Groundwater Flooding in Thames Catchment

Groundwater Flooding and

Sewers in the Thames Catchment

Steve Buss

stevebuss@esinternational.com

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.