THEME 2: NATURAL ENVIRONMENT — RIVERS & COASTS
Key:
Syllabus point
Definition or equation
Case study
2.2 Rivers
Global hydrological cycle
Key definitions
Inputs
When water is added to a system
Processes/Transfers
When water is moving within a system
Stores
When water is stationary within a system
Outputs
When water leaves a system
Precipitation: Any moisture that falls from the sky, e.g. rain and snow
Infiltration: When water travels from the surface of the earth into the ground beneath
Groundwater storage: water that is stored in saturated ground
Evaporation: liquid water from surface stores and rivers turning into water vapour (gas)
Percolation: when water travels from unsaturated ground into saturated ground
Soil-moisture storage: Water that is stored below the surface in unsaturated ground
Transpiration: Liquid water evaporating from vegetation
Throughflow: The horizontal movement of water through unsaturated ground
Surface storage: any water that is held on the surface of the earth eg. lake or pond. Some surface stores like puddles may only be temporary
River discharge: Eventually most rivers enter the sea and discharge the river’s flow into the sea
Groundwater flow: the movement of water through saturated ground
Overland flow/Surface runoff: When water travels across the surface of the Earth
Interception: When an object stops precipitation from infiltrating into the ground beneath
Drainage basins
Considered an open system with inputs, outputs, stores and processes (see table above)
Definitions:
Drainage basin: a catchment area drained by a river and its tributaries
Tributary: a smaller stream feeding into a larger river
Watershed: the area of high land forming the edge/boundary of a drainage basin
Confluence: the point at which more than one river/stream meet
Source of river: where the river originates (usually high up in mountains)
Mouth of river: where the river empties into a lake or ocean
Load: eroded material which the river transports
River profile
A river can be split into 3 main parts — upper, middle and lower courses
River characteristics change downstream — the 3 courses each have different characteristics
Cross-sections:
UPPER MIDDLE LOWER
Vertical erosion (downwards)
Less vertical erosion, more lateral (horizontal) erosion
Lateral erosion
Steep V-shaped valleys
Channel is deeper & wider; more U-shaped
Channel is at its widest and deepest
Load is larger and more angular (sharper)
Load becomes smaller and less angular (more rounded edges)
Very small and very rounded load
Bradshaw Model:
More definitions:
Discharge: the volume of water flowing through the river channel at a given point, per unit of time
Cross-sectional area: channel width channel depth
Wetted perimeter: Total length of the river bed and banks that is in contact with the water
Velocity: speed at which the water is flowing through the river
Hydraulic radius: cross sectional area wetted perimeter (measures efficiency of a river — ie. how fast water flows from source to mouth)
River processes - definitions
→ Erosional processes:
Hydraulic action: erosion that occurs when the motion of water against a rock surface produces mechanical weathering
Corrasion/Abrasion: when pebbles grind along a rock platform (works like sandpaper). Over time, the rock platform becomes smoother
Corrosion/Solution: when carbon dioxide dissolves in river water to produce a weak acid, which then dissolves rocks by chemical processes
Attrition: when eroded rocks collide and break into smaller fragments due to friction and become smaller and rounder over time
Rocks become smaller and rounder as they move downstream
→ Transportational processes
Traction: when large boulders are rolled along the riverbed
Saltation: when small stones and pebbles are bounced along the riverbed
Suspension: when fine, light material are suspended in the water/float on the surface
Solution: when material is dissolved in water and carried along by the river
→ Deposition: when the water ‘drops’ the eroded material it has been carrying onto a landform (eg. floodplain, delta)
River landforms
Waterfalls
Hard rock overlies soft rock
Soft rock is eroded as a result of abrasion and hydraulic action while hard rock is not
Differential rates of erosion
A plunge pool forms over the eroded area and starts to get deeper due to erosion. It begins to undercut the hard rock, causing an overhang
Eventually, due to erosion, weathering and effects of gravity, the overhang collapses
As erosion continues to occur, the waterfall retreats backwards, forming a gorge (a narrow valley)
Potholes
Meanders and oxbow lakes
Meanders are found in the middle and lower courses, where there are gentler gradients that allows lateral erosion to occur, widening the river channel
Form at slight curvatures in rivers
