Flood Risk Assessment in Marlow

Marlow sits on a sharp bend of the River Thames, and the river has left its banks into the town more than once this century. Buckinghamshire Council's own flood record confirms southern Marlow was under water in 2003, and an intense rainfall event in July 2007 brought the Thames and surface water together across the town within the same week. Rising groundwater through the chalk beneath the floodplain adds a third, slower-moving source of risk that can persist for weeks after a river level has already dropped.

Government data lists 287 Marlow properties as being at significant or very significant risk of flooding from the river and groundwater, concentrated around Marlow Bridge, Bisham village, Pound Lane and Gossmore, and the Thames Industrial Estate and Dedmere area east of the town centre. A separate, smaller watercourse, the Newt Ditch, adds its own surface-water risk to roads including Dedmere Road and Fieldhouse Lane, and is now the subject of a live Buckinghamshire Council flood alleviation scheme.

Whatever brings you to a site in Marlow — a planning application, a mortgage or insurance query, a BREEAM assessment or a drainage strategy for a new development — Unda's flood risk consultants combine that local flood history with live Environment Agency and Buckinghamshire Council data to show exactly what a site faces and what it takes to make it safe. Call 01293 214444 or email enquiries@unda.co.uk to talk through a site in Marlow.

Two of the last three notable Thames floods in Marlow — 2003 and 2007 — put water into the same low-lying streets south of the town centre, and the Environment Agency now formally designates Marlow a nationally significant flood risk area.

Flood risk assessments in Marlow — for planning, purchase, insurance and BREEAM

A flood risk assessment in Marlow can be commissioned for several different reasons, and each is treated as a complete piece of work in its own right. Unda prepares flood risk assessments in Marlow for planning applications, property purchase and mortgage conditions, insurance broking and underwriting, and BREEAM certification, alongside sequential and exception tests and surface-water drainage strategies for sites on the Thames floodplain and the chalk catchment that feeds it.

• Planning applications. Sites in Flood Zone 2 or 3, over one hectare, or affected by surface water on the Flood Map for Planning normally need a site-specific flood risk assessment for planning to satisfy the NPPF and Buckinghamshire Council as Lead Local Flood Authority.
• Property purchase and mortgages. When an environmental search flags a Marlow property near the Thames or the Newt Ditch, or a lender asks for more detail, a homebuyer flood risk survey goes beyond the national datasets to assess the actual building, its thresholds and its ground levels.
• Insurance. Where premiums on a Marlow property have risen sharply or cover has been restricted after a claim, an insurance flood risk assessment gives brokers and underwriters the technical detail behind the postcode-level rating.
• BREEAM. A BREEAM flood risk assessment is structured around Pol 03 and its pre-development risk classification, which is a different test to a planning submission and is scoped accordingly.
• Sequential and exception tests. Where a Marlow site sits in a higher flood zone, a sequential or exception test sets out whether a lower-risk site is reasonably available before the development can proceed.
• Drainage strategies and SuDS. A surface-water drainage strategy works out where runoff from a Marlow development goes and how discharge is kept within the rates Buckinghamshire Council requires.

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a Marlow planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go.

Marlow's flood risk — the Thames, groundwater and the Newt Ditch

Marlow is affected by three separate sources of flooding, and a site-specific assessment needs to look at all three rather than relying on a single national map. The River Thames is the dominant risk, groundwater rises through gravel deposits over the underlying chalk in low-lying parts of the floodplain, and surface water collects both generally across the town and specifically along the Newt Ditch, an ordinary watercourse whose culverts lack capacity in eastern Marlow.

• River Thames. Marlow sits on the Thames floodplain and is nationally designated by the Environment Agency as the Marlow, Thames Flood Risk Area, with 245 properties identified as being at very significant risk and a further 42 at significant risk of fluvial flooding.
• Groundwater. Groundwater flooding has occurred in low-lying parts of Marlow, tied to high river levels where the floodplain is underlain by gravel deposits over chalk — a risk a groundwater assessment within a wider flood risk assessment needs to capture, since it appears on no national flood map.
• Surface water. Heavy and persistent rainfall running off land and hard surfaces adds a further risk across the town, covered by a Marlow-specific Surface Water Management Plan.
• The Newt Ditch. This ordinary watercourse in eastern Marlow is fed by culverts that lack capacity, with roads including Dedmere Road, The Croft, Quarrydale Drive and Fieldhouse Lane historically affected and blockages recorded under the A404.

Where the modelled flood extent or levels on a Marlow site need more precision than the national datasets provide — a site right on the river, or one that could affect flow routes for a neighbouring property — flood modelling derives site-specific depths, extents and velocities to feed into the wider assessment. One striking local response to that risk sits on a small island in the Thames near Marlow: the Formosa amphibious house, designed to rise on its own foundations during a flood rather than resist the water — a rare but telling illustration of how seriously flood risk is taken on this stretch of the river.

Where flooding hits hardest in Marlow

The Environment Agency maintains two flood warning areas covering Marlow and neighbouring Bisham, and both name the streets most consistently affected. Together they give the clearest official picture of where a Marlow site is likely to need closer attention.

