to the next; human water management initiatives may also modify the system by creating reservoirs and dams affecting channel flow, by abstracting water for irrigation, domestic and industrial use or through cross-basin transfers to aid water shortages in adjacent areas.

Discharge relationships within drainage basins River discharge A river operates as a main conduit for water within a drainage basin. It is essentially the equivalent route for water, as motorways are for cars, offering the most efficient route for transportation. Precipitated water has a direct influence on the level of water in the river. The quicker the response the greater the influence on the existing flow. Additional water in the form of precipitation will raise the water level above its base level. As water enters the river the river level will rise. After a period with little or no water, river levels will fall. The volume of water moving past a point in a river per given time (usually cubic metres per second/litres per second) is called the discharge. Discharge can be calculated as: Q=A×V Where: Q = discharge, A = cross-sectional area, V = velocity The level of discharge is influenced by the rate of precipitation and the speed at which water is transferred to the river. Variations in discharge A river’s flow is inherently influenced by the characteristics of the area and the prevalent weather conditions acting on it. Different conditions in differing locations may produce very different discharges over the course of a year. This annual variation is known as its river regime. Using data from Sauquet et al. (2008), we can see the huge range in variation both over the year and from region to region throughout France. Rivers of similar characteristics have been categorised into twelve colour-coded types. From the data we can see there are some common trends. For example there is a decrease in summer runoff, with the exception of mountainous rivers to the south and east (see Figure 1.8 (a) and 1.8 (b)).

Hydrology and ﬂuvial geomorphology 15

River groups Group 1 Group 2 Group 3 Group 4 Group 5 Group 6

Group 7 Group 8 Group 9 Group 10 Group 11 Group 12

figure 1.8 (a) Drainage patterns in France. The map shows the drainage basins colour coded to their respective graphs on the facing page.

Storm hydrographs Hydrographs enable us to look at the relationship between rainfall and discharge after each rainfall event as river levels top up and subsequently drop over time. The response of a catchment to a rainfall event may be rapid or gradual depending on many factors (outlined below). The shape of the hydrograph may reflect the speed at which the water has travelled and the obstacles and stores in its way. Hydrographs are particularly important for identifying the potential risk of flooding to an area. There are several key features to any hydrograph. They represent the various stages to the graph and help to identify the nature of the discharge. Most hydrographs show time or duration on the x-axis followed by two scales on the y-axis – one for the rainfall/precipitation and one for discharge. Be sure to identify which is which. 16 Hydrology and fluvial geomorphology

Group 5

0.20

0.15

0.10

0.10 0.05

0.05

0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Group 6

Group 10

0.25

0.20

0.15

0.10

Zref

0.25

Zref

0.10

0.05

0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Group 7

Group 11 0.25

0.20

0.15

0.10

Zref

0.25

Zref

0.25

0.10

0.05

0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Group 4

Group 8

Group 12

0.25

0.20

0.15

Zref

0.25

0.10

Zref

0.10

0.05

Group 3

Zref

0.25

Group 2

Zref

Group 9

0.25

Zref

Group 1 0.25

0.10

0.05

0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

rainfall (mm)

50 40 30 20 10 0

cumecs 10

tle

gen

li ing

storm flow

ris

base flow/groundwater flow day 1

day 2

day 3

day 4

time

rainfall (mm)

long lag time

50 40 30 20 10 0

20 peak precipitation

limb rising

low peak discharge

throughflow

lag time

n ssio

ece

b/r

bankfull discharge

lim

peak flow/discharge

ing

runoff (cumecs) 50

fall

runoff (cumecs) 50

river in flood

figure 1.8 (b) Drainage patterns in France. The graphs show monthly variation in discharge for selected streams throughout France.

storm flow

cumecs

10 base flow/groundwater flow day 1

day 2

day 3

day 4

time

figure 1.9 Hydrographs showing low peak discharge and storm discharge.

Hydrology and ï¬&#x201A;uvial geomorphology 17

Base flow/groundwater flow – this is the ‘normal’ level of water in the channel determined by the groundwater flow prior to a rainfall event. Lag time – this is the period between the peak precipitation and the peak discharge. Peak flow/discharge – this is the maximum river discharge for any given event measured in cubic metres per second m3s-1 (cumecs). Rising limb – this is the part of the graph that initially rises, indicating the increasing level of water as determined by the combined rate of surface runoff, throughflow and groundwater flow following a precipitation event. Storm flow – this is the additional discharge created as a result of a precipitation event. Falling limb/recession – this is the part of the graph that shows the discharge decreasing and river levels falling back towards base level.

As the rain falls within the catchment it takes a variety of routes before some of it enters the river (see Figure 1.5). As that water joins the river the volume of water increases, thus increasing the discharge. Water that rapidly flows into a river will have a more rapid rise in discharge. Water that travels slowly to the river will have a more gradual effect on the level of discharge. Catchment hydrology Catchment hydrology refers to the movement, distribution and quality of water within a drainage basin. Whilst drainage basins vary in form there are common principles that will shape the response of the area to any given event. Infiltration rate Infiltration is the flow of water (precipitation, irrigation) through the soil surface into a porous medium under gravity action and pressure effects. The maximum rate of infiltration for an event is the infiltration capacity. Several factors control the rate of infiltration within the catchment/drainage basin.

The morphology of the drainage basin affects discharge in a number of ways. The larger the drainage basin the greater potential discharge but longer lag time as precipitation is caught over a wider area. Roughly circular shaped basins are more likely to result in a ‘flashy’ rapid response as precipitated water is more likely to reach the river at the same time having travelled an equal distance. Steeper drainage basins will have a short lag time as the influence of gravity will increase the rate of flow to the river.

18 Hydrology and ﬂuvial geomorphology

Types of precipitation Flooding most frequently occurs after prolonged periods of rainfall when soil stores are full and there is less drainage possible. The conditions preceding a rainfall event can be referred to as antecedent conditions. During cold conditions, water may be temporarily stored as snow or ice. This means there is less water circulating through the system. It also means that there may be a sudden release of water during times of thaw. Annual flooding in Bangladesh is largely attributed to the combined effects of monsoonal rain and seasonal snow-melt from the Himalayas to the north. There has been much speculation on the effects of climate change. Though storms are not necessarily increasing in frequency, there does seem to be a correlation with an increasing intensity. Intense storms are more likely to cause floods as the ground is unable to absorb high quantities of water in a limited amount of time. Relief The size and shape of the land affects the rate at which water can flow down it. Slopes with an angle of less than 5o will have significantly greater rates of infiltration. The greater the gradient, the greater the rate of surface runoff as there is less opportunity for infiltration. Higher in the catchment, rivers may cut steep incised valleys acting under the influence of gravity (as they seek to reach the lowest point). As they travel downstream this influence is lessened and rivers erode laterally creating flat, wide floodplains.

