The Thames and Tweed: a tale of two HELP basins

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Tom Ball, UNESCO Centre for Water Law, Policy and Science at the University of Dundee, comparing Tweed HELP basin with that of the Thames and beyond. Presentation to World Water Day workshop on 22 …

Tom Ball, UNESCO Centre for Water Law, Policy and Science at the University of Dundee, comparing Tweed HELP basin with that of the Thames and beyond. Presentation to World Water Day workshop on 22 March 2010.

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  • Emphasise flow control and engineering works – influence on this trend.
  • Sewage treatment work reform – Roding example. Importance of replacement of flow back into rivers – cf sewage effluent – impact of WQ measures – abstraction without replacement
  • Naturalised low flow show correction for abstractions – much more uniform pictrure, and more in line with summer rainfall trends.
  • Drought experience: in general, exceptional ability to manage drought episodes. Reservoir and aquifer recharge crucial
  • BUT 2004- 06 drought showed effect of 2x dry winters in succession.
  • Recent record contains few protracted droughts

Transcript

  • 1. Tom Ball Associate Fellow: UNESCO centre for Water Law, Policy and Science, University of Dundee The Thames and Tweed: a Tale of Two UK HELP Basins
  • 2. Acknowledgments
    • T. Marsh, Centre for Ecology and Hydrology, Wallingford
    • Tweed Forum
  • 3. Structure
    • Introduction to the basins
    • Hydrometry in the UK
      • Patterns and trends
    • Pressures and impacts
      • Impact of climate change?
      • ->Are high flows getting higher?
      • ->Are low flows getting lower?
  • 4. The Thames Basin
    • Low rainfall – 2/3 of which is lost to evaporation
    • Population 13 million and growing
    • ‘ Cleanest metropolitan river in the world’
    • Heavy water demand (met by reservoirs and groundwater)
    • Very heterogeneous catchment; significant & sustained land-use change
    • Low flow and pollution issues
    • Substantial floodplain development
    • Amenity and ecological expectations
    • Major water & river management challenges (including climate change)
    Average annual rainfall 600-800 mm Geology Land use
  • 5. The Tweed Basin
    • Medium to high rainfall in W part of catchment (1200 mm+) + much lower abstraction and evaporative loss
    • Tweed Forum identify the following issues
    • Water Quality – esp agriculture, forestry
    • Water resources
    • >12,000 Salmon catch annually
    • -contributes around £18m per year to the local economy and supports over 500 jobs
    Average annual rainfall - 700-1700 mm
    • Flood management
    • Riverworks
    • Tourism and recreation
  • 6. Hydrometry in the UK
    • Underpin UK capability to identify trends
    • Good hydrometric performance & limited artificial influence on flow patterns
    • Designated in collaboration with the UK measuring authorities
  • 7. Data source: Met Office/Hadley Centre
  • 8. High Flow Trends
    • Increases in flood frequency and high flow prevalence since the 1960s
    • Mainly confined to upland and maritime catchments
    • Association with positive phase of the NOA
    • Again, very sensitive to the timeframe examined
    • Generally, trends in Amax are less significant than those for flood frequency and high flow prevalence
    High Flow Magnitude, 1970 - 2004 Hannaford, J. and Marsh, T.J. 2008. High-flow and flood trends in a network of undisturbed catchments in the UK. Int. J. Clim. 28, 1325-38
  • 9. Scotland: increased incidence of floods last 30 years?
    • 1993 flood on the Tay – highest flow on the NRFA
    • Dec 1994 >200mm rain in 48 hrs. widespread event (lower Clyde basin). 27 river flow gauges exceeded previous maxima. Three fatalities and greatest damage since the 1968 flooding in southern England.
    • 2009: Elgin flooded for 4 th time in last 13 years
  • 10. [input dundee research on rivers] August 2008: Kirknewton, Tweed Catchment © Environment Agency
  • 11. Are summers getting more arid? Yes, but how does this impact on water resources stress? T. Marsh, CEH Wallingford
  • 12. Low flow trends
    • Trends sensitive to the timeframe examined
    • Few compelling trends in low flow magnitude or duration over the last 40 years
    • Exceptionally high minimum flows in 2007 & 2008
    • (Trends due to artificial influences are common)
    30-day min 1973 - 2002 Hannaford, J. and Marsh, T.J. 2008. An assessment of trends in UK runoff and low flows using a network of undisturbed catchments in the UK. Int. J. Clim. 26, 1237-1253
  • 13. Thames Basin: 1883-2009 water-yr max flows at Teddington 1894 1947 1974 T. Marsh, CEH Wallingford
  • 14. Low flows in the Thames basin
    • Many trends & discontinuities reflect artificial influences – particularly heavy water abstraction
    • But low flow alleviation strategies are also having a considerable impact
    • Rationalisation of STWs is also important
    Diversion of Luxborough effluent completed
  • 15. Low flows at Teddington/Kingston
    • Abstractions have increased by an order of magnitude
    • Minimum flow requirement at Teddington
    • Pre-1951 Poor hydrometric performance of Teddington weir during drought episodes
    T. Marsh, CEH Wallingford
  • 16. Naturalised low flows at Teddington/Kingston T. Marsh, CEH Wallingford
  • 17. 2003 Drought – ‘scenario compliant’
    • 2 nd driest Feb-Oct for Thames basin in 128-yr series
    • Warmest summer for E&W in a series from 1659
    • Very high evaporation rates
    • No water supply restrictions in Thames basin
    Water resources provision in the Thames basin is resilient to dry summers
  • 18. 2004-06 drought – ‘not scenario compliant’
    • Above average May-Sept 2006 rainfall in the Thames basin
    • But preceded by two successive dry winters
    • 13 million (E&W) affected by hosepipe bans
    • Restrictions on spray irrigation
    • Substantial water resources and ecological stress
    Water resources in the Thames basin are particularly vulnerable to clusters of dry winters T. Marsh, CEH Wallingford
  • 19. Some conclusions
    • Floods & droughts are naturally recurring features of the UK climate
    • Climatic variability has been a major factor influencing flood & drought severity over the last 200 years
    • Long hydrological records demonstrate appreciable resilience to climate change
    • That resilience has to be reinforced by improved scientific understanding of flood and drought risk
      • - the role of HELP / IHP-VII
    • Further reconciliation is needed between climate change scenarios and observational evidence
    • It is imperative that monitoring of meteorological and hydrological extremes is maintained