River Medway Flooding & Flood Risk

The River Medway is the river that shapes flood risk across west and mid Kent. It drains the largest catchment in the county, gathers the runoff of the whole Weald into a single valley, and has flooded the towns along it — Tonbridge, Yalding, East Peckham and Maidstone among them — roughly once a generation for as long as records have been kept. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Medway floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

Over the Christmas period of 2013–14, the Medway and its tributaries produced the largest flood ever measured across much of the catchment, putting more than 900 homes and businesses under water in Tonbridge, East Peckham, Yalding, Maidstone and the villages between them.

• The largest catchment in Kent. The Medway drains around 2,409 square kilometres of the Weald and the North Downs, so heavy rain anywhere across a huge area eventually arrives in the same valley.
• A Main River for its whole length. The Environment Agency regulates the Medway, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• Fluvial upstream, tidal downstream. Above Allington Lock near Maidstone the Medway floods from rainfall and runoff; below it the river is tidal, and a North Sea surge can push flood water up into Rochester, Chatham and Gillingham.
• Live Environment Agency gauges the length of the river. Tonbridge, the Leigh flood storage area, Teston, Maidstone and Allington all appear on the map below, with their latest levels updating in real time.

Check flood risk along the River Medway

The map below draws the Medway in coral, with the Environment Agency’s live river gauges positioned along it — click any gauge to see its latest level. Enter a postcode near the river to zoom in and screen that location against the Environment Agency’s flood data for rivers and sea, surface water, reservoirs and the flood zones used for planning. It is an indicative screening tool, not a flood risk assessment.

Why does the River Medway flood?

The Medway floods because of the shape and geology of the land it drains. It rises from springs near Turners Hill in the High Weald of West Sussex, close to Ashdown Forest, and runs for about 113 kilometres across Kent before reaching the Thames Estuary at Sheerness. Between source and sea it collects the water of the largest catchment in Kent, around 2,409 square kilometres of Wealden clay and greensand ringed by the chalk of the North Downs. Much of that ground is heavy clay that sheds rain rather than soaking it up, so after prolonged wet weather the runoff arrives in the river quickly and levels rise across the catchment at much the same time.

What makes the Medway distinctive is how its tributaries come together. Flowing west to east along the foot of the North Downs, the river gathers the Eden, then the Bourne, and a short way above Yalding it meets its two largest tributaries, the Beult and the Teise, before the Len joins at Maidstone. Those tributaries drain broad, low-lying Wealden ground, and when they peak together the flood water concentrates into a single valley. At the Beult confluence the river turns north and forces its way through the chalk of the North Downs at the Medway Gap near Rochester, beyond which it becomes a broad tidal estuary. The tidal limit is Allington Lock, just below Maidstone, first built in 1792.

• A large, clay-dominated catchment. Impermeable Weald clay across much of the 2,409 square kilometre catchment produces rapid runoff and a river that responds together across a wide area.
• Tributaries that peak as one. The Eden, Bourne, Beult, Teise and Len feed the Medway, and the Beult and Teise join it just above Yalding — the reason that village floods so readily.
• Two behaviours in one river. Above Allington Lock the Medway is a rainfall-driven main river; below it, through Rochester, Chatham and Gillingham, it is tidal and exposed to North Sea surges.
• Gauged the length of the valley. The Environment Agency records levels at Tonbridge, the Leigh flood storage area, Teston and Maidstone, and the National River Flow Archive gauge at Teston measures a catchment of about 1,261 square kilometres.

A history of flooding on the River Medway

The Medway has one of the longest and best-documented flood records of any river in the South East, and the pattern is unmistakable: a major flood roughly every twenty-five years, with numerous smaller events between. Serious floods are recorded in the 1920s and in 1947, 1955, 1958, 1960, 1963, 1968, 1974, 1979, 2000–01 and 2013–14, and the towns of the valley have learned to live with them.

The benchmark event of the twentieth century came in September 1968. A slow-moving trough brought exceptional rainfall to the South East, and on Sunday 15 September the Medway and its tributaries produced what was probably the severest inland flood the Home Counties had seen in a hundred years. Tonbridge was badly hit, Edenbridge was cut off as the river burst its banks, and the scale of the damage led directly to the building of a flood barrier upstream of Tonbridge. The other benchmark is far more recent. Over Christmas 2013 and into the new year, after weeks of saturated ground and heavy rain, the river produced the largest flood ever measured across much of the catchment, overtopping the design of the Leigh flood storage area and flooding more than 900 homes and businesses in Tonbridge, East Peckham, Yalding, Maidstone and the smaller communities around them.

No village illustrates the river’s behaviour better than Yalding, which sits at the meeting of the Medway, the Beult and the Teise. It has flooded in 1960, 1963, 1968, 1974, 1979, 2000–01, 2013–14, 2019–20 and again in 2021 — more often in living memory than almost anywhere else in Kent. These are the events an assessment for a site anywhere on the Medway floodplain has to reckon with.

Which areas does the River Medway put at risk?

Flooding from the Medway is concentrated on the valley floor, and the Environment Agency runs a formal warning service for the communities most exposed. Three catchment flood alert areas cover the non-tidal river — the Upper River Medway from Forest Row to Penshurst, the Middle River Medway from Penshurst to Hampstead Lock at Yalding, and the Lower River Medway from Yalding to Allington Lock — with more precise flood warning areas within them where the Environment Agency can warn individual properties. Below Allington the tidal Medway brings a different risk to the estuary towns. The table below summarises the main settlements at risk from source to sea.

