Rivers, Floods and Management AQA. Contents Hydrological cycle and river discharge River processes...

Rivers, Floods and Management

AQA

Contents

Hydrological cycle and river dischargeRiver processesProfiles and characteristicsLandforms of fluvial erosion and depositionCauses and impacts of floodingFlood management strategiesExam questions

The drainage basin

Lesson 1

The drainage basin and hydrological cycle (lesson 1)

Lesson objectives

• Students develop skills of OS map reading and identifying key river features on an OS map.

Success criteria

To recognise 4 different

patterns of drainage basin.

To apply knowledge to an

OS map and identify 3

different drainage

basins and their pattern

Starter: Identify the key features of the drainage basin

Watershed

Main river channel

Source

Confluence

Tributary

Mouth

Task: Annotate the drainage basin to define the key word labels.

The watershedmouth

Drainage basin or catchment area Definition: What is a drainage

basin?

An area of land (also called the catchment area) drained by a river and its tributaries

The drainage basin

Types of drainage basin

The rectangular drainage pattern. Streams follow the path of least resistance and thus are concentrated in places where exposed rock is the weakest.

Trellis drainage patterns look similar to their namesake, the common garden trellis.

Parallel drainage patterns form where there is a pronounced slope to the surface. Tributary streams tend to stretch out in a parallel-like fashion following the slope of the surface.

A dendritic drainage pattern is the most common form and looks like the branching pattern of tree roots.

Using Ordnance Survey maps

Tasks: 1.Using the OS map of Snowdonia, identify as three

different drainage basins.

2. Having achieved this, write down the name of the main river channel that the drainage basin belongs to.

3.Now, identify the drainage basin pattern for each identified.

Plenary:

In your books, drawn the four different drainage basin

patterns that you have learnt about this lesson (without

referring to your class notes!)

Annotate each one identifying:

• Watershed

• Main river channel

• Source

• Confluence

• Tributary

• Mouth

Please print the following reading from the shared area, make key notes revision cards for next

lesson: Drainage basin: River Ouse

The drainage basin and hydrological cycle (lesson 1)

Lesson objectives

• Students develop skills of OS map reading and identifying key river features on an OS map.

Success criteria

To recognise 4 different

patterns of drainage basin.

To apply knowledge to an

OS map and identify 3

different drainage

basins and their pattern

Drainage basin processes

Lesson 2


Lesson objectives

• To identify key processes of a drainage basin.

• Students be able to work out and construct soil moisture budget.

Success criteria

• To be able to identify drainage basin processes.

• To contribute thinking of geology and the soil moisture budget.

• Examine how this year’s summer weather will affect the usual British Isle moisture budget.

Starter:

Task:

Write down 5 words that you would use to

describe this summer’s weather.

Examine the impacts of this on our drainage

basins.

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

Outputs

Stores

Transfers

Can you define these terms?

Ways in which water enters the system

Ways in which water leaves the system

Ways in which water is held in the system

Ways in which water is moved through and within the system

Task:

Complete the handout of a typical drainage basin.

In order to complete this you will need 4 different colours.

Assign each shape in the key with a colour and then colour

the shapes accordingly in the diagram.

Now, label each shape with the correct process.

Elements of the systemName each store Name each transfer Name the Input Name each Output

Surface storage - lakes, rivers, sea, depression storage

The water table (position varies)

Saturated or impermeable rock

Interception by vegetation

Soil water storage

Groundwater storage

Throughfall + stemflow

Infiltration

Throughflow

Surface (overland) runoff

Percolation

Groundwater (base) flow

Precipitation

Evaporation

Transpiration

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 storageThe graph shows the main features of the water balance – average monthly precipitation and potential evapotranspiration levels

Task: Draw this model out, ensuring that it takes half an A4 page.

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

Understanding the water balance graph

Q. Why is a water deficit not shown on this graph?

A. In the UK, precipitation (nearly) always exceeds evapotranspiration

Q. How might water balance graphs be a) useful?, and b) difficult to construct?

A. They can be useful in predicting river regimes and managing drainage basins. It is however difficult to accurately measure evapotranspiration

Task: Underneath your diagram of the water balance graph,1.Explain what the graph shows (you must refer to each process along the course of the year)2.Strength and weakness of using this graph

Plenary:

Peer assess your study buddy’s work using this mark scheme


Lesson objectives

• To identify key processes of a drainage basin.

• Students be able to work out and construct soil moisture budget.

Success criteria

• To be able to identify drainage basin processes.

• To contribute thinking of geology and the soil moisture budget.

• Examine how this year’s summer weather will affect the usual British Isle moisture budget.

The Storm Hydrograph and factors affecting shape of

hydrographs

Lesson 3 Please bring the following reading and supporting revision cards to next

lesson:

River hydrographs

Lesson 3

Lesson objectivesTo understand the impact of

land use, time of year,

slope, intensity of rain,

geology, previous weather

on hydrograph shape.

