Flood Risk Assessment in Marlow

A flood risk assessment in Marlow has to deal with water arriving from three directions at once. The Thames runs along the south of the town, the river terrace gravels beneath it fill with groundwater from the Chilterns chalk, and the eastern streets drain through a small culverted watercourse, the Newt Ditch. We prepare these assessments for Marlow sites going to planning and for buyers, lenders and insurers, together with sequential and exception tests and surface water drainage strategies.

On this page Marlow means the town's built-up area in the SL7 postcode district, the town once called Great Marlow: the centre around the High Street and West Street, the riverside streets from Pound Lane to Mill Road, Dedmere and the Thames Industrial Estate in the east, and the estates on the rising ground north of the centre. Marlow Bottom and Little Marlow sit just outside it, Bourne End lies further down the river, and Bisham faces the town from the Berkshire bank. Buckinghamshire Council decides planning applications here and is the Lead Local Flood Authority, and the town sits within our Buckinghamshire coverage.

In February 2014 the Thames reached 1.24 m at Marlow Lock, its highest on record, while groundwater rose through the gravels and the sewers surcharged. The council's investigation put about 55 to 60 Marlow properties flooded inside.

To talk through a site in Marlow, call our flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

Marlow flooding: the Thames, the gravels and the Newt Ditch

Marlow flooding rarely has a single cause. The town is built on river terrace gravels laid over chalk, and the Environment Agency's Thames flood risk management plan names Marlow a Flood Risk Area for river flooding: 2,574 people live within reach of the Thames there, counting the Bisham bank. The table sets out each source and where it shows up.

The Thames drains more than 6,700 km² above Marlow, so it rises over a week or more and falls just as slowly, while the Newt Ditch and the street drains react within hours. Readings from Marlow Lock and the Temple and Hurley locks upstream appear on our live river levels map, and the river's wider record is on our River Thames page.

Is Marlow in a flood zone? Pound Lane to the Thames Industrial Estate

Its southern part is. On the Environment Agency maps used in the council's 2014 investigation, southern Marlow around Pound Lane, Gossmore and the riverside falls inside the 1% annual chance river outline, and the north of the eastern area inside the 0.1% outline. Enter an SL7 postcode in the tool below for a first screen, or open our full flood risk map. The coral line marks the built-up area.

Groundwater is the gap in that screen. Defra's groundwater mapping shows much of Marlow with water at or within 2 m of the surface in a winter like 2000/01, and no national flood zone shows it. A Marlow site near the river needs both checked, and the groundwater flooding explainer sets out why the two behave differently.

Winter 2014 and the Marlow floods before it

Marlow's record goes back to 1768, when letters from Great Marlow reported the Thames more than a mile wide. The modern events below come from the Environment Agency, the council's Section 19 investigation of the 2014 floods and the 2024 Strategic Flood Risk Assessment.

1. November 1894 and March 1947. The two highest Thames floods in the historic record. In 1947 the water was quite deep in the low-lying parts of the town.
2. 3 to 9 January 2003. The river and rising groundwater flooded southern Marlow for about a week; the Environment Agency counted at least 22 properties flooded, from Pound Lane and Firview Close to Gossmore Lane and Dedmere Road.
3. 20 July 2007. An intense storm on a saturated catchment brought surface water and the Thames together, with more than 30 streets reported from Chapel Street and the High Street to Marlow Bottom Road.
4. February 2009. Heavy rain and high groundwater closed roads across the south of the county, Marlow among them.
5. 2012. Thames flooding reached 8 properties; in December the Newt Ditch flooded Dedmere Road and The Croft.
6. January and February 2014. The Thames peaked on 10 January and again on 9 to 11 February. Soakaways stopped working, the Pound Lane pumps were overwhelmed, the Newt Ditch backed up and the sewers surcharged. Buckinghamshire Fire and Rescue put about 150 properties around Pound Lane as affected, Wycombe District Council estimated 55 to 60 flooded inside, Pound Lane was shut for nearly two weeks and 250 RAF personnel were sent to the town.
7. January 2024. After Storm Henk the Environment Agency found Thames levels from Marlow downstream similar to January 2014. Across the bridge, the Royal Borough's investigation recorded 16 homes and businesses in Bisham flooded inside, and residents along Bisham Road reported groundwater about four feet deep.