• Marlow Bridge and Bisham village. The Environment Agency's warning area for "River Thames at Bisham village and Marlow town" covers Bisham Abbey and Stoney Ware in Bisham, Marlow Bridge, Bream Close, Trout Close and Pound Crescent, plus the Thames Industrial Estate, Dedmere, Fieldhouse and Westhorpe Farm areas of Marlow.
• Pound Lane, Gossmore and Lock Island. A second warning area, "properties closest to the River Thames from All Saints Church, Bisham to Little Marlow", covers Marlow Bridge Lane and Quarry Wood Road in Bisham, Lock Island and Gossmore, Meadow Close and Pike Close, and Pound Lane and Garnet Court in Marlow — where the Marlow Flood Alleviation Scheme's defence wall now runs along the Thames Path.
• The Thames Industrial Estate and Dedmere. This commercial area east of the centre sits within the primary Marlow warning area alongside Fieldhouse and Westhorpe Farm, and is also within reach of Newt Ditch surface-water flooding.
• The Newt Ditch corridor. Dedmere Road, The Croft, Quarrydale Drive and Fieldhouse Lane have a documented history of flooding where the Newt Ditch's culverts, including a pinch point under the A404, cannot pass the flow.

Check the flood risk at a Marlow postcode

Enter a Marlow postcode below for an indicative screening of river, surface water and reservoir flood risk, drawn live from Environment Agency and Ordnance Survey open data, with the parish outlined in coral. It is a useful first look at a site — not a substitute for a site-specific flood risk assessment.

Planning policy and the Lead Local Flood Authority in Marlow

Buckinghamshire Council has been the Lead Local Flood Authority for Marlow since April 2020, when Buckinghamshire moved from a two-tier structure that included Wycombe District Council to a single unitary authority. Flood risk and drainage policy for Marlow now sits across the Buckinghamshire Local Flood Risk Management Strategy 2022–2028, a Buckinghamshire Level 1 Strategic Flood Risk Assessment, and the flood risk and SuDS policy carried over from the Wycombe District Local Plan, adopted in August 2019, pending a single Buckinghamshire-wide Local Plan expected to be in place by April 2027.

• Buckinghamshire Council as LLFA. The council reviews surface-water drainage strategies as a statutory consultee on major planning applications, expecting drainage calculations, a critical storm duration assessment and a whole-life maintenance plan.
• The Local Flood Risk Management Strategy. The current 2022–2028 strategy documents Marlow's fluvial, groundwater and Newt Ditch flood history and sets out how the council prioritises local flood-risk investment.
• Wycombe District Local Plan. Adopted in August 2019, the plan's flood risk and SuDS policy remains part of the development plan for Marlow pending the emerging Buckinghamshire-wide Local Plan.
• The Newt Ditch scheme. Buckinghamshire Council submitted a planning application in November 2024 for a flood alleviation scheme targeting the Newt Ditch's culvert capacity, evidence of continued investment in Marlow's drainage network.

Marlow is also constrained by the Chilterns National Landscape (formerly the Chilterns Area of Outstanding Natural Beauty) to its north and the Thames floodplain to its south, which together limit the scale of new development the town can absorb compared with larger Buckinghamshire towns — meaning the sites that do come forward tend to need a correspondingly careful flood risk case.

Flood risk assessments for every situation in Marlow

However a Marlow site reaches Unda, the same local knowledge of the Thames, the chalk and the Newt Ditch runs through every report Unda prepares.

• For planning: flood risk assessment for planning, guided by NPPF flood risk guidance.
• For property purchase: homebuyer flood risk survey.
• For insurance: insurance flood risk assessment.
• For BREEAM: BREEAM flood risk assessment.
• Locational testing: sequential and exception tests.
• Surface water: surface-water drainage strategy and SuDS design, preliminary drainage design and discharge of drainage conditions.
• Technical inputs: groundwater assessment and flood modelling where a site needs them.

Before commissioning a report, it is worth a first look at Unda's free screening tools: the Flood Risk Map for a postcode-level check, the Live River Levels Map for current Thames conditions, the Geology and Ground Conditions checker for soakaway potential on the Marlow chalk, and the Drainage Calculators and BRE 365 Infiltration Rate Calculator for early drainage sizing.

That 2003 event, and the intense rainfall that flooded the town again in July 2007, are the reason Marlow now carries a permanent Environment Agency flood risk area designation and a completed £8.5 million flood alleviation scheme along Pound Lane, Firview Close, Gossmore Lane and the Thames Path. Unda's flood risk assessments in Marlow draw on that same official record, rather than starting from a generic national dataset, so the report reflects what has actually happened on this stretch of the Thames.

Local flood resources and who to contact in Marlow

Marlow Town Council and Bisham Parish Council both run active flood-response arrangements alongside the statutory agencies, and it is worth knowing the right contact before a flood happens rather than during one.

• Environment Agency Incident Hotline: 0800 807060, free and available 24 hours, to report flooding, pollution or a watercourse problem.
• Floodline: 0345 988 1188, 24/7, for the latest flood warnings and advice for a Marlow address.
• Sign up for flood warnings: register a Marlow address for free Environment Agency flood warnings at gov.uk/sign-up-for-flood-warnings.
• Buckinghamshire Highways (flooding on roads): 01296 382416 in hours, 01296 486630 out of hours.
• Thames Water (sewer flooding): 0800 316 9800, 24-hour customer centre.
• Marlow Town Council: maintains the Marlow Community Flood Plan and is recruiting volunteer flood wardens — see the council's flooding page.
• Bisham Parish Council: runs a flood warden, coordinator and marshal team, with a Flood Bronze Hub opened at Bisham Abbey during flood events — see the parish council's flooding information.

Buckinghamshire sits within the Thames Regional Flood and Coastal Committee and the Thames Valley Local Resilience Forum, the two bodies that coordinate flood risk investment and emergency response across the wider Thames catchment. Further reading on the wider Thames defences shaping Marlow's flood risk includes Unda's coverage of the Jubilee River downstream at Maidenhead, the River Thames Scheme further downstream, and how flood risk affects house prices in areas like Marlow and Bisham.

Frequently asked questions