Flood Risk Assessment in Aylesbury

A flood risk assessment in Aylesbury is mostly about Bear Brook and the smaller brooks that feed it. They run in culverts and narrow channels under the middle of the town, while the River Thame curves round its northern and western edge. We prepare flood risk assessments in Aylesbury for planning applications, house purchases, mortgages and insurance, alongside sequential and exception tests and surface water drainage strategies, for sites from the town centre out to Berryfields, Watermead and Fairford Leys.

On this page Aylesbury means the town as the Office for National Statistics draws its built-up area: the centre and Kingsbury, Quarrendon, Buckingham Park, Watermead and Berryfields to the north, Elmhurst and Stocklake to the east, Bedgrove, Walton Court and Southcourt to the south, and the Willows and Fairford Leys to the west, across the HP19, HP20 and HP21 postcode districts. The newer estates at Kingsbrook and the villages of Bierton, Aston Clinton, Weston Turville and Stoke Mandeville sit just outside that outline in HP22 but share the same brooks and councils, so they are covered here too. Buckinghamshire Council decides planning applications and is the lead local flood authority; the rest of the county, Aylesbury Vale included, is on our Buckinghamshire page.

On 7 February 2014 the Stoke Brook and Sedrup Ditch overtopped beside the Willows estate in west Aylesbury, and 80 homes there flooded inside. Debris at the culvert under the Oxford Road held the water back.

Call 01293 214444 or email enquiries@unda.co.uk and one of our flood risk consultants will look at your Aylesbury address with you.

Bear Brook, the Thame and the brooks under Aylesbury's streets

Bear Brook rises near Aston Clinton, passes under the Aylesbury Arm of the Grand Union Canal and runs west through the town centre to join the Thame north-west of Aylesbury. The county's 2024 Strategic Flood Risk Assessment calls it one of the most urbanised watercourses in the whole Thame catchment, and notes that its floodplains through the town are tightly confined because the brooks are culverted for much of their length. Every brook in the table below is a main river, so the Environment Agency, not the council, regulates work in or beside it:

Which brook drains a plot decides most of what the report has to cover, and the answer is not always the nearest one on the map, because piped surface water often reaches a different channel. So the outfall is traced on the ground before any drainage layout is drawn.

Look up an address in Aylesbury, Watermead or Fairford Leys

Enter any Aylesbury postcode below to check it against Environment Agency data for rivers, surface water and the planning flood zones, or open our full flood risk map for anywhere else in England. Kingsbrook and the HP22 villages fall just beyond the coral outline but can still be searched.

What the gauges on Bear Brook, Southcourt Brook and the Thame record

Three Environment Agency gauges cover the town. Their records show how high each brook has risen, and each gauge has a level above which the agency expects homes nearby to flood:

Two flood warning areas follow Bear Brook through the town: one from Broughton to Haydon Mill Farm in Coldharbour, taking in Hilda Wharf and the California area, and one from Walton Court in the Ellen Road area to St Marys School, which also covers the Stoke and Southcourt brooks. The Thame has its own warning area from Long Marston to north Aylesbury. In 2023 Southcourt Brook came within 20 cm of its property flooding level. Our live river levels map shows the current readings at these gauges beside their normal ranges.

Floods on record in Aylesbury, from 1947 to Storm Alex

Aylesbury's record is a mix of brook floods, surface water and overloaded sewers rather than one big river event. The dates below come from the Environment Agency, the county's flood investigations and the two Strategic Flood Risk Assessments:

1. 1947, 1990, 1992 and 1993. Floods on the Thame and Bear Brook appear on the Environment Agency's historic flood map, as listed in the 2024 Strategic Flood Risk Assessment.
2. April 1998 and January 2003. Both are on the 2017 assessment's list of significant floods in the former district. On 2 January 2003 the Thame reached 2.51 m at the Aylesbury gauge, still its highest reading.
3. November and December 2006. Repeated heavy rain caused surface water flooding in the town centre and sewer surcharging in Manor Park, according to the 2017 Aylesbury Vale Strategic Flood Risk Assessment.
4. 24 December 2013 to 7 February 2014. The Willows estate flooded three times from the Stoke Brook and Sedrup Ditch. On the last date 80 homes across ten streets took water inside, after 9 mm of rain fell in two hours on saturated ground (Buckinghamshire County Council flood investigation).
5. 2 to 4 October 2020, Storm Alex. Up to 109 mm of rain fell in 60 hours after a dry summer. Bear Brook and Stocklake Brook flooded, Bear Brook hit its record 2.00 m at the gauge, and residents on Weedon Road reported sewage in their gardens.
6. 20 September 2023. Southcourt Brook reached 2.32 m at Churchill Avenue, the highest level on that gauge's record.