The Environment Agency's own 2014 count is 23 properties, taken from its river records. The council's figure is higher because it counts every source: the river was only one of four ways water reached Marlow homes that winter, and a report that looked at the Thames alone would miss the other three.

What the Marlow flood scheme changed, and what it left

The Environment Agency finished the Marlow Flood Alleviation Scheme in autumn 2018 at a cost of £9.5 million. It is designed to a 1 in 75 standard and reduces the risk to 287 properties from the river and from groundwater.

• Walls and embankments. Raised defences along Pound Lane, the Thames Path, Gossmore playing fields and the Pergola field, with flood gates at Millbank.
• Storage and pumping. Flood storage at Lower Pound Lane in place of lost floodplain, and a permanent pumping system that returns groundwater from the gravels to the Thames.
• What it does not do. Surface water was outside its scope, and the flood zones ignore defences, so a plot behind the walls is still judged against Flood Zone 3, as our piece on staying in a flood zone after defences are built explains. A defended site also has to show what happens if the defences are overtopped or the pumps fail.
• The Newt Ditch scheme. Buckinghamshire Council's Newt Ditch scheme opens up piped stretches of the ditch near Quarrydale Drive and Gunthorpe Road and regrades it east of the A404, for about 100 homes and 60 other properties. The planning application went in in November 2024 (24/07725/R9FUL), and in July 2025 the council expected completion in late 2026.
• Further ahead. The 2024 SFRA records a West Marlow study into surface water. In Bisham, a relief pipe south of Marlow Bridge was studied without finding an option that would lower peak levels.

Policy DM39 and the draft Local Plan: what a Marlow application must show

Applications in Marlow are decided against the Wycombe District Local Plan, adopted on 19 August 2019, until a Buckinghamshire-wide plan replaces it. Flood risk sits in Policy DM39, Managing flood risk and sustainable drainage systems, which treats medium and high surface water risk and the Wycombe Critical Drainage Areas as areas at risk alongside Flood Zones 2, 3a and 3b. The Level 1 SFRA of June 2024 confirms those drainage areas were drawn in Marlow.

• When an assessment is needed. Any development in an area at risk, and any site over a hectare, needs a site-specific assessment covering every source, safe access and escape, resilient construction, drainage and climate change.
• The sequential test. Required for development in an area at risk, apart from allocated sites (unless the use is more vulnerable than the allocation) and minor development.
• Runoff. All development includes SuDS: no material increase in runoff, greenfield rates on greenfield land, and brownfield rates brought as close to greenfield as reasonably practicable.
• The functional floodplain. Flood Zone 3b is kept for water, and inappropriate development in high-risk areas is refused.

The draft Local Plan for Buckinghamshire has been through its Regulation 19 consultation and is due to go to the Planning Inspectorate by the end of 2026. Its draft Policy CC1 asks for a flood risk assessment on all land at risk now or in future and every site over a hectare, bars building over culverts, and requires a strategic drainage strategy on schemes of 50 homes or more. National policy sits in chapter 18 of the NPPF, as revised in August 2026.

Drainage on the Marlow gravels

Most of Marlow already drains into the ground. The 2014 investigation found soakaways in use across the town, with sumps in the Pound Lane area pumping the excess to the Thames, and found them failing once groundwater rose. A soakaway that tests well in August may sit in water in February, which is why Buckinghamshire Council asks for evidence through the winter.

Our ground conditions and soakaway checker gives a first view of the geology, and the guide to groundwater monitoring for planning covers how long a winter record takes. Foul drainage matters as well: Thames Water is the sewerage company here, and the Secretary of State's November 2025 permission for Marlow Film Studios at Westhorpe bars occupation until network upgrades or a phasing plan agreed with Thames Water are in place.

Do I need a flood risk assessment in Marlow for an extension, flats or offices?

