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Deluges of grandeur

Numerous landslides across the country and the collapse of an abandoned mineshaft near Newcastle highlight the link betwen the weather and geohazards. Alan Forster provides a geological perspective on the recent flooding.

SEVERE THOUGH the autumn weather has been in many parts of the UK, it is not unprecedented.

Britain enjoys a maritime climate noted for its variability. It may even be considered as a succession of weather rather than a predictable climate.

The hurricane of 1987 wrought enormous damage across the south of England. The east coast floods of 1953 brought great devastation and loss of life.

Floods and extreme magnitude rainfall events, though infrequent, are far from unknown, as the British Hydrological Society's Chronology of British Hydrological Events website (www.dundee.ac.uk/geography/cbhe) shows.

For example, 1929 has entries showing very high rainfall in South Wales (1112 mm in 39 days at Pont Lluest Wen reservoir), major floods in Cumberland, Somerset and exceptional rainfall in Devon (345 mm in 12 days).

So while the date of the next event is unpredictable, it is not unforeseeable. Sooner or later extreme weather events will occur, and if predictions of the consequences of global warming are accurate, they will occur sooner and more frequently.

Nature's forces can rarely be stopped but they can be understood. The consequences of the extreme natural events they generate can be mitigated by careful planning.

In terms of ground conditions and geohazards, the engineering implications of intense rain and flooding are quite wide-ranging. Landsliding is the most likely and widespread result. It may be particularly frequent on soft rock coastal cliffs where saturated slopes containing active springs lines and surface water gullies, intensified by water draining from the land, are undercut by wave action during storms.

Other important hazards include foundation failure in saturated ground and sudden settlement in poorly compacted fill or in collapsible soils on which construction has imposed a critical load since the last time they experienced saturation.

Local flooding will occur if the capacity of rainwater soakaways is exceeded during episodes of intense rainfall, especially when ground is already saturated. Sudden, high flow through natural drainage systems may cause new drainage paths to form that may result in piping in uncemented sands and silts.

Perhaps some consolation may be found in the probability that the damage due to the shrinkage of clay soils caused by a succession of dry summers and winters will be less in the next few years.

If the various factors that cause ground condition problems are identified, they can be marked on maps and plans, allowing analysis of the level of risk that prevails at a site for each geohazard. The findings can be incorporated into land use planning.

For example, the occurrence of landslides may be regarded as a consequence of the combination of predisposing factors such as slope angle, lithology and hydrogeology, combined with a triggering event such as a high magnitude rainfall event.

A slope above its maximum long-term stable angle in a landslide-susceptible lithology may have pore water pressures raised due to a period of high rainfall, extending over weeks or months, that will bring the slope close to failure. But the triggering mechanism may be a single fall of rain, especially as a severe storm.This may raise pore water pressures above a critical value, increase the loading of the slope by saturating the surface, or, if adjacent to a stream, undercut the slope.

Artificial slopes in cuttings may be vulnerable to slope failure if they have had to be cut at a steeper angle than the ideal value to minimise land take or if they include adverse geological factors such as landslide susceptible lithologies or perched water tables.

High magnitude rainfall may also cause a sudden recharge to perched water tables that will result in unusually high discharge in unexpected areas, causing slope instability, back sapping or local flooding.

Flooding has important implications for the spread of pollution, with its ability to bypass engineered barriers built to isolate potential contaminants such as sewage, chemical wastes, landfill leachates and mine spoil from 'receptors' such as people, sensitive ecosystems, surface water and groundwater.

Examples include the flooding of protective bunds around solvent and fuel oil storage facilities, the dispersal of bacteria and other pathogens from overburdened storm flow systems, and the dispersal of chemical contamination associated with current and former industrial sites and landfills.

These examples are commonly found near rivers and urban areas which emphasises the need for them to be considered in a risk-based approach to flood alleviation schemes.

Although it is not immediately apparent, geological maps can help in planning for flood hazard. The mapped extent of alluvium represents the maximum extent of flooding that has occurred over the last 10,000 years or so.

The reported sudden collapse of the cover of an abandoned mineshaft near Newcastle on 10 November highlighted the problems that flooding can bring in areas of former mine workings. The downward flow into a shaft can wash away loose infill or weathered rockhead, thus eroding support if an engineered capping is in place and ultimately leading to collapse.

The flow of large volumes of floodwater into abandoned mine workings raises questions as to the implications for pillar stability and the movement of pollution and acid mine waters through the subsurface.

Similarly, in karstic limestone or gypsiferous ground the flow of floodwater into fissures may accelerate existing subsidence features or form new ones and introduce large volumes of potentially polluted surface water into the groundwater system.

The British Geological Survey (www.bgs.ac.uk) is creating and populating geohazard databases of the UK that will facilitate the assessment of geohazards for consideration in land use planning and construction decision-making processes.

Alan Forster is an engineering geologist and project manager in British Geological Survey's urban geoscience and geohazards programme.

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