Residential: high local water tables and flooding

Causes, effects and solutions

14 October 2015

Michael Parrett looks at problems caused by high local water tables and flooding

Water tables, substrata and flooding are inextricably linked. The effect of water tables depends on the substrata  beneath a building and there can be huge variations in levels, for example, low water tables at the bottom of a valley and high perched water tables at the top of a hill. And while clay is highly impermeable to water, gravel beds are highly permeable and can allow rapid rises in water tables.

An example of the link between water tables and flooding was in Carlisle in January 2005. Flooding caused £250m of damage, costing £38m to rectify. The main contributor was 175mm of rain falling in 36 hours over the high ground of the nearby Lake District and Pennines. The surrounding ground became saturated and the water table rose and could not drain away. Continued rainfall caused a vast volume of water to drain into the River Eden, on which Carlisle is situated.

Man-made factors

Deindustrialisation and the withdrawal of human intervention can cause flooding. Around Nottingham, for example, coal mines have largely been closed, leading  to much lower water extraction from aquifers. Water tables have therefore risen to create increased flooding risks at local and district levels.

There was widespread flooding in Nottingham in 2000 after the River Trent burst its banks causing extensive damage, which cost more than £45m to rectify. The city’s Left Bank project was completed in 2012 to reduce the risk of flooding to 16,000 homes and businesses along a 27km stretch of the river, which includes new flood barriers along vulnerable stretches.

Another cause of flooding could be poor maintenance of underground water mains, which have corroded over time and leaked into the ground. I know of a basement workshop studio that began  filling with water during a period of heavy and prolonged rainfall because its cementitious waterproofing protection failed. This was put under pressure because of a rise in the water table in the surrounding ground and an escape from a burst underground water main.

The British Standard BS 8102:2009 suggests that with full-height structures below ground or full-height earth-retaining walls, the surrounding water table could reach approximately two-thirds of the height of the structure. This is useful guidance for designers when constructing below ground or retrofitting basement waterproofing to existing structures.

Water mains supplying properties near railway lines often corrode much more quickly because an electrical discharge ‘earths’ onto metallic elements in the ground, which eventually leads to perforation of the pipes.

Many Victorian and Edwardian properties had lead water mains routed under the timber suspended floor from the front of the property through to the rear kitchen. I have found many incidents of water escape from these old main supplies, which have often been confused with rising damp caused by a failed damp-proof course (DPC).

Shows black mould on external walls

Figure 1: Black mould on external walls where damp had been caused by replacing the original timber suspended flooring with a solid floor. A localised excavation found the floor had been laid without any damp-proof membrane

pattern of internal flooding

Figure 2: Area of solid floor prone to internal flooding. A section of screed was removed to examine the make-up of the floor. This rapidly filled with ground water due to a high local water table and failure of the internal waterproofing

area of solid floor prone to internal flooding

Figure 3: Pattern of internal flooding caused by a rise in the local water table following sustained and heavy rainfall

Concrete and hard surfaces

New commercial or industrial developments are usually accompanied by copious amounts of concrete and tarmac. If this happens on a previously well-drained site, then water will just be displaced elsewhere.
This issue is considered in detail in CIRIA’s 2007 SuDS manual, which contains best practice guidance on planning, designing, constructing, operating and maintaining sustainable drainage systems. These are water-management practices and facilities designed to drain surface water in a slow sustainable way, e.g. using ponds with reed beds to encourage transpiration and underground water surge balancing tanks, rather than routing directly to a watercourse.

I found an example of this displacement during work for Hong Kong’s Antiquities and Monuments Office while surveying an ancestral home of the Tsang family from the Qing dynasty. Several high-rise apartments had been built nearby, as had the new Canton Railway into China.

The additional concrete in the ground from these developments had caused the water table to rise and completely cover the ground floor of the 19th century building. The best way to preserve the building was to recommend dismantling and reconstructing it on a raised platform so the water just flowed underneath, similar to a storm drain. The alternative, which was neither sustainable nor energy efficient, was an underground substation to pump the water away Another case I investigated was in Surrey with some newbuild properties in an area containing typical 1930s properties with semi-circular bays, cavity walls and timber suspended floors. Soon after the newbuild completion, damp was found in the 1930s houses and we discovered the timber suspended floors were full of water, with condensate dripping off the underside of the floorboards and joists.

