Fens 2100+ explained: what the Environment Agency’s evidence actually says

Fens 2100+ is the Environment Agency's long-term flood risk and water management programme for the Lincolnshire and Cambridgeshire Fens, run in partnership with Internal Drainage Boards, local authorities and water companies. In April 2026 it published a summary baseline report, seven catchment evidence reports and a Case for Change. Between them they put numbers on something the sector has discussed for decades: how long the current model of flood risk management can hold in a landscape that only exists because water is pumped out of it.

The reports do not introduce a new problem. They quantify one that has been building since the post-war investment period, and the quantities are unusually specific.

Emma Jeffery, Senior Flood Risk Consultant at Unda, notes that "the Fens are not just exposed to flood risk. They depend on active flood and water management every day, across a very complex system."

The evidence base values the assets protecting the Fens at £58.8 billion of benefits, and puts the cost of sustaining them at their current standard of service for the next 100 years at £6 billion to £9.4 billion.

What is Fens 2100+?

Fens 2100+ is a partnership programme led by the Environment Agency covering 4,333km² of the Lincolnshire and Cambridgeshire Fens across seven catchments: Steeping, East and West Fen, Lower Witham, South Forty Foot Drain, Lower Welland, Lower Nene and Great Ouse. It has a dedicated measure in the National Flood and Coastal Erosion Risk Management Strategy for England, which is what set it up in 2020.

The partnership structure follows from how the system is actually run. Responsibility for flood risk and land drainage in the Fens sits with the Environment Agency, 58 Internal Drainage Boards, local authorities, water companies and riparian owners. The Environment Agency owns around 35% of the flood risk assets; the remaining 65% belong to other risk management authorities under the Flood and Water Management Act 2010 and private owners. No single organisation controls the system, and no single organisation can fix it.

The programme's own summary is direct: given a network of more than 17,000 interconnected assets, "systems thinking is central to the future of the Fens. No single organisation can solve the challenge alone."

Two documents carry the argument. The Summary Baseline Report sets out the shared evidence: what exists, what it protects, what condition it is in, and what climate change does to it. The Case for Change and Partnership Action Plan sets out what the partnership intends to do about it between 2025 and 2027.

Why flood risk in the Fens is different

The Fens are one of England's most engineered landscapes, and the scale of the engineering is what makes the flood risk unusual. This is not a floodplain that occasionally floods. It is a drained basin held below water level by continuous mechanical intervention.

• Area. 4,333km² of low-lying land, with 89km of coastline.
• Population. More than 600,000 people.
• Assets. Over 17,000 flood risk and water management assets, including 318 pumping stations.
• Watercourses. Around 4,500km, much of it artificial channel.
• Elevation. 87% of the Fens lies below 3.9m AOD (the Mean High Water Springs level), with the lowest point at Holme Fen, 2.75m below sea level and 47km from the sea.
• Agriculture. 48% of England's Grade 1 agricultural land, producing roughly a third of the country's vegetables and supporting 80,000 jobs across the food supply chain.
• Infrastructure at risk. £8.1 billion of transport assets and £5.1 billion of utilities.

The baseline report is unambiguous about what those assets do: "Without critical flood risk management assets, the Fens would quickly flood and become uninhabitable."

Because the ground sits below the tide, water in much of the Fens does not drain to the sea under gravity. It is lifted, by pump, and released when the tide allows.

That is why catchment behaviour matters as much as site conditions here, in the way explored in spongy landscapes and flood risk. It is also why any credible site-specific flood risk assessment for planning in the Fens has to look past the red line and ask how the surrounding system performs, and for how long.

Emma explains: "In the Fens, the issue is not just whether water reaches a site. It is whether the wider system can keep managing water as conditions change."

The peat paradox: why the ground keeps sinking

The Fens have a self-reinforcing problem that most defended landscapes do not. Drained peat oxidises and shrinks. As it shrinks, ground levels fall, which increases the head the pumps have to lift against and reduces the windows in which water can be released by gravity. The response to lower ground is more drainage, which dries more peat.

The baseline report states the mechanism plainly:

Holme Fen is the clearest measure of it. In the mid-1800s the peat there was over 6.5m deep. In the 175 years since, ground level has dropped by around 4m. Peaty soils still cover 18% of the Fens, most of it degraded by long-term drainage and intensive agriculture, and "centuries of land subsidence due to the shrinking of drained peat as it dries and oxidises have left many rivers elevated above their floodplains."

Every Fens 2100+ catchment except Steeping now emits more carbon than it absorbs, at an assessed cost of £359 million a year.

The Steeping catchment report is the exception because it holds proportionally less drained peat. That figure links the flood risk case to the wider question of soils and flood resilience, and it is one reason the partnership's action plan includes a carbon strategy alongside the asset work: pumped drainage is energy-intensive, and the peat itself is a carbon liability.