Success criteriaTo be able to plot

precipitation and discharge

using two different scales.

To apply base flow, run off

and flood concepts to the

graph.

To identify the features of

flashy and subdued

hydrographs and suggest

reasons why a flashy

hydrograph or subdued

hydrograph would occur.

Starter: Identify the key features of the storm hydrograph

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

• Name the main features of the hydrograph

Time (hours)

Discharge (cumecs)

rainfall

?Rising limb

?Falling limb

?Peak discharge

?Lag time

?Storm flow

?Base flow

What factors affect river discharge?

Size and shape of drainage

basin

Porosity of soils underlying basin

Geology:permeability of underlying

rock

Drainage density

Vegetation: type and seasonal variations in

cover. Deforestation

Drainage basin relief

Factors affecting discharge

Precipitation: intensity and

duration. Antecedent rainfall.

Land use

How do these factors affect the storm hydrograph?

Factor Effects on Hydrograph

High intensity, long duration of rainfall, or antecedent rainfall

Snow melt

Porous soils and / or permeable rock

Impermeable rock / frozen ground

Small drainage basin

Elongated drainage basin shape

Steep slopes within drainage basin

Summer vegetation

Deforestation

Steep rising limb as infiltration capacity of soil exceeded

Greatly increased discharge, especially if ground frozen

Less steep or ‘flashy’ hydrographs

Reduced lag time and steeper rising limb

Faster response, so shorter lag time and steeper rising limb

Slower passage to river, so longer lag time

Faster passage to river, so shorter lag time and steeper rising limb

Interception higher - slower response, peak discharge lower

Faster response and higher peak discharge

time

discharge (cumecs) and rainfall (mm)

Shorter lag time as water quickly reaches the channel via surface runoff, through drains, sewers etc

Steeper rising limb due to impermeable surfaces

Urbanisation and the storm hydrograph

Higher peak flow as less water is ‘stored’; more water reaches the river

Rural

Urban

Flashy and subdued hydrographs

Task 1: Identify the differences between A and B.Task 2: Describe and explain the conditions under which A and B may occur

Plenary:

To complete the exam question handed out

to you. Answer this question on the

resource.

Lesson 3

Lesson objectivesTo understand the impact of

land use, time of year,

slope, intensity of rain,

geology, previous weather

on hydrograph shape.

Success criteriaTo be able to plot

precipitation and discharge

using two different scales.

To apply base flow, run off

and flood concepts to the

graph.

To identify the features of

flashy and subdued

hydrographs and suggest

reasons why a flashy

hydrograph or subdued

hydrograph would occur.

Valley profiles Lesson 4 and 5

Please print, read and produce revision

cards for the following:

River long profiles and valley cross

profiles

Lesson 4 and 5

Lesson objectives

• Students to be able to fit a cross profile to the long profile of a river.

• To understand Long profile (thalweg) and cross valley profiles and variation.

Success criteria

• To know the basic long and cross valley shapes.

• To understand what causes irregularities (knick points)

• To compare and contrast general profile with example of Afon Glaslyn.

Starter: Describe the changes to this channel from source to confluence along its long profile.

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 (a river will achieve this once it has achieved a balance between erosion and deposition), with a steeper gradient in the upper course becoming progressively gentler towards the mouth

Height (m)

60 Distance from sea (km) 0

A

B

C

How would the processes of erosion, transportation and deposition change from A, through B, and to C?

The changing cross-profile

A

B

C

Height (m)


A B C

Upper course Middle course Lower course

OS map skills

• Locate Afon Glaslyn on the OS map of Snowdonia.

• Identify:

Source, mouth, flow direction,

stores, valley shape and channel

pattern.

Plenary

• Draw the long profile of any river from the Snowdonia OS map on graph paper. You will need to ensure that your scale fits onto the graph paper!

Lesson 4 and 5

Lesson objectives

• Students to be able to fit a cross profile to the long profile of a river.

• To understand Long profile (thalweg) and cross valley profiles and variation.

Success criteria

• To know the basic long and cross valley shapes.

• To understand what causes irregularities (knick points)

• To compare and contrast general profile with example of Afon Glaslyn.

The River Processes

Lesson 6 and 7

Lesson 6 & 7

Lesson objectives

• To understand the significance concerning the river processes

Success criteria

• To know what the river processes are

• To examine the relationship between the river processes and river formations and landforms

Starter: Which of these sediments will be entrained first? Why?

River processes

1. FLUVIAL EROSION

The break-up of rocks by the action of the river

2. TRANSPORTATION

The movement of eroded material

3. DEPOSITION

The laying down of material which has been transported

by the river

Think…

What key processes do you think determine the size, shape and features of a river channel?

