[C2] Chapter 2: River and Coast
[C2] Chapter 2: River and Coast
Characteristics of Sections in Rivers
Upper | Middle | Lower | River mouth | |
Altitude | High | Medium | Low | Very low |
Gradient | Steep | Medium | Gentle | Very gentle |
Water amount | Small | Medium | Large | / |
Roughness | Very rough | Rough | Smooth | / |
Shape | Narrow → v-shaped | Wider → v-shaped | Very wide | / |
Overall energy | Little | Medium | High | Very little |
Erosion |
Types of Erosion
Headward | Strength slope of bank of river source → overhanging rocks fall down → lengthen river channel / river valley → river source extend backward |
Vertical | River removes rock from its bed in upper course → deepen river channel / river valley |
Lateral | River removes rock along the bank in middle + lower courses → widen river channel / river valley |
Factors Affecting Rate of Erosion
Load | - Higher load → more erosion - Heavier / angular load → higher potential for erosion |
River energy | - Steep gradient → higher velocity / more water → higher energy → more erosive power |
Water quality | Acidic water → more corrosion |
Rock | More cracks / soluble materials → more erosion |
Deforestation | Less vegetation cover → higher overland flow → more rocks / stones washed into river |
Urbanization | - More surfaces covered with cement → less infiltration → more river discharge → higher energy - Slope cutting → steeper slope gradient → higher overland flow → more channel water → higher energy |
Industralization | Sewage pollute river water → more corrosion |
Transportation |
Types of Transportation
Suspension | Fine / light materials carried along → does not touch river bed |
Traction | Large / heavy boulders / rocks → rolled → dragged along river bed |
Saltation | Small stones / pebbles → bounce along river bed |
Solution | Soluble materials → dissolved → carried downstream |
Factors Affecting Rate of Transportation
Load | Larger size → higher energy → more transportation |
River energy | Higher energy → larger amount of heavier load → more transportation |
Erosion | Higher rate of erosion → more eroded materials |
Rock Resistance | Higher resistance → more eroded / weathered materials |
Climate | Hot / wet climate → more weathered materials |
Gradient | Steep → more surface runoff // more soil erosion |
Vegetation | Less vegetation → bare surface → more soil erosion |
Lakes | Fewer lakes → sediments cannot be trapped |
Deposition |
Factors Affecting Rate of Transportation
Water amount | Less water // more water leaves basin // high evaporation rate → less deposition |
Velocity | River flows into a large body of water // high channel friction // high amount of load // gentle channel gradient → lower water flow speed → less deposition |
Sorting of Sediments
Top layer | Lightest / finest load → last to deposit |
Bottom layer | Heaviest load → first to deposit |
Factors Affecting Fluvial Processes and Downstream |
Upper Course
Energy | Small amount of channel water // rough channel bed or banks // large or angular rock fragments → higher channel friction → lower water flow speed → less discharge → lower river energy |
Erosion | - High elevation → high potential energy - Vertical erosion being dominant → lateral erosion // headward erosion at river source |
Transportation | Less energy for transportation // less discharge → insufficient energy to remove large rock fragments |
Deposition | - Insufficient energy → deposition - Sudden decrease in river energy → deposition - Less discharge after rainstorms → river cannot transport large fragments → deposit on river bed |
Middle Course
Energy | More channel water // smoother channel bed // smaller and rounder rock fragments → more discharge → greater river energy |
Erosion | Steeper channel gradient → more lateral erosion than vertical erosion |
Transportation | - Larger load → transported by traction / saltation |
Deposition | - After flooding → river deposits sediments on valley floor - Decrease in energy → deposition |
Lower Course
Energy | - Large amount of channel water → more discharge - Smoother channel bed and banks // fine and rounded rock debris → smaller channel friction → higher water flow speed → greater river energy |
Erosion | Outer bank → lateral erosion → less vertical erosion |
Transportation | Mainly transported by suspension |
Deposition | Occurs in river mouth // inner banks // river bed during dry season // on valley floor during flooding |
Landforms |
Upper Course
V-shaped valleys | Upper course → active vertical erosion → river valley becomes deeper → v-shaped valleys |
Interlocking spurs | Little energy in upper course → river cannot go through obstacles → forced to zigzag from side to side around obstacles → vertical erosion → less river channel → distinct spurs |
