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

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Contents

Introduction

How to use this book

Locations of case studies used in the book

1: Hydrology and fluvial geomorphology

The drainage basin system

Discharge relationships within drainage basins

River channel processes and landforms

The human impact

2: Atmosphere and weather

Diurnal energy budgets

The global energy budget

Weather processes and phenomena

The human impact

3: Rocks and weathering

Plate tectonics

Weathering

Slope processes

The human impact

4: Population

Natural increase as a component of population change

Demographic transition

Population-resource relationships

The management of natural increase

5: Migration

Migration as a component of population change

Internal migration (within a country)

International migration

The management of international migration

6: Settlement dynamics

Changes in rural settlements

Urban trends and issues of urbanisation

The changing structure of urban settlements

The management of urban settlements

7: Tropical environments

Tropical climates

Landforms of tropical environments

Humid tropical ecosystems and seasonally humid tropical ecosystems

Sustainable management of tropical environments

8: Coastal environments

Coastal processes

Characteristics and formation of coastal landforms

5

6

7

8–37

10–15

15–20

20–29

29–37

38–59

40–41

41–48

48–52

52–59

60–85

62–68

68–74

74–79

79–85

86–107

88–96

96–98

98–106

106–107

108–129

110–119

119–124

124–129

114–115

130–155

132–137

137–147

147–153

153–155

156–177

158–162

162–166

166–173

173–177

178–205

180–187

187–197

3


Contents

Coral reefs

Sustainable management of coasts

9: Hazardous environments

Hazards resulting from tectonic processes

Hazards resulting from mass movements

Hazards resulting from atmospheric disturbances

Sustainable management in hazardous environments

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

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

12: Environmental management

Sustainable energy supplies

The management of energy supply

Environmental degradation

The management of a degraded environment

13: Global interdependence

Trade flows and trading patterns

International debt and international aid

The development of international tourism

The management of a tourist destination

14: Economic transition

National development

The globalisation of economic activity

Regional development within countries

The management of regional development

15: Geographical skills

Diagrams and graphs

Maps

Satellite images and aerial photographs

Data types

Glossary

Index

Acknowldegements

Key concepts

197–201

201–205

206–231

208–215

215–219

219–225

225–231

232–255

235–242

242–250

250–254

254–255

256–281

258–268

268–271

271–278

278–281

282–307

284–291

291–295

295–302

302–307

308–337

310–318

318–324

324–334

334–337

338–367

340–351

351–359

359–362

362–367

368–384

370–373

373–380

380–380

380–384

385–399

400–416

417–418

419

4


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.

5


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.

6


1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Locations of case studies used in the book

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

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

22

15,29

3

15

2,5,13,31

37

40

11

10

1

28

6,8,33

34

32

25,39

26

35

7 9

36

38

4 18

23

14,17

27

24

20

19

12

30

16

21

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).

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

The drainage basin system

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.

Rectangular

Dendritic

Radial

fractures

ridge

valley

Trellised

figure 1.4 Drainage basin morphology

Hydrology and fluvial geomorphology 11


storage in

ice and snow

precipitation

on land

surface runoff

(overland flow)

moisture over land

evapotranspiration

evaporation from land

condensation

precipitation

on ocean

soil layer

permeable

rock layer

impermeable

rock layer

percolation

freshwater

storage

lake

throughflow

evaporation

lake

surface outflow

evaporation from ocean

water table

zone of saturation

groundwater outflow

ocean

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


transpiration

evaporation

precipitation

input

output

interception

transfer

stemflow/

leaf drip

store

surface storage

surface runoff

(overland flow)

infiltration

vegetation

storage

soil moisture

storage

throughflow

channel storage

channel flow

variable level

percolation

water table

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

unsaturated zone

river

(dry in summer)

