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Rivers, floods and management

 

  Rivers

Objective 1 – Describe and explain the drainage basin hydrological cycle: the water balance.

Experts’ Tips

• As in all parts of the specification the knowledge of this section can be tested in short, low scoring questions (2 – 4 marks). Command words could include ‘define’, outline’, ‘describe’.

• It could be tested in structured questions such as ‘Explain the likely effects of …’
What you must know:

• You need to know what happens to water from the time that it leaves the atmosphere until the time it leaves the drainage basin either through runoff, evaporation or transpiration. The route taken by the water involves a series of STORES and TRANSFERS. You must know what the stores and transfers are and understand the relationship between the various stores and transfers.

• You need to understand how changes in local conditions can change the way that water flows through the system. E.g.
A. Deciduous forests intercept more precipitation in summer than winter because they shed their leaves in winter.
B. Vegetation stores, uses and transpires much more water in spring and summer than it does in winter.
C. Urban areas have many more impermeable surfaces than rural areas and so infiltration is low and over-land flow is much higher.

•You must understand the concept of the water balance. The drainage basin hydrological cycle is an open system. Although the relationship between the inputs and outputs can be very complex, it can be summed up by the equation:

P = Q + E +/- S where:
P is precipitation
Q is runoff (measured in m3s-1or CUMECS)
E is evapotranspiration
ΔS is the change in storage (in surface, soil, vegetation or the bedrock).

Once you have understood the above concepts then you move to the next part of the specification.

Objective 2 – Which factors affect river discharge: the storm hydrograph.

Experts’ Tips

• Questions can be short (2 – 4 marks) definitions etc OR they may test a skill by asking you to complete a storm hydrograph.

• Understanding may be tested using structured questions.

• This topic may be included in an extended prose question worth 15 marks.

What you need to know:

• How water reaches a channel in a drainage basin.

• Overland flow, groundwater flow, throughflow and direct precipitation vary according to the environmental conditions.

• The speeds of these transfers differ. Direct precipitation reaches the river first. This is followed by overland flow and then the much slower throughflow and groundwater flows.

Once this is understood then you have to:
• Be able to describe, explain the shape of and sketch a storm hydrograph.

• You must be able to describe and explain how the shape of a storm hydrograph may vary because of variations in the drainage basin hydrological cycle. E.g.
A. The storm hydrograph for an urban area is more flashy than one for a rural area.
B. The storm hydrograph for a river in deciduous woodland is flashier in winter than summer.
C. The intensity of precipitation affects the steepness of the rising limb.

Questions:

Describe the various ways in which water reaches a river channel. [4 marks]

2. Define the term ‘interception’ (this could be: infiltration, percolation, throughflow, groundwater flow etc) [2 marks]

3. Explain how a drainage basin dominated by deciduous forest may affect the discharge of a river. (this could be: dominated by permeable rock or impermeable urban surfaces etc)[5 marks]

4. Describe and explain the likely differences in the shapes of storm hydrographs for a stream before and after an area has become urbanised. [7 marks]

Objective 3 – Describe the long profile – changing processes: types of erosion, transportation and deposition, types of load; the Hjulstorm curve.

Experts’ Tips

• The most common questions on this section are of the short definition/explanation type. Command words could include ‘define’, outline’, ‘describe’.

• An understanding of the processes is important when longer structured questions are asked.

What you need to know:

• You need to know the various processes that take place in a river and where they dominate in the river’s flow from source to mouth.

• The types of erosion are:
A. Abrasion (including pothole drilling)
B. Hydraulic action
C. Attrition.

You must be able to say how each process operates and what effect(s) it has on the river bed, banks and load.

• The types of transportation are:
A. Traction
B. Saltation
C. Suspension
D. Solution

You must, once again, be able to state how they operate and what type of load they move.

• The load varies in size from large boulders to very fine clay. Dissolved load only occurs at the ionic level.

• You must be able to understand the Hjulstrom curve.

Objective 4 – Valley profiles – long profile and changing cross-profile downstream, graded profile, potential and kinetic energy.

Experts’ Tips

• Questions can be short (2 – 4 marks) definitions etc OR a skill could be tested by asking you to interpret an OS map.

• Understanding may be tested using structured questions.

