This document discusses river processes and landforms. It begins by explaining the hydrological cycle and components of a drainage basin such as precipitation, evapotranspiration, surface runoff and groundwater flow. River discharge is influenced by several factors like basin size, geology and land use. Meanders, floodplains, levees and deltas are landforms created by fluvial erosion and deposition. The document also examines causes of flooding and different flood management strategies.

1. Rivers, Floods and
Management
AQA

2. Contents
Hydrological cycle and river discharge
River processes
Profiles and characteristics
Landforms of fluvial erosion and deposition
Causes and impacts of flooding
Flood management strategies
Exam questions

3. The drainage basin
Drainage basin or
catchment area
What is a drainage basin?
An area of land (also called the
catchment area) drained by a river and
its tributaries
What is the name for the
boundary which divides one
drainage basin from another?
The watershed
mouth

4. The drainage basin as an open system
The drainage basin forms part of the hydrological cycle, and can be
described as an ‘open’ system involving a series of;
Inputs
Ways in which water enters the system
Outputs
Ways in which water leaves the system
Can you define these terms?
Stores
Ways in which water is held in the system
Transfers
Ways in which water is moved through and within
the system

5. Elements of the system
Name each store Name each transfer Name the Input Name each Output
Transpiration
Precipitation
Throughfall +
stemflow
Interception by
vegetation
Infiltration
Throughflow
Soil water storage
The water
table
(position
varies)
Groundwater
storage
Surface (overland)
runoff
Evaporation
Percolation
Groundwater
(base) flow
Saturated or impermeable rock
Surface
storage lakes,
rivers, sea,
depression
storage

6. The water balance
This is the balance between inputs
and outputs in the drainage basin.
It is expressed as:
P + Q +E (+/-), where;
P = precipitation
Q = run-off
E = evapotranspiration
(+/-) = change in storage
The graph shows the main features of the
water balance – average monthly precipitation
and potential evapotranspiration levels

7. Understanding the water balance graph
In which month(s) is there a water
surplus, and why is this?
Jan, Feb, March, April, Oct, Nov, Dec,
when precipitation exceeds potential
evapotranspiration
What is soil recharge and when does
this occur?
The replacement of soil moisture lost
during drier periods; during autumn in
the UK (Oct and Nov)
Field capacity is attained after soil recharge. What exactly is field capacity?
It’s the maximum amount of water soil can hold

8. Understanding the water balance graph
Why is a water deficit not shown
on this graph?
In the UK, precipitation (nearly)
always exceeds
evapotranspiration
How might water balance graphs be a) useful?, and b) difficult to construct?
They can be useful in predicting river regimes and managing drainage
basins. It is however difficult to accurately measure evapotranspiration

9. Discharge and the storm hydrograph
• A hydrograph
shows changes in
a river’s discharge
over time
• A storm
hydrograph shows
this information
following a rainfall
event
Peak
?
discharge
Discharge
(cumecs)
Rising
?
limb
Lag time
?
rainfall
Falling
?
limb
Storm
?
flow
Base
?
flow
• Name the main
features of the
hydrograph
Time
(hours)

10. Factors affecting discharge
Porosity of soils
underlying basin
Geology:per
meability of
underlying
rock
Drainage
density
Drainage basin
relief
Land use
What factors affect river
discharge?
Precipitation:
intensity and
duration. Antecedent
rainfall.
Size and
shape of
drainage
basin
Vegetation: type
and seasonal
variations in
cover.
Deforestation

11. How do these factors affect the storm hydrograph?
Factor
Effects on Hydrograph
High intensity, long duration of
rainfall, or antecedent rainfall
Steep rising limb as infiltration capacity of soil exceeded
Snow melt
Greatly increased discharge, especially if ground frozen
Porous soils and / or permeable
rock
Less steep or ‘flashy’ hydrographs
Impermeable rock / frozen
ground
Reduced lag time and steeper rising limb
Small drainage basin
Faster response, so shorter lag time and steeper rising limb
Elongated drainage basin shape
Slower passage to river, so longer lag time
Steep slopes within drainage
basin
Faster passage to river, so shorter lag time and steeper
rising limb
Summer vegetation
Interception higher - slower response, peak discharge lower
Deforestation
Faster response and higher peak discharge

