Rivers, floods and management


Published on

Rivers, floods and management revision slideshow for aqa as sudents

Published in: Education, Technology
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Rivers, floods and management

  1. 1. geography revision rivers, floods and management
  2. 2. drainage basins • an area of land which is drained by a river and its tributaries. • there is a drainage basin hydrological cycle which looks like this... the cycle is an OPEN system infiltration is the movement of water downwards from surface into soil throughflow is the water that moves down slope through soil percolation is the downward movement of water from soil into rock below groundwater storage is the water stored in the rock pores underground
  3. 3. the storm hydrograph • the graph is of river discharge against time, mainly leading up to or after storm or rainfall event. • they can help predict a storm, helps in managing the river • different variables can affect shape of graph, for example: o basin shape (can take varying times to reach river) o steepness (water gets into river more quickly) o land use (towns have impermeable surfaces so overland flow will increase)
  4. 4. valley profiles types of river erosion: • hydraulic action - force of air and water on sides and cracks of river • abrasion - wearing away of bank by the load of the river • solution/corrosion - removal of chemical ions, eg. calcium. • attrition - wearing away of the load in the river, loads gets smaller and smaller types of river transportation: • suspension - carried within flow, largest contribution • saltation - small stones bouncing along river bed • traction - bigger rocks/boulders rolled along river bed
  5. 5. valley profiles the hjulstrom curve relates the work done by a river to its velocity competence is the max load a river can carry capacity is the maximum load carried In general larger particles require more velocity to be lifted off the bed Larger particles will be deposited at higher velocities where smaller particles will remain in transport.
  6. 6. valley profiles
  7. 7. valley profiles In the upper course of a river the velocity is low due to roughness of bed, it mainly has potential energy. further down the long profile the energy turns into kinetic energy due to river is bigger and moving more quickly. wetted perimeter is the length of the banks and the bed that are in contact with the river. The greater the channel roughness the greater the wetted perimeter. the hydraulic radius is a measure of the rivers efficiency, it equals CSA/WP. The higher the hydraulic radius the more efficient the stream.
  8. 8. river landforms landforms found in upper course include. • potholes - caused by drilling (abrasion), pebbles collect in depression and the flow causes them to rotate and create a hollow • waterfalls - caused by a change in geology with underlying softer rock, waterfalls migrates upstream due to lip collapsing. • rapids - steeper sections of the long profile, usually have very turbulent flow, caused by changes in geology. middle/lower course landforms • meanders - there are pools (deeper) and riffles (shallower) parts of the river and they cause a change in speed, the river flows from one side to another eroding the banks, on the inside of the meander where there isnt enough energy there is deposition
  9. 9. river landforms flood plains are flat areas of deposition close to river channel many rivers have natural embankments called levees, formed during floods, they can act as a natural flood defence. deltas are formed when a river enter the sea, as the river slows down it loses competence and deposits. the fresh water mixes with salty water and they coagulate called flocculation. braided channels are created when a river loses competence and look like small islands. rejuvenation occurs when its landscape is dominated by erosion, causes include uplift of land due to melting land ice or uplift of land caused by tectonics.
  10. 10. river landforms knickpoints, river terraces and incised meanders are created because of the difference of height between the river and the sea (caused by rejuvenation), this causes river to gain potential energy thus eroding vertically. knickpoint are sudden changes in the rivers gradient, river terraces occurs when a river cuts into its floodplain. Incised meanders are a result of established meanders cutting vertically into the landscape.
  11. 11. flooding physical causes: • excessive rainfall = increased overland flow and soil saturation • intense rainfall, when its > rate of infiltration, overland flow occurs, can be exacerbated by impermeable surfaces. • rapidly melting snow • coastal storm surges human causes • urban areas - impermeable surfaces increase overland flow • deforestation - removes interception store which decreases evaporation. vegetation store removed, soil has lower infiltration rate. sediment can also get into the rivers. • failure of dams
  12. 12. flooding areas at risk from flooding get mapped, flood prediction is not reliable due to it is based on historical data. recurrence interval can be calculated and tells us how often a flood will occur. physical impacts of flooding: • area of land flooded • sediment deposited or washed away • change in rivers’ course eg. meander turns into oxbow lake human impacts • economic costs to householders, insurance, businesses, agriculture, organizing help and aid • social costs like deaths, homelessness, health issues
  13. 13. flooding case studies 1 Bangladesh LEDC july - september 2004 very populated country is south asia and is subject to annual monsoon floods and cyclones. most of the country consists of a huge floodplain and delta, 70% of country is below 1m sea level, also there is snow melt from the himalayas. the 3 rivers ganges, brahmaputra and meghna all exceeded their flood levels during heavy intense rains following monsoon weather. deforestation in the himalayas and urbanisation also increased run off the level of sea in the bay of bengal was at a record high
  14. 14. flooding case studies 1 short term impacts • 1 million km^2 flooded • 1050 dead • 30 million left homeless long term impacts • exports decreased by 20% • 14,000 schools flooded • 26,500 livestock lost • 1 million houses destroyed short terms relief measures ● volunteers/aid workers ● rapid creation of shelters ● international food programmes Long term relief measures ● creation of embankments ● flood protection shelters ● flood proof storage ● suggested dam construction
  15. 15. flooding case studies 2 carlisle MEDC january 2005 located at confluence of rivers eden, caldew and petteril short term impacts • 3 dead • 120 injuries • roads were impassable long term impacts • 1925 homes and business flooded • 3000 homeless • high financial costs
  16. 16. flooding case studies 2 flood management strategy the SPAR decided their best option was to perform upstream managed realignment: • involved moving existing line of defence back from river to provide larger area of floodplain • also raise existing flood defences this was selected because t was seen an environmentally acceptable, with it providing more opportunity for environmental enhancement. this would provide protection to an estimated 1 in 200 year standard.
  17. 17. flooding flood management strategies can be divided into hard and soft engineering strategies. hard engineering - either requires construction or landscape change, includes dams, river straightening, levee construction. they are generally found in LEDC’s as they are expensive and are protecting high value property. soft engineering - floodplain management, wetland conservation and river banks conservation. Also forecasting comes under it and better forecasting in bangladesh would have reduced death toll due to more time to react. river restoration is about enhancing the environment rather than reducing flood risk, but when rivers are returned to their original state floods may be less catastrophic and less frequent.