Pure Geography SA2 Overall Revision NotesChapter 4: Rivers and
Coasts
The River System
1. 2. 3. 4. 5. 6. 7. 8.
3 Courses: Lower, Middle, Upper. Watershed: A stretch of land
that surrounds drainage basin (a.k.a. boundary) Drainage Basin: A
river system that collects rainwater and comprises of streams and
rivers Tributaries: Streams that flow in the river which is mainly
found in the upper course. Distributaries: Branch out from the
river which is mainly found in the river delta. Confluence: Area
where the stream meets the river River Mouth: Place where the river
meets the ocean River Delta: Landform created at the mouth of the
sea where the river flows into. It is formed by the deposition of
the sediment carried by the river as it flows into the sea. 3
Courses of a River Place where the river begins Many tributaries
join up to form larger streams which in turn join to create a river
River starts to meander (curve) Many tributaries join the river
Meanders are common Many distributaries River flows towards the
river mouth and enters the sea Delta (land formed by sediments) may
form at river mouth
Upper Middle Lower
Pure Geography SA2 Overall Revision NotesChapter 4: Rivers and
Coasts
Factors affecting River Speed and VolumeFactors affecting Speed
of water in River Definition High Speed / Volume Low Speed / Volume
How steep a river is. River is steeper (usually at River is gentler
(lower course) upper course) How rough a river channel is Fewer
obstacles More obstacles More friction (affected by obstacles in
river) Smoother channel Reduced speed Faster Area in contact with
the river. Shape of river channel affects the wetted perimeter
(Length and Breadth in contact with the river) A smaller wetted
perimeter Lesser friction Faster A larger wetted perimeter More
water More friction Slower
Gradient Roughness of River Channel Wetted Perimeter
Factor Permeability of Rocks Vegetation Cover
Climate
Size of Drainage Basin
Factors affecting Volume of water in River Definition High Speed
/ Volume Permeability is the ability of Permeable rocks (holds more
rocks being able to hold water) Lesser surface runoff water
Vegetation: Plants at the Sparse vegetation Less side of the river
rainwater infiltrate ground More surface runoff Large volume How
rainfall / temperature Hot / Wet (High Evaporation) affects the
river High volume of water Wet/dry weather Water level fluctuates
(drop/unstable) Drainage Basin: An area Bigger Drainage Basin
Larger which collects water Surface area Higher volume (comprises
of rivers) of water
Low Speed / Volume Less Permeable rocks More surface runoff
(more volume of water) More vegetation More rainwater infiltrate
ground Lesser surface run off Smaller Volume Dry Low volume of
water throughout the year
Smaller drainage basin Smaller surface area Smaller volume of
water
High Speed + High Volume = High Energy!
Pure Geography SA2 Overall Revision NotesChapter 4: Rivers and
Coasts
Erosion, Transportation and Depositional Processes
Pure Geography SA2 Overall Revision NotesChapter 4: Rivers and
Coasts
River Landforms
Author Comments: This is a very important and crucial part of
the chapter. Remember to know how to draw the various river
landforms.
Floodplains and Leeves (Example: Floodplain of River Wyre,
England)
1. Floodplains are found at the lower course of a river. 2. When
river overflows due to heavy rain, flooding occurs and water
spreads over a larger area, losing speed. 3. When water recedes,
river deposits material carrying on the land, leaving behind
sediments on river banks and beds. 4. Coarser sediments are
deposited closer to river edges and fine particles are deposited
away from river. The deposited materials build up on both sides of
river due to flooding. 5. Over time, the accumulation of sediments
at the flat plains called floodplains is formed. Coarser and
heavier materials accumulated nearer to the river form raised banks
called levees.
Valleys and Gorges (Example: 1. Valleys and gorges are usually
found in the upper course where vertical erosion is prominent. 2.
In the upper course, the volume of water is small but the gradient
is steep. 3. The swift flowing water exerts a powerful force on the
river bed, forming a V-shaped valley through hydraulic action,
abrasion and solution. 4. A gorge is an exceptionally deep and
narrow valley formed when the river erodes vertically through
resistant rocks leaving very steep valley walls. Waterfalls
(Example: Iguazu Falls, border of Argentina and Brazil)
1. River flows through across rocks of different resistance, 2.
eroding the less resistant rocks more rapidly, 3. causing a change
of gradient in river course. 4. Over time, river plunges from great
height to hit river bed below with tremendous force, forming a
waterfall. 6. 7. 8. 9.