Parent material The parent material refers to the underlying geology of an area and the origins of the formed soil. The characteristics of the geology will determine the permeability and ultimately how well the ground will drain. Rock type Rocks can be classified into three types based on their formation. Sedimentary rocks are formed through the deposition of sediment and the subsequent compression as additional layers are deposited above. They often are porous (with air spaces), such as sandstone, or pervious (with cracks and bedding planes), such as limestone. This means that water can pass through sedimentary rocks. Rocks that allow water to pass through them are termed permeable. Metamorphic rocks are sediments and rocks that have been transformed by heat and pressure. The permeability of metamorphic rocks will depend on the nature of the transformation. Igneous rocks are formed by extreme heat and pressure in magmatic environments and are more simply referred to as volcanic rocks. Examples include basalt, usually formed in ocean environments, and granite, more commonly found on land. Rocks such as these do not let water pass through them and are called impermeable. Soil type, structure and density Soil is composed of rock fragments, organic matter, water, air, organic material and organisms in varying proportions. The greater the clay content, the more water retentive the soil is as clay particles bond together tightly restricting the flow of water. A sandy soil is free draining as the larger sand particles provide gaps and spaces for water to pass through. Most soils contain a mix but soils become saturated easily when there are greater proportions of clay. Compare the waves draining on a beach to boggy areas surrounding a river, for example. Often floodplains contain a lot of small particles deposited by floods known as alluvium. Beaches are almost exclusively sand. Drainage density The drainage density refers to the number of rivers and streams in an area. The greater the number of rivers, the more easily the catchment will be able to drain. This may produce a quick rise in the hydrograph and a greater probability of flooding.

low

Antecedent conditions These relate to the previous conditions that have affected an area such as precipitation rates. An area that has experienced a high amount of precipitation may have partially or fully saturated soil, increasing the rate of surface runoff. Dry conditions would allow for greater water storage but too dry may mean the ground has a baked impermeable crust, which makes infiltration difficult. In this scenario water may run off the land creating a flashy response hydrograph.

high

Land use The land use of an area may be hugely influential in determining catchment response. ‘Land use’ simply refers to how the land is used or managed. Urbanisation Settlements are often heavily concreted spaces very different to those on open moorland or arable farms. World urban populations are growing, resulting in greater urbanisation and an increase in the risk of flooding. Water cannot infiltrate through tarmac and concrete and, combined with gutters and drains that channel and direct runoff, water can be carried at great speed to the nearest waterways. Often runoff from roads and urban landscapes contains pollutants and waste that are unnatural to a river environment and this causes damage to the freshwater ecosystem.

T figure 1.10 These diagrams show the relationship between drainage density and discharge.

Hydrology and ﬂuvial geomorphology 19

Vegetation Vegetated areas have a greater capacity to intercept precipitation and absorb soil moisture. The type, nature and extent of vegetation will determine its ability to retain moisture. Estimates suggest that tropical rainforests intercept up to 80 per cent of rainfall (30 per cent of which may later evaporate) whereas arable land may only intercept 10 per cent. In the United Kingdom, large broadleaved deciduous trees have a larger biomass and expansive canopy in the summer months leading to greater interception rates than in winter where intake is greatly reduced due to the loss of leaves in autumn months. Deforestation is an activity widely associated with flooding. The removal of vegetation whether for the clearance of land for development or harvesting of a cash crop often has negative consequences and widespread implications on a river regime. Flows can be considerably faster. In addition, the stability of soil profiles can be compromised by logging trails and disturbed ground with further areas vulnerable to erosion by the fast flowing surface flows. The resultant runoff is often heavily silted, which makes rivers thick and dirty with sediment. Areas heavily reliant on rivers for washing and drinking are the first to suffer. Tides and storm surges The daily rise and fall of the tides affects the relative base level to which a river flows. High spring tides may prevent water from discharging into the sea, increasing the potential for flooding. Low pressure systems such as depressions and tropical storms reduce the amount of air pressure acting on sea level leading to a slight rise in water level at these times. This coupled with strong winds create further pressure on low-lying coastal areas. Storm surges occur when strong wind conditions affect a coastline, forcing waves landward and inland through estuaries.

River channel processes and landforms The long profile The long profile is the name given to the gradient of a river from the start of the river (source) to its mouth. Rivers always work under the influence of gravity, cutting a path downhill through the landscape. The higher up a river’s UPPER COURSE

MIDDLE COURSE

LOWER COURSE

Vertical erosion with hydraulic action, abrasion and attrition dominant processes

Channel is deeper and wider

Channel is at its widest and deepest, and may be tidal

Vertical erosion decreasing in importance, more lateral erosion and deposition

Deposition more important than erosion

cross profiles

characteristics and processes

Height above sea level

Traction and saltation at high flow

500

Load size is large and angular

400

V-shaped valleys

Suspension is the main transportation type

Fine material deposited Large amount of load but the size is very small and very rounded

Load becomes smaller and less angular

300 200 100

Long profile is the change in gradient with distance. It starts off steep but reduces with distance from source, and has a concave profile sea or ocean

0 –100 Source

Increasing distance downstream

figure 1.11 Long and cross proﬁles on a typical river.

20 Hydrology and ﬂuvial geomorphology

Mouth

source is, the higher the gravitational potential. As a result the upper reaches of a 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 tend to expel their energy by eroding laterally across the landscape. A graded profile shows 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 the underlying geology and human influences (such as dams) may distort this idealised view. As water flows downhill under gravity it seeks the path of least resistance. In the higher reaches the river has greater potential energy but channels are often rough and poorly formed. Further downstream channels become wider, deeper and more efficient as more water joins from tributaries and is able to shape a smoother route. The upper course The upper course is a high-energy environment that experiences a high level of erosion and turbulent flow. The source of the river can often be found in boggy upland areas with no distinct channel or form. As water accumulates it starts to carve out shallow paths in the soil and vegetation before descending more rapidly under the influence of gravity. At altitude the combined processes of weathering and fluvial erosion contribute to the high level of bedload (sediments that lie on the riverbed) and large angular material including frost shattered boulders and scree. Partly as a result of the large material, traction (the largest stones, boulders and cobbles rolled along the riverbed by strong turbulent flow) and saltation (a transportational process where smaller bedload such as pebbles bounce along the riverbed) are common.

figure 1.12 Characteristic turbulent ﬂow of the upper course, showing large rock debris.

figure 1.13 A sweeping curve of the middle course. Rivers become more sinuous as they have more energy to expel downstream.