12. Urbanisation and the storm hydrograph
Steeper rising limb
due to impermeable
surfaces
Higher peak flow as less water is
‘stored’; more water reaches the
river
discharge
(cumecs) and
rainfall (mm)
Shorter lag time as water
quickly reaches the channel via
surface runoff, through drains,
sewers etc
Rural
time
Urban

13. River processes
FLUVIAL EROSION
The break-up of rocks by the
action of the river
TRANSPORTATION
The movement of eroded
material
DEPOSITION
The laying down of material
which has been transported
by the river
Abrasion /
corrasion
Attrition
What are the
main processes
of fluvial
erosion?
Corrosion /
solution
Hydrauli
c action

14. Definitions
Can you define the following terms?
What effect will each have on the river?
EROSIONAL
PROCESS
DEFINITION
EFFECT ON
RIVER
Corrasion /
Abrasion
Scraping, scouring and rubbing action
of the load carried by the river
Attrition
Process where pebbles knock together Sediment load size
and get smaller and more rounded
decreases
Hydraulic action
Removal of loose
Frictional drag and pressure created by material - river
moving water as it is forced into cracks banks undercut and
collapse
Corrosion
River banks wear
away
Where the naturally weak acidic water An increase in the
dissolve some carboniferous rocks
dissolved load
such as limestone and chalk
(solution)

15. River transportation
A river transports it’s load in four main ways. Can you describe them?
Suspension: Very
small particles of
sand and silt (0.001
– 0.99 mm diam)
carried in the flow
Traction: Large stones
and boulders (> 100 mm
diam) rolled along river
bed
Saltation: Small stones
(1.0 – 99.99 mm diam)
bounced along river bed
Solution: Dissolved
minerals within
water

16. Deposition
In flood
conditions, when
a river overflows
onto its
floodplain
When river discharge is
reduced due to a period of
dry weather
Under what conditions will a
river deposit its load?
There is
shallow
water, e.g.
on the inside
of a
meander
bend
When there is a decrease in
the gradient and / or velocity of
the river e.g. at river mouth, or
when entering a lake
When
there is
an
increase
in the size
of the
sediment
load
caused by
a
landslide
or
tributary
delivering
larger
particles

17. The Hjulström curve
The Hjulström curve shows
the relationship between
river velocity and the size of
particles which can be
eroded, transported and
deposited.
F. Hjulström collated data
from 30 experimental studies
into the competence of
different flow velocities
(competence is the maximum
particle size which can be
transported at specific
velocities)
2
3
1
4
1. Silt/sand are picked up (entrained) at the lowest
velocities.
2. Clays are as difficult to pick up as pebbles although they are small particles they are very
cohesive and the claybed is very smooth.
3. Large boulders are dropped very easily.
4. Clay particles can be transported in suspension at
very low velocities.

18. The long profile
•
The long profile illustrates changes in altitude from the source of
a river along its entire length to the mouth
•
The idealised model is smoothly concave, with a steeper
gradient in the upper course becoming progressively gentler
towards the mouth
How would the processes of
erosion, transportation and
deposition change from A, through
B, and to C?
A
Height (m)
B
C
60
Distance from sea (km)
0

19. The long profile – changing processes
Erosion
A (Upper
course)
B (Middle
course)
C (Lower
course)
Hydraulic, attrition.
Vertical erosion
dominant
Transportation
Deposition
Mostly large boulders.
Some in suspension +
solution
Large bed-load only
Mostly attrition, some
hydraulic. Vertical
erosion decreases +
lateral begins
Saltation and traction of
smaller bed-load.
Suspension increased.
Some in solution
Coarser material
builds up in braiding,
slip-off slopes and
floodplain
Some lateral erosion
on outside of
meanders
Smaller sized bedload of sand and
gravel, transported in
suspension
Mostly fine material
deposited on levees,
floodplain and slip-off
slopes