• This topic may be included in an extended prose question worth 15 marks.
What you need to know:

• This section is clearly about the valley, not the channel. Many river valleys have a steep gradient close to the source and this gradient reduces as the river moves from source to base level. An ideal version of this profile is the graded profile.

• For convenience, rivers are divided into three stages: upper, middle and lower.

• Near the source of a river the water velocity and mass is low and the load of the river is coarse. The river’s energy is dominated by potential energy (a result of the river’s mass and the effect of gravity). This causes the river to erode vertically.

• As the river flows into its lower stages it gets bigger and faster. This results in an increase in kinetic energy (a function of mass and velocity). Erosion can be both vertical and lateral. Transportation increases because of an increase in the kinetic energy as well as a decrease in the size of the load.

In the lowest part of the river, load is very fine, but there is a lot of it. Any erosion that occurs is lateral. Deposition is dominant.

Questions:

5. Describe the various ways in which a river erodes its banks and bed [or transports its load]. [4 marks]

6. Define the term ‘hydraulic action’ (this could be: abrasion, attrition, traction, saltation, suspension, etc) [2 marks]

7. Explain why the cross profile of a river changes downstream. [5 marks]

Objective 5 – Changing channel characteristics – cross profile, wetted perimeter, hydraulic radius, roughness, efficiency and links to velocity and discharge.

Experts’ Tips

• This section is about the channel rather than the valley. Try not to get the two mixed up.

• Much of this is background information which helps you understand other aspects of river studies but direct questions can still be asked about it.

What you need to know:

The cross profile refers to both the shape and the area of the river cross section.

The wetted perimeter (WP) is the length of the river bed in cross section, in contact with the moving water. In the upper reaches of the river this is high in relation to its size because of all the boulders etc on the stream bed.

The hydraulic radius (HR) is a measure of the efficiency of a stream. It is calculated by the following equation:

HR=A/WP

The higher the HR the more efficient is the stream. This means that there is less energy used in overcoming the friction between the moving water and the river banks and bed.

The roughness of a river depends on the amount of coarse bed load it contains. The rougher a river bed, the higher the hydraulic radius and the less efficient the stream.

Questions:

8. How can the efficiency [or hydraulic radius] of a stream be measured? [4 marks]

9. Define the term ‘wetted perimeter’ (this could be: hydraulic radius, roughness, efficiency etc) [2 marks]

10. Describe and explain how channel roughness and efficiency affect the velocity and discharge. [7 marks]

Objective 6 – Which landforms are associated with flu vial erosion and deposition – potholes, rapids, waterfalls, meanders, braiding, levees, flood plains and deltas.


Experts’ Tips

• It is a good idea to have case studies of at least one of each of the above named landforms. For each one you must be able to describe the landform, explain its formation and explain its location within the landscape.

• Experts do not give many marks for generic descriptions of landforms.

There are a variety of questions that could be asked about this section. These include:

• Interpretation of photographs. i.e. Describe the landform in a photograph and explain how it was formed.
• Description and explanation of a named landform; this would be a structured question.
• Description and explanation of a collection of landforms in a landscape; this could be an extended essay.
What you need to know:

• You must be able to describe examples of all the above named landforms.

• You must be able to locate the landforms with respect to the long profile of a river.

• You must be able to explain the formation of the landform by the processes named in part [3].

Questions:

11. Erosion is important in the formation of some river landforms. Describe the role of erosion in the formation of river landforms. [7 marks]

12. Describe one landform that is made by a combination of erosion and deposition. Explain the roles of erosion and deposition in the formation of your chosen landform. [7 marks]

Objective 7 – Processes and impact of rejuvenation – knick points, waterfalls, river terraces and incised meanders.

Experts’ Tips

Questions could be asked that are very like those in part [6]. That is:

• Interpretation of photographs. i.e. Describe the landform in a photograph and explain how rejuvenation has led to its formation

• Description and explanation of a named landform caused by rejuvenation; this would be a structured question.

• Description and explanation of a collection of landforms in a rejuvenated landscape; this could be an extended essay.

What you need to know:

• You should be able to explain why sea levels have fallen

• Link the fact that changes in potential energy result in vertical erosion

• Changes to the long profile of a valley: know what is meant by knick point and how that can lead to the development of waterfalls.

• Changes to the valley cross profile: paired and asymmetrical river terraces (with examples) and incised meanders (both entrenched and ingrown).