The middle course The middle course is a longer section of river characterised by a decreasing gradient and greater lateral erosion. As a result the valley sides are less incised than the upper reaches and the river starts to become more sinuous (winding). The river itself here becomes more established with a greater number of tributaries bringing additional water. There is a high proportion of suspended load and bedload is smaller and less angular than upstream. The lower course The lower course is the low-lying portion of the river that joins with the sea. It is characterised by wide flat sweeping floodplains and large meander bends. It is the depositional zone of the river, featuring small rounded stones that have been worked on by fluvial action and erosion. There is a high proportion of suspended material in the low profile. Flow A 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 underlying geology. The flow of the river is the manner in which the water travels. There are three types of flow: •

•

Laminar flow is characterised by a smooth horizontal motion often too simplistic for complex natural river environments that have many changes, steps and gradients. A laminar-style flow may be found in carefully managed channellised sections on a relatively small scale where there are few additional influences. Turbulent flow is characterised by a series of erratic horizontal and vertical spiral flows (known as eddies) that disturb the smooth appearance of the water. Turbulent flow is the dominant method of flow in a river

figure 1.14 The lower course where the river joins the sea at the depositional zone.

figure 1.15 Turbulent glacial water in Norway.

Hydrology and ﬂuvial geomorphology 21

•

environment. The amount of turbulence varies depending on the velocity of the flow as well as the influence of friction and the energy available. The greater the velocity, the greater the amount of spare energy after friction and so the greater the turbulence. Helicoidal flow is a corkscrew-like flow that is mainly found as water travels around river bends. It is associated with meanders and the formation of sediment bars and slip-off slopes.

The thalweg is the name given to the path of least resistance where water flows the fastest. In a straight channel it can be found in the middle of the channel under the surface of the water furthest from the influence of friction from the riverbanks, riverbed and the air. On a bend, however, the fastest flow will continue in a straight line before hitting the outside of the bend and being reflected downstream. Factors affecting river velocity The velocity of a river is not determined by one single factor. There are many factors that impact a river’s ability to transport water and sediment downstream. Gradient, efficiency and bed roughness all determine how well the water flows. The differing velocity will in turn affect the erosive and depositional capacity of the river and its potential to shape the channel. Drainpipes and waterslides are built the way they are for an efficient flow to move water quickly. The closer the river is to a smooth semicircular form the more efficient it will be. Man-made channels are often much more efficient than natural ones. The measure of efficiency can be determined by calculating the hydraulic radius (HR). HR =

cross-sectional area wetted perimeter

(the width of the river across the contours of the riverbed)

It is a ratio and has no units.

0.1

0.2

0.3

0.4

velocity isovels in m/sec

0.2 0.1 0.4 0.3

0.4 0.3

0.2

figure 1.16 Cross section showing velocity at a meander.

22 Hydrology and ﬂuvial geomorphology

0.1

Erosion The power of the water and the material that is carried will continually shape and wear away the bed and banks of a river channel. There are four main processes important in ﬂuvial (water) environments: •

•

Hydraulic action is the force of the water pushing into cracks and hitting against the river’s banks. This repeated action weakens the riverbank as air in the cracks is compressed and pressure builds up. Collapsing air bubbles create small shock waves in a type of hydraulic action known as cavitation. Unlike coastal environments where waves may be large and powerful, hydraulic action is a slow and ineffective process of erosion. Corrasion occurs when sediment in the river is thrown into or scraped along (abrasion) the banks and bed of the river. This process is extremely common and is the main form of erosion within a river. During times of high flow or flood the river has a greater capacity to transport larger material, which results in the greatest amount of damage. Potholes may form as stones become trapped in depressions and hollows and are continually swirled around by eddies in the turbulent flow. Attrition is the process by which stones and sediment within the river become increasingly rounded. As material is transported it collides with other objects in the river. The collisions cause the stones to break into smaller pieces and the edges and points of the stones to break off. Corrosion or solution is a continuous chemical process that occurs independently from river flow. Water that has slightly acidic properties, for example as a result of decomposing organic material (humic acid) or acid rain (carbonic acid), will chemically dissolve and weaken certain types of rock. Limestone is composed of calcium carbonate and is particularly vulnerable to corrosion.

Transport In addition to the movement of water, rivers also become important conduits for the transport of sediment. Rivers transport sediment in a number of ways. The mode by which sediment is transported is related to the speed of flow and its size. Unsurprisingly, faster flows can transport larger material. This is perhaps most noticeable in times of flood when large boulders, trees and even cars may be carried by a river. Material carried by a river is referred to as its load. Rivers can only carry so much load depending on their energy. Capacity is the name given to the total load of material actually transported. Competence is the name given for the maximum size of material that a river is capable of transporting. The load is transported by four main processes: •

•

Traction is when the largest stones, boulders and cobbles are rolled along the riverbed by strong turbulent flow. Often these sediments will lie undisturbed on the riverbed until sufficient discharge is reached to displace them. Saltation is where smaller bedload such as pebbles, stones and gravel are lifted and carried temporarily in the flow in a hopping or bouncing motion. As turbulent flow is not constant the river will have varying amounts of energy to lift and carry the load. Suspended load is when very fine particles of sand and silt are carried in suspension in fast flowing water. The faster and more turbulent the water, the greater the amount and size of material that can be transported. Suspended load is easier to see in the lower reaches of a river or after a rainfall event where the water has a muddy brown appearance. Dissolved load or solution is the process by which small dissolved sediments and minerals are transported within the river. They form just a small proportion of the total load but are significant as corrosion (or solution) is constantly occurring.

Hydrology and ﬂuvial geomorphology 23

Deposition If the river no longer has energy to transport material it will be deposited. As the competence (maximum particle size) and capacity (maximum load) to carry material falls the largest boulders will be deposited ﬁrst followed by progressively smaller material. The amount of energy that a river has and the likelihood it will deposit material is closely linked to ﬂow conditions. Deposition is more likely to occur: • • • •

following low periods of precipitation where river levels drop where the river flow meets the sea in areas of slow flow within a channel, such as on meander bends when the load suddenly increases above the capacity, for example following a landslide when the water has carried the material outside of the channel, such as in times of flood.

•

With the exception of material in solution, which will never be deposited, river deposits tend to become smaller and more round closer to the sea. However it must be noted that larger stones may be present along the entire course of the river as the bed and banks are constantly being acted on by other processes such as weathering and erosion. Hjulstrom’s Curve The relationship between particle size and velocity can be seen using Hjulstrom’s Curve (Figure 1.17). The mean or critical erosion velocity curve shows the approximate velocity needed to pick up and transport (in suspension) particles of various sizes. The capacity of the river is responsible for most of the subsequent erosion. The mean fall or settling velocity curve shows the velocities at which particles of a given size become too heavy to be transported and so will fall out of suspension and be deposited. There are three important features of Hjulstrom’s curves: •

The smallest and largest particles require high velocities to lift them. For example, particles between 0.1 and 1 mm require velocities of around 100 mm/sec to be entrained, compared with values of over 500 mm/sec to lift clay and gravel. Clay resists entrainment due to cohesion, gravel due to weight. Higher velocities are required for entrainment than for transport. When velocity falls below a certain level those particles are deposited.