As we consider these processes, remember that a river’s discharge and velocity will have an impact on rates of erosion, transportation and also upon deposition

Tip

Put a new title in your book: River Processes

Key question:

What two types of energy will dictate the

rate of erosion, transportation and

deposition?

At any one time the dominant process operating within the river depends on the amount of energy available. This is governed by the velocity of the flow and the discharge within the channel.

Entrainment velocity: the critical erosion velocity (velocity required to pick up and transport sediment)

Remember...

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

What are the main processes

of fluvial erosion?

Attrition Abrasion / corrasion

Corrosion / solution

Hydraulic action

Definitions

EROSIONAL PROCESS

DEFINITION EFFECT ON RIVER

Corrasion / Abrasion

Attrition

Hydraulic action

Corrosion

Task: Using the cards, complete the table.

Scraping, scouring and rubbing action of the load carried by the river

Process where pebbles knock together and get smaller and more rounded

Frictional drag and pressure created by moving water as it is forced into cracks

Where the naturally weak acidic water dissolve some carboniferous rocks such as limestone and chalk

River banks wear away

Sediment load size decreases

Removal of loose material - river banks undercut and collapse

An increase in the dissolved load (solution)

Abrasion

Hydraulic Action

Corrosion

Attrition

ErosionAbrasion: The scrapping, scouring and rubbing action of materials carried along by the river.

Rivers carry rock fragments in the flow of the water or drag them along the bed of the river channel.

Abrasion is the most effective in short turbulent periods when the river is at bankfull or in flood.

During times when the river level is low, the load consists of small particles, e.g. Sand grains and these tend to smooth the surface of the river channel.

ErosionHydraulic Action: caused by the power of moving water. It is the movement of loose unconsolidated material due to the frictional drag of the moving water on sediment lying on the channel bad.

As velocity increases, turbulent flow lifts a large number of sand-sized particles from the channel floor.

Hydraulic action is particularly successful at removing loose material in the banks of meanders , which can lead to undercutting and collapse.

May also cause rocks to fragment along joints and other lines of weakness.

Erosion

Corrosion: Is the most active on rocks that contain carbonates such as limestone and chalk.

The minerals in the rock are dissolved by weak acids in the river water and carried away in solution.

Erosion

Attrition: Reduction in size of fragments and particles within a river due to actions like corrosion.

The fragments strike one another as well as the river bed. Particles become smoother, smaller and more rounded as they move along the river channel.

These smaller fragments therefore are found more commonly downstream.

Erosion• Vertical erosion: dominant in

the upper course of the river where the river is trying the cut down to reach either its base level of sea level.

• Depending upon meters above sea level will depend on its potential energy and ultimately the shape of the landscape.

• When the discharge level and velocity of the river is high, the river cuts down into its valley mainly by abrasion and hydraulic action. Such rivers often produce steep-sided valleys.

Erosion• Lateral erosion: Dominant in the

middle to lower course of the river channel.

• As the cannel reaches flatter land, the river’s energy erodes laterally.

• This may result in the formation of meanders.

• The channel will also become wider and deeper.

• Hydraulic action is prevalent here. • The strongest current of the water

is on the outside of the meander bend and causes undercutting, resulting in the banks collapsing.

2. River Transportation

Solution: Dissolved minerals within water

A river transports it’s load in four main ways.

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

Extension: Write a small paragraph underneath your diagram, explaining why you think different types of sediment are transported in a particular way.

Transportation

• River competence: the maximum size (calibre) of load a river is capable of transporting.

• River capacity: total volume of sediment a river can transport .

What calibre of sediment would you expect to be transported in the

upper, middle and lower course of a river?

What factors would you need to take into account?

Under what conditions will a river deposit its load?

When there is an increase in the size of the sediment load caused by a landslide or tributary delivering larger particles

In flood conditions, when a river overflows onto its floodplain

There is shallow water, e.g. on the inside of a meander bend

When river discharge is reduced due to a period of dry weather

3. Deposition

When there is a decrease in the gradient and / or velocity of the river e.g. at river mouth, or when entering a lake

Task: In your book, give yourself a heading ‘Deposition’. In the middle of your book write the following question. In pairs, develop some ideas that considers this question. You have 5 minutes!!

Deposition• A river deposits when it not longer has the competence (maximum size

(calibre)) of load a river is capable of transporting.• Any reduction in river velocity will reduce competence and the material will

begin to be deposited (starting with the coarsest sediment as this requires a lot of energy to remain suspended), e.g. gravel, pebbles, boulders.

• Very fine sediment may never be deposited in the channel and may be carried all the way out to sea via the estuary.

Deposition occurs when:

1. Sudden reduction in gradient

2. River enters a store

3. Discharge has been reduced

4. Shallow water (e.g. Meander bend = build up of sediment)

5. Sudden increase in calibre

Profile of dynamic equilibrium

Relationship between erosion and

deposition.