Question:

13. Describe the landforms that are the result of rejuvenation. Explain how they were formed.[15 marks]

Objective 8 – Magnitude – frequency analysis of flood risk.

Experts’ Tips

• You could be asked to add to or read off a logarithmic graph (flood frequency graph).

• The interpretation of flood frequency graphs may be asked in structured questions.

• This section could be part of an extended essay about flooding.

What do you need to know:

• How past discharge records can be used to predict the likelihood of a flood of a particular magnitude.

• The concept of recurrence interval and flood return period.

Understand both the construction and interpretation of a flood frequency graph.

Questions:

14. Outline what is meant by ‘flood return period (recurrence interval)’.[4 marks]

15. Using two case studies from recent flooding events from contrasting parts of the world discuss how flood prediction has affected both the impacts of floods and the responses to floods. [15 marks]

Objective 9 – What are the physical and human causes of flooding – two case studies of recent flooding events should be undertaken from contrasting areas of the world.

Experts’ Tips

• Case studies are clearly asked for here.

• Experts will never give higher level marks for generic answers. Any details that you give about a flood must be with respect to your chosen case studies.

• Questions could range from short ‘outline’ type to extended essays.

• There are often demands to ‘discuss’; this means that you should present more than one side of an argument (using evidence from your chosen case studies). There may be positive and negative aspects to the discussion, or winners and losers.

What you need to know:

All of the following should be undertaken with respect to your chosen case studies:

• Physical causes of flooding:
• Excessive rainfall over a long time period and/or very intense rainfall.
• Impermeable ground surfaces e.g. frozen surfaces, baked surfaces.
• Snow/ice melt
• Tropical revolving storms
• Abnormally deep mid-latitude depressions.

• Human causes of flooding:
• The growth of urban areas. These increase infiltration and increase rapid overland flow. Urban surfaces are designed to shed water quickly (e.g. pitched roofs and cambered roads) and urban rivers are often constricted.
• Deforestation. This takes away the interception layer, reduces evapotranspiration and removes the anchoring effect of roots. Soil is more easily washed away leaving bare surfaces, encouraging overland flow. The soil chokes river channels.

Questions:

16. Outline one physical cause and one human cause of flooding. [4 marks]

17. With specific reference to recent case studies of flooding from contrasting areas of the world, assess the relative importance of their physical and human causes. [15 marks]

Objective 10 – What are the impacts of floods – two case studies of recent flooding events should be undertaken from contrasting areas of the world.

Experts’ Tips

• Note that this section is about impacts and not responses.

• Use the same two case studies as for part [9]

• As mentioned above, impacts must relate directly to your case studies. Experts do not like generic material

• This section could be part of an extended essay about flooding.

What you need to know:

Impacts can be classified as

• Physical impacts (e.g. How big an area was flooded or how much alluvium was deposited)
• Social impacts (e.g. Deaths, disease, displacement, homelessness, loss of infrastructure etc).
• Economic impacts (e.g. Insurance bill, total costs, loss of jobs, loss of crops, commodity price changes etc.)

Question:

18. For two floods in contrasting parts of the world, compare and comment on the physical, economic and social effects of flooding. [15 marks]

Objective 11 – Discuss flood management strategies – to include hard engineering – dams, straightening, building up levees, diversion spillways and soft engineering – forecasts and warnings, land use management on flood plains, wetland and river.


Experts’ Tips

• The best way to cover this section is to use case studies. Generic descriptions of flood management strategies will rarely get a mark above a level 1.

• For each of the strategies you need to be able to answer the following questions

• What does it look like (if appropriate)?

• How does it alleviate or stop flooding?

• How much did it cost?

• How successful has it been?

• Does it have any negative effects?

• Questions will ranges from short tasks to extended essays.bank conservation and river restoration.
What do you need to know:

The specification is very clear about the range of strategies that you have to know.

Questions:

19. Outline one hard [or soft] engineering strategy that can be used to manage flooding. [4 marks]

20. Discuss the advantages and disadvantages of soft engineering as a flood management strategy.[15 marks]

 

Fieldwork

Use the website below which will help in studying for the final exams (paper 2 re-cap)

http://www.rgs.org/OurWork/Schools/Fieldwork+and+local+learning/Fieldwork+techniques/Rivers.htm

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