Where a site sits on shrinking peat, the ground investigation and topographical survey work behind a drainage design carries more weight than usual, and infiltration is rarely the answer. In this landscape a surface water drainage strategy has to answer where the water goes next, and which pump eventually moves it.

Flooding still happens in defended landscapes

The Fens 2100+ baseline includes a historic flood timeline, and it is the part of the evidence base that most directly contradicts the assumption that a defended area is a safe one. These are events that occurred despite the defences, not before them.

Emma notes that "there is sometimes an assumption that defended means safe. The Fens record shows it is more complicated than that."

This is the same argument that runs through below-standard flood defences and the question of why a property can remain in a flood zone after new flood defences have been built. What the Fens add is scale and repetition: eight centuries of it, in one place, with the mechanism recorded each time.

The timeline is becoming more difficult

The baseline report's "now and next" timeline sets recent operational stress against projected pressure. Read together, the two halves are the case for change.

Recent system stress

• 2020. Measure 1.5.4 of the National FCERM Strategy establishes the Fens 2100+ Partnership.
• 2023 to 2024. The wettest winter on record for the UK pushed flood risk assets in the Fens "to their limits and beyond".
• January 2025. Water levels in the South Forty Foot Drain were the highest ever recorded.
• 2024 and 2025. Property flooding in both years, in a defended system operating as designed.

Future pressures

Climate change does not act evenly across the Fens. Working from the UKCP18 projections that underpin current planning guidance, the baseline evidence gives a per-catchment picture of peak river flow increase by the 2080s, which matters because the catchments are hydraulically linked but individually appraised.

On the tidal side, the picture is starker. Government Upper End allowances project 1.2m to 1.6m of sea level rise by 2125. Applying a 1.1m rise to the current 0.5% annual exceedance probability event produces an eight-fold increase in the area at tidal flood risk across the Fens, and a twenty-fold increase in the East and West Fens specifically.

By 2040, 31% of the Fens' flood risk assets will be beyond their foreseeable design life. By 2075, 89% will be.

Emma highlights the issue: "The system works because it is managed intensively. As conditions change, that becomes harder to sustain."

The allowances themselves are set out in the Environment Agency's climate change allowances guidance, and how they apply to a planning submission is covered in Unda's guide to climate change allowances for flood risk assessments. What happens after 2125 is a separate question, and the subject of sea level rise and planning beyond 2100.

Why the current flood risk model is under pressure

Flood risk management in the Fens has historically worked asset by asset: maintain what exists, upgrade what fails, prioritise by consequence. The Case for Change argues that approach can no longer keep pace, for three connected reasons.

Ageing infrastructure

Most of the 17,000-plus assets were built in the post-war decades and are reaching the end of their design life together, because they were built together. Maintenance investment is prioritised for the assets that would cause the most damage if they failed, which is rational in isolation and leaves everything else on a fix-on-fail footing. The report's assessment is that "the current 'fix-on-fail' approach limits strategic renewal."

Reduced gravity drainage

Sea level rise and continuing land subsidence together shorten the tidal windows in which water can be discharged by gravity. Less gravity discharge means more pumping, more energy, more carbon and more dependence on the assets that are ageing fastest.

System interdependency

The Fens behave as one network rather than a set of independent schemes. The Case for Change puts it directly: "Because these assets are interconnected, failure in one part can cascade across the system." A decision about a sluice or a washland in one catchment changes water levels in another, which is why Internal Drainage Boards are so central to how the Fens work and why the split between main rivers and ordinary watercourses is more consequential here than almost anywhere else in England.

Emma notes: "As the system becomes more interconnected, managing assets in isolation becomes less effective."

What it costs to keep the Fens working

The baseline report models what happens under two ends of the investment spectrum. This is the section of the evidence base that has had the least attention and carries the most weight.

Under the do-nothing case the modelled losses break down as £36.6 billion of property (99,000 residential and 29,000 non-residential), £35.5 billion of local economic output, £8.1 billion of transport, £7.9 billion of agriculture with 2,020km² of arable land written off, £5 billion of utilities and £217.5 million of heritage.

Sustaining the current standard of service for 100 years needs £6 billion to £9.4 billion of investment, and half of it is needed within 40 years.

For context, the report estimates that building an equivalent system from nothing today would cost £25 billion to £30 billion. The existing network, whatever its condition, is an asset the country could not now afford to create. The Case for Change is candid that the bill will not be met from one source: "The scale of investment cannot be met by public finances alone."

That places the Fens in the same funding conversation as the wider picture set out in England's flood defence performance and the NAO findings, but with a sharper edge, because in the Fens the alternative to investment is not a higher residual risk. It is an uninhabitable landscape.

