Topic 4 - River process and Pressures

Question 1

What is a drainage basin?

Answer 1

An area of land drained by a river an it’s tributaries.

Question 2

What are the Features of Drainage basins?

Answer 2

Watershed: Highland or hill that separates one drainage basin from another
Confluence: the point where two rivers/streams meet/join
Tributary: a smaller stream or river that joins a bigger stream or river
Source: the starting point of a river or stream
Mouth: the point where a river leaves the drainage basin and enters the sea

Question 3

Weathering

Answer 3

Weathering is the breakdown of rock by natural processes. There are three key
weathering processes that affect river valley’s

Question 4

Physical
(Freeze thaw)

Answer 4

water enters cracks in rocks and freezes when temperatures drop
below zero, the water expands, putting pressure on the rock. This
process of expanding and contracting causes the rick to break
into smaller pieces.

Question 5

Chemical (acid rain)

Answer 5

slightly acidic rainfall, polluted by factories and vehicles, reacts with weak minerals causing them to dissolve and decay.

Question 6

Biological weathering

Answer 6

the roots of plants grow in cracks and split the rock apart.

Question 7

Mass movement

Answer 7

is the transfer of material down the valley/slope due to gravity.

Question 8

Soil creep

Answer 8

Individual particles soil move slowly down a slope due to gravity

Question 9

Slumping

Answer 9

At the bottom of a valley slope the river erodes the valley side.
Material above slides downwards rotating as it does often after
times of heavy rain saturating the rock and soil making it heavy.

Question 10

River erosion

Answer 10

The action of water wearing away rocks and soil at times of flood and on steep
gradients. There are four key processes of erosion.

Question 11

Abrasion

Answer 11

Load is dragged by water wearing away the banks and bed of the river and causes most erosion.

Question 12

Attrition

Answer 12

Load collides with load and wears down/breaks up

Question 13

Solution

Answer 13

Weak acid dissolves rocks such as Limestone

Question 14

Hydraulic Action

Answer 14

The shear force of the water trapping air in cracks fracturing the rock on the banks and bed of the river

Question 15

Traction

Answer 15

large, heavy pebbles are rolled along the river bed. This is most common near the source of a river, as here the load is larger.

Question 16

Saltation

Answer 16

pebbles are bounced along the river bed, most commonly near the source.

Question 17

Suspension

Answer 17

lighter sediment is suspended (carried) within the water, most commonly near the
mouth of the river.

Question 18

Solution

Answer 18

the transport of dissolved chemicals. This varies along the river depending on the presence of soluble rocks.

Question 19

Deposition

Answer 19

When a river loses it’s energy deposition occurs. Heaviest material is deposited first.

Question 20

Interlocking spurs

Answer 20

At the source rivers have less power and flow around valley slopes (spurs) instead of eroding them. The spurs then inter
lock from one side to the other.

Question 21

Waterfalls

Answer 21

Occur where water flows over bands of rock with differing resistance. Weaker less resistant rock erodes quicker due to
increased velocity and creates a step in the river bed gradually undercutting the more resistant rock. Continued abrasion and
hydraulic action creates an overhang and a plunge pool is created at the base from abrasion and the force of falling water. Eventually the overhang will collapse and make the waterfall steeper. Repetition causes the waterfall to retreat upstream forming a steep-sided gorge.

Question 22

Meander

Answer 22

Large bends that swing from side to side (sinuosity) on the floodplain. Faster flowing water erodes the outside of the bend
through lateral erosion creating a steep bank (river cliff) whilst the inside of the bend due to slower shallower water
deposition takes places creating a gently sloping bank (slip-off slope).

Question 23

Oxbow Lake

Answer 23

When a meander grows its neck narrows then at times of flood the river simply cuts straight through it leaving an old meander
cut off (horseshoe-shaped lake). Deposition blocks up the old bend.

Question 24

Levees

Answer 24

Levees are natural embankments formed by the deposition of
sediment at times of flood. Large sediment is dropped first as the
river floods onto the floodplain and loses velocity. Smaller
sediment is deposited afterwards and when this process is
repeated the banks get higher forming Levees.

Question 25

Flood plains

Answer 25

The area of land at the side of a river in the lower course. Lateral
erosion on the outside bend cause meanders to migrate across
the valley floor so the valley floor becomes wide and flat. During
floods rivers deposit fine sediments called alluvium.

Question 26

Deltas

Answer 26

Water speed decreases near the sea. Material is deposited. Over time this
builds up to create an area of new land - a delta. Because the river is now flowing slowly the channel fills up with sediment and the river splits into different streams, distributaries.

Question 27

Upper Level

Answer 27

610m above SL,
2500mm rainfall
Hard, impermeable
geology e.g. shales

Question 28

Mid-course

Answer 28

700mm rainfall
Softer permeable rock
e.g. sandstone

Question 29

Lower course

Answer 29

soft geology e.g.
mudstones, River 70m
wide E.g. River Severn

Question 30

Gradient

Answer 30

Decreases: Steep-source (hills) Gentle-mouth due to a shift from vertical to lateral erosion.

