Groundwater Flooding andSewers in the Thames Catchment            Steve Buss      stevebuss@esinternational.com
Summary•   Groundwater flooding mechanisms•   Groundwater flooding in SE England•   Mapping groundwater flood risk•   Base...
Groundwater Flooding MechanismsBedrock (Clearwater) Flooding                                        1. Ambient conditions ...
Groundwater Flooding viaPermeable Superficial Deposits                        Normal conditions in the                    ...
Recognising Bedrock vs. PSDGroundwater FloodingBedrock                     PSD• Upper catchment           • Lower/middle c...
Groundwater Flooding Mechanisms                      1965: Mines and heavy                      industry are active. Mines...
Transient Response to Abstraction
Needed an additional         50 ML/day to stabilise         groundwater levels:         Phase 1: Re-         commissioning...
Groundwater FloodingMechanisms
Groundwater Flooding Mechanisms
Groundwater Flooding Mechanisms                           High : 0.042305                           Low : -0.0400162
Groundwater flooding in the SE
MappingGroundwaterFloodingSusceptibility
Mapping Groundwater FloodingSusceptibility        What about risk?
Methodology (e.g.)       Water table elevation                            Depth to groundwaterWater table + 20 m > ground ...
Methodology (e.g)   Remove flood plains                          Final groundwater emergence map (GEM)© ESI Ltd.
Sewer Infiltration
Potential for InteractionSewer above TillSewer within TillSewer below Till
Mapping Potential forSewer Infiltration
Modelling Groundwater-SewerInteraction                   Ú                   Û     Ú     Û
Modelling Groundwater-SewerInteraction
Modelling Groundwater-SewerInteraction                         Aquifer gain                         (m3/day)              ...
Summary• GW flooding occurs frequently in the Thames  catchment.  – From Chalk and limestone aquifers  – Over permeable su...
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Groundwater Flooding in Thames Catchment

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A description of the relevance of groundwater flooding in the Thames catchment, and some thoughts on predicting sewer flooding.

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Groundwater Flooding in Thames Catchment

  1. 1. Groundwater Flooding andSewers in the Thames Catchment Steve Buss stevebuss@esinternational.com
  2. 2. Summary• Groundwater flooding mechanisms• Groundwater flooding in SE England• Mapping groundwater flood risk• Basement impact assessments in London• Groundwater and sewers
  3. 3. Groundwater Flooding MechanismsBedrock (Clearwater) Flooding 1. Ambient conditions in the aquifer: the water table slopes at a shallow angle and Ground surface groundwater discharges from a spring line at the break of slope. Water table Spring line 2. After a long period of heavy rainfall, the water table has risen close to the surface but the spring line continues to discharge groundwater, albeit at a higher rate. The spring line may also have moved up slope. 3. Following further heavy rainfall the water table can rise to intersect the ground surface and springs emerge further up the slope. Alternatively, as there is a very shallow water table, rainfall cannot percolate the ground and all rainfall turns to runoff.
  4. 4. Groundwater Flooding viaPermeable Superficial Deposits Normal conditions in the alluvial aquifer: water table slopes gently towards river. River rises: groundwater is forced back into the aquifer, raising levels near the river. Groundwater may move beneath flood defences. River rises further and overtops banks
  5. 5. Recognising Bedrock vs. PSDGroundwater FloodingBedrock PSD• Upper catchment • Lower/middle catchment• Driven by high recharge & coastal aquifers• Only form of flooding • Driven by high rainfall• May feed into fluvial/ • Often a precursor of surface water flooding fluvial/tidal flooding
  6. 6. Groundwater Flooding Mechanisms 1965: Mines and heavy industry are active. Mines are dewatered to the deepest level. Concentrations of industry uses unsustainable volumes of groundwater. 2012: Mines and heavy industry are gone. Water table has risen in the mines and aquifer. Ochreous discharge results from sulphate-rich mine waters where drifts come to surface. Cellars and underground infrastructure are flooded by rising groundwater beneath former industrial area.
  7. 7. Transient Response to Abstraction
  8. 8. Needed an additional 50 ML/day to stabilise groundwater levels: Phase 1: Re- commissioning disused sources, 20 Ml/day Phase 2: Developing proven existing boreholes, 21.5 ML/day Phase 3: Private, commercial boreholes, 13GARDIT ML/day Phase 4: New borehole sites in central area, 4.2 ML/day www.ukgroundwaterforum.org.uk
  9. 9. Groundwater FloodingMechanisms
  10. 10. Groundwater Flooding Mechanisms
  11. 11. Groundwater Flooding Mechanisms High : 0.042305 Low : -0.0400162
  12. 12. Groundwater flooding in the SE
  13. 13. MappingGroundwaterFloodingSusceptibility
  14. 14. Mapping Groundwater FloodingSusceptibility What about risk?
  15. 15. Methodology (e.g.) Water table elevation Depth to groundwaterWater table + 20 m > ground surface Remove confining layers Morris, S.E., Cobby, D. and Parkes, A., 2007. Towards groundwater flood risk mapping. Quarterly Journal of Engineering Geology and Hydrogeology 40, 203-211. © ESI Ltd.
  16. 16. Methodology (e.g) Remove flood plains  Final groundwater emergence map (GEM)© ESI Ltd.
  17. 17. Sewer Infiltration
  18. 18. Potential for InteractionSewer above TillSewer within TillSewer below Till
  19. 19. Mapping Potential forSewer Infiltration
  20. 20. Modelling Groundwater-SewerInteraction Ú Û Ú Û
  21. 21. Modelling Groundwater-SewerInteraction
  22. 22. Modelling Groundwater-SewerInteraction Aquifer gain (m3/day) -31 - -30 -30 - -25 -25 - -20 -20 - -15 -15 - -10 -10 - -5 -5 - 0 0-5 5 - 10 10 - 15
  23. 23. Summary• GW flooding occurs frequently in the Thames catchment. – From Chalk and limestone aquifers – Over permeable superficial aquifers• GW flooding susceptibility may be linked intimately with likelihood of sewer infiltration.• Sewers can be modelled in groundwater models. But issues of scale and timescale and data for calibration remain.

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