• •

1000 500 100

River velocity (cm/sec)

particles eroded

mean 1 o or critical er

n sio

l oc

i ty oc

particles transported

1 – particles of sand picked up 2 – clay needs a greater velocity as particles stick together 3 – gravel also needs higher velocities due to size and weight 4 – small particles in transport require very little velocity 5 – for larger material only a small drop in velocity may lead to sedimentation

0.5 4

0.1 0.001

0.01 clay

0.1 silt

1.0 sand

10.0 gravel

Particle diameter (mm) figure 1.17 Hjulstrom’s Curve

24 Hydrology and ﬂuvial geomorphology

rve cu

l ve ng tli t e rs ll o fa particles n ea deposited m

10 5

3 rve

u yc

100.0

1000.0

pebbles cobbles boulders

Fluvial features: erosion V-shaped valleys and interlocking spurs The upper reaches of a catchment often experience large seasonal variations and as a result the rate of erosion can vary greatly. Large angular boulders often choke the upper channel, creating more friction and disrupting flow. During times of peak discharge, such as periods of snow-melt, vertical erosion will be high as there is a greater capacity for erosion. Though the generalised image of a V is common, the extent and angle of incision will be dependent on local factors such as rock type. Interlocking spurs As the river flows downstream it may be forced to wind through the landscape creating protrusions of the riverbank in the valley known as spurs. As the river continues to wind downstream in a zig-zag pattern the view along the course of the river may be restricted as the spurs appear to knit together like clasped fingers.

figure 1.18 Interlocking spurs, Oxendale, England, UK

waterfall retreats

hard rock steep-sided gorge develops as waterfall retreats

overhang

position of waterfall after retreat

plunge pool

ridges of hard rock create an uneven slope; this creates rapids

soft rock hard rock

fallen rocks

gorge left by retreat

original position of waterfall

figure 1.19 Gorge formation

Rapids, waterfalls and pools Rapids are areas of high velocity, turbulent flow. They are created by a sudden change in gradient or a narrowing of the river. Contrastingly, pools are areas of slow moving deep water that have low erosive capability and greater deposition. Waterfalls are large steps in the river as a result of differential erosion usually attributed to bands of hard and soft rock. Water flowing over hard rock will have relatively little impact erosively. Once it then meets a band of softer rock there will be greater erosion. Over time the amount of erosion will be so great that a noticeable step in the profile may be created. Continued erosion may cause undercutting of the rock layers eventually resulting in rock collapse. The fallen material is often large and angular and is forced to swirl around scouring out a depression known as a plunge pool. As the process is repeated waterfalls migrate upstream, leaving a deep steep-sided gorge, for example the falls at Niagara are retreating at a rate of 1 m a year. Fluvial features: erosion and deposition Meanders Meanders are created as the result of both erosion and depositional activities. The snake-like path of a river (sinuosity) increases downstream. Sinuosity =

actual channel length straight-line distance Hydrology and ﬂuvial geomorphology 25

figure 1.20 Horseshoe Falls, part of Niagara Falls on the USA/Canadian border.

figure 1.21 Retreat of Niagara Falls, 1678–2015

A low sinuosity river has a value of 1.0 (straight) whereas a high sinuosity river may have a value above 4.0. A meander is the term used for a bend in the river with a sinuosity greater than 1.5. Though no agreed explanation for their formation occurs, it is generally considered to relate to the energy balance of the river and not the result of an obstruction within the channel or ﬂoodplain.

figure 1.22 A sweeping meander

26 Hydrology and ﬂuvial geomorphology

Meander form Meanders have an asymmetric cross section (Figure 1.23). On the outside of the bend, where flow is fastest, erosion deepens the channel. On the inside of the bend, where flow is slower, deposition occurs. Helicoidal flow occurs where surface water flows towards the outer banks while the bottom flow is towards the inner bank. Variations in the flow create differences in the river cross sections. The most characteristic features of meanders are river cliffs and slip-off slopes or point bars. River cliffs are formed on the outside of the bend where erosion is greatest. The combined effect of hydraulic action and abrasion weaken the riverbank causing it to collapse. Over time a steep bank will be formed with some of the collapsed material remaining on the riverbed. Conversely, on the inside of the meander bend where discharge is at a minimum and friction is at its greatest, deposition is greatest. Sediment accumulates to create a gentle sloping bar known as a slip-off slope or point bar. The particles are usually graded in size with the largest material being found on the upstream side of the bar. Riffles and pools are a sequence of alternating fast and slow flows as a result of the differing energy states of the river. Riffles are shallow areas of fast flowing oxygenated water. Pools are deeper areas with slow moving water. Not all meanders have a regular form but they do have several key characteristics: • The meander wavelength tends to be 10 times the channel width (λ ≈ 10 – 14 W). • Riffles and pools are spaced 5–7 times the channel width (riffle spacing ≈ 5 – 7 W or ≈ ½ λ).

• •

The radius of curvature of the bend is proportional to 2–3 times that of the channel width (rc ≈ 2 – 3 W). Meander amplitude is 5–7 times the channel width (MA ≈ 5 – 7 W).

Meanders over time Meanders constantly change and evolve. Whilst these changes may be relatively gradual, the curvature of a meander grows with time. As continued erosion occurs the river cliff will migrate back as deposition on the inside becomes more stabilised, leading to movement of the river across the landscape. Meander bends become more pronounced so that the path of the river no longer becomes the most efficient route. The river may continue to erode the outside of the bend before eroding a shortcut between meander bends, causing a temporary straightening of the channel. Where this occurs a bend may eventually become redundant. Isolated bends will become detached creating a feature known as an oxbow lake or cutoff, which, due to its lack of fluvial input, will dry up. Evidence of past meanders may be visible on the landscape as meander scars. A tributary that runs parallel to a river within the same valley for some distance before eventually joining it is known as a yazoo tributary.

meander scars

slip-off slope

fastest current

bank will eventually collapse

slowest current

deposition on the inside of the bend

lateral erosion moves the meander sideways

figure 1.23 Cross section of a meander showing its asymmetric shape.

meandering, graded stream meander scars oxbow lake

yazoo tributary cutoff point bar

bluffs

alluvial deposits natural levees backswamp figure 1.24 The middle course of a river highlighting the life cycle of a meander and oxbow lakes.