A river is always trying to reach a profile of

dynamic equilibrium.

If a river achieved this, how would the long

profile appear?

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 (a river will achieve this once it has achieved a balance between erosion and deposition), with a steeper gradient in the upper course becoming progressively gentler towards the mouth

Height (m)


A

B

C

How would the processes of erosion, transportation and deposition change from A, through B, and to C?

Ways in which changing processes affect the valley profile

Erosion Transportation Deposition

A (Upper course)

B (Middle course)

C (Lower course)

Hydraulic, attrition. Vertical erosion dominant

Mostly attrition, some hydraulic. Vertical erosion decreases + lateral begins

Some lateral erosion on outside of meanders

Mostly large boulders. Some in suspension + solution

Saltation and traction of smaller bed-load. Suspension increased. Some in solution

Smaller sized bed-load of sand and gravel, transported in suspension

Large bed-load only

Coarser material builds up in braiding, slip-off slopes and floodplain

Mostly fine material deposited on levees, floodplain and slip-off slopes

Task: Using the cards, complete the table.

The changing cross-profile

A

B

C

Height (m)


A B C

Upper course Middle course Lower course

Bradshaw’s model Bradshaw’s model illustrates the main changes expected down the long profile of a river

Upstream Downstream

dischargedischarge

occupied channel widthoccupied channel width

water depthwater depth

water velocitywater velocity

load qualityload quality

load particle sizeload particle size

channel bed roughnesschannel bed roughness

gradientgradient

Task: Examine Bradshaw’s

model of changing channel

characteristics based on your knowledge of

river processes

Upstream Downstream

dischargedischarge

occupied channel widthoccupied channel width

water depthwater depth

water velocitywater velocity

load qualityload quality

load particle sizeload particle size

channel bed roughnesschannel bed roughness

gradientgradient

Plenary:

With reference to Afon Glaslyn, annotate the

diagram to show the different processes at

work.

Lesson 6 & 7

Lesson objectives

• To understand the significance concerning the river processes

Success criteria

• To know what the river processes are

• To examine the relationship between the river processes and river formations and landforms

The Hjulstrom Curve

Lesson 8

Lesson 8

Lesson objectives

To be able to apply

knowledge of erosion,

transportation and

deposition to a theoretical

model

Success Criteria

Complete an annotated

diagram of the Hjulstrom

Curve

To know how to use and

understand a logarithmic

scale.

Starter:

1. Identify the three river processes.

2. Define ‘River competence’

3. Define ‘Entrainment velocity’

4. Define ‘Profile of dynamic equilibrium’.

The Hjulström curve shows the relationship between river velocity and the size of particles which can be eroded, transported and deposited.

The Hjulström curve

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)

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.

1

2 3

4

Task: Draw an enlarged version of the Hjulstrom curve in your book, take up half a page (you can use the image in your text books).

Task:

Annotate your Hjulstrom curve to explain what the model shows and refer to

examples.

Semi-log graphs are used to show data that have a particularly wide range of values. For example, a semi-log graph allows us to compare things that differ enormously in size. This example allows us to compare Oceania and East Asia in terms of their trends in absolute population growth. This comparison would not be feasible using ordinary graph paper.

The logarithmic scale compresses the range of values. It gives more space to smaller values and reduces the amount of space for the larger values. Thus it can show relative growth quite clearly.

On the scale there are ‘cycles’ of values. Each cycle increases by a larger amount, usually to the power of 10. With this example, the first cycle increases by 10 each time to 100, the second cycle increases by 100 to 1000 and so on.

Plenary:

Complete a detailed A5 revision card to

demonstrate what you have learnt this lesson.

Lesson 8

Lesson objectives

To be able to apply

knowledge of erosion,

transportation and

deposition to a theoretical

model

Success Criteria

Complete an annotated

diagram of the Hjulstrom

Curve

To know how to use and

understand a logarithmic

scale.

The River Tees

Lesson 11In order to support your work on the River Tees,

you will need to print and use the following

reading:

Flood and River basin management: River

Tees

Lesson 11

Lesson objectives

• To understand the formation of and profile of the River Tees

Success criteria

• Produce key notes from the video to inform your case study

River Tees Landforms case study

• Watch the 30 minute video about river landforms and features along the River Tees.

1.Location (source and mouth)

2.Features along the profile of the Tees

3.Landforms along the profile of the Tees

4.Management and reasons for management

River Tees case study

• Location• Length of the River Tees• Upper course formations and landforms• Middle course formations and landforms• Lower course formation and landforms• Management

Tip

You must show evidence of research and further reading.

You must also provide annotated images specific to the formation

and landforms along the river Tees.