Airborne moisture was also wicking into the walls, giving the illusion of a failed DPC. It transpired the builder had gone bankrupt before the development was completed, and had not installed a dewatering substation as part of an agreed water table attenuation scheme. This caused localised flooding to properties.

As a pumping substation would take time to install, we laid perforated pipes in external trenches in an attempt to drain off the water, but this was futile. We had to install sumps with mains and batterypowered self-priming pumps underneath the suspended floors to continuously siphon into the existing drainage system to keep the water table from rising above oversight level below the timber suspended floor.

Some residents replaced their timber suspended floors with solid concrete, but this did not address the issue of a high water table and just pushed the water elsewhere. Their damp problems were exacerbated because the damp-proof membranes in the solid floors were never properly interleafed into the existing horizontal DPC in the walls so moisture was easily pushed up around the perimeter of the floor into the walls.

These examples illustrate the risks of increasing the density of buildings in areas without fully considering their impact on the water table and associated properties.

Properly designed attenuation schemes can help manage the water table

Warning signs

Timber suspended floors and cellars may show signs of excessive hydration, e.g. water marks on walls and timbers. A water table pushing up under a suspended floor would so hydrate the subfloor area that the timber flooring and skirtings will absorb moisture. Older properties in which timber skirtings are set off the wall on timber ‘soldiers’ have a void where moisture can condensate and wick up into the plaster, giving the illusion of rising damp, or a missing or failed DPC.

What can be done?

Properly designed attenuation schemes can help to manage the water table and/or surface water flooding to minimise or eliminate collateral damage to property, but these can be complex and expensive, as evidenced in Carlisle and Nottingham. Even in areas less prone to flooding, a fluctuating water table can still be problematical for ground-floor areas and basements in buildings.

Consider excavating trenches to install perforated drainage pipes that are either connected into the existing underground drainage system, or led to soakaways some distance from the property, as a means of attenuating and managing water levels in the ground. Typically, perforated drainage pipes would be set below ground at the base of walls at the toe or top edge of the foundation or basement slab level to prevent the water table rising higher in the ground.

Narrow trenches can also be dug and backfilled with washed gravel or pea shingle to encourage better drainage around a building and perimeter channels can be installed at least 150mm below the height of the physical horizontal DPC. These avoid bridging the DPC and encourage any excess rainwater to drain away into the existing drainage system. In rural areas, ditches or dykes are often dug to drain the land and alleviate flooding from water tables and periods of intense rainfall.

External abutting ground levels should be kept at least 150mm below a DPC or 200-250mm below the internal finished floor level. The most important aspect is to reduce the amount of additional hard surfaces introduced around a building, e.g. driveways, concrete hardstands and solid patios. Think about draining the land in the spirit of the SuDS manual. If a solid floor gives high moisture readings from electrical resistance or capacitance meters, use a floor hygrometer to measure how much moisture is being exuded. If the relative humidity is consistently above 75%, suspect either a missing or impaired damp-proof membrane. Should solid floors exhibit relative humidity in excess of 80%, the next step would be to recommend a localised excavation of the solid floor to determine the existence of any damp-proof membrane and then carry out appropriate remediation works to repair or renew.

Even in areas less prone to flooding, a fluctuating water table can still be problematical for ground-floor areas and basements

Information sources

The Environment Agency website has interactive maps showing flooding predictions by location. Local authorities, neighbours, builders and shopkeepers are also good sources of information about flooding events in the area. The British Geological Survey has an app that uses GPS coordinates to give details about the substrata under a location. Insurers can advise on areas prone to regular flooding because this would be reflected in premiums charged or, in extreme cases, cover being declined due to a perceived high risk or past history.

Groundwater monitoring is increasingly important to understand the flood risks to proposed newbuilds or redevelopments. The British Standards for site investigations and geotechnical investigations and testing require groundwater levels to be monitored for 12 months, but this is rarely done.

A holistic approach

The spirit of the Victorian ‘three-foot rule’, which aimed to reduce the density of buildings by maintaining a gap between them, is now long forgotten with today’s trend for high-density building. But a lack of intelligent and considered site investigations may have consequences for nearby properties, neighbourhoods, districts and towns.

Michael Parrett is a Building Pathologist, Chartered Building Surveyor and Founder of Michael Parrett Associates. He is an eminent Fellow of RICS

The next article will cover physical blockages under suspended floors and blocked external vents to ground floors.

Further information