Rejuvination and sea level change The lowest point of a river’s course is known as its base level. In most cases this is the sea but on a localised scale it may be a pond, lake or reservoir. The river is constantly trying to produce the most efficient route to its base level whilst continually being influenced by the energy balance and outside factors. Changes in base level affect the energy balance and a river’s ability to erode. Over our history there have been many changes to our sea levels. During the last interglacial, 125 000 years ago, sea level was approximately 4 metres higher (eustatic rise) than the present day due to thermal expansion and ice melt. During the last ice age, 18 000 to 10 000 years ago, sea level was much lower (eustatic fall) due to thermal contraction and as water was trapped as ice on the land. Sea levels reduced by up to 120 metres on the west coast of England, which encouraged deep vertical erosion. As a result many parts of Britain have very deep estuaries known as rias that were scoured out when the sea level was much lower, such as at Dartmouth in Devon.

figure 1.25 Dartmouth Ria. A ria is a drowned river valley formed in glacial periods with characteristic deep channels.

Hydrology and ﬂuvial geomorphology 27

figure 1.26 An entrenched meander on the San Juan tributary of the Colorado River, USA.

Effect on fluvial features In situations where a meandering river has been influenced by a change in base level then entrenched meanders or incised meanders may form. The distinction between the two forms relates to the speed of erosion. Incised meanders are asymmetrical in shape as they are eroded more slowly. As the river channel erodes vertically as well as laterally it will start to undercut on the outside of the bend creating an overhang in the river cliff. The inside of the bend, due to the continued deposition, will take the form of a gentle sloping bar. Entrenched meanders are formed, geologically, more rapidly. As a result the meanders tend to take a more symmetrical shape as they carve out a deep winding gorge across the landscape such as the Grand Canyon. Entrenched and incised meanders are more visual where they have cut through different layers of bedrock. Gooseneck on the San Juan river, a major tributary of the Colorado River, is a well known example of an entrenched meander heavily influenced by the distorted uplift (or upwarp) of the Monument Plateau. River terraces are areas of higher ground surrounding a river. They are the former floodplains of the river that were carved out when it was higher up, which are now above the current levels of flooding. Due to a change in base level an increase in vertical erosion creates a newly cut river. Fluvial features: deposition Deposition of sediment occurs when there is a decrease in energy or an increase in capacity that makes the river less competent to carry its load. Deposition can occur at any stage along the river but it is most common in the lower reaches.

figure 1.27 The river terraces of the River Dovey, Wales, UK.

Floodplains Floodplains are large areas of flat land surrounding a river channel. They are the areas most susceptible to flooding. Initially cut by a river, a floodplain is made up of a large amount of alluvial deposits (silt) dropped during times of flood. As a result they are often fertile and used extensively for agriculture. As the river spills over the floodplain in times of flood, there is an increase in friction, a loss of energy and resultant deposition of material. Repeated flooding causes the deposits to build up in height forming a series of layers high above the bedrock. The edge of the floodplain is marked by a slightly raised line known as a bluff. Levees When a river floods its banks the coarsest material is often deposited first creating a ridge along the edge of the river channel. Over time more sediments may be added to the ridge thus creating a natural preventative barrier to flooding. In low lying areas such as in Holland and New Orleans artificial levees have been built in response to the threat of flooding.

figure 1.28 Braiding on the White River, Washington, USA.

figure 1.29 The Nile Delta, Egypt, ﬂowing into the Mediterranean Sea.

28 Hydrology and ﬂuvial geomorphology

Braiding Braiding occurs when there is a high proportion of load in relation to the discharge. This may be the result of seasonal changes and snow-melt, such as in the Alps. At times of low flow the river may be forced to cut a series of paths that converge and diverge as they weave through large expanses of deposited material. Braiding begins with a mid-channel bar that grows downstream as the discharge decreases following a flood. The coarse bedload is deposited first. This forms the basis of bars and, as the flood is reduced, finer sediment is deposited. The upstream end becomes stabilised and over time can become vegetated. These islands can alter subsequent flows, diverting the river and increasing friction. Deltas Deltas are formed when large amounts of river load meet the sea and are deposited. Deltas are usually composed of fine sediments that are dropped during low energy conditions and are so called because they are triangular in shape, which is similar to the shape of ‘delta’, the fourth letter of the Greek alphabet. As freshwater and saltwater mix, clay particles coagulate (stick together) and settle to the seabed in a process known as flocculation. The finest sediments are carried furthest and are the first to be deposited as bottomset beds. Slightly coarser material is transported less far and deposited as foreset beds, while the coarsest material is deposited as topset beds.

There are three main types of delta: • Arcuate delta – having a rounded convex outer margin, such as the Nile River. • Cuspate delta – where material is evenly spread on either side of the channel, such as the Ebro Delta, Spain. • Bird’s foot delta – where the sediment is distributed around many branches of the river (distributaries) in the shape of the claw of a bird’s foot, such as the Mississippi Delta.

The human impact The influence of humans on the hydrological cycle Water resources are important to both society and ecosystems. As humans we depend on reliable and clean supplies of freshwater water to sustain our health. We also need water for agriculture, energy production, navigation, recreation and manufacturing. Many of these uses put pressure on water resources and these stresses are likely to be exacerbated by climate change and population growth. In many areas, climate change as well as population expansion is likely to increase water demand, while shrinking water supplies. Spatially, in some areas, water shortages will be less of a problem than increases in runoff, flooding, or sea level rise. Human influences on the hydrological cycle may be both intentional and unintentional. We have been naïve in our approach to resource management and continue to mismanage many of our resources such as water. There are many components to the hydrological cycle and humans can have an impact at each stage, affecting both water quantity and water quality. Water quantity simply refers to the amount of water available. The flows of the hydrological cycle vary both spatially with location – latitude, altitude and continentality – and temporally, through seasonal changes. It has long been documented that the climate has fluctuated and changed since our atmosphere formed some 4 billion years ago, but there is more and more evidence to suggest that human activities on the planet have increased global temperatures by 0.8 oC over the last 30 years bringing about greater disturbances. Whilst our understanding of weather and climate mechanisms has never been better, the unpredictability of the weather means there is greater potential for extreme events such as drought or flooding. Water quality refers to the cleanliness and ultimately the usefulness of water to our societies and environment. Humans are harnessing more water than ever before and not all the practices we use to do this are efficient, clean or sustainable.

figure 1.30 The bird’s foot shape of the Mississippi Delta, USA.

figure 1.31 Water polluted by copper mining at Geamana Lake, Romania.