5. Waterfall can also be formed by faulting, where rocks are
uplifted, causing displacement of rocks where 1 layer is higher
than the other. When the river flows across an area where faulting
occurred, gradient drops suddenly.
Repeated pounding of river bed leaves a depression at base of
waterfall. Depression is further deepened by rocks and boulders
swirling around, forming a plunge pool. Over time, as the harder
rock is undercut it produces an overhang which will eventually
collapse. As the rock collapses the waterfall retreats upstream,
leaving a gorge.
Meanders and Oxbow Lakes 1. Meanders can occur anywhere along
the course of the river, however they are more prominent in the
middle/ lower course. 2. As the river travels downstream, it tends
to find the easiest route, thus it will bend. 3. Therefore, bends
of the river at middle/lower course allows erosion to occur at the
outer bank of a river. 4. Eroded materials are deposited at the
inner bank where water moves in a spiral fashion. Along the outer
bank, speed of river is high due to less friction and greater
volume. 5. Over time, the concave bank gets undercut1 and a
steep-sided cliff known as a river cliff is formed. 6. As the bends
become more obvious, a meander is formed. Formation of Oxbow Lakes
7. Constant erosion at 2 consecutive concave banks will cause the 2
meanders to more closer, forming a loop. Over time, it becomes more
distinct, separated by land. 8. As deposition and erosion
continues, the 2 meanders eventually meet. 9. Sediments deposited
at convex banks will dam up the river, forming an oxbow lake which
will be separated from the main river. The main river will flow in
a straight path. 10. Due to deposition the old meander bend is left
isolated from the main channel as an ox-bow lake.
1
The process in which material at the bottom of the cliff or
river bank is eroded away. This may cause the upper portion of the
cliff river bank to collapse.
Formation of Deltas 1. Deltas are found at the lower course of
the river. 2. As a river approaches sea/lake, speed decreases, thus
deposition occurs. 3. Although sediments can be removed by tides,
if rate of deposition is faster than removal, sediments will build
up at mouth. 4. Over time, more sediment is deposited and delta
extends outwards into the sea. Sediments carried will block flow of
main river. 5. As a result, smaller channels known as
distributaries are formed to find their way out to reach the sea or
lake. 6. Deltas only form under certain conditions a. The river
must be transporting a large amount of sediment b. The sea must
have a small tidal range and weak currents c. The sea must be
shallow at the river mouth
Pure Geography SA2 Overall Revision NotesChapter 4: Rivers and
Coasts
River Management Strategies
1. Key Principles: a. Aimed to improve the speed of flow (take
note: This can increase the amount of erosion, reducing the ability
of the river to hold water). b. Increase the capacity of the river.
(N.B. By increasing the volume of water, more water will flow
downstream, causing flooding there) River Channelization Processes
such as resectioning is the widening or deepening of a river to let
it carry more water, whereas Realignment deals with straightening
the river channel to increase the speed of flow. Although this can
solve the problem upstream, however it will increase the chances of
flooding downstream. This is because when there is greater speed
and volume of the water in the river, more erosion will occur and
thus more sediment will be deposited at the river bed downstream,
reducing the capacity of the river to hold water. When the huge
volume of water reaches downstream, it will eventually cause
flooding.
Pure Geography SA2 Overall Revision NotesChapter 4: Rivers and
Coasts
IntroductionChapter 7.1: Wave Energy 1. It is the energy
produced by movement of waves 2. Factors that can affect wave
energy: Wind Energy: Wind is air moving from region of high to low
pressure. The faster wind speed, the greater wave energy. Duration
of wind: The longer wind blows, the larger the waves. Fetch:
Distance of sea over which winds blow to generate waves. The longer
the distance, the more energy the waves have. 3. The wave energy
determines what kind of coastal processes the waves possesses.
Chapter 7.2: Wave Movement 4. Waves move in a series of circular
looplike motions. As the waves approach the shore, the water
becomes shallower; the friction of the waves against the sea floor
slows down wave movement. The frictional force cause the waves to
rise and bend forward, and eventually collapse to foaming water.
