Tackling Flooding From the Ground Up

Previously overlooked by the media in favour of more dramatic cases, groundwater flooding actually causes two to four times the level of damage to buildings and can have a serious impact on infrastructure.

Previously overlooked by the media in favour of more dramatic cases, groundwater flooding actually causes two to four times the level of damage to buildings and can have a serious impact on infrastructure. This article looks at the science behind groundwater flooding and introduces an innovative new tool in the fight to better predict and prevent resultant issues.

By Mark Fermor

Groundwater flooding occurs when sub-surface water emerges from the ground at the surface, or into made ground and structures. This could be as a result of persistent rainfall that recharges aquifers until they are full, or may be a result of high river levels or tides driving water through near-surface deposits. Compared to surface water flooding, groundwater flooding can last considerably longer, with incidents enduring anything from a week to several months, which is why it can prove substantially more costly to businesses and property owners.

Although emergent groundwater tends to be clear and relatively clean compared to muddy fluvial flood waters, it has the potential to be contaminated by sewers and brownfield sites, and it is the discharge of untreated sewage effluent during storm events which forms one of the primary mechanisms causing pollution of surface waters in England and Wales.

We have seen wastewater service providers recently come under increasing pressure from the Environment Agency to further understand and manage the ingress of groundwater to their sewer networks, and they have been tasked with reducing the frequency of emergency discharge to surface waters.

Water companies also have an intrinsic desire to better understand mechanisms occurring in their catchments which affect their sewerage network.

Yet despite local authorities, water companies and commercial organisations being highly reliant on current and accurate flood risk guidance to predict groundwater flooding, until now there has been no national-scale authoritative map of groundwater flood risk.

In October 2013 ESI addressed groundwater flooding issues in England and Wales by publishing the first national authoritative Groundwater Flood Risk Map. The map was developed using best practice algorithms and calibrated risk predictions using site-specific evidence of real flooding events from many parts of the country to achieve an authoritative national map of groundwater flood risk.

Already proving popular with a variety of companies in the water and utilities sectors, as well as infrastructure owners and investors, the new Groundwater Flood Risk Map and related GIS data should see a step-change in the way that groundwater flooding is considered.

By providing the very best and most accurate data we are able to allow water and wastewater professionals to mitigate and manage potential issues as they wish. Groundwater flooding issues are more localised than previously thought, but projects within risk zones need groundwater to be properly considered in order to avoid problems.

In the face of the prolonged flooding witnessed recently, I would also recommend water and wastewater professionals to revisit their flood risk management and continuity plans. When floods occur it's often too late to make effective plans, which is why good preparation is key.

Mark Fermor is an expert hydrogeologist and managing director of ESI. Email: becky@zen-communications.co.uk

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