20. Bradshaw’s model
Upstream
Downstream
discharge
occupied channel width
water depth
water velocity
load quality
load particle size
channel bed roughness
gradient
Bradshaw’s model
illustrates the main
changes expected
down the long
profile of a river

21. The changing cross-profile
A
Height (m)
B
C
60
Distance from sea (km)
A
B
Upper course
Middle course
0
C
Lower course

22. Landforms of fluvial erosion
What are the
features of
fluvial erosion?
Erosional
Features
Upper
Middle and
course
lower course
Waterfalls
Potholes
& rapids
River
Meanders
terraces
Interlocking
Ox-bow
spurs
lakes

23. V-shaped valleys and interlocking spurs
Predominantly vertical erosion
results in a v-shaped valley.
In areas of resistant rock (e.g. on
Exmoor), meandering streams and rivers
will be incised into the landscape, forming
interlocking spurs.
Interlocking
spurs
Rapids are formed by
variations in rock resistance

24. Potholes
Potholes provide evidence of fluvial erosion
What is the most dominant type of
erosion here?
Abrasion
Where there are depressions /
fissures, fine particles and larger
boulders (‘grinders’) may become
trapped and swirled around by the
current
Bourke’s Luck Potholes, South Africa
In resistant rock, potholes require
hundreds to thousands of years to
form

25. Waterfalls
Where do waterfalls form?
Usually where there is varying resistance in the types of rock or where
there is a fault running across a river.
Which processes operate?
Soft rock
Hard rock
River
Usually there is considerable
hydraulic action due to the
falling water. Abrasion is also
likely to occur to create the
plunge pool at the base of the
waterfall
Soft rock
Broken pieces
of hard rock
Which feature is formed as
the waterfall retreats
upstream?
A gorge or canyon

26. High Force Waterfall, upper Teesdale,
Yorkshire
•
Tallest waterfall in England at 22m
high
• 500m gorge downstream
How was it formed?
Resistant band of igneous rock (Whin
Sill) overlying softer sandstone,
limestone, shale and coal seam
Water erodes softer rock more quickly,
creating an overhang and plunge pool
This eventually collapses and the
waterfall retreats upstream

27. Rejuvenation
Significant breaks in slope (Knick points) along a river’s long
profile may be due to rejuvenation.
What is rejuvenation?
It occurs when there is a fall in sea level (relative to land) or the land surface rises.
Vertical erosion increases and, starting from the sea, the river adjusts to the new
base level. The knick point (where the old profile joins the new) thus moves upstream
knick point
original long profile
new long profile
What feature will be found here?
A waterfall
original sea level
relative fall in sea
level (or rise of land
surface)
new sea level

28. Rejuvenation
Rejuvenation may also be caused by the sea eroding through and
creating a breach in the coastal geology, e.g. River Lyn on the north
Devon coast.
Knick point waterfalls
Original course of River Lyn
Breach in
coastal
geology
‘Valley of
the rocks’
Present – day village
of Lynmouth

29. Meanders
• Sinuous bends in the river
• Surface flow to outer bank and subsurface return to inner bank = helicoidal
flow
• Velocity across the meander varies and is
related to depth
• The fastest flowing water (the ‘thalweg’) is
near the outside of the bend, where the
water is deepest
• Here, erosion occurs creating a river cliff
• On the shallower inside slower moving
water allows deposition to occur and a
slip-off-slope or point bar forms
0.2 m/sec
0.1 m/sec
Straight sections contain riffles or
bars in the middle of the channel,
where a ridge of bed load has been
deposited in the middle of the river’s
bed because the water velocity is
slower here.
Outside of
meander
Inside of
meander

30. Ingrown and entrenched meanders
Renewed energy from rejuvenation results in increased vertical erosion
and incised (deepened) meanders
When incision is less rapid and lateral erosion is occurring, meanders become
ingrown, e.g. the River Wye at Tintern.
When incision is rapid and
vertical erosion dominates,
Steep river
an entrenched meander is
Inside of
cliff
formed, with steep sides
meander slopes
and a gorge-like
gently
appearance
What are the differences in
form between entrenched and
ingrown meanders?
FLOW
Entrenched meanders have
a symmetrical cross-profile
(rapid uplift) whereas
ingrown meanders are
more asymmetrical (slower
uplift)

