Flood Risk Assessment in Buckinghamshire

Our team at Unda prepares clear, compliant flood risk assessments and drainage strategies for commercial and residential sites across Buckinghamshire. A flood risk assessment in Buckinghamshire shows your local planning authority that a development will stay safe from flooding for its whole lifetime, and that it will not push flood risk onto land or property nearby.

Buckinghamshire sits across two of England's major river basins. The north of the county drains to the River Great Ouse around Buckingham; the south is shaped by the River Thames and the chalk streams of the Chilterns, the Wye, the Misbourne and the Chess. We assess every source that matters here, from those rivers and the surface water that overwhelms drains in the towns to the groundwater that rises through the Chiltern chalk, then deal directly with the Environment Agency and Buckinghamshire Council on your behalf. We work with architects, planning consultants, developers, landowners, home buyers and businesses, whether the report is needed for planning, a property purchase, a mortgage or insurance, BREEAM, or a surface-water drainage strategy.

If you need a quote or want to talk a site through, contact one of our experienced flood risk consultants on 01293 214444.

Flood risk assessment in Buckinghamshire at a glance

• Two river basins, one county. The Great Ouse drains the north around Buckingham, while the Thames and the Chiltern chalk streams shape the south, so the flooding problem changes markedly from one part of Buckinghamshire to the next.
• 18,380 properties are at risk from surface water at a 1% annual chance, against 3,357 at risk from rivers, on Buckinghamshire Council's own figures. Surface water, not the rivers, is the county's most widespread flood risk.
• Chesham and High Wycombe are nationally significant Flood Risk Areas for surface water flooding, recognised in the council's strategy as among the worst-affected places in the county.
• Buckinghamshire Council is the Lead Local Flood Authority, responsible for surface water, groundwater and ordinary watercourses since it became a unitary authority in 2020. Milton Keynes, immediately to the north, is a separate authority.
• Flood Zone 1 is not a free pass. Since the 2025 update to national Planning Practice Guidance, surface water and groundwater risk can trigger an assessment even on a site that sits entirely in Flood Zone 1.

Check flood risk anywhere in Buckinghamshire

Buckinghamshire is outlined below. Screen any postcode in the county against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass and a useful starting point, not a substitute for a site-specific assessment.

Flood risk in Buckinghamshire: the Thames, the Great Ouse and the Chiltern chalk

Flooding in Buckinghamshire comes from several sources at once, and which one dominates depends on where you are. The county spans the Thames and Great Ouse catchments and five Environment Agency management catchments in all, with the underlying geology swinging from limestone in the north to chalk under the Chilterns and clay in the far south. That variety is why a flood risk assessment here has to look beyond the nearest river.

The most extensive surface water flow paths run through the low-lying Aylesbury Vale catchments of the Thame and the Ray, and through the steeper Chess, Misbourne and Wye valleys in the east. The Chalk of the Chilterns forms a major aquifer, which is why groundwater rises to the surface in winter across Amersham, Chesham, the Chalfonts, Monks Risborough and Princes Risborough, and why it lingers for weeks once it arrives. The Great Ouse and its tributaries carry the fluvial risk in the north, and the Thames flanks the whole southern boundary.

A robust flood risk assessment in Buckinghamshire has to weigh all of these. We assess rivers and the sea, surface water, groundwater, sewers and reservoirs together, and where the national mapping is too coarse for the site we model flood levels and flow routes directly. Buckinghamshire also contains 19 reservoirs regulated under the Reservoirs Act, and the Grand Union Canal runs raised above the surrounding ground in places along its Aylesbury Arm, both of which can feature in a site's risk picture.

• Surface water dominates. 18,380 Buckinghamshire properties sit in a 1% annual chance surface water flood extent today, rising by a further 28,702 under climate change.
• Groundwater is a chalk problem. The Chiltern aquifer drives winter groundwater flooding that can persist for months, and it routinely undermines soakaway-based drainage on chalk and gravel sites.
• Climate change is already costed in. The Environment Agency's allowances point to peak river flows rising by 25% to 47% by the 2080s across the county's management catchments, and peak rainfall by around 25%.