Hydrology and ﬂuvial geomorphology 29

Precipitation In heavily industrialised areas and urban spaces precipitation rates are as much as 10 per cent higher due to an increased number of pollutants and particulate matter creating a greater extent and frequency of clouds. For moisture to fall as rain, water vapour must attach to small particulate matter in the atmosphere known as hygroscopic nuclei. As water vapour accumulates and condenses to form clouds, droplets of water increase in size before falling under the influence of gravity. According to Colorado’s National Centre for Atmospheric Research (NCAR) there are over 150 legitimate weather modification programmes taking place in 37 countries, though their complexity and cost vary greatly. Cloud seeding is one strategy designed to encourage precipitation. Cloud seeding injects more particulate matter into the atmosphere in order to create rain. Silver iodine, carbon dioxide and ammonium nitrate are used and dispersed either by aircraft or more commonly fired by cannon or rocket into the air. The result of cloud seeding is largely inconclusive. In Australia it has been suggested that precipitation has increased by 10–30 per cent on a small scale and short-term basis. China is investing heavily in the technology with the introduction of 40 000 field operatives. Land use change Urbanisation An increase in urbanisation creates large impermeable surfaces, which reduce the amount of interception and infiltration. Urbanisation has a close relationship with flashy hydrographs. As water runs over impenetrable surfaces and into drains it is carried rapidly resulting in a quicker response in the river, raising levels and increasing flood risk. An increase in urban surfaces increases runoff and the potential for flooding. Deforestation and afforestation The effect of vegetation removal on hydrology and streams, through land clearance, is a common theme on populated landscapes. Now less than 1 per cent of Britain is covered by natural woodland due to the expansive activities of humans. Whether for land clearance, development or crop harvesting, the removal of vegetation can have profound effects on the hydrological balance of an area. Where clearance is large in relation to the vegetative coverage the effects will be heightened. The rates of interception are determined by the type and extent of vegetative cover. Much of the land’s surface has experienced some level of clearance and modification, resulting in widespread deforestation. Deforestation reduces evapotransipiration rates and increases surface runoff, resulting in a flashier response and shorter lag time. Afforested areas will have a greater capacity to absorb moisture and help bind the soil. Afforested areas are largely planted for

figure 1.32 Forest removal, Derbyshire, UK

30 Hydrology and ﬂuvial geomorphology

commercial reasons though there are additional benefits in the form of habitat creation and flood management. Infiltration is up to five times greater under forest compared to pasture. Forested areas intercept precipitation before funnelling it ground-ward. Bioturbation (the reworking of soil by animals, for example earthworms, or plants) is often high in fertile forest with macro-invertebrates constantly aerating the soil. Pore spaces are often larger and more plentiful than pastoral land where the ground is heavily compacted where animals have trodden. Storage Dams and reservoirs Although the impact is relatively small in relation to the rest of the hydrological cycle, the effect of dams and reservoirs on evaporation and evapotranspiration is significant. Large stores of open water such as reservoirs increase the potential for evaporation. Where temperatures are high evaporation rates are also high. Lake Nasser, for example, behind the Aswan Dam, loses up to a third of its water per year due to evaporation. Water loss through evaporation can be reduced by creating underground and covered storage using plastics or by using sand-filled dams, both of which can be impractical for large applications. In warmer environments and drought-prone areas many underground storage containers and water tanks are used. In Africa they are known as jo-jo tanks and in China they are called shuijiao. Water abstraction Water abstraction is the removal of water either temporarily or permanently from lakes, rivers, canals or from underground rock strata. The redirection of this water from the natural flows within a drainage basin can be done for commercial, industrial or domestic purposes. In many countries the use of water resources are closely regulated. In the UK the Environment Agency is responsible for assessing the impact of activities using their Catchment Abstraction Management Strategy to ensure a sustainable approach to water usage. Water abstraction laws in the UK are based on weather and climatic predictions and trends. There are many different reasons for water abstraction including irrigation, groundwater withdrawal and inter-basin transfer/trans-basin diversion.

figure 1.33 Lake Nasser behind the Aswan High Dam, Egypt.

Irrigation Irrigation is used to increase the productivity of an area through water redirection, though the amount of water must be carefully managed to suit the crop. The Ica Valley is a desert area in the Andes and one of the driest places on Earth. The asparagus beds developed there in the last decade require constant irrigation, with the result that the local water table has plummeted since 2002 when extraction overtook replenishment. Two wells serving up to 18 500 people in the valley have already dried up. Traditional small- and medium-scale farms have also found their water supplies severely diminished. Groundwater withdrawal per sector on the Peruvian coast The rate of extraction for large-scale commercial agricultural purposes is rapidly exceeding that of domestic and industrial use. As a result many local people are suffering from a lack of accessible water in their neighbouring aquifers as many large farms redirect the flow in order to ready their produce for export and profit. Agriculture consumes 50 per cent of all water withdrawn. Little of this is for smallscale subsistence farming. Conversely, the reduction in agricultural and industrial extraction in some areas has led to an excess of water at groundwater level. There are several associated problems with this: • • • • •

figure 1.34 Freshly cut asparagus

an increase in spring and river flows surface flooding and saturation of agricultural land flooding of basements and underground tunnels re-emergence of dry rivers and wells chemical weathering of building foundations.

Hydrology and ﬂuvial geomorphology 31

Case Study The Aral Sea

figure 1.35 Aral Sea catchment area

The Aral Sea is one example of how irrigation can have signiﬁcant consequences on an area. Formerly the fourth-largest lake in the world, spanning 68 000 sq km, the Aral Sea has been steadily shrinking since its waters were ﬁrst redirected by Soviet irrigation projects in the 1960s. The loss of water from the Aral Sea to a catchment some 500 km away has meant there has been a reduction in the amount of evaporation and evapotranspiration in the basin, contributing to a lack of cloud cover and resultant rain. The frequency and intensity of rainfall is thought to have declined over the past 30 years. The drying up of the Aral Sea is often considered to be one of the greatest management disasters in history. Between 1954 and 1960 the government of the former Soviet Union ordered the construction of a 500 km-long canal that would take a third of the water from the Amudar’ya River to an immense area of irrigated land in order to grow cotton in the region. Some 5 per cent of the nearby reservoirs and wetlands have become deserts and more than 50 lakes from deltas, with a surface area of 60 000 hectares, have dried up. Although irrigation made the desert bloom, it devastated the Aral Sea.

2001

2015

figure 1.36 The shrinking waters of the Aral Sea.