Lesson 11

Lesson objectives

• To understand the formation of and profile of the River Tees

Success criteria

• Produce key notes from the video to inform your case study

River Landforms

Lesson 12 and 13

Lesson 12 and 13

Lesson objectives

To understand and explain

the formation of river

landforms

Success Criteria

Produce a 3 min

PowerPoint presentation

and revision hand out for

next lesson

Starter: Annotate this image as best you can from what you learnt last lesson

Presentation task

Aim:

In pairs or groups of three, prepare a 5 minute presentation

that explains the formation of a river landform.

Requirements:

1. You must produce a 3 minute presentation.

2. You must produce a key point revision handout for each member of the class (including images).

3. You must print out your PowerPoint presentation slides onto a handout page and submit to me with a copy of your revision handout.

What your assessed work must include:Presentation:

1. Identification of landform and location along the profile of the river

2. What it is

3. What they look like

4. Geographical formation (river processes)

5. An example

6. Include relevant annotated images

Revision handout

1. The step by step formation of the landform

2. Images of the formation

3. River processes

4. Example

This must be ready to

present next lesson

– please save work

onto a memory

stick

Landforms

• Upper course: V-shaped valleys and interlocking spurs Potholes and Rapids

• Upper/ Middle course: Waterfalls and Gorges

• Middle course: Meanders and Oxbow lakes

• Middle/ Lower course: Channel braiding and Eyots (island bars)

• Lower course: Floodplains, levees and deltas

Plenary:

Assess each group’s presentation using the

peer/ self assessment feedback sheets.

Provide feedback to each group.

Lesson 12 and 13

Lesson objectives

To identify and explain


Landforms along the profile

of a river

Success CriteriaSynthesise knowledge using tree

diagram.

Create a story board or flow diagram that

examines the formation of river landforms

from source to confluence of a river

Challenge: Incorporate Bradshaw’s

model.

Assess own progress and what you

learnt by completing a past exam

question using the Zones of Relevance

Challenge: Evaluate whether these

features are affecting a river’s profile of

dynamic equilibrium

Synthesising information: Landforms of fluvial erosion

Erosional

Features

Meanders

Interlocking

spurs

PotholesWaterfalls

& rapids

Middle and

lower course

Upper

course

River

terraces

Ox-bow

lakes

Lesson 12 and 13

Lesson objectives




of a river


diagram.





model.






dynamic equilibrium

In areas of resistant rock (e.g. on Exmoor), meandering streams and rivers will be incised into the landscape, forming interlocking spurs.

Interlocking spurs

V-shaped valleys and interlocking spurs

Predominantly vertical erosion results in a v-shaped valley.

Rapids are formed by variations in rock resistance

Bourke’s Luck Potholes, South Africa

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

In resistant rock, potholes require hundreds to thousands of years to form

Potholes

Soft rock

Soft rock

Broken piecesof hard rock

RiverHard rock

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?

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

Which feature is formed as the waterfall retreats upstream?

A gorge or canyon

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

Can you remember how to describe the formation of ox-bow lakes?

What are these features, and what do they represent?

Ox-bow lakes. These show the former course of the river

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 ox-bow lake

Meanders

Meanders

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.

0.2 m/sec0.1 m/sec

• Sinuous bends in the river• Surface flow to outer bank and sub-

surface 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

Outside of meander

Inside of meander

They are unstable and mobile features. When discharge and velocity increase they are easily eroded, and their position changes

The area where the sand or gravel is deposited is known as a point bar (also knows as an eyot)

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

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

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 ‘tooth-like’ delta; material spread evenly on either side of channel, e.g. Tiber

Lesson 12 and 13

Lesson objectives




of a river


diagram.





model.






dynamic equilibrium

Plenary:

Mark scheme

Lesson 12 and 13

Lesson objectives




of a river


diagram.





model.






dynamic equilibrium

Process and impact of rejuvenation – knick points,

waterfalls, river terraces and incised

meanders.

Lesson 14, 15 and 16


Lesson objectives

To examine the causes of

rejuvenation and how a

rejuvenated valley can be

recognised.

Success criteria• Produce a 3 min PowerPoint

presentation and revision hand out for next lesson

• To be able to define ‘rejuvenation’.

• To produce an annotated diagram showing the causes of and effects of rejuvenation.

• To relate coastal terms Isostatic recovery and Eustatic Change.

• To be able to draw and annotate between Ingrown and Entrenched form of Incised meanders.

Starter:

Draw diagram showing formation the of a

meander & Ox Bow lake, a Delta and a

Waterfall, then annotate.

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

original sea leveloriginal long profile

What feature will be found here?

A waterfall

relative fall in sea level (or rise of land surface)

new sea level

knick point

new long profile

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.

Original course of River Lyn

Breach in coastal geology

Knick point waterfalls

Present – day village of Lynmouth

‘Valley of the rocks’

Rejuvenation

Renewed energy from rejuvenation results in increased vertical erosion and incised (deepened) meanders

Steep river cliff

Inside of meander slopes

gently

When incision is less rapid and lateral erosion is occurring, meanders become ingrown, e.g. the River Wye at Tintern.