If the site is near the river or the Newt Ditch, inside a Critical Drainage Area or larger than a hectare, almost certainly. The cases below are the ones that come up most in the town.

Send us the address and a short description of the scheme, and we will say which of these applies.

Reports for Marlow sites, from Pound Lane to Globe Park

We work on Marlow sites for homeowners and buyers, architects, planning agents, developers and businesses. A flood risk assessment asks whether a site is safe, the sequential test whether it should be there at all, and a drainage strategy where its rainwater goes; a riverside scheme often needs more than one.

• Every source on one site. Where the Thames, groundwater in the gravels and the Newt Ditch can each reach a plot, a flood risk assessment looks at all of them and at what happens if the defences are overtopped.
• Applications to Buckinghamshire Council. From a single house on Pound Lane to flats near the High Street, a flood risk assessment for planning sets out the evidence Policy DM39 asks for, Critical Drainage Areas included.
• Buying in SL7. Pound Lane, Garnet Court and Quarrydale Drive all flooded in 2014, and a flood risk survey for property purchase tests the building itself against those events rather than a postcode band.
• Riverside plots. A Marlow site in Flood Zone 2 or 3 normally needs a sequential and exception test, unless it is allocated or minor development.
• Schemes of every size. DM39 asks all development for SuDS, so a drainage strategy belongs in Marlow applications from a single dwelling upwards.
• Soakaways that still drain in February. A surface water drainage strategy sizes infiltration on winter groundwater readings and BRE 365 tests, and finds another outfall where the gravels fill.
• After the decision. Where Buckinghamshire Council attaches a drainage condition, we prepare the information for the discharge of drainage conditions before work starts.
• Water rising from below. Groundwater flooded parts of Marlow in 2000/01, Dedmere Road and The Acre in 2010/11 and the Pound Lane area in 2014, so a groundwater flood risk assessment is worth having for any basement or soakaway there.
• Getting out before Pound Lane closes. Two Environment Agency warning areas cover the town, and a flood warning and evacuation plan sets out when people leave a riverside building and by which road.
• Levels for one plot. Where the national outlines are too coarse for a site near Marlow Lock or behind the scheme's walls, flood modelling of the Thames at Marlow gives depths and extents for the site itself.
• Homes with a claim history. After 2003 and 2014, insurance flood risk assessments give owners the levels and protection for their own house to put to an insurer.
• Firms in the warning area. The Thames Industrial Estate and Fieldhouse Lane sit inside the main warning area, and firms there protected their own premises in 2014; business continuity flood assessments plan for the next high river.
• Buying commercial property. Policy MR7 keeps Globe Park for business use and backs its regeneration, and due diligence flood risk assessments help buyers and lenders judge surface water and drainage before exchange.
• New employment buildings. Office and studio schemes at Globe Park and Westhorpe often need BREEAM flood risk assessments to score under Pol 03.
• Sewers under strain. Groundwater surcharged Marlow's sewers in 2014, and a foul drainage strategy shows how a scheme connects and what capacity it needs from Thames Water.

Who to call when Marlow floods, and how to get warnings

Ring 999 where floodwater threatens life, and 105 when it has knocked out the electricity. Beyond that, the contact depends on the source: the Thames is the Environment Agency's, while groundwater, surface water and the Newt Ditch fall to Buckinghamshire Council.

The council's flood investigations page holds the 2014 reports for Marlow, Bourne End and Medmenham, and its Local Flood Risk Management Strategy for 2022 to 2028 sets out the programme that includes the Newt Ditch work.

What owners, buyers and developers ask about flooding in Marlow

To discuss a site anywhere in Marlow, from Pound Lane to Globe Park, call our chartered flood risk consultants on 01293 214444 or email enquiries@unda.co.uk. A senior consultant checks and signs every report before it leaves us, and Unda Consulting Limited is a CIWEM Business Partner. Along the river we also cover Henley-on-Thames upstream and Taplow and Maidenhead downstream, with High Wycombe up the A404 and Bisham's bank in our Berkshire coverage.