Management

• Differences in the shape of flood hydrographs are caused by both human and physical factors
• Flood hydrograph


  • Shows changes in river discharge after a storm event
  • Graph shows a short period of time, usually 24 hours
  • Features: base flow, peak rainfall, rising limb, peak discharge, lag time, recessional/falling limb
• Factors increasing surface runoff/overland flow

  Short lag time, steep rising limb, high discharge, increased flood risk
• Factors decreasing surface runoff/overland flow

  Longer lag time, gentle rising limb, lower discharge, lower flood risk
• Factors increasing flood risk

  • Steep slopes
  • Impermeable rocks
  • Deforestation
  • Urbanisation
  • Saturated/frozen soil
  • Heavy/prolonged rainfall
• Factors decreasing flood risk

  • Gentle slopes
  • Permeable rocks
  • Lots of vegetation/afforestation
  • Rural areas
  • Deep, dry soils
  • Light rainfall
• It is not enough to say 'steep slopes', you need to explain why this leads to a steep rising limb
• This is a levelled answer, so marks are awarded for the answer overall rather than for individual points
• Austwick Beck has much more discharge than Clapham Beck this may be due to impermeable surfaces, steep slopes or lack of vegetation. Clapham Beck may have more vegetation and permeable surfaces.
• The key cause of the flooding is the amount and duration of precipitation: This cannot be altered
• There are a number of methods of managing floods and reducing the severity and/or impact
• The two main categories of flood management are hard and soft engineering:
• Hard engineering involves building structures or changing the river channel
• Soft engineering works with natural processes of the river and surrounding environment
• Flood plain zoning aims to work with the environment
• the type of land use is based on closeness to the river
• Land nearest to the river is used for grazing and land furthest away is used for housing
• this means that when the river floods there is less damage to the 'expensive' land use
• Planting trees or afforestation increases interception and storage
• this reduces surface run off which means that the river discharge is lower and flooding is less likely
• River restoration removes any hard engineering restoring the river to its natural state
• the natural processes of the river such as meanders slow the river flow which reduces the risk of flooding downstream
• York regularly floods: Serious flooding has occurred in 1947, 1978, 1991, 1995, 2000 and 2015
• The floods in 2000 were the worst on record
• The Ouse rose 5.5 meters above its normal level
• 540 properties were flooded and 320 were at serious risk
• 18,700 hectares of farmland were affected
• The overall cost of the floods was estimated to be over £12 million including lost income from tourism
• The Foss Barrier was built to prevent water from the River Ouse backing up the River Foss. This barrier failed in 2015 and led to the flooding of many areas which had not been flooded since 1978. It has since been upgraded at a cost of £38 million
• Embankments at Leeman Road increase the capacity of the river channel
• Clifton Ings has been left as an open, undeveloped floodplain. It can store 2.3 million m3 of water
• Flood relief channels around Acomb to divert water directly into the Ouse downstream of York
• Flood gates such as those on Marygate: These can be removed when there is no flood risk, flood gates are also on individual properties
• The flood management strategies have reduced the risk of flooding for many properties in York
• Transport disruption during times of flood has been reduced
• Sustain cycleway over Clifton Ings made wider and resurfaced
• Some habitat destruction in the creation of the embankments and flood relief channels
• Two ponds created connected to the River Foss which have created new wetland habitats
• Foss Barrier upgrade cost £38 million rather than the estimated £17 million
• Total cost of flood management strategies estimated at £83 million