Flood Risk Assessment in Tonbridge

A flood risk assessment in Tonbridge deals with a town built across the River Medway's floodplain and protected upstream by the Leigh flood storage area, which the Environment Agency finished enlarging in September 2026. We prepare flood risk assessments in Tonbridge for planning applications, house purchases, mortgages and insurance, alongside sequential and exception tests and surface water drainage strategies, for sites from the riverside streets below the castle to the estates around Higham Wood and Cage Green.

On this page Tonbridge means the town as the Office for National Statistics draws its built-up area: the High Street and the riverside around Medway Wharf Road and Avebury Avenue, Barden to the west, Trench Wood, Cage Green and Higham Wood to the north, and the streets towards Somerhill in the south-east, across the TN9 and TN10 postcode districts. Hildenborough and Leigh lie just outside that line, in TN11, but share the river, the flood warning and the defences, so the page covers them as well. Tonbridge and Malling Borough Council decides planning applications and Kent County Council is the lead local flood authority; the rest of the county is on our Kent page.

On Christmas Eve 2013 the Medway ran into the Leigh storage area at 342 cubic metres a second, the highest peak ever recorded above it. The barrier held back 5.5 million cubic metres and cut the flow into Tonbridge to 160 cubic metres a second, and flooding still reached about 80 homes around Barden Road and Danvers Road.

Call 01293 214444 or email enquiries@unda.co.uk to talk a Tonbridge site through with one of our flood risk consultants. Reports for sites in Tonbridge come from Unda Consulting Limited, a CIWEM Business Partner.

Tonbridge behind the Leigh barrier and the town walls: what the defences leave for a site

Tonbridge relies on defences, and each has a limit. Floodwater from the upper Medway is held back at Leigh, just west of the town, while walls along the river through the centre and open floodplain at the sports pitches take what is let through. A flood risk assessment in Tonbridge has to show how a site fares once a defence is beaten, as well as in an ordinary wet winter.

• Leigh flood storage area. Completed in 1982, a 1.3 km earth embankment five metres high, with three steel gates, stores water above the town and lets it out slowly. The Environment Agency had operated it 75 times by summer 2026. Its £42 million expansion raised the top water level by 55 cm and the capacity by about 23%, to roughly 7.4 million cubic metres.
• Hildenborough. The same scheme added a 98 m embankment across the floodplain, a sheet-piled wall and a pumping station on the Hawden Stream, for a village that flooded badly in 2013. The works protect 1,870 homes and 575 businesses in all.
• The town walls. Flood walls along the Medway through the centre belong to the same defence system as Leigh, the Environment Agency says, together with the natural floodplain at the sports pitches. Like any wall, they are built to a height a bigger flood can exceed.
• Avebury Avenue. After the river overflowed its bank here in December 2013, the Environment Agency built a 320 m defence behind the Avebury Avenue flats, working with the town-centre walls and the storage at Leigh.

None of this changes the flood zones. The Flood Map for Planning shows land as it would flood without defences, so the ground beside the river through the centre stays in Flood Zones 2 and 3. The council's 2025 Strategic Flood Risk Assessment also treats Leigh as a reservoir, warning that a breach could have notable implications for Tonbridge. Our guide to residual flood risk explains how an assessment handles water that gets past a defence, and the reservoir flooding explainer covers the breach case.

Look up an address in Tonbridge, Higham Wood or Cage Green

The map below checks any Tonbridge postcode against the Environment Agency's river, surface water and reservoir data; for an address anywhere else in England, our flood risk map does the same job. The coral line follows the ONS built-up area for Tonbridge, from Barden and the station north across the river to Trench Wood, Cage Green and Higham Wood. Hildenborough, Leigh, Haysden and the open floodplain upstream sit outside it; their flood zones and records come from the same Environment Agency and council sources.

The Medway at Town Lock: normal levels, warnings and the Christmas 2013 peak

The Environment Agency's gauge at Town Lock, in the middle of Tonbridge, gives the reading that matters for most riverside sites in the town. It normally sits between 0.88 m and 1.66 m, and above that low-lying land can flood. On 25 December 2013 it reached 2.34 m, the highest level recorded there, a day after the storage area upstream took its record inflow.

Our live river levels map shows Town Lock, the Leigh gauge and the stations upstream on the Medway and the Eden in one view. For how the whole river behaves between the Weald and the estuary, see our River Medway page; downstream, the same floods reach Yalding and Maidstone.

Tonbridge's recorded floods and the defences that followed them

Tonbridge's defences were each built after a flood, and the council's 2025 Strategic Flood Risk Assessment lists the big Medway floods that reached the town: 1953, 1968, 2000 and the winter of 2013/14. The Environment Agency, Kent County Council and the borough council record what happened in each of the recent ones.

1. September 1968. One of the major Medway floods the SFRA records in Tonbridge. The storage area at Leigh followed, completed in 1982; our article on the great flood of 1968 follows that storm across the South East.
2. October 2000. The borough council records that Tonbridge narrowly escaped severe flooding from the Medway, while the Botany Stream flooded streets off the High Street.
3. 23 to 27 December 2013. Up to 76 mm of rain fell in 24 hours on saturated ground. About 80 homes were affected around Barden Road and Danvers Road and about 180 flooded in Hildenborough; across the borough, 335 homes and 101 businesses flooded and 720 more businesses were affected.
4. 29 December 2015. The Environment Agency announced the 320 m Avebury Avenue defence.
5. 30 September 2026. The Environment Agency announced the enlarged storage area and the Hildenborough defences open.

December 2013 is the flood a Tonbridge site is usually measured against. It set the record at Town Lock, and both the Avebury Avenue wall and the Leigh expansion came after it.