Which parts of Buckinghamshire flood?

No single part of Buckinghamshire holds all the risk, but several places appear again and again in the flood record. Chesham and High Wycombe are designated nationally significant Flood Risk Areas for surface water. Buckingham and the villages of the upper Great Ouse flood from the river. The Chiltern towns flood from groundwater. Marlow sits against the Thames. A site-specific assessment is what separates a genuine constraint from a postcode that simply shares a town name with a flood map.

• High Wycombe and Chesham. Both are nationally significant surface water Flood Risk Areas, with steep Chiltern valleys funnelling runoff into built-up centres.
• Buckingham and the upper Great Ouse villages. Leckhampstead, Thornborough, Thornton, Gawcott and Tingewick flooded from the river and ordinary watercourses such as the Cowerde Brook and the River Leck in December 2020.
• Amersham, Chesham, the Chalfonts and the Risboroughs. These Chiltern chalk locations have a long history of winter groundwater flooding from the aquifer.
• Marlow and the Thames corridor. The southern bank of the Thames at Marlow and Medmenham has flooded repeatedly, most recently when Storm Henk pushed the river out of bank in January 2024.

A history of flooding in Buckinghamshire

Buckinghamshire has a long and well-documented flood record, drawn together in the council's Section 19 investigations and its Strategic Flood Risk Assessment. The dates matter for an assessment, because a planning authority will expect to see that local history reflected in how a site is judged.

• Winter 2000 to 2001. An unusually wet winter raised the Chalk aquifer across the county, lifting the River Misbourne and flooding the Chiltern valleys. Groundwater stayed high for months, flooding properties, roads and public areas.
• July 2007. Intense rainfall after weeks of wet weather flooded 96 properties, with the Thames out of bank at Marlow and the Great Ouse at Buckingham.
• January to February 2014. Six weeks of rain saturated the south. The Thames overtopped at Medmenham to around half a metre deep, and groundwater flooded Old Amersham and the Wycombe and Chiltern areas.
• September 2014. Surface water and a rising River Chess and Vale Brook flooded at least 34 properties internally in Chesham.
• October 2020 (Storm Alex). Up to 109mm of rain fell in 60 hours, flooding the Aylesbury area, Ickford and Quainton.
• December 2020. 112mm of rain through the month, then 52mm in 16 hours on already saturated ground, flooded Buckingham, Leckhampstead, Thornborough, Thornton, Gawcott and Tingewick from the Great Ouse and its brooks.
• June 2023. Localised, intense summer rainfall overwhelmed highway drainage and flooded homes and businesses in several towns and villages across the county.
• January 2024 (Storm Henk). The Thames burst its banks again, flooding parts of Marlow.

More than 18,000 Buckinghamshire properties already sit within a 1% annual chance surface water flood extent, and that figure roughly trebles once climate change is allowed for.

Planning and drainage policy for a flood risk assessment in Buckinghamshire

In Buckinghamshire your assessment is judged against national policy and a set of local requirements. The Environment Agency advises the planning authority on main river and sea flood risk, while Buckinghamshire Council, as Lead Local Flood Authority, leads on surface water, groundwater and ordinary watercourses and is a statutory consultee on surface water drainage for major development. Getting both on side early is usually what keeps an application moving.

• National policy. A flood risk assessment must satisfy the National Planning Policy Framework and its Planning Practice Guidance, demonstrating the development is safe for its lifetime and does not increase risk elsewhere. Since the 2025 update, the sequential, risk-based approach applies to all sources of flooding.
• The Lead Local Flood Authority. Buckinghamshire Council sets local expectations for surface water drainage through its developer guidance, and an unresolved drainage objection will usually hold up a decision.
• Local discharge and climate standards. The council asks for runoff to be restricted to agreed rates, with climate change allowances of 35% for the 1 in 30 year event and 40% for the 1 in 100 year event, and a half-drain-down time for the 1 in 30 year event within 24 hours.
• The drainage hierarchy. Schemes must work down the SuDS hierarchy, with rainwater reuse and infiltration ahead of any discharge to a watercourse, and sewers only as a last resort.
• Local plans and the SFRA. The Vale of Aylesbury Local Plan and the former district plans for Wycombe, Chiltern and South Bucks carry the flood and drainage policies a site must meet, all underpinned by the council's Strategic Flood Risk Assessment.