Results in large, horseshoe-like bends
As the river bends, the flow of water is faster on the outer bend, resulting in greater erosion
Flow of water is slower in the inner bend, resulting in greater deposition
This causes the rivers to become more curved
As the river continues to bend, an oxbow lake can form
During a flood event, the river cuts through the neck of the meander
Material is deposited at the neck, eventually cutting off the curve from the main channel
The curve is known as an oxbow lake
Floodplains and levees
Floodplains are found in the lower courses of a river, and are made of deposited clay, silt and alluvium
When a river floods, its velocity is reduced, causing the river to deposit some of its load
Coarser, heavier material is dropped first, which forms levees on the river banks
Over time, lighter material like silt and clay is deposited, forming floodplains (flat pieces of land on both sides of the river)
Deltas
Three types of deltas:
When a river meets a body of water with low velocity, the river discharge also decreases in velocity
If the river has a high sediment load (alluvium, silt etc.) that is too much for the water body’s current/tides to carry away, the sediment will build up
Heaviest sediment deposited first; lighter sediment deposited further downstream
Sediment collects to form deltas
Deltas typically have many distributaries (‘branches’); some have lagoons
Hazards of living near a river or delta/on a floodplain
Flooding:
→ Causes:
Heavy rainfall/monsoon season
Snow melt
Deforestation
Less trees for plant interception = more surface runoff entering river
Impermeable rock (allows no/little fluid to pass through)
Less infiltration of rainwater into ground = more surface runoff entering river
Urbanisation
Causes deforestation
More impermeable surfaces (eg. concrete, brick) = less infiltration of rainwater = more surface runoff entering river
→ Effects:
Death/injury
Loss of homes/personal belongings
Crops destroyed, which could lead to starvation
Disruption to work/education
Destruction of important transport networks, which impedes rescue/evacuation efforts
Spread of water-borne diseases eg. malaria
Financial burden for governments due to evacuation/rescue missions, emergency aid for affected people etc.
Erosion of riverbanks — impacts:
Homes and other buildings collapse/become unstable
Crops destroyed/reduction in area of farmland
Opportunities of living near a river or delta/on a floodplain
Fishing
Recreational sports activities eg. canoeing, swimming
Irrigation and alluvium (rich in nutrients) for agriculture
Transportation and trading
Potential for construction of dams and hydroelectricity generators
Tourism
Managing the impacts of flooding
Flood prediction — estimating the location, time and intensity of future flood events so that people can be evacuated in time/other resources can be mobilised in time
Satellites/GPS imaging
Weather forecasts
Use of past flood data to predict future floods
Flood mitigation — how to minimise the impacts of floods (preventing floods or reducing their intensity)
→ Hard engineering strategies: making permanent physical changes to the river
Dams: structures built across rivers that have gates which can be opened and closed to control the volume of water flowing downstream
Man-made dams create artificial lakes called reservoirs, which stores river water
Channel modifications
Straightening, widening, dredging (smoothing the riverbed by removing sediment), creation of new river channels
Embankments (man-made levees): similar to a wall which prevents flood water from flowing onto the floodplain
→ Soft engineering strategies: works with natural river systems and processes so that any changes made are not permanent or damaging
Afforestation: planting more trees to increase plant interception
Sandbags: temporary barriers/walls
Planning and preparing for flood events:
Educating locals on the impacts of floods, running evacuation drills
Training emergency personnel (eg. rescue, healthcare) for potential flood events
Investing in prediction and warning systems
Setting aside money and other resources to be mobilised in flood emergencies
Land-use zoning: urban planning which prevents important infrastructure (eg. housing, hospitals, schools) from being built on flood-prone land
Case study: Bangladesh — causes and effects of flooding (hazards) + management of hazards + opportunities
→ Background information:
South Asian LEDC located on the Bay of Bengal
80% on floodplain
Most of the country made up of the Ganges river delta
Confluence of three major rivers and their tributaries: Ganges, Brahmaputra and Meghna
Over 230 rivers in total
→ Causes of flooding:
During monsoon season, some parts of the country receive up to 500mm of rainfall a day
