F761 rivers case studies

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  • Created by: charlie
  • Created on: 16-02-14 19:13

BOSCASTLE flooding in urban area

AUGUST 2004 

INPUT 

  • 5 inches of rain in few hours
  • 2m rise in river levels 
  • air arriving from SW as anitclockwise winds around low pressure area = tropical storm 
  • 'brown willy' effect - highest point 420m above sea level - vapour rises + cools 

BASIN 

  • steep valley, thin soils - accelerated rainwater + concentrated in small area 
  • impermeable shale (clay) increased overland flow + basin lost tree coverage 
  • confluence of JORDAN + VALENCY 
  • lack of flood control system + small bridge structure 

EFFECTS 

  • no quick emergency exit, properties damages, quick accomodation, inc. insurance,
  • 6 weeks of WITCHCRAFT MUSEUM season lost 
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erosion landforms of upper course

INTERLOCKING SPURS - ashes hollow (tributary of river seven) - little stretton, Shropshire 

  • upstream source called 'boiling well' 475m above sea level 
  • 4 km long 

POTHOLES (evorsion hollows) - river clyde (Scotland)

  • porous limestone/sandstone type rocks 
  • 105 miles long 

RAPIDS + HIGH FORCE WATERFALLS - river tees (north east england) 

  • hard rock - Dolerite 
  • soft rock - Limestone 
  • gorge more than 30 metres wide 
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bangladesh floods 1998

INPUT - Antecedent conditions 

  • onshore winds bring monsoon season - 1000mm  per day 
  • path of tropical cyclone - low pressure systems HEAVY RAIN + SEA STATES 
  • global warming - ICE RETREATED 30M in 30 YEARS 
  • all 3 rivers have peak flow at same time  BRAHMAPUTRA -- GANGES -- MEGHNA 

BASIN 

  • vast - 1.7 km2 - LOTS OF RAINWATER drained into rivers 
  • low lying relief - 70% OF LAND <1M 

HUMAN 

  • 282% POP. INC. 1961 - 2011
  •  - higher demand for wood (DEFORESTATION) - more ploughed land (TOP SOIL REMOVED REDUCING RIVER CAPACITY) 
  • cities grow (URBANISATION) - water reaches river channel quicker 

POST SCHEMES  - cluster villages, NGO help, fish culture farming (not affected by floods)

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Mekong River - conflicts

Hydropower Vs fishing 

  • Xayaburi dam being built in Laos said to generate 1,260 megawatts of power 
  • 1,000 freshwater species 41 possibly become endangered 
  • 300-megawatt Don Sahong dam across Hoo Sahong - 25m high will block fish migratory spawning pathway 

Hydropower Vs tourism 

  • sting rays, fresh water dolphin, + mekong giant catfish (worlds largest fresh water fish 3 metres long ) now endangered by IUCN 

hydropower Vs agriculture 

  • hold too much water back - kill rice industry - 16 million metric tonnes of rice per year 

hydropower vs recreation 

  • families live on the water + rely on it as a source 
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Mekong River - sustainable development (MRC)

Irrigation + drought management 

  • surveys carried out to inc. irrigation efficiency as less discharge = less rice yield 

flood management 

  • 2008 - 300 people died cost US $100 M (accurate forecasting put in place) 

hydropower 

  • cross cutting initiatives - long term + mutually beneficial development 

fisheries 

  • MRC takes research + gives info to policy makers along river 

environemnt 

  • communities given understanding of how their daily routines affect + water quality tested 
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Mekong River - multi-use resource

AGRICULTURE - still dominant in lower basin - Thai. 20% GDP     Laos 52% GDP

  • water for irrigation 
  • alluvium from flood water 
  • debris - sheltered body of water forming delta 

FISHING - worth $1.4 Bn per year 

  • climate - flooding of tonle sap 
  • wide + big river 

HEP - 4,450/17,000 Mw currently exploited 

  • steep river valley - large vol. of water 
  • growing pop. in adjacent countries = growing demand for electricity 
  • high head of water falling on turbines - maximum energy 

INDUSTRY - trade valued at $4.7 Bn passed through the basin 

  • HEP provides energy + deep + wide = large vessels + economy of scale (cheaper costs)
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Los Angeles river - flood control

why is there a need? - 3000 km2, soft + easily eroded rocks 

  • 60 miles of high urbanisation - deforestation - little interception + fast run off 
  • steep slopes - thin soils - poor vegetation (CHAPARRAL)
  • gulleying of rocks forming hills - smaller cpaacity 

URBANISATION 

  • tall buildings act as mountains :  inc. rainfall by 20% + inc. lightning strikes (friction) 
  • agredation leads to depostion from mountans high up 

DEFENCES 

  • widespan bridges - straemlined support stands 
  • Rivers semi circular shape - high HYDRAULIC RADIUS 
  • concrete smoothed - reduce friction + inc. river speed 
  • DEBRIS DAM --> FLOOD CONTROL BASIN/DAM (san gabriel) --> SPREADING GROUND --> IMPROVED CHANNEL 

1915-LA flood control authority took sediment from sea-reduces beach building-causes erosion

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Jubilee river - drainage diversion system 2002

  • 11.6 km long, cost £84 M to build - protecting 4,800 private properties + 12,500 people + M4
  • leaves Thames at Boulter's weir + rejoins at Black Potts Viaduct 

need for it?

  • major Thames flood 1947 - small management schemes post 
  • flood 1954 - more serious measures needed + inc. urban development - Jubilee river 2002 
  • 6 gates along Jubilee control the flows (3 stationary + 3 moving)

priorites 

  • look natural + add to already established ecosystems + habitats 
  • build new habitats longside that have been destroyed by the river Thames 

reasons 

  • Thames only holds 285 m3/s at bankfull 
  • Jubilee + Thames can hold 515 m3/s 
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