Buckinghamshire Council publishes a flood investigation whenever five or more homes in one square kilometre flood inside; inside the town the Willows report is the one on file. It found the brook overtopping at low points near Dormer Close and Parrot Close, gullies backing up, and debris in the channel, which is why a report for a Willows address looks hard at culverts and the drains as well as the brook itself.

Storage at Broughton and low embankments through the town

The town's main protection sits upstream, not along the streets. In the mid-1990s the Environment Agency built the Aylesbury Vale Flood Alleviation Scheme, and most of the embankments inside the town are built to a much lower standard than the storage behind them:

• Two flood storage areas at Broughton. Clay embankments hold back Bear Brook (with Bedgrove Brook) over about 105,000 m² and Stocklake Brook over about 30,000 m², cutting peak flows by 27% and 53%. The Environment Agency maintains both, and they protect the urban reaches against a 1 in 100 year flood.
• Embankments in the town. Raised banks at Hilda Wharf, the railway station and Station Way, and Coldharbour Way give only about a 1 in 5 year standard against Bear Brook. That is a low bar. The 2017 assessment rated the Hilda Wharf bank fair and the others good or very good.
• The Southcourt Brook floodwall. A wall on the west bank protects the neighbouring housing to about a 1 in 25 year standard.
• Culverts that matter if they block. Blockage modelling picked out four: Southcourt Brook at Churchill Avenue, the crossing under the canal, Bear Brook at the High Street, and Bedgrove Brook at Broughton Avenue. Two of them, under the canal and at the High Street, put Park Street and the health centre at risk if they block.

Both assessments warn that the storage only works as designed if new development keeps its runoff in check, and that climate change will push flows higher. For a site behind these defences the assessment tests what happens when they are overwhelmed or a culvert blocks; our article on why a site can stay in a flood zone after defences are built explains the mapping behind that.

Clay, limestone and the sewers: drainage on Aylesbury sites

Aylesbury's old centre stands on a low hill of Portland limestone, while most of the estates around it sit on Kimmeridge Clay. That split, clear on the British Geological Survey's maps, decides whether water can soak away on a plot. Mostly it cannot:

Our free geology and ground conditions map and BRE 365 infiltration rate calculator give a first view before any trial pits are dug.

Thames Water runs the sewers, and Aylesbury has a history of sewer flooding. The 2024 assessment lists HP19, which covers north-west Aylesbury, among the Thames Water postcode districts with the most recorded sewer flooding, and the October 2020 storm left sewage in gardens on Weedon Road. Policy I4 of the Vale of Aylesbury Local Plan accepts surface water into a combined or surface water sewer only in exceptional cases, so a scheme that cannot infiltrate normally discharges to a watercourse.

Which policies must a flood risk assessment in Aylesbury satisfy?

Buckinghamshire Council decides Aylesbury applications against the Vale of Aylesbury Local Plan, adopted on 15 September 2021, while a single plan for the whole county moves towards examination. As lead local flood authority it also comments on the drainage of major schemes. The documents that shape an Aylesbury report are these:

The 9 m trigger in Policy I4 can bring an Aylesbury plot into scope even in Flood Zone 1, because so many brooks and ditches run between the houses. Where a scheme needs floodplain compensation, our note on level-for-level storage sets out how it is measured.

Garden Town growth: South Aylesbury, Kingsbrook, Berryfields and the A41 sites

Most of Aylesbury's new housing comes from six strategic allocations in the Local Plan, on farmland drained by the same brooks that later run through the town. That is why the council asks so much of their drainage:

