Groundwater Flooding andSewers in the Thames Catchment Steve Buss firstname.lastname@example.org
Summary• Groundwater flooding mechanisms• Groundwater flooding in SE England• Mapping groundwater flood risk• Basement impact assessments in London• Groundwater and sewers
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.
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
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
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.
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.