zone of intermittent saturation

winter water table

summer water table

water table

river

aquifer

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

figure 1.7 (b) Perched water table

14

Hydrology and fluvial geomorphology


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


0.25

Group 1

0.25

Group 5

0.25

Group 9

0.20

0.20

0.20

Zref

0.15

0.10

Zref

0.15

0.10

Zref

0.15

0.10

0.05

0.05

0.05

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

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

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

0.25

Group 2

0.25

Group 6

0.25

Group 10

0.20

0.20

0.20

Zref

0.15

0.10

Zref

0.15

0.10

Zref

0.15

0.10

0.05

0.05

0.05

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

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

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

0.25

Group 3

0.25

Group 7

0.25

Group 11

0.20

0.20

0.20

Zref

0.15

0.10

Zref

0.15

0.10

Zref

0.15

0.10

0.05

0.05

0.05

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

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

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

0.25

Group 4

0.25

Group 8

0.25

Group 12

0.20

0.20

0.20

Zref

0.15

0.10

Zref

0.15

0.10

Zref

0.15

0.10

0.05

0.05

0.05

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

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

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

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

runoff

(cumecs)

50

runoff

(cumecs)

50

river in flood

peak

flow/discharge

bankfull

discharge

40

40

falling limb/recession

rainfall (mm)

50

40

30

20

10

0

30

20

10

cumecs

long lag time

gentle rising limb

low peak

discharge

day 1 day 2 day 3

time

storm flow

base flow/groundwater flow

day 4

rainfall (mm)

50

40

30

20

10

0

30

20

peak

precipitation

10

rising limb

lag time

cumecs

throughflow

storm flow

day 1 day 2 day 3

time

base flow/groundwater flow

day 4

figure 1.9 Hydrographs showing low peak discharge and storm discharge.

Hydrology and fluvial 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 m 3 s -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.

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 5 o 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.

18

Hydrology and fluvial geomorphology


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.

low

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.

D

T

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.

D

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

cross profiles

characteristics

and processes

Height above sea level

500

400

300

200

100

0

Vertical erosion

with hydraulic

action, abrasion and

attrition dominant

processes

Traction and

saltation at high

flow

Load size is large

and angular

V-shaped valleys

Channel is deeper and

wider

Vertical erosion

decreasing in

importance, more

lateral erosion and

deposition

Suspension is the

main transportation

type

Load becomes smaller

and less angular

Channel is at its widest and

deepest, and may be tidal

Deposition more important

than erosion

Fine material deposited

Large amount of load but the

size is very small and very

rounded

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

–100

Source Increasing distance downstream Mouth

figure 1.11 Long and cross profiles on a typical river.

20

Hydrology and fluvial geomorphology


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.

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 1.12 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.

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).

cross-sectional area

HR =

wetted perimeter

It is a ratio and has no units.

(the width of the river across the contours

of the riverbed)

0.2 0.4 0.2

0.1 0.3 0.4 0.3 0.1

velocity isovels in m/sec

0.4

0.30.2

0.1

figure 1.16 Cross section showing velocity at a meander.

22

Hydrology and fluvial geomorphology


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

River velocity (cm/sec)

500

100

50

10

5

1

0.5

0.1

0.001

2

particles

transported

clay

4

0.01

silt

figure 1.17 Hjulstrom’s Curve

particles

eroded

1

mean or critical erosion velocity curve

3

particles

deposited

mean fall or settling velocity curve

0.1 1.0 10.0 100.0 1000.0

sand gravel pebbles

cobbles boulders

Particle diameter (mm)

5

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

24

Hydrology and fluvial geomorphology


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

overhang

plunge pool

steep-sided gorge

develops as waterfall

retreats

ridges of hard rock

create an uneven slope;

this creates rapids

position of waterfall

after retreat

fallen rocks

soft rock

hard rock

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.

actual channel length

Sinuosity =

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

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).

26

Hydrology and fluvial geomorphology


• 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).

slip-off

slope

fastest

current

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.

deposition on the

inside of the bend

slowest

current

lateral erosion moves

the meander sideways

figure 1.23 Cross section of a meander showing its

asymmetric shape.

bank will

eventually

collapse

meander

scars

meandering, graded

stream

meander

scars

oxbow lake

yazoo

tributary

cutoff

bluffs

point bar

alluvial deposits

natural levees

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.

backswamp

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 o C 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

figure 1.35 Aral Sea catchment area

The Aral Sea

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.

Tides can add to the height

of flood waters, increasing

the area flooded.

Major cities built on

floodplains also experience

floods.

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.

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.

36

Hydrology and fluvial geomorphology


• 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

2

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

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