Question 31

Velocity and Discharge

Answer 31

Both increase due to tributaries feeding more water into main channel and reduced friction.

Question 32

Channel width/depth/roughness

Answer 32

Becomes wider/deeper/smoother and more efficient
with less friction.

Question 33

Sediment/load

Answer 33

Smaller: Source =boulders/cobbles then pebbles/sand finally mouth= silt/clay due to abrasion and attrition. Capacity increases downstream.

Question 34

How do physical factors and human activities affect storm hydrographs?

Answer 34

A hydrograph is a way of showing how a river responds to a rainfall event showing
the relationship between rainfall (mm) and discharge (m3/cumecs). The shape of a
storm hydrograph varies due to a number of factors

Question 35

Rising limb

Answer 35

indicates discharge increases a few
hours after rainfall.

Question 36

Peak flow

Answer 36

Discharge reaches max levels.

Question 37

Recession (falling) limb

Answer 37

indicates a fall in discharge
once the water has passed
downstream.

Question 38

Lag time

Answer 38

time from peak rainfall to peak discharge

Question 39

How do Human and physical processes interact to cause of flooding?
E.g. Yorkshire, UK,2007

Answer 39

2007 saw very heavy rainfall in June and July with rivers level at their
highest for over 100yrs. Rainfall doubled the average for these months and
reached 140mm on one day, 20th July in just a few hours caused by a series
of depressions and a strong jet stream. Antecedent weather conditions led
to immediate run-off over saturated soils into already swollen rives. Flash
flooding in urban areas e.g. Sheffield caused flash flooding. Areas at the
confluence of two tributaries e.g Tewkesbury along the Severn and Avon.
It’s Abbey flooded for the first time in 250yrs.

Question 40

Why is the flood risk in the UK increasing?

Answer 40

Flooding is a natural occurrence but since 1998 severe flooding has oc
curred somewhere in the UK every year sometimes twice in a year. The
main reasons for this are as follows:
1. Increased population = more housing. Building on the cheaper land of
the flood plain has put 2.3million houses at risk of flooding.
2. Land use changes with urban developments = more impermeable surfac
es which increases surface run-off.
3. Changes in weather patterns linked to climate change making extreme
weather more likely as a result of the changes in the behaviour of the jet
stream. Storms that once occurred every 100yrs are now more likely to
happen every 80yrs in southern UK.

Question 41

How does the Environment Agency manage flood risk?

Answer 41

The Environment Agency makes Catchment Management Plans, manages
rivers and land use, controlling developments in flood plains, building flood defenses as well as helping people to prepare and giving warnings.

Question 42

How is flooding reduced through Catchment Management Plans?

Answer 42

The EA works out the chances of a flood happening for example:
Nr. The river Severn has a 1% chance of flooding which would put 60,000
people and 29,000 business at risk as well as infrastructure including roads
and power supplies. The plan would then include the following actions:
1. reduce run-off by improving land use and restoring flood plains
2. prevent unsuitable developments on the flood plain
3. improve flood defence in urban areas and protect vulnerable buildings
4. work with natural flood processes where few people live.

Question 43

(HE) Embankments– high
banks (levees)

Answer 43

Advantages
Stop overflowing, covered
in grass can blend with
the environment.
Disadvantages
Can burst under
pressure, water can
flow over the top.

Question 44

(HE) Flood walls

Answer 44

Advantages
Prevent water spreading
in high impact areas e.g.
housing
Disadvantages
Expensive, cause
flooding downstream,
look unnatural

Question 45

(HE) Demountable flood
barriers

Answer 45

Advantages
Put up and taken down,
replace ugly permanent defences
Disadvantages
Risk of timing issues,
can only be used
where deployed

Question 46

(HE) Flood barriers or storm
surge barriers

Answer 46

Advantages
Protect large areas, can
be used at high tide or
storm surge is forecast
Disadvantages
High construction
costs and regular
maintenance needed

Question 47

(SE) River restoration-
rivers original course
including meanders

Answer 47

Advantages
More attractive for
recreation, creates
natural habitats
Disadvantages
Some flood banks
often still needed.

Question 48

(SE) Floodplain retention -
land use according to
flood risk

Answer 48

Advantages
Low risk areas are used
for building, high risk land
is used for parks/
recreation.
Disadvantages
Poor public accessibility to some areas

Question 49

What decisions are made before building flood defences?

Answer 49

Because flood defences are so expensive the EA works out which would be
most effective with limited environmental damage by conducting an impact
assessment (residents, business, transport, wildlife and habitats) and a cost-benefit analysis (value for money). In 2000, severe flooding of the river
Severn affected 140 in Bewdley so local residents and businesses want to
improve the flood defences. The EA worked a number of possible options
including the costs and benefits:
Costs:
1. Do nothing therefore £0
2. Maintenance of banks £0.2m
3. Storage dams 1km upstream
£15m
4. Demountable aluminium flood
defences, 2.7m high costing £6.9m
Benefits:
1.Little benefit except £0 spent
2. Bank collapse prevented
3. Volume of water retained wouldn’t
prevent a 100yr event £0.5m
4. 150 properties protected, 24hr
warning required £7.5million