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When incessant, persistent rainfall occurs, in some areas the water table can rise to the surface causing groundwater flooding.
This happens usually in areas that have chalk under the ground. There are large strata of chalk in the South and South East. The chalk acts like a large sponge which soaks up the water. When the ‘sponge’ becomes saturated by excessive rainfall which cannot permeate the hard bedrock beneath it, the water table rises causing flooding of low lying land and surface structures.
One of the major problems with high groundwater is that the sewerage network is affected as groundwater infiltrates it, usually through leaking joints. Sewer manhole covers pop up and toilets back up, potentially causing releases of sewage into streets, gardens and properties.
Without immediate action many communities cannot drain their domestic wastewater – including flushing toilets, taking baths and showers. Southern Water responds temporarily to such emergency situations by tankering (pumping wastewater – heavily diluted by groundwater - from a sewer into a tanker to convey it to a wastewater treatment works) but when this response is not sufficient, to avoid flooding properties it also over-pumps (pumping wastewater – heavily diluted by groundwater - from a sewer into a tank where it is passed through a filtration system before being discharged into nearby rivers, streams or ditches).
As a longer term remedy, the company is enacting a major £multi-million investment programme to survey and seal its sewer network in vulnerable areas such as St Mary Bourne - to help resolve the problems caused by groundwater infiltration of the sewers.
The Environment Agency requires wastewater companies to publish Infiltration Reductions Plans (IRPs) in catchments prone to groundwater and surface water flooding of the sewer system. For details of how we are managing the problem in St Mary Bourne and the work we have undertaken, please click on this link to see the IRP.