Rapids / Waterfall / Gorges | River flows over more resistant rocks and less resistant rocks → erode the less resistant rocks → small = rapids → large = waterfall → continued undercutting of less resistant rocks → overhanging resistant rocks lose support → collapse → waterfall retreat → gorges |
Middle Course
Meanders | - Outer bank → erosion → higher water velocity → lateral erosion → undercut outer bank - Inner bank → deposition → higher water velocity → deposited load → slip-off slope → meander |
Lower Course
Oxbow lakes | Meanders → lateral erosion → meander bends → grow closer → flood → river cuts through bends to join → new channel → oxbow lakes = abandoned channel sealed by deposition |
Floodplains / Levees | Flood water carry load in river → form valley floor → repeated flooding / deposition → build up floodplain → flood water overflow from channel → lower water flow speed → larger and heavier load deposited close to river bank → levees (ridge-line) |
Braided streams | - Dry season → lower water level → deposition → more river bed becomes low ridges - Wet season → river water returns to previous levels → river flow blocked by ridges → braided streams = river forced to split into several separated channels → flow around ridges |
River Mouth
Deltas / Lagoons | River enters the sea → lower speed of flow → river deposits its load layer upon layer → gently sloping platform of sediment builds up → extends into sea → delta → sediments block river channel → river split into small channels → lagoons |
Human Activities |
Conservation of Reservoirs and Catchwaters
- Catchwaters → divert water from streams to reservoirs - Examples: Upper Shing Mun Reservoir / Tam Lam Chun Reservoir | |
Positive Impact | Negative Impact |
More freshwater for consumption / farming at lower course | - Less channel water amount at lower course - Dry season → more cutoff → pollutants difficult to disperse → more water pollution → degrade fish habitats - Less salt amount and nutrients → degrade farmlands |
Farming and Fisheries
Some wetlands on floodplains are reclaimed → become ponds / farmlands | |
Positive Impact | Negative Impact |
- More farming / freshwater fisheries - Attract more migratory birds → fish bonds become feeding ground | - Higher risk of flooding: wetlands originally absorb water → conversion of wetland into fish bonds / farmland → less floodwater will be absorbed |
Development of New Towns / Infrastructure
New towns need more land → large-scale reclamation at river mouth / floodplains → low-lying areas of river valleys become roads / railway networks | |
Positive Impact | Negative Impact |
- Provided land for development - Old urban areas have less over-concentration of population | - Higher risk of flooding → natural land surface become impermeable → more overland flow → higher amount of channel water - Destroy natural habitats |
Strategies Against Flooding |
Hard Strategies
Channelization | Positive | - Deepen and widen river channel → higher water-holding capacity - Straightening river channel → higher river velocity - Cover with concrete → less bed / bank erosion → less deposition / friction |
Negative | - Expensive to build - Concrete lining destroys river habitats - Less natural river banks | |
Dredging | Positive | - Deepen river channel → higher water-holding capacity - Less channel roughness → higher river velocity |
Negative | - Suitable for small section of river only → costly and time consuming for large-scale implementation - Needs to carry out maintenance periodically - Stirs up sediments → disturb aquatic lives - Not effective against large-scale flooding | |
Dykes / Embankments | Positive | - Larger height of river banks → higher water-holding capacity - More recreational space → cycling / walking trails |
Negative | - Expensive to build - Dykes collapse → flooding occurs - Look unnatural | |
Bypass floodway | Positive | Catch runoff → divert water → less threat of flooding in populated areas |
Negative | - Expensive to build - Large area of land is needed → suitable area is not always available - Damage original natural habitats - More chance of flooding in areas in the floodway | |
Village flood pumping scheme | Positive | - Embankments surround village → less flood water flows into villages - Stormwater drainage system → store stormwater runoff → increase time lag → more time to regulate discharge → less risk of flooding |
Negative | - Expensive to build and maintenance - Requires a large area of land |
Soft Strategies
Land use planning | Positive | - Less casualties and property losses - Store flood water - Less worsening drainage during heavy rainstorms |
Negative | - Difficult to restrict development - Difficult to enforce: complex land ownership | |
Afforestation | Positive | - More interception → less overland flow - Less soil erosion → less supply of load / silting - Creates natural habitats - More recreational grounds |
Negative | - Requires a lot of time to grow trees - Trees cannot intercept all rainwater → cannot prevent serious flooding - Requires a lot of space - High money cost for management and afforestation | |