31. Meanders
What are these
features, and
what do they
represent?
Ox-bow lakes.
These show the
former course of
the river
Can you remember how to describe the
formation of ox-bow lakes?
Erosion (E) and
deposition (D)
around a
meander (a
bend in a river)
More erosion
during flood
conditions.
The meander
becomes
bigger
The river breaks
through during a
flood. More
deposition causes
the old meander
to become an oxbow lake

32. Floodplains and terraces
River terraces result from
which process?
Are floodplains features of erosion or
Both
deposition?
Rejuvenation
forms river
terraces
How are they
formed?
• Lateral erosion
What are they?
Remnant of
former
floodplain
Floodplain
Terrace
• Meander
migration
• Valley widened
Bedrock
How are they formed?
• Deposition of
sand and silt
during floods
• Following rejuvenation, river sinks into former valley
• Old floodplain left at higher level
• Several stages of rejuvenation can create several terraces, e.g. lower course
of the River Thames

33. Fluvial deposition – braided rivers
What is braiding and under what conditions does it occur?
It describes a section where the river has been forced to split into several
channels separated by islands. It occurs in rivers supplied by large amounts of
sediment load and / or rivers with variable / rapidly fluctuating discharges
The river becomes very wide in
relation to its depth
The area where the sand or gravel is
deposited is known as a point bar
(also knows as an eyot)
They are unstable and mobile features.
When discharge and velocity increase
they are easily eroded, and their
position changes

34. Levees
Describe the process of levee formation shown in diagrams (a) – (d)
(a) During times of high discharge, the
river floods. The competence of the
river decreases as velocity is
reduced when the river breaks the
banks. Heavier, coarse material is
deposited first
(b) Small banks of deposited material
build up
(c) Subsequent floods result in further
deposition on these banks and the
bed of the river
(d) Raised banks, called levees are
created and the river flows at a
higher level than the floodplain

35. Deltas
• Feature of deposition at river mouth
• Velocity (and therefore competence)
decreases on entering a lake or the
sea
• They occur where river sediment load
is very large, and where there are
weak currents and / or small tidal
range. The rate of deposition thus
exceeds the rate of removal.
• Flocculation occurs as sea water and
fresh water mix
• Clay coagulates and settles on the
river bed
• Sorting of deposits occurs into topset
beds (larger heavier material),
foreset beds (medium graded
particles) and bottomset beds (finest
particles)
Curving shoreline and
dendritic pattern of
drainage, e.g. Nile
Fingers of deposition
build out into sea along
distributaries, e.g
Mississippi
Pointed ‘toothlike’ delta;
material spread
evenly on either
side of channel,
e.g. Tiber

36. Causes of river flooding
Flooding occurs when a
river exceeds its bankfull
discharge
Human factors
Excessive, prolonged
rainfall
Physical factors
Short, intense rainfall
event
Saturated soil
Deforestation
Local relative rise
in sea level / storm surge
What physical and human
factors contribute
to flooding?
Urbanisation
High drainage
density
Snow melt
Impermeable rock
Frozen soil
River management
Steep gradient

37. High risk areas; Shrewsbury, UK
Shrewsbury lies inside an incised meander loop of the River Severn.
The town centre lies above the floodplain but some of the main
transport routes lie vulnerably low, and recent development has taken
place on low lying floodplain

38. Boscastle, Cornwall
Study the photograph which is looking upstream from
Boscastle harbour towards the village of Boscastle.
Suggest the
ways in which
the physical
geography of the
area may
increase the
speed of onset
and severity of
flooding

39. Impact of flash flooding in Boscastle, - August
2004
•
•
•
•
•
•
•
Intense low pressure weather system caused
localised heavy thundery downpours
200mm rain fell in 24 hours (most between
midday and 5pm on the 16th) on high ground to
the east.
Already saturated catchment – rapid runoff
Boscastle lies in a deep valley just
downstream of the confluence of the rivers
Valency and Jordan
2m rise in river levels in one hour
Debris caught under narrow bridge caused 3m
high wave of water which burst down main
street when bridge collapsed
70-80 cars swept away, significant structural
damage, 100 people air lifted to safety but no
loss of life