Swash is the forward movement of waves to the shore. It carries
sediments to the shore. Backwash is the flow back to the sea due to
gravity. It carries sediments back to the sea. Chapter 7.3:
Constructive and Destructive Waves Wave Height Wave length Wave
Frequency Breakers Powerful Swash or backwash Place of occurrence
Prominent Process Constructive Waves Low Long 6-8 per minute
Spilling Swash Gentle costal slope, sheltered coast Deposition
Destructive Waves High Short 10-14 per minute Plunging Backwash
Steep slope, open coast Erosion
Chapter 7.4: Tides Refers to the daily alternate rising and
falling of sea level seen along coasts. Mainly caused by gravity of
the Moon. The pull produces high tides on opposite sides of the
Earth. Each costal area receives 2 high tides and 2 low tides
daily. The difference in water level between the 2 tides is known
as tidal range. The rise and fall of sea level affects erosion and
transportation. Areas located between low and high tides will
experience more weathering and erosion as it have continuous drying
and wetting.
Chapter 7.5: Currents Refers to the large-scale and persistent
movements of water in the ocean. Driven mainly by winds. Longshore
currents is a ocean current that flow parallel to a coast and are
formed by waves that approach the coast at an oblique angle.
Pure GeographyChapter 7: Coastal System and Processes
Coastal ProcessesCoastal Erosion 1. Corrasion (Abrasion): Rock
fragments are hurled at cliffs by breaking waves, gradually
scraping away at the cliff face. 2. Attrition: Rock fragments
carried by the waves hit each other and wear down to form sand and
silt. 3. Solution (Corrosion): Occurs when the salt water can
dissolve some chemicals in the rocks. 4. Hydraulic Action: Waves
break against the cliff face, the pressure of the breaking wave
compresses air in the cracks, which gradually forces open the crack
in the rock. It becomes weakened as the process continues. 5.
Scouring: This occurs at the base of the cliff as the waves break
and swirl around, gradually removing loose rock. 6. Wave Pounding:
The sheer force of waves hitting against the cliff face. Coastal
Deposition 1. 2. 3. 4. Deposition occurs when the material is too
heavy to be transported is left behind, building up the beach. The
largest material is deposited first. Distinctive features may form
due to deposition. Factors: Deposition of sediment along the coast
is dependent on: Supply of sediment: Most sediment is transported
down to the coast by rivers, from coastal erosion or from coasts.
When the waves cannot carry their load of sediment, deposition
takes place. Gradient of slope: On gentle slopes, the wave energy
is spreaded out and reduced due to the friction with the shore and
gravity, resulting in constructive waves as they deposit materials
more than eroding them. Position of the coast: Coasts that are
sheltered from strong winds and the calm coastal condition allow
deposition to take place. It also occurs when waves enter a region
of shallow water. Coastal Transportation
1. Waves usually approach the coast at an angle in the direction
of the prevailing wind. 2. This causes the waves to break on the
beach. 3. The swash then carries materials up the beach at an
angle. The backwash then flows back to the sea in a straight line
at 90o due to gravity. 4. This movement of material is called
transportation. 5. The combined effect causes material to be
transported sideways along the coast known as longshore drift. 6.
The 4 ways that waves can transport sediment (solution, suspension,
saltation, traction applies here too!)
Pure GeographyChapter 7: Coastal System and Processes
Coastal LandformsCliffs and Wave cut platforms 1. Cliffs are
produced by the action of waves undercutting a steep rocky coast.
2. Hydraulic action and abrasion may erode a crack on the rock
surface. 3. The crack can enlarge to form a notch, which is further
deepened to produce a cave. Further undercutting by the waves will
cause the roof of the cave to collapse. 4. An overhanging cliff is
formed, which will eventually collapse and its sediments deposited
at the foot of the cliff. 5. Those sediments might be picked by
crashing waves and thrown against the base of the cliff, causing
further erosion. A cliff is formed. 6. Continuous erosion may cause
the cliff to retreat further inland and over time, a wave-cut
platform is formed at the foot of the cliff.
Headlands and Bays 1. A headland is made of resistant rocks
which extend outwards into the sea and formed due to differential
erosion of coastal rocks. 2. Coasts with alternate strips of
resistant hard rock and less resistant soft rock will cause the
soft rock to be eroded faster than the hard rock. 3. The result is
the formation of an indented coast with the remaining hard rocks
forming headlands extending into the sea and soft rocks forming
bays.