FLOW

Entrenched meanders have a symmetrical cross-profile (rapid uplift) whereas ingrown meanders are more asymmetrical (slower uplift)

Ingrown and entrenched meanders

What are the differences in form between entrenched and ingrown meanders?

When incision is rapid and vertical erosion dominates, an entrenched meander is formed, with steep sides and a gorge-like appearance

Floodplains and terraces

Are floodplains features of erosion or deposition?

River terraces result from which process?

BothRejuvenation forms river terraces

How are they formed?

• Lateral erosion

• Meander migration

• Valley widened

• Deposition of sand and silt during floods

How are they formed?

• 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

What are they?

Remnant of former floodplain

Terrace

Floodplain

Bedrock

http://www.bbc.co.uk/learningzone/clips/river-terraces/402.html

Plenary:


Lesson objectives

To examine the causes of

rejuvenation and how a

rejuvenated valley can be

recognised.

Success criteria• Produce a 3 min PowerPoint

presentation and revision hand out for next lesson

• To be able to define ‘rejuvenation’.

• To produce an annotated diagram showing the causes of and effects of rejuvenation.

• To relate coastal terms Isostatic recovery and Eustatic Change.

• To be able to draw and annotate between Ingrown and Entrenched form of Incised meanders.

What causes flooding?Lesson 19

Lesson 19

Lesson objectives

To understand and be able

to calculate flood frequency

and recurrence interval and

apply to concept of flood

magnitude.

Success criteria

• Design a mind map showing the causes of flood (human and physical)

• To complete flood

frequency calculations.

Starter: Causes of river flooding

What physical and human factors contribute

to flooding?

Human factors

Excessive, prolonged rainfall

Saturated soil Deforestation

Urbanisation

Physical factors

Snow melt

Frozen soil

Local relative rise in sea level / storm surge

Steep gradientImpermeable rock

High drainagedensity

Flooding occurs when a river exceeds its bankfull discharge

River managementShort, intense rainfall event

Plenary:

Complete the flood risk and recurrence

interval calculations.

Lesson 19

Lesson objectives

To understand and be able

to calculate flood frequency

and recurrence interval and

apply to concept of flood

magnitude.

Success criteria

• Design a mind map showing the causes of flood (human and physical)

• To complete flood

frequency calculations.

Flooding in an MEDC

Lesson 20 and 21

Lesson 20 and 21

Lesson objectives

• To assess the causes an impacts of flooding in an MEDC.

Success criteria

• Design MEDC case study.

High risk areas; Shrewsbury, UKStarter: Examine the map below and identify causes of flooding in Shrewsbury town and the socio-economic impacts.

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

Boscastle, Cornwall

Suggest the ways in which the physical geography of the area may increase the speed of onset and severity of flooding

Study the photograph which is looking upstream from Boscastle harbour towards the village of Boscastle.

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

Southern Britain, July 2007

Normal Jet stream

June – 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;

• 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

MEDC flooding case study structure.1. Title2. Images3. Map location (annotated identifying

obvious human/ physical causes, floodplain and flood risk area)

4. Causes (explained, interdependent links)

5. Impacts – Short term(social, economic, environmental)

6. Impacts – Long term (social, economic, environmental)

7. Management strategies – What has been put in place to reduce risk and impacts of flood in future?

TIP

Include statistics e.g. cost of damage and claims made, numbers injured/ killed.

Plenary:

Identify 1 cause and 3 impacts of flooding in

your chosen MEDC case study to share with

the class.

Lesson 20 and 21

Lesson objectives

• To assess the causes an impacts of flooding in an MEDC.

Success criteria

• Design MEDC case study.

Flooding in an LEDC

Lesson 22 and 23

Lesson 22 and 23

Lesson objectives

To assess the causes and

impacts of flooding in

Bangladesh (2007).

Success criteria

• Design LEDC case study.

Starter: High risk areas - Bangladesh

Major rivers converge?

?Himalaya Mountains; (monsoonal) rainfall and snow melt

?

Storm surges, especially during cyclones / hurricanes. Also local sea level rises of 7mm/year

80% of country occupies low-lying delta < 1m above sea level

What are the human influences?

• Deforestation

• Agricultural practices

• 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

?

Explain why these factors have been identified as causes of flooding?

Flooding in Bangladesh, 2004Exceptionally high rainfall totals in the monsoon of 2004 led to widespread flooding in July and August

Consequences of flooding

38% land area flooded – worst floods for 6 years

800,000 ha agricultural land flooded – small scale farmers severely affected

Capital city, Dhaka Flooded.