• South Aylesbury (D-AGT1). About 95 ha between the town and Stoke Mandeville, for at least 1,000 homes and a link road. The railway splits it in two: the east drains to Bedgrove Brook and the west to Southcourt Brook. The 2023 planning document keeps built development out of land still at risk in a 1 in 1,000 year flood after mitigation, requires hydraulic modelling, and limits discharge to greenfield rates.
• South west Aylesbury (D-AGT2). West of Lower Road, separated from South Aylesbury by newer housing in Stoke Mandeville parish and joined to it by relief roads.
• North and south of the A41 (D-AGT3 and D-AGT4). Either side of the A41 on the eastern side of the town. The southern site takes in Hampden Fields, east of Wendover Road, and already has planning permission; Bedgrove Brook crosses the A41 on its way into Bedgrove.
• Berryfields and Kingsbrook (D-AGT5 and D-AGT6). The north-western and north-eastern extensions, towards the Thame meadows and Bierton respectively.
• Town centre sites. Smaller allocations such as the Cambridge Street sorting office, Buckingham Street and Hampden House sit on the old town's limestone, where surface water and the culverted Bear Brook matter more than the Thame.

Upstream allocations feed the same storage that protects the town centre, so the drainage strategy for one of them is judged partly on what it does downstream.

Which report an Aylesbury proposal usually needs

The flood zone, the distance to the nearest brook and the size of the scheme between them decide the paperwork. These are typical starting points, not rules:

Our guide to Flood Zones 1, 2 and 3 explains what each zone means, and the note on drainage objections from the lead local flood authority covers the common reasons a drainage submission is sent back.

Reports we prepare for sites across Aylesbury

Our chartered consultants write each of these for the brook, the ground and the policy that apply to the address, and a senior consultant signs off every report. Unda Consulting Limited is a CIWEM Business Partner.

• Applications near the brooks. Homes and extensions within 9 m of Bear Brook, Southcourt Brook or the Stoke Brook, or in their flood zones, need a flood risk assessment for planning that meets Policy I4.
• All sources in one report. A flood risk assessment for an Aylesbury site covers the brooks, the Thame, surface water, groundwater, the canal and the two flood storage reservoirs upstream, and tests a blocked culvert.
• Buying in the Willows or Watermead. A flood risk survey for property purchase looks at one building, its floor level and its flood history, which matters on an estate where 80 homes flooded in 2014.
• Whether the site is the right one. For new homes in Flood Zones 2 or 3, the sequential and exception test shows whether a lower-risk site in or around Aylesbury could take the scheme instead.
• Where the water goes. On Kimmeridge Clay most sites cannot soak water away, so the drainage strategy works out where runoff can go without overloading the brooks or the Broughton storage.
• Runoff at greenfield rates. A surface water drainage strategy sizes storage and controls discharge to the limits the council and the South Aylesbury rules expect, using SuDS where the layout allows.
• Conditions after approval. We prepare the calculations, plans and maintenance details needed for the discharge of drainage conditions attached to an Aylesbury permission.
• Brooks that need modelling. Policy I4 asks for nearby ordinary watercourses to be modelled, and South Aylesbury needs hydraulic modelling, so flood modelling sets levels where national mapping stops short.
• Warnings on Bear Brook. With two warning areas along Bear Brook and Policy I4 asking for evacuation plans where needed, a flood warning and evacuation plan sets out who leaves and by which route.
• Homes that have flooded. After the Willows in 2014 and Storm Alex in 2020, a flood risk assessment for insurance gives an insurer evidence about a single address.
• Water close to the surface. Ground investigation at South Aylesbury found groundwater near the surface and the terrace gravels can hold perched water, so a groundwater flood risk assessment checks basements and soakaways.
• Sewers under pressure. With HP19 among Thames Water's worst postcode districts for sewer flooding, a foul drainage strategy shows how a new scheme connects without adding to it.
• Land before an offer. On the Garden Town fringe a flood risk feasibility study shows early how much of a parcel can be built once buffers, storage and the 1 in 1,000 year outline are drawn.

Aylesbury flood contacts: council, Environment Agency, Thames Water and the canal

In a flood that threatens life, dial 999; a power cut goes to 105. For everything else, the right contact depends on where the water came from:

The Thame flows on into Oxfordshire, covered on our Oxfordshire page. North of the Vale, Buckingham drains to the Great Ouse instead, and over the Chilterns High Wycombe sits on the River Wye. Our areas we cover page lists the rest.

Aylesbury flood questions from buyers, builders and owners

To talk through a house near the Stoke Brook, a plot by Bear Brook or land on the Garden Town edge, call 01293 214444 or email enquiries@unda.co.uk. One of our experienced flood risk consultants will look at the site and come back with a fixed fee.