The blowing dust from the exposed lakebed, contaminated with agricultural chemicals, became a public health hazard. The salty dust blew off the lakebed and settled onto fields, degrading the soil. Croplands had to be flushed with larger and larger volumes of river water. The loss of the moderating influence of such a large body of water made winters colder and summers hotter and drier. As the lake dried up, fisheries and the communities that depended on them collapsed. The increasingly salty water became polluted with fertilisers and pesticides. In 2005 the World Bank and the government of Kazakhstan constructed a 13 km dam at a cost of US$85 million. By 2008 fish stocks had returned to their 1960 levels. In 2008 the North Aral was subject to a US$250 million project to rejuvenate the area, though progress is slow. figure 1.37 Boats in what is now desert around the Aral Sea, Uzbekistan.

32 Hydrology and ﬂuvial geomorphology

Groundwater Human activity has seriously reduced the sustainable potential of groundwater in some parts of the world. If the use of groundwater exceeds the recharge of groundwater, the water table will drop. Many groundwater stores are in a stable state of equilibrium where recharge and discharge are equal. One of the main problems of groundwater abstraction is in coastal areas, namely saltwater intrusion. This is the movement of saltwater into an aquifer that previously held freshwater. For decades many coastal communities around the United States have experienced saltwater intrusion. Overextraction can lead to subsidence. As water is moved from the rock, sediment particles fill pore spaces previously filled with water. The result is a compression of the land and a reduction in height of the land. This can be particularly problematic when occurring under structures and buildings. Railway lines and pipes can be ruptured. Industrial usage Mining Mining can deplete surface and groundwater supplies. Groundwater withdrawals may damage or destroy streamside habitat many miles from the actual mine site. In Nevada, the driest state in the United States of America, the Humboldt River is being drained to benefit gold mining operations along the Carlin Trend. Mines in the northeastern Nevada Desert pumped out more than 580 billion gallons of water between 1986 and 2001 – enough to feed New York City’s taps for more than a year. Mining can affect water quality in a number of ways, for example heavy metal contamination, such as arsenic being leached out of the ground, sulphide-rich rocks reacting with water to create sulphuric acid, chemical agents designed to separate minerals that leak into nearby water bodies, erosion and sedimentation from ground disturbance that can clog waterways and smother vegetation and organisms as well as silting up fresh drinking water. Energy generation Hydropower uses the force of water to turn turbines. This has little impact on the quantity and quality of water as it is largely returned with little change in state. Less sustainable energy uses involve the use of water for fossil fuel and nuclear energy production. In each, water is converted to steam that powers the turbine in order to generate electricity. This water is then returned to surrounding bodies of water, rivers and lakes with a lower oxygen content at differing temperatures, threatening fish populations and freshwater habitats.

Structures like dams can reduce the impact of a flood in downstream areas. Major cities built on floodplains also experience floods. Tides can add to the height of flood waters, increasing the area flooded.

Floods occur in rural areas. They can happen quickly or slowly. Floods occur in urban areas. They can happen quickly or slowly.

figure 1.38 Human inﬂuence on the hydrological cycle.

Hydrology and ﬂuvial geomorphology 33

Several types of data can be collected to help hydrologists predict when and where ﬂoods might occur: •

Monitoring the amount of rainfall occurring on a real-time basis.

•

Monitoring the rate of change in river stage on a real-time basis, which can help to indicate the severity and immediacy of the threat.

•

Knowledge about the type of storm producing the moisture, such as duration, intensity and aerial extent, which can be valuable for determining the possible severity of the ﬂooding.

•

Knowledge about the characteristics of a river’s drainage basin, such as soilmoisture conditions, soil saturation, topography, vegetation cover, impermeable land area and snow cover, which can help to predict how extensive and damaging a ﬂood might become.

In the UK the Met Oﬃce collects and interprets rainfall data and works with the Environment Agency to issue ﬂood watches and warnings as appropriate.

Recurrence intervals refer to the probability of a flood occurring based on past flow states compiled over at least a 10 year period. Often people use them to infer magnitude where a 1 in 100 year flood will exceed that of a 1 in 40 year flood. Hydrologists determine the recurrence interval based on previous flow states and the probability that the discharge will exceed that able to be contained by the channel. A 1 in 100 recurrence interval refers to a 1 per cent probability that the river will reach a certain discharge for that river. Several 100 year floods could still occur within 1 given year as the data is based on averages. A 100 year storm over a catchment may not necessarily equate to a 100 year flood as many factors will influence the rate of drainage.

34 Hydrology and ﬂuvial geomorphology

Causes of flooding Flooding can be classed as an inundation of water covering the land’s surface. Most commonly flooding is the result of excessive precipitation caused by low pressure depressions that bring storm clouds with great vertical extent. Flooding occurs when water exceeds the capacity of a river channel although it can be the result of a rising water table or coastal inundation. In situations where floodwater travels at great speed there is increased likelihood of damage. In the case of the Boscastle flood (2004), the extreme nature of the flood uprooted trees and carried cars into a narrow channel, further exacerbating the flood. Prediction: forecast and warning Floods are considered the most serious type of natural disasters in the world due to their frequency and intensity affecting widespread populations. On average flooding contributes to 10 000 deaths per year globally with projections showing an increase due to climatic instability and population growth. Much of modern flood prediction utilises technology and relies on computer models and simulation software that use algorithms (mathematical formulas) based on the characteristics of an area. The use of precipitation data as well as relief, land use and saturation rates may all be used to help forecast flow rates from a few hours to a few days. Due to recent technological advances such as greater computing capability, reduced errors and better physical modelling, more effective use of data, flood forecasting and warning has never been better. However, despite this, due to the unpredictable nature of our weather there is still a high percentage of risk in many areas. Satellites, radar and climate modelling have all helped to track global weather systems and statistical models are used with flood histories to try to predict the results of expected storms. In the UK the Environment Agency has thousands of monitoring stations across many major river networks. Most of the measurements used to make predictions are taken electronically by sensors in the river, stored on site and then automatically sent back to databases used by forecasting systems. River and seawater level measurements are now also sent from telemetry systems and published online. Despite this, due to the flashy nature of many of our river systems, many properties in England and Wales have less than six hours of flood warning time. In the case of Boscastle in 2004, the town had less than three hours’ warning. Scale and impact Large drainage basins often provide greater opportunity for warning as the water has further to travel, delaying its impact. In the case of the Brahmaputra and Ganges rivers that run into Bangladesh, bringing meltwater down from the Himalayas, settlements may have up to 72 hours to prepare for a flood event. However the extent of the flood has the potential to be more severe. In the 2007 Bangladesh flood 1000 people lost their lives and 9 million more were made homeless. Prevention and amelioration Extreme weather events only become hazardous when there is a population that may be affected. As the global population grows more and more people are marginalised and forced to live in hazardous areas simply due to a lack of space. This, combined with the greater frequency and intensity of some weather events, increases a population’s vulnerability and their capacity to cope. Often in Middle Income Countries (MICs) economic losses exceed social losses as more and more buildings are built on floodplains. Floodplains are desirable places to build because of their building potential as easily accessible flat land. However this is not without risk. Flood protection can take a number of forms, such as loss-sharing adjustments and event modifications. Loss-sharing refers to mechanisms designed to help cope with a flood. They include insurance payments and disaster aid, the latter of which may take the form of money, equipment and technical assistance. In MICs insurance is an important loss-sharing strategy though not all houses will be eligible for insurance and many homeowners underestimate the impact of flood damage.

