Evidencing groundwater flooding in a flood risk assessment

Groundwater flooding evidence is the part of a flood risk assessment most often reduced to a single sentence saying the risk is low. On a chalk catchment, or a valley floor with permeable superficial deposits over an aquifer, that sentence will be challenged by the lead local flood authority, and correctly. Groundwater is a named source of flooding in national flood risk policy, and an assessment that addresses rivers and surface water while ignoring it is incomplete.

The difficulty is that there is no national groundwater flood map equivalent to the flood zones. The evidence has to be assembled from geology, from records and, where the decision needs it, from monitoring.

Why groundwater is missed

Three reasons, and they compound each other.

There is no single authoritative map, so the search for evidence has no obvious starting point. Groundwater flooding is slow, persistent and undramatic compared with a river bursting its banks, so it generates fewer photographs and less local memory of a specific date. And it emerges in places that look dry for years at a time, because it follows the multi-year rhythm of aquifer recharge rather than the timing of a single storm.

The result is that a site can have flooded from groundwater in a wet winter a decade ago and carry no trace of it in any dataset the assessor consulted.

The consequence when it is missed is expensive. Groundwater emergence into a basement or a below ground car park is difficult and costly to remedy after construction, and permanent pumping to keep a structure dry is usually unacceptable to both the authority and the eventual owner.

Geology that makes it likely

Start with the geology, because it narrows the question quickly.

Clearwater flooding from a major aquifer is the classic case, associated with chalk and, to a lesser extent, other limestone. Water levels rise through a wet winter, emerge from springs and from the beds of intermittent streams, and can stay high for weeks or months. The intermittent streams that appear in these settings have their own local names and their behaviour is often recorded in parish memory long before it appears in any database.

Permeable superficial deposits over a less permeable bedrock produce a shallower version of the same effect. River terrace gravels in a valley floor are the common setting, and levels there respond to the river as well as to rainfall, so groundwater and fluvial flooding can coincide.

Made ground and former workings are the third setting. Backfilled quarries, mineral workings and dock basins create zones of high permeability surrounded by lower permeability material, and water collects in them. Mining legacy adds the possibility of rising mine water where dewatering has ceased.

The British Geological Survey publishes geological mapping and a national susceptibility dataset for groundwater flooding. Treat the susceptibility layer as a screening indicator rather than as a site level answer, in the same way the surface water map is treated.

Sources of evidence

Assemble these in order, because each one narrows what the next has to do.

• Published geological mapping at the largest scale available, bedrock and superficial.
• The national groundwater flooding susceptibility dataset, with its limitations stated.
• The council's strategic flood risk assessment, which usually has a groundwater section and sometimes identifies affected areas specifically.
• The lead local flood authority's flood investigation reports, which record investigated incidents and their attributed source.
• Environment Agency groundwater level records where a monitoring borehole exists nearby.
• Borehole logs from the national geological records, which give strike water levels and rest water levels from earlier investigations on or near the site.
• Site investigation data from any previous scheme, including standpipe readings.
• Local accounts, including the parish council, the highway steward and long-standing residents.

Borehole logs are underused and often decisive. A log recording water struck at 1.2 metres in February tells you something specific about the site that no regional dataset can.

Monitoring boreholes and seasonal data

Where the geology suggests a risk and the scheme puts anything below ground, install monitoring.

A single dip on the day of the site investigation is not evidence of anything. Groundwater levels vary seasonally, with the highest levels typically late in the winter recharge period and the lowest in early autumn, and a reading taken in September can be metres below the winter maximum.

Install standpipes or piezometers in the phase 2 investigation and read them at intervals through at least one winter. Longer is better, because a single winter may be dry. Where the programme cannot accommodate a full winter, a shorter record can sometimes be extended by correlation with a long-running Environment Agency observation borehole in the same aquifer, provided the correlation is explained rather than asserted.

Report the actual readings with dates in a table, alongside the rainfall or groundwater level record for the same period, so a reviewer can see whether the monitoring period was wet, dry or average. That context is what turns a set of numbers into evidence.

Design responses to groundwater risk

The design answers differ from those used for surface water, and substituting one for the other is a common error.

Raise floor levels above the assessed maximum groundwater level with freeboard, and set the level from monitoring rather than from a nominal allowance. Avoid habitable accommodation below ground where levels are high, and avoid it entirely where sleeping accommodation is proposed and the risk is material.

Infiltration drainage is the design element most affected. Soakaways and infiltration systems need unsaturated ground beneath them to function, and the widely applied guidance is that the base of an infiltration system should sit a clear vertical distance above the seasonal maximum groundwater level, commonly taken as one metre. Where that clearance does not exist, infiltration is not available and the drainage strategy has to find another outfall, which changes the whole approach and sometimes the site capacity. The consequences for the drainage strategy are usually larger than the flood risk consequences.

Below ground structures need waterproofing designed for a permanent water table rather than for occasional wetting, with the grade of protection chosen for the intended use. Passive measures are strongly preferred over pumped systems, for the same reason that applies to any mitigation depending on maintenance.

Ground gas and contamination interactions are worth checking at the same time, since a rising water table can mobilise contaminants and can drive gas.

Interaction with basement design

Basements concentrate every groundwater issue into one element of the scheme.

Three points decide most basement applications on permeable ground. Whether the structure sits below the seasonal maximum water level, in which case it needs to resist hydrostatic pressure and buoyancy rather than simply exclude occasional water. Whether construction dewatering will be needed, which affects neighbouring structures and may need an abstraction licence. And whether the completed structure will alter groundwater flow, creating a dam effect that raises levels on the upstream side and affects neighbouring properties.

That last point is the one councils in basement-sensitive areas focus on, and it is properly answered with a groundwater flow assessment rather than with a paragraph.

Where the scheme also sits in a fluvial floodplain and involves raising ground levels, the compensatory storage requirement runs alongside, and the method is covered in compensatory flood storage and level for level design.

Recording residual risk

State what remains after the design measures, in terms a non-specialist can act on.

Give the assessed maximum groundwater level with its basis and its uncertainty. Give the finished floor levels relative to it. Say what happens in an event exceeding the design assumption, which for groundwater usually means a wetter winter than the monitoring period. Identify who maintains any element the design depends on, and set out the monitoring or maintenance regime if one is needed.

Then make sure the flood risk assessment addresses groundwater as a source in its own section rather than in a single line, and that the conclusions match what the drainage design assumes. Where the two documents disagree about the water table, the consultee will notice, and the things they check are set out in what a consultee checks in a drainage model output.

One honest uncertainty. Groundwater level records for a single site are almost always short relative to the variability of the aquifer, so the assessed maximum carries real uncertainty. Say so, and design with freeboard rather than to the measured maximum.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.