Beach
1. A beach is a zone of deposition along the coast. It is formed
when materials carried by waves and currents is deposited on gentle
slopes and constructive waves which allows materials to be
deposited by the strong swash. 2. They are formed at protected bays
along an indented coast due to wave refraction, where waves
approach the shallow sea in front of the headland first. 3. As wave
energy tends to concentrate at the headlands, erosion takes place
there. 4. Along the bays, waves are diverged, thus their energy is
spread out and weakened. Deposition occurs, and over time beaches
are formed.
Spit and Tombolo
1. Spits are beaches joined to the coast at just one end. 2.
Formed by longshore drift. 3. When the direction of the coastline
changes, it continues to deposit materials in the original
direction which accumulate in the sea forming a spit. 4. The spit
continues to grow as materials are continuously deposited, joining
a nearby island to the mainland, forming a tombolo.
Pure GeographyChapter 7: Coastal System and Processes
Coastal Protection Measures1. 2. 3. 4. The rationale and
principle for coastal protection usually rest on the economic
values of these coastal areas. Often, the implementation of these
measures led to more problems rather than solving the threats from
coastal processes. The 2 underlying principles are to reduce the
energy of waves and/or retain coastal materials. Types of
Approaches: Hard engineering: Construction of physical structures
to defend against the erosive power of waves. Soft engineering:
Focuses on planning, management and changing individual attitudes
towards coastal protection.
Measure Seawalls (e.g. Kallang River in Singapore)
AdvantagesThey are built along the coast which absorbs the
energy of waves before they can erode away loose materials.
Disadvantages1. However, they do not prevent the powerful
backsplash of refracted waves from washing away the beach materials
between the walls. 2. The backsplashes will eventually undermine
the base of the seawall and eventually it collapses. 3. They are
costly to build and maintain as repairs have to be made to prevent
their collapse. However materials deposited in the zones behind the
breakwater are protected but the zones located away from the
breakwater are not, and will be eroded away.
Breakwaters
1.
Groynes
They help protect the coast and harbour by reducing the force of
high energy waves before they reach the waves. 2. They create a
zone of calm water behind them. 1. They are built at right angles
to the shore to prevent longshore drift. 2. They absorb/reduce the
energy of the waves and cause materials to be deposited on the side
of the groyne facing the longshore drift.
3. As no fresh materials are deposited on the downdrift side,
that part will gradually be eroded. 4. To prevent this, the tips of
groynes are sometimes angled about 5 to 10, but if failed, erosion
can still take place on unprotected areas.
Gabions
1.
They are wire cages filled with crushed rocks that are piled up
along the shore to reduce coastal erosion by weakening wave
energy.
Beach Nourishment Relocation of Property
Planting of Mangroves Coastal Dunes
Refers to the constant replenishment of large quantities of sand
to the beach system, which leads to the improvement of beach
quality and storm protection. 1. Coastal planners are trying to
protect manmade structures by relocating them and let nature
reclaim the beach slowly. 2. No building of new structures is
allowed in coastal areas vulnerable to coastal erosion. 1.
Mangroves can help to trap sediments and reduce coastal
erosion.
2. They need to be maintained as they are easily corroded. 3. If
not properly maintained, the wire baskets become unsightly and pose
a threat. However it is expensive to constantly transport sand to
fill up the beach as it is continuously eroded. Coral reefs may be
in danger as the sand covers the corals. It can also led to muddy
and polluted water. This approach is likely to be opposed by people
who want to invest in the coastal areas.
2. 3.
1.
Growth of Coral Reefs
1. 2.
Coastal dunes can be found along the shore where lots of sand
and strong winds exist. The winds carry and deposit the land on the
coast further inland, forming coastal dunes gradually which help
defend the coast from the sea. They can weaken wave energy.
Artificial reefs are created along the coast to help enhance
fishing opportunities, serve as undersea barriers and replace
damaged coral reefs.
2. 4.
However some areas with violent destructive waves may not
support mangroves. The depth of the coast may also become
shallower, affecting coastal transportation and port activities.
However they are very fragile and thus access points to the beach
are controlled and designated. Shrubs and trees are planted to
stabilise the dunes to anchor the sand.
3.
Tourists can damage these coral reefs.