Mapa mental
Rivers, Floods & Management
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Rivers, Floods & Management
Impacts of Flooding
- Human
- Urbanisation
- Deforestation
- Poor River Management
- Climate Change
- Physical
- Excessive levels of precipitation
- Intensive rainfall in a short period of time
- Melting snow/ice
- Climatic Hazards
- Vegetation
- Soil Type
- Drainage Density
- Human
Flooding Management
- Hard Engineering
- Dams
- Embankments & Levees
- Channel Straightening
- Channel Enlargement
- Flood Relief Channels
- Flood Storage Resevoirs
- Soft Engineering
- Afforestation
- Land Use Zoning & Management
- Wetland & River Bank Conservation
- River Restoration
- Weather Forecasting
- Hard Engineering
Drainage Basin Hydrological Cycle
- Infiltration - Downward movement of water into soil
- Runoff - Flow of water into river
- Precipitation - Forms of moisture from Atmosphere
- Transpiration - Water loss from vegetation
- Evaporation - Transformation from liquid to gas
- Interception - Barrier stopping water reaching river (vegetation)
- Throughfall - Water moving downhill through soil
- Stem Flow - Flow of water down stem of plant
- Surface Storage - Precipitation retained on ground surface
- Soil Water Storage
- Vegetation Storage
- Channel Storage
- Percolation - Flow of water through permeable rock
- Ground Water Storage
- Ground Water Flow - Flow of water through underlying rock
Factors Affecting Discharge
- Rock & Soil Type
- Permeable rocks and soils (sandy soils) absorb water easily, so surface run-off is rare
- Impermeable rock and soils (clay soils) are more closely packed. Rainwater can’t infiltrate, so water reaches the river more quickly
- Pervious rocks (like limestone) allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles
- Land Use
- In urban areas, surfaces like roads are impermeable, it runs into drains, gathers speed and joins rainwater from other drains – eventually spilling into the river
- In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster increasing discharge
- Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
- The amount and type of rainfall
- When the ground is already saturated, rain will then flow over the grounds surface
- Heavy continual rain, or melting snow, means more water flowing into the river
- Relief
- Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate. On more gentle slopes infiltration is more likely
- Weather Conditions
- Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into the river
- High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge
- Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in
- Rock & Soil Type
Channel Processes
- Erosion
- Hydraulic Action - The force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away
- Abrasion - Rocks carried along by the river wear down the river bed and banks
- Attrition - Rocks being carried by the river smash together and break into smaller, smoother and rounder particles
- Solution - Soluble particles are dissolved into the river
- Deposition
- When a river loses it’s energy and can no longer hold carrying capacity
- Transportation
- Solution - Minerals are dissolved in the water and carried along in solution
- Suspension - Fine light material is carried along in the water
- Saltation - Small pebbles and stones are bounced along the river bed
- Traction - Large boulders and rocks are rolled along the river bed
- Erosion
Valley Profiles
- The longitudinal course of a river from head to mouth, showing only vertical changes
- Gradient being steeper in the upper course and becoming progressively gentler towards the mouth
- Rapids, waterfalls, knickpoints, rough bedload, meanders flood plains or levees can change shape
Changing Channel Characteristics
- Channel Cross profile: Shows the shape of the channel and valley
- Roughness: The smoothness of the bed of a river affecting friction
- Discharge: The volume of water within a river
- Efficiency: Ability of channel to conserve energy, lost as friction
- Hydraulic Radius: Cross sectional area / wetted perimeter
- Velocity: Speed of the flow of water
- Wetted Perimeter: Part of the bed/bank in contact with water
- Braided Channels: Where a river becomes subdivided into many separate channels
Rejuvination
- Incised Meander
- Lateral Erosion
- Valley Floor Deepened
- Asymmetrical Cross Section
- Entrenched
- Vertical Erosion
- Deep Cut
- Symmetrical Cross Section
- Knick Points
- Sharp change in channel gradient
- Long profile lengthened and land rises from sea
- River Terraces
- Remains of former floodplain
- Incised Meander
Landforms of Fluvial Erosion & Deposition
- Potholes
- Cylinder holes drilled into rock bed of river
- High velocity water loaded with pebbles
- Grind hole in rock by abrasion
- Attrition rounds and smooths bedload
- Vary from cm to meters
- Upper and early-middle course
- Oxbow Lakes
- Hydraulic action narrows neck of meander
- 2 outer bends meet and river cuts through
- Deposition seals of old meander
- Meanders
- Water flows faster on outer bend
- Channel deeper – less friction
- Increase in erosion forming steep river cliff and gradual slip off slope
- Helicoidal Flow
- Waterfalls
- Upper course
- Band of hard rock ontop of soft rock
- Plunge pool forms (hydraulic action and abrasion) undercutting hard rock causing it to collapse
- Steep sided valley left behind (gorge)
- Floodplains
- Middle and Lower Course
- Width determined by meander migration and lateral erosion
- Depth depends on the amount of flooding and deposition
- Levees
- Middle and Lower Course
- Small raised banks along side of channel
- Deltas
- Mouth
- Rate of deposition exceeds sediment removal
- Potholes
Causes of Flooding
- Physical
- Heavy Rainfall: Raise in water level
- Snowmelt: Raise in water level, temp and global warming
- Relief: Low-lying areas
- Costal Flooding: High tides raises water level
- Human
- Deforestation: No interception
- Poor water management: poorly constructed dams
- Poor farming: Over grazing, people want more food and money
- Population pressure: increase in soil erosion
- Physical