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The Tweed

Introduction:

  • River Tweed flows through Scotland and Northern England- Stretches 156km
  • Poses a flood risk to Eddleston – Village in Scottish Borders

 

Factors causing Flooding: Precipitation is an important factor but other factors arguable more important

  1. Precipitation: Pluvial flooding: refers to floodingevents that are caused by extreme rainfall.
    1. Average rainfall is 2000mm pa
    2. Frequent winter storms such as Storm Frank in December 2015 which doubles monthly rainfall and caused a flood in Peebles
    3. High annual average PPT means soil stores remain saturated antecedent soil= slows lag time= Flashy hydrograph
  2. Physical factors:
    1. Tweed has 9 major tributaries & dense network of smaller channels = High drainage density
    2. Lies on land which used to be volcanically active and has impermeable igneous rock e.g Glacial clays and granite in Cheviot Hills = water moves rapidly through the basin
    3. Cheviot Hills have steep gradients so more overland flow or channel flow vs. soil storage
  3. Human Factors:
    1. 19th Century straightening of river for agriculture drainage for sheep pastures = greater flood risk
    2. More recently: attempts to slow water transfers and increase stores: Tweed Catchment Plan 2015-2022 has made local scale changes to slow lag time between events and peak discharge – use any example such as Lake Wood
  4. Future of Tweed: Atlantic depressions crossing Scotland bring relativity high summer rainfall which keeps soil moisture store charged throughout out summer – Climate change will exacerbate this

 

Characteristics: Human: 10% of properties at risk of flooding

  • Tourism: Salmon fishing: Flow levels need to be maintained to keep fish stocks high
    • Nitrate fertilisers aren’t allowed
    • Salmon fishing contributes £24 million annually to local economy and supports over 50 jobs

 

Mitigation to Flooding:

  1. Tree Planting Scheme at Craigburn Farm:
    1. 66 hectares of Riparian woodlands have been planted
    2. Lots of interception, evapotranspiration, soil infiltration and decrease speed of overland flow
    3. On average 30% of rainfall caught in leaves =+ biosphere store
  2. Remeandering of Lake Wood
    1. Remeandered in response to risks of flooding caused by:
      1. 19th C: River straightened for agriculture purpose and reduce foot rot
      2. 20th C: Locals realised straight river increase flow rates and flood risk downstream
    2. Since Remeandering there has been a 50% increase in fish population and reed which re sustainable housing for Atlantic Salmon
  3. Flow restrictors in Middleburn:
    1. Encourage out of bank flow ad holds water back = reduce flow
  4. Off Stream Pond at Lake Wood:
    1. Acts as storage and decreased discharge and flow
    2. Dredging areas of land around stream reduce Bankful Discharge

 

Water Abstraction in London Basin:

  • Provides 40% of London’s population with water
  • 1960s peak abstraction = groundwater levels beneath central London dropped 88m below sea level, creating a large depression in the water table
  • How chalk aquifers form a syncline under London:
    • Aquifers are like giant sponges that fill up with water
    • A hole is drilled down below the water table
    • A Pressure builds up which causes water to flow up the pipe to the surface via spring
    • Water is stored in tanks and transferred to areas by pipeline
  • +: Unpolluted long term supply that doesn’t dry out
  • -: Finite supply that takes years to be replaced