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
4
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.
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
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
3
15,29
1
37
15
40
10
11
28
26
7
35
9
4 18
38
6,8,33
34
32
25,39
36
27
23
19
24
14,17
20
12
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
21
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
s
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
0.20
0.20
0.15
0.15
0.15
0.10
0.10
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
Group 6
Group 10
0.25
0.25
0.20
0.20
0.20
0.15
0.15
0.15
0.10
Zref
0.25
Zref
0.10
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
Group 7
Group 11
0.25
0.20
0.20
0.20
0.15
0.15
0.15
0.10
Zref
0.25
Zref
0.25
0.10
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
Group 4
Group 8
Group 12
0.25
0.25
0.20
0.20
0.20
0.15
0.15
0.15
Zref
0.25
0.10
Zref
Zref
0.10
0.05
Group 3
Zref
Zref
0.25
Group 2
Zref
Group 9
0.25
Zref
Zref
Group 1
0.25
0.10
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
rainfall (mm)
50
40
30
20
10
0
mb
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
20
50
40
30
20
10
0
20
peak
precipitation
limb
rising
low peak
discharge
throughflow
lag time
n
ssio
30
ece
30
b/r
40
bankfull
discharge
lim
40
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 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 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
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
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
2
50
mean 1
o
or critical er
n
sio
ve
l oc
it
i ty
oc
particles
transported
1
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
5
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.
2
Suggest reasons why two hydrographs in adjacent catchments may
show different characteristics for the same rainfall event.
Hydrology and fluvial geomorphology 37