Event modifications refer to actions that limit the ability of the flood to do damage and impact on people’s lives. River management Rivers can be managed in a variety of ways but are most commonly managed to minimise flood risk. There are several approaches to river management that can be categorised into hard engineering and soft engineering. Hard engineering requires the use of rock or concrete structures that have been purposely constructed to protect an area. Often these are less in keeping with the natural aesthetics of an area but are much more responsive to flood risk and erosion though not without consequence. Types of hard engineering include dams, channelisation, levees, storm drains and culverts, and barrages. Channel modification is the term used to describe a change in stream flow as a result of human activities. In many cases channel modification is the result of hard engineering and channelisation but in some instances channel modification may include a softer approach and the inclusion of natural features such as riffles and pools. Soft engineering tends to follow a more sensitive approach to maintaining and controlling river flow. Approaches seek to utilise the natural environment where possible and use natural and local materials to modify the river whilst still maintaining its character.

Case Study River Harbourne: Harnessing the Harbourne As far back as 1938 the rural Devon village of Harbertonford has recorded regular flooding. In the past 60 years the village has been flooded 21 times. The River Harbourne flood defence scheme was constructed in 2002 to combat regular flooding of properties and access roads to the village. Though not a large scale construction, it is perhaps one of the best examples of sustainable river management in Southwest England. Flow in the River Harbourne varies from less than 1 cumec at low flows, to 28 cumecs for a 10-year flood flow, through to 300 cumecs for a PMF event (Probable Maximum Flood). The flashy nature of the catchment means there is little warning for the residents of the village to prepare for the flooding and the misery it may cause. One elderly resident of the village had resorted to living solely on the upper floor of her house

Examples of soft engineering approaches include afforestation, washlands and riffle and pool sequences. Afforestation refers to the planting of water tolerant trees to stabilise soil and slopes whilst increasing the potential for interception and absorption. Though not as aggressive as many hard engineering techniques they often are utilised as part of an integrated management strategy which has the added benefit of habitat construction. Washlands are areas of land that are periodically allowed to flood in order to reduce pressure on settlements further down river. The land is often agricultural where loss of earnings may be in some part subsidised. Riffles and pools can be ‘manufactured’ much like a weir to encourage the river to respond differently. Fast flowing areas can be created to move water quickly from an area and pool sequences can be used to reduce the erosive capacity.

figure 1.39 The Palmer Dam is an earth mound dam designed to control the ﬂow of water entering Harbertonford, South Devon.

Hydrology and ﬂuvial geomorphology 35

Why does the river flood? The River Harbourne is a small river tributary of the River Dart, in Devon. There are a number of reasons for flooding. Physical factors • There has been an increased frequency in the number of intense rainfall events. • The river starts 350 m above sea level on the impermeable granite bedrock of Dartmoor. • Dartmoor receives 2020 mm of rainfall annually, twice as much rain as lower surrounding coastal areas. • From the moor the river cuts through steep narrow valleys on to slate bedrock descending 300 m in 12 km. • For the size of catchment the river has a high drainage density. • The village of Harbertonford lies at the confluence of three rivers – the River Harbourne, the Harberton Stream and the Yeolands Stream. Human factors • Many properties are built on the low-lying floodplain in the central area of the village. • The A381 road has been widened over the years to cope with traffic pressures, thereby increasing the amount of runoff flowing directly to the river. • Traditionally some water was extracted along mill leats to power the local mills, which have since closed. How is the river managed? Harbertonford is designated as a Conservation Area and several listed structures, including the village bridge, are contained within it. Atlantic salmon, bullhead, sea trout and brown trout occur in the river and protected species are also present within the catchment, including otter and common dormouse. With this in mind it was important that any flood management works must be sensitive to the environment. The river is managed using a variety of hard and soft engineering techniques. The aim of the scheme was to provide a range of flood defence measures whilst enhancing the local environment. As a result it was decided that the scheme should use natural local materials where possible in keeping with the surroundings with minimum need for maintenance. The scheme has two main features – an upstream flood storage reservoir, and flood defence works through the village. This option has reduced the risk of flooding from one in three years to a minimum of once in 40 years. Upstream • Wetland area and flood storage area: 1 km upstream from the village of Harbertonford a wildlife area was created containing flood-resistant trees and shrubs. The area directly upstream from the dam will become a 41 000 sq m water storage area in times of flood. Local schoolchildren will monitor the afforested area as part of an ongoing partnership. • The Palmer Dam: Built to control the flow of the river, this earthen mound was constructed using locally excavated materials. The dam gates can be controlled to restrict river flow in times of flood. A culvert was created to allow the free movement of fish up and downstream of the dam, whatever the flood conditions.

figure 1.40 Students measuring the channel at Harbertonford village green.

36 Hydrology and ﬂuvial geomorphology

Through the village • Bed-lowering: In order to keep the aesthetic quality of the central village green, the riverbed was lowered to increase the river’s carrying capacity without the need for ﬂood walls. • Channelisation: Throughout the lower sections of the village, along Bow Road, a 200 m wall has been created to protect the residential area from overtopping. The river is now twice as wide. The wall on the bend of the river is reinforced to reduce erosion.

• •

•

Storm drains: Storm drains have been added to reduce the impact of flood water entering the main channel from Harberton Stream. Riffles and pools: Due to the extensive work a system of riffles and pools were created to maintain the river’s natural flow whilst providing habitats for macro-invertebrates. New culvert: A new culvert to allow water to flow under the main road was installed to relieve pressure on the existing drainage network.

figure 1.41 Plan of the Harbertonford ﬂood defence scheme.

Flood hazard mapping Food hazard mapping is used to identify areas that are susceptible to flooding when the discharge of a stream exceeds the bankfull stage. Using historical data on river stages and the discharge of previous floods, along with topographic data, maps can be constructed to show areas expected to be covered with floodwater for various discharges or stages. They can also be used to highlight properties and infrastructure at risk, which allows planners and insurance companies to produce cost benefit analysis.

now investigate 1

Suggest reasons why a hydrograph for one location will experience changes over time.

Suggest reasons why two hydrographs in adjacent catchments may show diﬀerent characteristics for the same rainfall event.

Hydrology and ﬂuvial geomorphology 37