Groundwater, heritage, and the cost of getting flood risk wrong

A recent Upper Tribunal decision involving The King’s Lodging in Sandwich has brought an often-overlooked aspect of flood risk into sharp focus: groundwater.

The case centred on whether Environment Agency flood defence works altered subsurface conditions enough to cause damage to a historic property. The Tribunal concluded that they did — finding that groundwater levels rose significantly following construction, leading to both existing and ongoing damage.

The building itself — The King’s Lodging in Sandwich, a Grade II listed property — is a rare and historically significant structure, which makes the implications of the case more acute.

Flood risk is often framed in terms of rivers overtopping or coastal flooding. In reality, water does not simply disappear when a barrier is introduced. It moves, redistributes, and can re-emerge in less visible but equally damaging ways.

What the Tribunal actually decided about groundwater and flood risk

The decision was a preliminary ruling on causation, not a final determination of compensation.

The Tribunal was asked to decide whether the Environment Agency’s flood defence works caused damage to the claimants’ property. Liability for some garden impacts had already been accepted. The central issue was whether the works also caused damage to the house itself.

A detailed legal summary of the findings is also set out in analysis of the Upper Tribunal decision on flood defence works and listed buildings, which confirms that the Tribunal identified a material change in groundwater conditions as the cause of the damage.

This is a clear finding that the issue was not traditional flooding, but a change in groundwater behaviour caused by the works.

Why this case turned on groundwater, not fluvial flooding

Although widely described as a flood defence dispute, the case is more accurately understood as a groundwater flooding case.

The works involved installing a new sheet piled wall, with infill material and a concrete capping layer. While effective at controlling river levels, such interventions can significantly alter groundwater flow and pressure.

Groundwater systems respond dynamically to change. When barriers are introduced, water can be redirected, slowed, or trapped, leading to localised increases in groundwater levels.

This is the critical point for planning and design: reducing fluvial flood risk does not remove water from the system — it changes its behaviour. The distinction between surface flooding and subsurface processes is central to understanding groundwater processes such as those described in groundwater flooding causes, risks and planning implications.

The evidential gap in flood risk assessment - no baseline groundwater data

One of the most consequential aspects of the case was the absence of robust pre-construction groundwater data.

The Tribunal had no direct measurements of groundwater levels before the works were undertaken, meaning historic conditions had to be reconstructed indirectly.

This introduces significant uncertainty. Without baseline data, causation becomes harder to establish, expert interpretation carries greater weight, and disputes become more complex.

Situations like this illustrate why technical rigour is critical in flood risk assessment for planning and why expertise matters.

How the Tribunal determined groundwater levels had risen

The Tribunal relied on multiple strands of consistent evidence rather than a single dataset.

Historic and pre-works evidence

Earlier site investigations indicated groundwater levels were materially lower prior to construction.

Physical site behaviour

The ability to drain and maintain a swimming pool without groundwater ingress strongly suggested lower historic groundwater levels.

Post-works monitoring data

Borehole monitoring showed groundwater levels around 2m AOD, significantly higher than pre-works indications.

Changes in ground conditions

The garden became persistently waterlogged, with saline and anaerobic conditions, consistent with a raised groundwater regime.

Design assumptions

Pre-construction assumptions appear to have underestimated groundwater behaviour, treating it as broadly aligned with river levels.

Taken together, the Tribunal concluded groundwater levels had increased by approximately 0.5m to 1m.

Why listed buildings are highly sensitive to groundwater flooding

The same groundwater change might have had a lesser impact on a modern structure. That was not the case here.

Historic buildings are particularly vulnerable due to breathable materials such as lime mortar, shallow or irregular foundations, and limited damp protection in floor structures. They are also more susceptible to salt migration and long-term decay.

Damage from groundwater is often gradual, appearing as persistent damp, structural movement, and progressive deterioration rather than sudden failure.

Even relatively small changes in groundwater conditions can therefore lead to significant and often irreversible impacts in heritage assets.

What the judgment reveals about causation in flood risk cases

The Tribunal rejected simplistic explanations and focused on mechanism.

It did not accept that timing alone proved causation, nor that the building’s age explained the damage. Instead, it relied on hydrological evidence as the primary driver.

Surveying and engineering evidence described the damage, but groundwater behaviour explained it.

This reinforces a key principle: flood risk must be understood as a process, not just an outcome.

What this case shows about getting flood risk wrong

The case highlights recurring weaknesses in flood risk assessment and design.

Groundwater is frequently under-assessed compared to more visible risks such as river flooding, despite the close interaction between processes described in fluvial flooding causes and planning implications.

Other issues include reliance on assumptions rather than measured data, absence of baseline monitoring, and incomplete understanding of subsurface conditions.

Flood risk management also involves multiple organisations with different responsibilities, reflected in the structure of managing flood risk responsibilities in England.

The lesson is not that flood defence schemes are flawed, but that they must be based on a complete understanding of how water behaves.

Implications for planning, flood risk assessment, and development

Flood risk assessment for planning cannot rely solely on mapping or surface-level analysis.

Flood Zones and mapping tools are essential, but they do not capture groundwater processes or site-specific interactions. The Environment Agency plays a central role in this system, particularly in how flood risk is considered within planning decisions and guidance on Environment Agency flood risk planning.

Groundwater risk should be considered where sites are adjacent to rivers, involve structural interventions, or include sensitive assets such as historic buildings. Some works may also require environmental consents, including those set out in flood risk activity permits.

Watercourse classification can also influence both risk and regulatory requirements, particularly in relation to main rivers and ordinary watercourses.

Getting flood risk right before damage occurs

The most significant flood risk issues are often the least visible.

Groundwater is slower, less obvious, and harder to interpret than surface flooding — but its long-term effects can be just as damaging.

Proper flood risk assessment requires a robust conceptual understanding of the site, supported by appropriate data and specialist expertise from the outset.