Rivers, Floods & Management

Impacts of Flooding
1. Human
  1. Urbanisation
  2. Deforestation
  3. Poor River Management
  4. Climate Change
2. Physical
  1. Excessive levels of precipitation
  2. Intensive rainfall in a short period of time
  3. Melting snow/ice
  4. Climatic Hazards
  5. Vegetation
  6. Soil Type
  7. Drainage Density
Flooding Management
1. Hard Engineering
  1. Dams
  2. Embankments & Levees
  3. Channel Straightening
  4. Channel Enlargement
  5. Flood Relief Channels
  6. Flood Storage Resevoirs
2. Soft Engineering
  1. Afforestation
  2. Land Use Zoning & Management
  3. Wetland & River Bank Conservation
  4. River Restoration
  5. Weather Forecasting
Drainage Basin Hydrological Cycle
1. Infiltration - Downward movement of water into soil
2. Runoff - Flow of water into river
3. Precipitation - Forms of moisture from Atmosphere
4. Transpiration - Water loss from vegetation
5. Evaporation - Transformation from liquid to gas
6. Interception - Barrier stopping water reaching river (vegetation)
7. Throughfall - Water moving downhill through soil
8. Stem Flow - Flow of water down stem of plant
9. Surface Storage - Precipitation retained on ground surface
10. Soil Water Storage
11. Vegetation Storage
12. Channel Storage
13. Percolation - Flow of water through permeable rock
14. Ground Water Storage
15. Ground Water Flow - Flow of water through underlying rock
Factors Affecting Discharge
1. Rock & Soil Type
  1. Permeable rocks and soils (sandy soils) absorb water easily, so surface run-off is rare
  2. Impermeable rock and soils (clay soils) are more closely packed. Rainwater can’t infiltrate, so water reaches the river more quickly
  3. Pervious rocks (like limestone) allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles
2. Land Use
  1. 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
  2. In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster increasing discharge
  3. Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
  4. The amount and type of rainfall
  5. When the ground is already saturated, rain will then flow over the grounds surface
  6. Heavy continual rain, or melting snow, means more water flowing into the river
3. Relief
  1. 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
  2. Weather Conditions
  3. 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
  4. High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge
  5. Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in
Channel Processes
1. Erosion
  1. 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
  2. Abrasion - Rocks carried along by the river wear down the river bed and banks
  3. Attrition - Rocks being carried by the river smash together and break into smaller, smoother and rounder particles
  4. Solution - Soluble particles are dissolved into the river
2. Deposition
  1. When a river loses it’s energy and can no longer hold carrying capacity
3. Transportation
  1. Solution - Minerals are dissolved in the water and carried along in solution
  2. Suspension - Fine light material is carried along in the water
  3. Saltation - Small pebbles and stones are bounced along the river bed
  4. Traction - Large boulders and rocks are rolled along the river bed
Valley Profiles
1. The longitudinal course of a river from head to mouth, showing only vertical changes
2. Gradient being steeper in the upper course and becoming progressively gentler towards the mouth
3. Rapids, waterfalls, knickpoints, rough bedload, meanders flood plains or levees can change shape
Changing Channel Characteristics
1. Channel Cross profile: Shows the shape of the channel and valley
2. Roughness: The smoothness of the bed of a river affecting friction
3. Discharge: The volume of water within a river
4. Efficiency: Ability of channel to conserve energy, lost as friction
5. Hydraulic Radius: Cross sectional area / wetted perimeter
6. Velocity: Speed of the flow of water
7. Wetted Perimeter: Part of the bed/bank in contact with water
8. Braided Channels: Where a river becomes subdivided into many separate channels
Rejuvination
1. Incised Meander
  1. Lateral Erosion
  2. Valley Floor Deepened
  3. Asymmetrical Cross Section
2. Entrenched
  1. Vertical Erosion
  2. Deep Cut
  3. Symmetrical Cross Section
  4. Knick Points
    1. Sharp change in channel gradient
    2. Long profile lengthened and land rises from sea
  5. River Terraces
    1. Remains of former floodplain
Landforms of Fluvial Erosion & Deposition
1. Potholes
  1. Cylinder holes drilled into rock bed of river
  2. High velocity water loaded with pebbles
  3. Grind hole in rock by abrasion
  4. Attrition rounds and smooths bedload
  5. Vary from cm to meters
  6. Upper and early-middle course
2. Oxbow Lakes
  1. Hydraulic action narrows neck of meander
  2. 2 outer bends meet and river cuts through
  3. Deposition seals of old meander
3. Meanders
  1. Water flows faster on outer bend
  2. Channel deeper – less friction
  3. Increase in erosion forming steep river cliff and gradual slip off slope
  4. Helicoidal Flow
4. Waterfalls
  1. Upper course
  2. Band of hard rock ontop of soft rock
  3. Plunge pool forms (hydraulic action and abrasion) undercutting hard rock causing it to collapse
  4. Steep sided valley left behind (gorge)
5. Floodplains
  1. Middle and Lower Course
  2. Width determined by meander migration and lateral erosion
  3. Depth depends on the amount of flooding and deposition
6. Levees
  1. Middle and Lower Course
  2. Small raised banks along side of channel
7. Deltas
  1. Mouth
  2. Rate of deposition exceeds sediment removal
Causes of Flooding
1. Physical
  1. Heavy Rainfall: Raise in water level
  2. Snowmelt: Raise in water level, temp and global warming
  3. Relief: Low-lying areas
  4. Costal Flooding: High tides raises water level
2. Human
  1. Deforestation: No interception
  2. Poor water management: poorly constructed dams
  3. Poor farming: Over grazing, people want more food and money
  4. Population pressure: increase in soil erosion

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Rivers, Floods & Management

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