36 million people made

homeless (nearly 29% of total population)

800 dead by mid-September

Spread of disease Flood waters mixed with

raw sewage caused diarrhoea outbreak

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)

LEDC flooding case study structure.1. Title2. Images3. Map location (annotated identifying

obvious human/ physical causes, floodplain and flood risk area)

4. Causes (explained, interdependent links)

5. Impacts – Short term(social, economic, environmental)

6. Impacts – Long term (social, economic, environmental)

7. Management strategies – What has been put in place to reduce risk and impacts of flood in future?

TIP

Include statistics e.g. cost of damage and claims made, numbers injured/ killed.

Add a paragraph at the bottom explaining why the impacts suffered from floods are different between MEDCs and LEDCs (based on our examples).

Plenary:

Identify 2 causes and 3 impacts of flooding

in Bangladesh ready to present to the class.

Lesson 22 and 23

Lesson objectives

To assess the causes and

impacts of flooding in

Bangladesh (2007).

Success criteria

• Design LEDC case study.

Flood management strategies

Lesson 24

Lesson 24

Lesson objectives

• To examine the use of hard and soft flood management strategies.

Success criteria

• To complete hard and soft engineering worksheet

• Begin Case Study

Starter:

Think of 5 questions beginning with the letter ‘W’

that you could ask about this image

Task 1:Hard and soft engineeringHow would you define the following key terms?

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.

Soft engineering:

Abatement strategies which aim to work with natural

processes, and be more sustainable solutions to flooding

Task 2:

• Using your text book and completing further research, complete the handout showing you different methods of hard and soft engineering to manage river flooding.

• You must include the following in each annotation box:

1.What it is

2.Aim

3.Named river where used

4.Advantages

5.Disadvantages

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?

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

Lynmouth – flood controls

What are the arguments for and against hard engineering? Outline the impacts for each.

Plenary: Hard engineering

FOR AGAINST

Reduction in flooding and therefore protects property

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 lead to destruction of habitats along river bank

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 re-establish itself

Straightening courses can lead to greater upstream erosion and downstream deposition

Lesson 24

Lesson objectives

• To examine the use of hard and soft flood management strategies.

Success criteria

• To complete hard and soft engineering worksheet

• Begin Case Study

Case study: The Three Gorges Dam, China.

Lesson 25

Lesson 25

Lesson objectives

• To examine the use of a dam in managing flood risk

Success criteria

• Produce a detailed case study

Starter:Identify as many impacts as a result of this

method of hard engineering.

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

The Three Gorges Dam

Hard Engineering case study structure

• Title

• Method of engineering

• Map location and images

• Name of river

• Aim: Why it was built, what was the need? (include flood magnitude and frequencies – any socio-economic/ environmental issues that prompted it?

• Size and cost of dam

• Labour and cost in its maintenance

• Benefits (socio-economic, environmental)

• Disadvantages (socio-economic, environmental)

• Conclusion

Plenary 1: Past exam question (15 marks)

Plenary 2: Keyword test

Define:

Hard engineering

Soft engineering

Afforestation

Rejuvenation

Profile of dynamic equilibrium

Erosion

Hydraulic radius

Percolation

Base level

Drainage basin

Lesson 25

Lesson objectives

• To examine the use of a dam in managing flood risk

Success criteria


Case study: Restoration schemes, River Quaggy

Lesson 27

Lesson 27

Lesson objectives

• To examine the impacts for the use of river restoration schemes

Lesson criteria


Starter: What are the impacts associated with this method of engineering?

What is soft engineering?

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

River restorationThe 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

River Restoration: River Quaggy, UK

http://www.qwag.org.uk/quaggy/restoration.php

Soft Engineering case study structure

• Title

• Method of engineering

• Map location and images

• Name of river

• Aim: Why it was constructed, what was the need? (include flood magnitude and frequencies – any socio-economic/ environmental issues that prompted it?

• Size and cost of project

• Labour and cost in its maintenance

• Benefits (socio-economic, environmental)

• Disadvantages (socio-economic, environmental)

• Conclusion

Plenary: Plan out your answer for this 15 mark question

MarkschemeThere is a need to make clear why soft engineering strategies are preferred to hard engineering or vice versa.

This is the likely route so there should be reference to the advantages of soft engineering and possibly also the

disadvantages of hard engineering. There will probably be some description of the relevant strategies that may

be adopted. Alternatively, candidates may disagree with the statement and provide

advantages of hard engineering and disadvantages of soft engineering. The final option is to perceive the

complementary nature of the two approaches and discuss this aspect. Advantages of soft engineering are

likely to refer to its greater sustainability, its limited interference with a natural system, the ability to improve

the environment at times and to work with natural systems so that wetlands and habitats may be

restored/created, the relative affordability. Disadvantages of hard engineering relate to the extent to which

there is change to the natural system and questions over its sustainability – the large scale of building dams and

their environmental impact, as well as economic and social costs. Similarly, channelisation means that the flood

risk may be increased downstream and habitats destroyed. Advantages of hard engineering

may relate to their effectiveness, especially in the short term, associated schemes for HEP, irrigation which

give other advantages. Disadvantages of soft engineering relate to ineffectiveness in already built-up

areas, the fact that flood warnings allow preparation but are not preventing damage from flooding. They will be

seen as reducing the scale of risk rather than preventing flooding. The actual content will depend on the

specific strategies considered and whether there is exclusive discussion of soft Engineering strategies only.