Botany Stream, Hilden Brook and the town's smaller watercourses

Not all of Tonbridge's flooding comes over the Medway's banks. Several smaller streams cross the town to join it, and a high river can hold them back. Heavy rain causes its own problems wherever the drains and sewers are overwhelmed.

Our guide to main rivers and ordinary watercourses explains which body answers for which channel. With TN9 so high on Southern Water's list, sewer capacity is worth checking early on a town-centre site.

CP10, CC3 and the 2025 SFRA: the rules a flood risk assessment in Tonbridge must satisfy

A flood risk assessment in Tonbridge is judged against the borough's adopted policies, the county's drainage requirements and chapter 18 of the National Planning Policy Framework. The Core Strategy dates from 2007, so the 2025 Strategic Flood Risk Assessment and the county's drainage policy carry much of the practical detail officers look for.

The new Local Plan is not settled. The council decided on 3 September 2026 not to take its plan forward, and the next day the Housing Minister directed it to begin consultation on the plan by 11 September and to submit it to the Government by 31 December 2026. Until a new plan is adopted, the 2007 and 2010 policies apply alongside national policy. Kent County Council comments on surface water for major schemes, and the Environment Agency on sites in Flood Zones 2 and 3.

North-east Tonbridge and the town centre: where the draft plan puts new building

Tonbridge is one of the borough's principal service centres, and the November 2025 draft Local Plan gives it, with Hilden Park, 3,214 new homes and 47,200 square metres of employment space. The council cannot currently show a five-year supply of housing land, 2.89 years by the figure in the Minister's letter, and that tends to bring forward applications on land the plan does not allocate.

• Land north east of Tonbridge (TO1). The largest draft allocation, for 1,671 homes on the north-eastern edge of the town. A scheme of that size needs its surface water drainage worked out from the first layout, with any watercourses and overland flow routes on the land mapped before the streets are drawn.
• The town centre. The draft plan keeps Tonbridge as the borough's main shopping and service centre and looks to regenerate the Angel Centre and neighbouring sites east of the High Street. The borough's own policies describe the centre as sitting on the floodplain, so redevelopment there meets the SFRA's floor-level and floodplain-storage rules.
• Land the plan does not allocate. Homes proposed in Flood Zone 2 or 3 outside the allocations need the sequential test, as the SFRA spells out, and where it is passed the exception test may follow.

Building, buying or insuring in Tonbridge: which report answers which question

Three reports come up most often on Tonbridge sites, and they answer different questions. A flood risk assessment shows the building will be safe and will not push water onto neighbours. A sequential test asks whether a lower-risk site could take the scheme instead. A drainage strategy deals with the rain that falls on the site itself.

Reports we prepare for sites on the Medway floodplain at Tonbridge

We prepare each of these for sites in Tonbridge, Hildenborough and Leigh, matched to the river, the streams and the policies that apply at the address:

• Every source, one report. A flood risk assessment for a Tonbridge site covers the Medway and its side streams, surface water, sewers, groundwater and a breach at Leigh, and tests what happens if the town walls are overtopped.
• Applications on the floodplain. Homes, conversions and shops in Flood Zones 2 and 3 along the river need a flood risk assessment for planning that meets Policy CP10 and the SFRA's 600 mm floor-level advice.
• Buying after 2013. A flood risk survey for property purchase looks at one building's floor levels, defences and flood history, which matters on the streets around Barden Road and Danvers Road that flooded in December 2013.
• Whether the site is the right one. For homes in Flood Zone 2 or 3 on land the plan does not allocate, the sequential and exception test shows whether a reasonably available site at lower risk could take the scheme.
• Where the water goes. Our drainage strategy work covers surface water and foul flows for Tonbridge schemes, from infill in Trench Wood to the larger allocations.
• Policy CC3 and the county's rules. A surface water drainage strategy shows runoff held to the rates Kent County Council sets, with storage sized for the 1 in 100 year storm plus climate change and 2 l/s as the usual floor for a small site.
• After permission. The discharge of drainage conditions covers the levels, calculations and maintenance arrangements the borough council and Kent County Council ask for before building starts.
• Depths if a defence fails. Where national mapping cannot show what a site behind the town walls would see if they were overtopped, or if the Leigh embankment breached, flood modelling works out the depths and flood levels for the assessment.
• A plan for the warning. A site in the Tonbridge and Hildenborough warning area that relies on warning and a dry way out needs a flood warning and evacuation plan setting out who leaves, when and by which road.
• Homes with a flood history. After flooding reached about 80 homes around Barden Road and Danvers Road in 2013, a flood risk assessment for insurance gives an insurer evidence about one address and the defences in front of it.
• Keeping a business open. In 2013, 101 businesses flooded across the borough and 720 more were affected; a business continuity flood risk assessment plans how a shop or unit in the town centre keeps trading when the river rises or the drains back up.
• Land before an offer. With 1,671 homes proposed north-east of the town in the draft plan, a flood risk feasibility study shows early how much of a parcel can be built on and how it can be drained.

Who handles flooding in Tonbridge: councils, the Environment Agency and Southern Water

Several bodies share the work of managing flood risk in Tonbridge, and the one to call depends on where the water is coming from. If anyone is in danger in a flood, call 999 first.

After the 2013 flood the Environment Agency backed volunteer flood wardens in central Tonbridge and Hildenborough, according to Kent County Council's review of that winter. Our overview of who manages flood risk in England shows how these roles fit together.

Questions about flood risk assessments, defences and flooding in Tonbridge

To talk through a house near the river, a unit on an industrial estate or land on the edge of town, call 01293 214444 or email enquiries@unda.co.uk. One of the chartered consultants at Unda Consulting Limited will tell you which reports the site needs.