40. Southern Britain, July 2007
Causes;
• Abnormal track of jet stream
• Rainfall totals for May-July highest since 1776
• Infiltration / percolation capacity minimal
• Exceptional rainfall on 20th July – event only expected
once in several hundred years
Consequences;
Normal Jet
stream
June –
July 2007
• Flash floods across southern England; especially lower Severn and upper Thames
catchments
• Drainage systems overwhelmed and transport networks severely disrupted - £25 million
damage to Gloucestershire’s road system – the year’s budget!
•
45, 000 households lost power; 350,000 lost running water – £1billion cost to water
industry
• £3 billion damage covered by insurance. Equivalent amount uninsured loss
• 50% crops lost in affected areas – shortages and price increases
• 3 people died

41. High risk areas - Bangladesh
What are the physical causes of flooding?
Himalaya
?
Mountains;
(monsoonal)
rainfall and snow
melt
Major rivers converge
?
What are the human
influences?
• Deforestation
• Agricultural practices
80% of country
?
occupies low-lying
delta < 1m above
sea level
Storm surges,
especially during
cyclones /
?
hurricanes. Also
local sea level rises
of 7mm/year
• Densely populated
• Urbanisation – Dhaka
population over 1 million
• Embankments built
(road and river) – have
prevented back-flow of
flood water and increase
siltation in drainage
channels
• Low GDP and lack of
investment

42. Flooding in Bangladesh, 2004
Exceptionally high rainfall totals in the monsoon of 2004 led to widespread
flooding in July and August
36 million
people made
homeless
(nearly 29% of
total population)
Spread of disease
Flood waters mixed with
raw sewage caused
diarrhoea outbreak
38% land area flooded –
worst floods
for 6 years
Consequences of
flooding
800,000 ha agricultural
land flooded – small scale
farmers severely affected
800 dead by
mid-September
Capital city, Dhaka
Flooded.
Infrastructure severely
Damaged – damage to
roads, bridges, school
and hospitals
estimated at $7 billion
$2.2 billion
estimated cost
of damage
(4% of GDP
for 2004)

43. Flood management
Hard engineering strategies involve
the building of structures or alteration of
the course / structure of the river
The aim is to reduce the frequency and
magnitude of flood events, and
therefore reduce the damage that floods
cause
1. Can you describe the measures shown
in the diagram opposite?
2. What might be the advantages and
disadvantages of hard engineering
methods?

44. Lynmouth – flood controls
After the disastrous Lynmouth floods of 1952, the river was managed.
What management can you see in the photograph?
Cottages now
set back from
river bank
Entry of East Lyn
tributary is under a
wide bridge to
prevent damming
Channel has been straightened
Building of
concrete
revetment to
increase speed
of river flow in
time of flood

45. Hard engineering
What are the arguments for and against hard engineering?
FOR
AGAINST
Reduction in flooding and therefore Can lead to destruction of habitats
protects property
along river bank
Takes water away from towns
more quickly
Increase in water supply e.g. on
the Nile
Improved navigation e.g.
Mississippi
Allows energy to be created e.g.
hydroelectric power on the
Colorado
Can be visually intrusive
It can dramatically increase peak
discharge, duration and timing of
floods downstream
Where meanders have been
straightened, the river will try to reestablish itself
Straightening courses can lead to
greater upstream erosion and
downstream deposition

46. Impact of the Three Gorges dam, China
•
•
•
Largest ever river hard engineering project
Dam completed in 2006; 2.3 km long and over 100m high, with reservoir (660km
by 1km) behind
Project due to complete in 2009, at a cost of an estimated £25 billion
Defence
Criticism
Produce HEP for 13 million people
Cost
Reduce the flood risk for 15 million
people
1.2 million people relocated to newly built
settlements following reservoir creation
Improve navigation for faster,
cheaper and safer shipping
Cultural monuments lost due to creation of
reservoir
Boost local economic growth
Downstream sediment supply decreased by
50% - increased erosion
Improved water supply
Disruption to habitats and environment