Three rivers converge in Bangladesh, the Ganges, Meghna and Brahmaputra, causing an increase in river discharge
Majority of Bangladesh’s land is low-lying
Heavy deforestation and urbanisation → less plant interception and more impermeable surfaces → greater surface runoff
Snow melt from Himalayas mountains in the north
Sand from mining in India (upstream) clogs up rivers
→ Effects of flooding:
Over 4 million people displaced from their homes and needed emergency relief during 2022 floods; over 100 dead
Erosion of land has caused houses and other buildings to collapse; cropland to be destroyed
Decrease in crop harvests (which has led to food shortages) — floods have contributed to a 61% decrease in wheat harvests
Spread of diseases eg. malaria
Caused landslides/mudslides
Destruction of transport networks made it challenging for relief efforts to reach those affected, even though the government set aside over 300 million Bangaldeshi taka
→ Flood management:
1990 Flood Action Plan: planned and set aside money for various hard and soft engineering mitigation strategies, but limited funding has delayed their execution
Construction of man-made embankments
Distribution of water purification tablets
Construction of raised flood shelters
Evacuation drills and warning systems
Land-use zoning
Planning for building of more flood prevention dams
Construction of more raised houses that can withstand floods
→ Opportunities that the rivers provide:
Irrigation (71% of Bangladesh is agricultural land; some crucial agricultural exports are wheat, rice and jute)
Alluvium — between 1-1.4 billion tonnes of fertile silt transported by all rivers every year
Rivers are the main source of water for domestic consumption (80% of the country is floodplain)
Factories also use water for industrial purposes
Inter-district transportation for locals and tourists
Trade of goods within and out of the country
Fishing — Bangladesh is 3rd in the world for inland fish production; fish is one of the staple foods for locals
2.3 Coasts
Waves
Wave size affected by 3 factors:
Wind speed
Wind duration — how long the wind has blown over the ocean
Fetch — distance the wind blows over open water
The greater the fetch = the bigger the wave = the greater the wave’s energy and erosional power
Wave movement:
Swash: movement of waves up the beach
Backwash: movement of waves down the beach
2 types of waves:
Constructive:
Stronger swash and weaker backwash
Less energy and therefore do not cause erosion
Deposit their load/material, which helps to build beaches
Destructive:
Stronger backwash and weaker swash
Build up over a long period of time with strong winds and long fetch
More energy = high erosional capacity
Swash and backwash move along the beach in the direction of the wind
Backwash is always at right angles to the beach — water runs back down the beach because of gravity
This movement of material along the beach is called longshore drift
Coastal erosion processes
Definitions:
Hydraulic action — occurs as waves hit or break against cliff face. Air trapped in joints or cracks is put under pressure. The increasing pressure of water and air causes rocks to crack
Solution — when rocks are dissolved by sea water through a chemical reaction
Corrasion/abrasion — Caused by waves picking up materials such as pebbles or shingle, and throwing them against a cliff face, wearing away the surface (like sandpaper)
Attrition — Process by which material carried by waves collide into each other, breaking up and becoming rounder and smaller over time
Coastal landforms
Cliffs, wave-cut platforms and notches
Cliffs are areas of rock formed by erosion and weathering
Soft rock erodes more quickly, forming gently sloping cliffs. Hard rock erodes more slowly, forming steep sloping cliffs
A wave cut platform is a wide gently sloping surface found at the foot of a cliff
When waves crash onto a cliff, a wave-cut notch (undercut in the cliff) is formed through abrasion and hydraulic action
This notch increases in size, causing the cliff to become unstable and eventually collapse, leading to a retreat in the cliff
Backwash carries away eroded material, leaving behind a wave-cut platform
This process repeats and the cliff continues to retreat
Caves, arches, stacks and stumps
Commonly found on headlands
Cracks in the headland are widened through erosion until they form caves
Continuous erosion causes caves to become larger, eventually forming arches
Continuous erosion widens arches until the roofs become too heavy and collapse into the sea, forming stacks
Stacks are undercut at their base by waves before collapsing to form stumps
Bays and headlands
Headlands are formed when the sea attacks a section of coast with alternating bands of hard and soft rock.