There may be reference to case studies – such as River Quaggy, London, Lincolnshire, Oxfordshire (Cherwell),

Ouse, Jubilee River Channel, Carlisle, Three Gorges Dam, Colorado etc.15 marks)

• Level 1 (Basic) 1-6 marks

Identifies soft and/or hard engineering strategies.

Refers to simple reasons why soft engineering is better.

Some use of appropriate terminology present at the higher end.

Coastal flooding response – if relevant, generic aspects.

• Level 2 (Clear) 7-12 marks

Describes strategies and advantages and / or disadvantages of soft and / or

hard engineering.

Begins to discuss why soft engineering strategies are better (or an alternative

option).

Uses strategies to illustrate points – will illustrate one aspect only or with

imbalance e.g. advantages of soft engineering may be discussed with no

reference to hard engineering.

Case study material may be included in a descriptive way.

Appropriate geographical terminology is used.

• Level 3 (Detailed) 13-15 marks

Clear, purposeful discussion that seeks to put a case for/against soft engineering

or is aware of the complementary nature of the strategies.

Advantages and disadvantages of soft and hard engineering are discussed.

Strategies are effectively used to illustrate concepts.

Case studies are used to make points.

Specific terminology is used throughout.

Lesson 27

Lesson objectives

• To examine the impacts for the use of river restoration schemes

Lesson criteria


Flood prediction and warning – Floodplain zoning, role of

Environment Agency(also methods of soft

engineering)

Lesson 28

Lesson 28

Lesson objectives

• To assess the use of flood prediction and warning and the importance of it, with specific reference to the Environment Agency (EA).

Success criteria

• Complete floodplain zoning diagrams

Starter: How useful is this floodplain informationchart in informing flood risk?

Floodplain zoning

Draw and annotate your own floodplain

zoning map, annotate each zone and

explain the importance of a floodplain zone

map.

Land use management on the floodplain (method of soft engineering)

PredictionFeet

above SLMagnitude

RecurrenceInterval (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

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

The Environment AgencyWhat 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

The Environment AgencyFlood prediction software• Helps to model likely outcomes, warnings may be issued in terms of the

potential severity of the flood risk and the areas that could be affected.• The Environment Agency’s England and Wales flood mapping system

is based on the National Flood Risk Assessment (NaFRA), which uses a method called Risk Assessment for Strategic Planning (RASP) – Risk-based probabilistic approach which factors the location, type, condition and performance of flood defences into the risk assessment.

• People concerned can call Floodline Direct to receive a service called Flood Warnings Direct (automatic telephone warnings).

• People in high risk areas e.g. Cumbria, receive automatic phone warnings to alert inhabitants in potential flood risk zones. Flood wardens ensure temporary defences are in place and evacuations carried out when necessary.

Plenary: Exam question 1How can planners help to restrict flood damage? (6)

They can stop the water getting to the settlement, by building a

reservoir and controlling the discharge. This was done when the Aswan Dam was built on the River Nile. They can also make sure that towns and cities are

not built too near a river which is likely to flood.They can also

make sure that there are enough sandbags to stop the floods

affecting the nearby buildings.

Good use of appropriate term; could just have said “water”

Suspect choice of example; prior to

Aswan being built the farmers relied on floods to provide

fertile alluvium. How much damage was

done?

Naïve; there are very few ‘new’

settlements whose location

can be determined by

planners. Impossible for every riverside

settlement have a stock - again, rather naive

Over-simplistic answer, with some poorly

chosen examples (2/6)

Where? needs to be upstream

Typical answer

Exam question 1

How can planners help to restrict flood damage? (6)

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.

Useful additional

detail

Always a good idea to add a named

exampleAgain, complex sentence

allows point to be developed

fully

Two examples combined in the one sentence

Although only three sentences, each is full,

with examples.

Would score as 3 x 2.

Specimen answer

Exam question 2Examine 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.

Lesson 28

Lesson objectives

• To assess the use of flood prediction and warning and the importance of it, with specific reference to the Environment Agency (EA).

Success criteria

• Complete floodplain zoning diagrams

Rivers, Floods and Management AQA. Contents Hydrological cycle and river discharge River processes...

Documents

Transcript of Rivers, Floods and Management AQA. Contents Hydrological cycle and river discharge River processes...