If your site sits within one of these areas, the Environment Agency’s flood map for planning is likely to place it in Flood Zone 2 or 3, which triggers the need for a site-specific flood risk assessment. You can screen any address against the live Environment Agency data using the map above, and the towns along the river are covered in more depth on our local pages for East Grinstead, near the source, and downstream at Yalding, Maidstone, Rochester, Gillingham, Sheerness and the Isle of Sheppey, or across the county on our Kent page.

Is the River Medway a Main River — and what that means for your site

Yes. The Environment Agency has designated the Medway a Main River along its length, which means the Agency, rather than the local council, is the regulator for the channel and its immediate margins. That distinction matters if you are developing nearby. Works within 8 metres of the top of the bank generally need a Flood Risk Activity Permit from the Environment Agency before they start, sought alongside — not instead of — planning permission, and a site in the river’s floodplain will normally need a flood risk assessment to support a planning application. The many tributaries and field ditches that feed the Medway are usually ordinary watercourses instead, regulated by the lead local flood authority and by the riparian owners whose land they cross. Getting that classification right at the outset saves time later.

• The 8 metre byelaw margin. Building, excavation or structures near the Medway usually require a Flood Risk Activity Permit from the Environment Agency.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3, or a larger site in Flood Zone 1, needs a flood risk assessment for planning, prepared against the NPPF and its climate change allowances.
• Know which watercourse you are on. The Medway itself is Environment Agency territory; its tributaries are ordinary watercourses under the lead local flood authority. We confirm this for every site.

How flood risk on the Medway is managed

The Medway is one of the most actively managed river catchments in the South East. The centrepiece is the Leigh flood storage area, completed in 1982 between the villages of Leigh and Penshurst: a 1.3 kilometre embankment, five metres high, that holds back flood water above Tonbridge and releases it slowly, protecting around 1,200 homes and businesses in Tonbridge and Hildenborough. The Christmas 2013 flood exceeded even that defence, and in response the Environment Agency has expanded the storage area, raising its maximum water level from 28.05 to 28.60 metres above ordnance datum and increasing its capacity by around a quarter to more than seven million cubic metres, alongside a new embankment at Hildenborough that together will better protect more than 1,400 properties.

Downstream, the Middle Medway flood resilience scheme took a different approach, fitting property flood resilience measures — flood doors, barriers and airbrick covers — to 256 homes across the villages around Yalding by late 2020. Both schemes sit within the Medway Flood Action Plan, run by the Medway Flood Partnership formed in 2017. The catchment lies in the South East River Basin District, and its strategy is set out through the Environment Agency’s Medway management catchment plans. Kent County Council is the lead local flood authority for most of the catchment, with Medway Council covering the unitary area around the estuary towns; the individual planning authorities along the river each hold their own strategic flood risk assessment, and our town pages carry that local detail.

How Unda helps with flood risk on the River Medway

Unda understand how a large Wealden river like the Medway behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. We know how the tributaries combine above Yalding, where the floodplain bites hardest along the Tonbridge and Maidstone reaches, and how the character of the river changes from fluvial to tidal below Allington Lock. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the valley, and we deal directly with the Environment Agency and the lead local flood authority on your behalf.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping, the river’s gauged levels and modelled flows, and the local planning policy that applies to the site. If you are working on a site near the Medway, talk to one of our experienced flood risk consultants about what your application will need.

Flood risk and drainage services for sites near the River Medway

A development near the Medway can call for several different reports, and it helps to be clear that they answer different questions. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they address separate things: whether the site is safe from flooding, whether the development should be located here, and where the surface water goes. Unda prepares each of them for sites across the Medway catchment.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements for development in the river’s flood zones — see our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent reports for buyers, owners and lenders through our flood risk survey for property purchase and flood risk assessment for insurance, which go well beyond the conveyancing flood reports a search throws up.
• Drainage strategies and SuDS. Surface-water designs that manage runoff on heavy clay and meet the discharge limits set by the lead local flood authority — see our sustainable drainage strategy service, backed by our drainage calculators and BRE 365 infiltration calculator.
• Sequential and exception tests. Locational justification where policy requires it, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and a groundwater flood risk assessment where a high winter water table on the clay or greensand is in play — more on that in our guide to groundwater flooding.

Where national mapping is not enough — a common situation for sites right beside a Main River — we can bring in hydraulic flood modelling to derive accurate flood levels and extents, prepare the preliminary drainage design that keeps a scheme viable, and produce the detailed work needed to discharge a drainage condition. For infiltration on the Wealden clay we also assess ground conditions and soakaway potential.

Live river levels, flood warnings and emergency contacts

Several bodies share responsibility for flooding on the Medway, and it is worth knowing who does what and where to look when the river is rising.

• Live river levels. Check the Medway’s gauges on the Environment Agency service — Teston, Maidstone and Allington Lock — or use our own live river levels map.
• Flood alerts and warnings. Sign up for Environment Agency flood warnings and check the Middle River Medway and neighbouring alert areas. Call Floodline on 0345 988 1188.
• Screen a location. Try our free flood risk map to check any address against the Environment Agency’s flood data before you commit to a site.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

River Medway flood risk: your questions answered