What Fens 2100+ changes

Fens 2100+ moves the unit of decision from the individual asset to the system. The Case for Change commits the partnership to treating "assets not as isolated components but as parts of an interconnected network". Around that it builds four pillars of resilience: plan, protect and maintain, respond, and recover. Each was tested against four plausible futures for the region.

The Partnership Action Plan sets 17 actions across a two-year window to 2027, in three groups.

1. Place-keeping (actions 1 to 6). Near-term asset management: supplementary baseline evidence, a value narrative, a communications strategy, a costed "decade of action" pipeline, a funding and finance strategic outline case, and position statements on asset management and decommissioning.
2. Transition interventions (actions 7 to 13). Programme design and governance, a monitoring and evaluation framework, and strategic appraisals of five critical points in the system: the River Nene tidal limit, the Denver Complex, Witham recovery, Hobhole pumping station replacement, and the Crowland and Cowbit Washes.
3. Place-making (actions 14 to 17). Longer-horizon work: spatial futures, a growth and water position statement developed with the mayoral authorities, a coastal strategy alignment roadmap, and updated farmland reservoir guidance.

It also accepts a limit on what engineering alone can deliver. Land management and natural processes have a part to play, in the way set out in the Environment Agency's own research on natural flood management, but the partnership is explicit that they operate within constraints in a landscape this heavily modified.

Emma summarises: "This is about making better decisions across the whole system, not just maintaining individual assets."

The plan also draws its own boundary. Fens 2100+ "is not the forum for making political and democratic decisions regarding regional growth and spatial planning". It produces the evidence those decisions should be made against, and stops there.

What this means for planning and development

For anyone promoting or determining development in the Fens, the practical consequence is straightforward. Flood risk here is a function of how the system performs over time, and the current mapping is a snapshot of one point in that.

Policy and decision making

The Sequential and Exception Tests already require future risk to be weighed, not present conditions alone, and the case law has moved the same way. See applying the Sequential Test post-Mead and what the Gladman judgment means for the Sequential Test. A published, catchment-level evidence base stating that 89% of the local flood risk assets reach end of life by 2075 is now part of the context in which those judgments are made. It does not change the Flood Zones. It does not change the NPPF tests or the planning practice guidance behind them. It changes what a well-informed assessment is expected to have read.

Technical evidence

In a pumped, defended catchment the technical burden is to show how the system performs across the lifetime of the development, rather than on the day it is validated.

• Levels and datum. Where the ground is below Mean High Water Springs, everything turns on accurate levels tied to Ordnance Datum. A commissioned topographical survey does what LiDAR cannot.
• System context. GIS analysis of the surrounding drain, sluice and pumping network establishes what the site actually depends on.
• Modelling. Hydraulic flood modelling is often the only way to show breach and overtopping behaviour behind an embanked defence.
• Discharge. Drainage boards set discharge rates in their districts, and land drainage consent under the Land Drainage Act 1991 runs separately from planning permission.
• Residual risk. Behind a defence, residual flood risk is the assessment, not a footnote to it.

In practice, a well-argued flood risk assessment for planning and an evidenced surface water drainage strategy are what turn this evidence base into a decision a case officer can defend. That applies as much in Fenland and South Holland as it does in Boston, Wisbech, or anywhere along the tidal Nene and the Great Ouse.

Why this matters beyond the Fens

The Fens are unusual. The pressures Fens 2100+ identifies are not.

Across England, flood risk systems face the same four things: infrastructure built in a single post-war push and ageing in a single wave, rainfall intensity rising faster than the design assumptions, surface water risk growing outside the mapped zones, and governance split across authorities that each hold part of the answer. The Fens make all four visible at once, because there the system has no slack. Remove the active management and the landscape does not degrade gradually. It floods.

That is what makes this evidence base useful well outside Lincolnshire and Cambridgeshire. It is a costed, catchment-scale statement of what happens when a defended system reaches the end of its design life, produced by the authority that maintains it. Very little else in English flood risk management is documented to that standard. The same gap between institutional evidence and local experience runs through the flood knowledge gap between communities and institutions.

Emma's final view is measured: "There is no immediate failure. But the current approach becomes harder to sustain without long term planning. The decisions made now will shape how manageable flood risk is in the future."

Fens 2100+ is not a warning about a single location. It is an indication of how flood risk management is changing, and where its limits are starting to show.

Frequently asked questions

If you are assessing a site anywhere in the Fens, or in any catchment that depends on pumped drainage and embanked defences, Unda's chartered consultants can prepare the flood risk assessment for planning and the surface water drainage strategy your application needs. Call 01293 214444 or email enquiries@unda.co.uk for a fixed-fee quote within 60 minutes.