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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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Summary
• Groundwater flooding mechanisms
• Groundwater flooding in SE England
• Mapping groundwater flood risk
• Basement impact assessments in London
• Groundwater and sewers
Groundwater Flooding Mechanisms
Bedrock (Clearwater) Flooding
1. Ambient conditions in the aquifer: the water table slopes at a shallow angle and groundwater discharges from a spring line at the break of slope.
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.
Water table
Ground surface
Spring line
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
Recognising Bedrock vs. PSD
Groundwater Flooding
Bedrock
• Upper catchment
• Driven by high recharge
• Only form of flooding
• May feed into fluvial/
surface water flooding
PSD
• Lower/middle catchment
& coastal aquifers
• Driven by high rainfall
• Often a precursor of
fluvial/tidal flooding
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.
Transient Response to Abstraction
GA
RD
IT
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
ML/day
Phase 4: New borehole
sites in central area, 4.2
ML/day
www.ukgroundwaterforum.org.uk
Groundwater Flooding
Mechanisms
Groundwater Flooding Mechanisms
Groundwater Flooding Mechanisms
High : 0.042305
Low : -0.0400162
Groundwater flooding in the SE
Mapping
Groundwater
Flooding
Susceptibility
Mapping Groundwater Flooding
Susceptibility
What about risk?
© ESI Ltd.
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.
Methodology (e.g)
Remove flood plains
Final groundwater emergence map (GEM)
Sewer Infiltration
Potential for Interaction
Sewer above Till
Sewer within Till
Sewer below Till
Mapping Potential for
Sewer Infiltration
Modelling Groundwater-Sewer
Interaction
ÛÚÛÚ
Modelling Groundwater-Sewer
Interaction
Modelling Groundwater-Sewer
Interaction
-31 - -30
-30 - -25
-25 - -20
-20 - -15
-15 - -10
-10 - -5
-5 - 0
0 - 5
5 - 10
10 - 15
Aquifer gain
(m3/day)
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.