Do you need a flood risk assessment in Buckinghamshire?

If you, your business or your development is in Buckinghamshire and you have been asked to provide a flood risk assessment, it is usually for one of a handful of reasons. The table below sets out the common triggers and what each one typically calls for.

How Unda helps with flood risk in Buckinghamshire

Unda is not a local county firm, but we know Buckinghamshire's flooding and its planning landscape well, and we have prepared flood risk assessments and drainage strategies for sites right across it. Our role is to translate accurately between your scheme and what the Environment Agency, Buckinghamshire Council and national policy actually require, then to stay involved past the report to answer any objection. A Senior Flood Risk Consultant oversees and signs off every project.

It helps to be clear about what you are commissioning. A flood risk assessment, a sequential or exception test and a drainage strategy are three different reports. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. Groundwater and flood modelling are not separate reports of their own; groundwater is a source every assessment examines, and modelling is a technique that feeds the assessment when the national maps are not enough.

Flood risk assessments for every situation in Buckinghamshire

People come to us for a flood risk assessment in Buckinghamshire for many reasons, and we cover the full range. Each report below is tailored to the site and the question it has to answer.

• Flood risk assessment for planning. The site-specific report submitted with a planning application to show the development is safe and does not increase risk elsewhere.
• Flood risk survey for property purchase. An independent, property-level survey for home buyers and their advisers when a search flags flood risk.
• Flood risk assessment for insurance. A property-specific report for owners and brokers where cover has been restricted, declined or priced on flood grounds.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 so a scheme can claim its flood and surface water management credits.
• Sequential and Exception Tests. The locational justification a higher-risk site needs when development is steered away from lower-risk land.
• Surface water drainage strategy and SuDS. Designing how runoff is managed and restricted to keep discharge within the rates the council expects.
• Groundwater flood risk. A dedicated look at the Chiltern chalk and gravel sites where the water table, not a river, is the constraint.
• Flood modelling. Hydraulic modelling that defines flood levels and flow routes where national mapping is too coarse for the site.

We also handle the discharge of drainage conditions after permission and preliminary drainage design for outline schemes. You can see the full range on our flood risk assessment and drainage assessment pages, or read more about our flood risk consultants.

Free flood risk tools for Buckinghamshire

Before you commission an assessment, our free tools give you an early read on a Buckinghamshire site. They use the same open Environment Agency and British Geological Survey data that planners rely on, and they are screening aids rather than a formal assessment.

• Flood Risk Map. Check flood risk by postcode across rivers and sea, surface water, reservoirs and the planning flood zones, with a climate outlook.
• Live River Levels Map. See the latest Environment Agency gauge readings on the Thames, the Great Ouse and the Chiltern streams near you.
• Geology and Ground Conditions Map. Read the geology beneath a postcode for an early view of whether a soakaway is likely to work, which matters on the Chiltern chalk.
• Drainage Calculators. A free suite for sizing and checking drainage, from soakaways to SuDS flow controls.
• BRE 365 Infiltration Rate Calculator. Turn soakage trial-pit results into a design infiltration rate.

Local flood risk management and emergency contacts in Buckinghamshire

Several organisations manage flood risk in Buckinghamshire, and which one to contact depends on the source. Buckinghamshire Council leads on surface water, groundwater and ordinary watercourses; the Environment Agency covers main rivers and reservoirs; and the water companies deal with sewer flooding. In a flood that threatens life, always call 999.

• Report and check flooding. Use Buckinghamshire Council's flood investigation pages and the Local Flood Risk Management Strategy to understand local risk.
• Sign up for flood warnings. The Environment Agency's check for flooding service gives live warnings and a five-day forecast, and you can register for free alerts.
• Check river levels. The gov.uk river and sea levels for Buckinghamshire show how local gauges are running.
• Developer guidance. The council's guidance for developers sets out what a surface water drainage strategy must contain.

Flood risk assessment in Buckinghamshire: frequently asked questions