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

1. Groundwater Flooding and
Sewers in the Thames Catchment
Steve Buss
stevebuss@esinternational.com

2. Summary
• Groundwater flooding mechanisms
• Groundwater flooding in SE England
• Mapping groundwater flood risk
• Basement impact assessments in London
• Groundwater and sewers

3. Groundwater Flooding Mechanisms
Bedrock (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. Groundwater Flooding via
Permeable 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. Recognising Bedrock vs. PSD
Groundwater Flooding
Bedrock 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. 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. Transient Response to Abstraction

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, 13
GARDIT
ML/day
Phase 4: New borehole
sites in central area, 4.2
ML/day
www.ukgroundwaterforum.org.uk

9. Groundwater Flooding
Mechanisms

10. Groundwater Flooding Mechanisms

11. Groundwater Flooding Mechanisms
High : 0.042305
Low : -0.0400162

12. Groundwater flooding in the SE

13. Mapping
Groundwater
Flooding
Susceptibility

14. Mapping Groundwater Flooding
Susceptibility
What about risk?

15. Methodology (e.g.)
Water table elevation Depth to groundwater
Water 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. Methodology (e.g)
Remove flood plains
 Final groundwater emergence map (GEM)
© ESI Ltd.

17. Sewer Infiltration

18. Potential for Interaction
Sewer above Till
Sewer within Till
Sewer below Till

19. Mapping Potential for
Sewer Infiltration

20. Modelling Groundwater-Sewer
Interaction
Ú
Û
Ú
Û

21. Modelling Groundwater-Sewer
Interaction

22. Modelling Groundwater-Sewer
Interaction
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. 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.