Flood warning systems | Positive | - Give flood warnings → more time to evacuate / take precautionary measures - Raise people's awareness of flooding - Less casualties / property loss |
Negative | - Cannot prevent flooding - Some people without communication devices cannot receive flood warning in time |
Waves |
Types of Waves
Constructive waves | Destructive waves | |
Properties | Swash > backwash | Backwash > swash |
Formation | - Calm weather condition → weak wind | Strong winds / storms |
Energy | Dispersed / weak | Concentrated / strong |
Work | Wave deposition (with shore materials) | Wave erosion (without shore materials) |
Wave form | - Lower wave height - Longer wavelength - Lower wave frequency | - Higher wave height - Shorter wave length - Higher wave frequency |
Beach shape | Wide / flat | Narrow / steep |
Factors Affecting Wave Characteristics
Constructive Wave | Destructive Wave | |
Wind speed | Lower → lower energy | Higher → higher energy |
Fetch | Shorter → obstructed by island | Longer → facing open sea |
Wind duration | Shorter | Longer → prevailing wind |
Offshore gradient | Gentler: widely-spaced submarine contour lines | Steeper: closely-spaced submarine contour lines |
Location | Sheltered → at bays | Exposed → at headland |
Seabed roughness | Rougher: mangrove | Smoother |
Rock resistance | More | Less |
Coastal process | Deposition | Erosion |
Coastal Landforms |
Constructive Waves
Beaches | Building up of sand in sheltered areas |
Spits / Bars / Lagoons / Tombolos | Long and narrow deposits of sediments attached to land → extend out into sea → spits formed by longshore drift → Ridges of sediments parallel to the coast → extension of spits → completely blocked off a bay / river mouth → bars // Spits build up → join on island to mainland → 2 islands connected → 2 spits connected → tombolos water trapped behind bars → lagoons |
Destructive Waves
Notch / Sea cliffs / Wave-cut platforms | Wave erosion concentrated at area between high tide and low tide → land undercut to form notch → undercutting continue → notch enlarged → overhanging parts collapse → steep rock face adjacent coast → sea cliff → flat rock surface: wave-cut platform |
Sea cave / sea arches and stacks | Lines of weakness exposed to wave erosion → enlarged → form cracks → grow and develop into sea cave |
Human Activities |
Reclamation
More land for urban development and economic development | |
Positive Impact | Negative Impact |
More land for development → increase quality of life | - Disturb marine ecosystem → bury marine habitats - High cost - Look unnatural and unattractive: straighter coastline - Less safety of navigation: harbor → narrower // higher water velocity - More water pollution: higher concentration of human activities → kill marine lives |
Recreation
Beautiful coastal landscapes for development of recreation / tourism // more grounds for water sports | |
Positive Impact | Negative Impact |
More place for people to relax | - Damage marine habitats: mangroves and coral reefs are removed - Water and land pollution: more number of tourists and human activities along the coast |
Strategies for Coastal Protection |
Hard Strategies
Sea walls | Positive | - Reflect and deflect waves → protect coast from wave erosion - Strong and durable - Effective and efficient to reflect and dissipate wave energy - More recreational areas - Protect coastal structures from coastal flooding |
Negative | - Expensive to build and maintain - Walls are not durable → waves may undercut the wall → collapse - Look unnatural | |
Breakwaters | Positive | - Break incoming waves → less erosion power - Effective and efficient to reflect wave energy - Safe for ships to anchor |
Negative | - Expensive to build and maintain - Visually unattractive - Easily destroyed in storms - Water pollution - Cannot limit erosion elsewhere: waves are only deflected | |
Rock armor / rip-rap | Positive | - Easy to build - Cheap and easy to maintain - Local boulders can blend with local geology - More place for fishing |
Negative | - Looks unnatural / ugly - Limits access to beaches - Requires maintenance | |
Gabions | Positive | - Absorb energy of waves - Cheap and flexible - Can blend with local geology - Can blend with environment → vegetation can grow |
Negative | - Not as effective as sea walls - Look unnatural - Cages are easily corroded → require regular maintenance - Risk of injury - Shorter life span |
Soft Strategies
Beach nourishment | Positive | - Looks natural - Slows down removal of beach materials - More recreational ground → favor development of tourism and economy - Cheap and easy to carry out with local sand |
Negative | - Ineffective against strong waves - Short term solution only: beach erosion is a continuous process - Imported sand: higher cost // harmful to local species // contains toxic materials |