The bands of soft rock, such as sand and clay, erode more quickly than those of more resistant rock, such as chalk.
This leaves a section of land jutting out into the sea called a headland. The areas where the soft rock has eroded away, next to the headland, are called bays.
Beaches
Formed when material (eroded from elsewhere) is washed ashore by constructive waves
This eroded material accumulates over time, increasing the size of the beach
Form in sheltered areas like bays where waves have little energy
Beaches can’t form in places where waves have high energy because there will be more erosion and less deposition
Form in places where the sea is shallow
Spits
Extended stretch of sand or shingle jutting out from land
Prevailing wind approaches the beach from an angle
Swash comes in at an angle
Backwash is vertical / perpendicular to the sea
Material is moved in a zig-zag manner (longshore drift)
Spit develops where there is a change in direction of coastline
Sand dunes
Mounds/hills of sand, formed by the accumulation of sand via longshore drift
As the tide goes out, sand on the beach dries, allowing prevailing winds to blow loose, dry sand up the beach
If sand encounters an obstruction (e.g. driftwood), then it begins to accumulate around that obstruction, forming a little hill
The first dunes which are formed are called embryo dunes (young dunes, less than 1m in height)
Embryo dunes grow bigger as sand accumulates to form foredunes. Pioneer plant species (eg. marram grass) start growing
These plant species trap even more sand, allowing the dunes to grow bigger into yellow dunes
Yellow dunes are formed when pioneer species change the composition of the dunes
Plant species die and become organic matter, enriching the sand until it becomes soil
As more species of plants grow and more sand accumulates, a yellow dune becomes a grey dune
Grey dunes — soil is damper and richer, allowing mosses, lichens and flowering plants to grow, eventually forming mature dunes which are found several hundred metres away from the shore
Mature dunes can be over 10m in height
Coral reefs
Composed of hundreds of thousands of individual animals called polyps
Polyps get 90% of their energy from zooxanthellae, which are algae that photosynthesise
Both have a symbiotic relationship — coral reefs provide zooxanthellae with shelter; zooxanthellae provide coral reefs with energy
Distribution of coral reefs:
Between Tropics of Cancer and Capricorn; around the equator
Coastal areas (including islands)
Conditions needed for survival:
Shallow water to receive enough sunlight (typically not found deeper than 50-60m)
Warm water between 18 and 27 deg C
Clear water that is free from sediment / not polluted
High salinity
Abundant supply of oxygen
Abundant supply of nutrients and plankton
Gentle/slow waves and calm water
Alkaline water (high pH)
Mangroves
Wetland ecosystems of salt-tolerant trees
Found in intertidal (between high and low water tide marks) regions
Tropical and subtropical
Cover around 25% of the world’s coastlines
Distribution of mangroves:
Mostly between Tropics of Cancer and Capricorn; around equator
Coastal areas
Conditions needed for survival:
Muddy water with low oxygen content
Rich in nutrients
High salinity
Tides (as mangroves are regularly submerged)
Sheltered areas with low wave action
Warm air and water temperatures (usually 20 deg C and higher)
Coastal hazards
Coastal erosion
Natural process – when the transportation of material away from the shore is less than the addition of new material deposited on the shore
Can cause beaches and other coastal landforms to recede/shrink/disappear
Amplifies the threat of coastal flooding
Tropical storms
Also known as hurricanes, cyclones and typhoons
Heavy rain and strong winds
Almost all tropical storms occur over tropical seas
Coastal opportunities
Tourism
Recreation (eg. water sports)
Fishing
Trade and transportation
Ecosystems eg. coral reefs and mangroves, which are rich in biodiversity
Deep sea mining for natural resources (oil/gas/other minerals found under the seabed)
Managing impacts of coastal erosion
*can be categorised into soft and hard engineering strategies (for an explanation, see ‘flood management’ section above).