47. The Three Gorges Dam

48. Soft engineering
What is soft engineering?
Abatement strategies which aim to work with natural processes, and be
more sustainable solutions to flooding
• Afforestation
• Contour ploughing and strip farming to reduce
runoff
• Floodplain zoning to allow (economically less
valuable) areas to flood naturally
• Conservation and restoration schemes;
returning rivers to their original state and
protecting, e.g. bales to improve water quality
• Forecasting and early warning, e.g.
Environment Agency flood watch and risk
maps. Some small-scale community projects
in Bangladesh have resulted in early warning
systems and lives are being saved

49. River restoration
The River Cole near Swindon underwent a restoration project between 1994
and 1996. The aims were to change the water course back to a more natural
state, improve water quality and manage bank side vegetation and habitats. The
main strategies are shown below

50. Prediction
All flood protection methods
are designed to cope with
certain magnitudes and
frequency of floods
Hydrologists try to predict
the likelihood of future
flooding by examining
historical discharge and
flood records
What is flood recurrence?
The average number of
years between floods of a
certain size is the
recurrence interval or return
period
Study the data shown to
understand the recurrence
intervals
Feet
above SL
Magnitude
Recurrence
Interval
(years)
796.8
1
57.00
792.3
2
28.50
791.4
3
19.00
791
4
14.25
789.9
5
11.40
789.8
6
9.50
789.7
7
8.14
789.6
8
7.13
789.5
9
6.33
789.4
10
5.70
785.1
20
2.85
782.8
30
1.90
779.5
40
1.43
774.1
50
1.14

51. The Environment Agency
What does The Environment Agency do?
• Monitoring of water levels and flows
• Building and maintenance of flood
defences on coast and rivers
• Use of radar rainfall data combined with
discharge data to predict flood risk and
issue flood warnings through; media,
automated messages to risk groups,
flood-line for the public
• Implements and coordinates incident plans
• Produce flood risk maps
• Advises planners and developers on flood risk
• Develops information, e.g. on the website about flood risk and what to do in the
event of, and aftermath of a flood

52. Exam question 1
How can planners help to restrict flood damage? (6)
Typical answer
Good use of
Where? needs to be upstream
appropriate term;They can stop the water getting
Suspect choice of
could just have
to the settlement, by building a
example; prior to
said “water”
reservoir and controlling the Aswan being built the
farmers relied on
Naïve; there are discharge. This was done when
floods to provide
very few ‘new’ the Aswan Dam was built on the
fertile alluvium. How
settlements
River Nile. They can also make much damage was
whose location
sure that towns and cities are
done?
can be
determined by not built too near a river which is
Over-simplistic
likely to flood.They can also
planners.
make sure that there are enough answer, with
Impossible for
some poorly
sandbags to stop the floods
every riverside
chosen examples
settlement have a affecting the nearby buildings.
(2/6)
stock - again,
rather naive

53. Exam question 1
How can planners help to restrict flood damage? (6)
Specimen answer
Useful
additional
detail
Again,
complex
sentence
allows point to
be developed
fully
Two examples
combined in the
one sentence
Planners can restrict the amount
of building on the floodplain of
river, or of buildings near to the
river (as at Lynmouth). They can
alter the channel of the river the
river to straighten and speed up
its flow. They can allow water
meadows to flood, storing water,
which can also be achieved by
upstream retention e.g.
reservoirs.
Always a
good idea to
add a named
example
Although
only three
sentences,
each is full,
with
examples.
Would score
as 3 x 2.

54. Exam question 2
Examine the physical factors responsible for flooding.
(25)
Points to consider
1. Introduction could attempt to classify the causes, e.g. climatic,
geomorphological, to indicate range of factors, and structure of
essay.
2. Range of detailed examples needed, from MEDCs and LEDCs
3. Essay should demonstrate that many floods are multi-causal.
4. Maps and diagrams integrated into text discussion
5. Answer should be well structured and well written, with
appropriate use of specific vocabulary. Conclusion
could consider changing physical factors likely to make
flooding more likely, e.g. global warming.