Seawalls
Built at the edge of a coastline
Protects the beaches/base of cliffs/other coastal landforms/buildings from erosion
Deflect waves
Absorbs wave energy
Prevents flooding
Expensive and hard to maintain
Also an eyesore
Groynes
Wooden barriers built perpendicular to the coastline
Prevents longshore drift
Traps material and prevents it from being transported away
Allows beaches to retain as much material as possible
Unattractive and costly
Gabion boxes
Cages filled with rocks
Absorbs wave energy
Stabilise cliffs (prevents erosion of cliffs)
Endurant (resistant to erosion) but an eyesore
Expensive
Revetments
Sloped structures (composed of planks) that absorb wave energy
Mostly made of concrete, but can also be made of wood etc.
Inexpensive
Rock armour/rip rap
Concrete/large boulders/rubble from demolished buildings that armours a coastline
Absorbs wave energy
Can be more natural-looking (depending on the material used)
Very durable
Animals may find it difficult to walk across the rocks
Beach nourishment
Replenishing eroded sand on beaches
Sands/sediment is taken from other sources, and transported by pipes or ships
Very repetitive process - requires multiple rounds of replenishment
Short-term solution - the replenished sand will still be prone to erosion
Expensive, and deprives other sources of sand
Case study: Gold Coast, Australia — opportunities, hazards and management
→ Background information
Coastal city in Brisbane, Queensland
More than 70km of coastline
→ Opportunities
Tourism
Around 10 million tourists visit every year
Biggest industry in the region - contributes more than $4.4 billion to the economy yearly
Over 13,000 available guest rooms
Recreation
Gold Coast Recreation Centre at Tallebudgera Beach is one of the many amenities that cater to both tourists and locals
Has some of the most popular surf breaks in the world eg. Surfers Paradise and Broadbeach
Other water sports eg. kayaking, jet skiing, scuba diving
Film production - many films are shot in Gold Coast because of its many coastal features
3rd largest film production centre
$150 million per year in expenditure for film production
Some movies shot here include Thor: Ragnarok and Aquaman
Queensland government supports the industry financially
Biodiversity
Home to over 700 species of animals and 1,800 species of plants
More than 2000 parks covering 21,000 hectares and 100,000 hectares of world heritage-listed rainforests
→ Hazards
In 1967, 11 cyclones (coupled with natural erosional processes) removed 8 million cubic metres of sand from beaches
More recent storms: 2009 Hamish etc
500,000 cubic metres of sand from beaches are lost annually due to longshore drift
More than 350,000 residents live in areas less than 10m above sea level and are thus vulnerable to coastal flooding
Residents of sea cliffs have sometimes had their homes/other infrastructure collapse due to cliff erosion
Tropical storms can cause storm surges (when tides are higher than usual)
Sea level rise caused by climate change also poses a threat
→ Management strategies
Gold Coast City Council launched a Three-point Plan in 2013, involving various hard and soft engineering strategies
Beach nourishment
Between 1995 and 2000, sand was dredged from the River Tweed Bar and placed offshore of the beaches on the Southern Gold Coast
A permanent pumping system was also constructed which has pumped 500,000 cubic metres of sand each year since 2000, from New South Wales across the border onto the Gold Coast beaches.
Helpful but costly
Seawall Construction Project
$6.8 million spent on constructing new seawalls
Most notable seawall: Kurrawa Seawall (completed in 2016)
Almost 4 kilometres of existing seawalls were recertified (means that inspections were carried out on old seawalls to ensure their quality), saving $15 million in seawall construction costs
Sand dunes as coastal protection
Protect inland areas from coastal water intrusion
Gold Coast council invested money in the BeachCare programme to ensure the health of the sand dunes