River Wey Flooding & Flood Risk

The River Wey is one of the most flood-prone rivers in the South East, and its floods are written into local memory. Draining 904 square kilometres of Surrey, Hampshire and West Sussex on its 140-kilometre journey to the Thames, the Wey gathers water from two branches, a string of tributaries and a broad chalk-and-greensand catchment, then pushes it through the low valley floors of Godalming, Guildford, Byfleet and Weybridge. In the winter of 2013–14 it flooded around 250 homes and businesses along that corridor. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the River Wey floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

In the winter of 2013–14 the River Wey flooded roughly 250 properties across Godalming, Guildford, Byfleet and Weybridge, and the Environment Agency counts 1,261 properties at risk of a 1-in-100-year flood along that stretch alone.

• A long, groundwater-fed chalk river. The Wey is the longest tributary of the Thames after the Medway, sustained through dry spells by baseflow from the chalk but capable of rising hard after prolonged winter rain.
• A designated Main River. The Environment Agency regulates the Wey through its populated reaches, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• Flooding from several directions at once. Along the valley floor the river arrives together with surface water off the slopes and groundwater rising through the chalk and greensand — rarely a single, tidy source.
• Development pressure the length of the valley. From Farnham through Guildford to Weybridge, sites in and beside the floodplain almost always need a flood risk assessment for planning.

Check flood risk along the River Wey

The map below draws the River Wey in coral, with the Environment Agency’s live river gauges positioned along it — click any gauge to see its latest level, its typical range and its record peak. 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.

A history of flooding on the River Wey

The Wey has a long and well-documented flood record, and the Environment Agency singles out 1947, 1968 and 2013–14 as the three most severe events on the river. The September 1968 flood is the one older residents still measure everything against. Torrential rain over the weekend of 14–15 September pushed the swollen river over its banks as it passed through Guildford, and by the early hours of Monday 16 September the bottom of the town’s High Street sat under around six feet of water. The same storm flooded communities the length of the valley and remains a key reference point in the Environment Agency’s records for the catchment.

The river set fresh gauge records in the autumn of 2000. During the widespread flooding of that November, the Wey reached its highest recorded level at Guildford — 3.88 metres on 7 November — and peaked at Weybridge the following morning. Thirteen years later came the benchmark modern event. Through the exceptionally wet winter of 2013–14, prolonged rain fell on an already saturated catchment and the river ran high for weeks, reaching its highest level on record at Tilford on Christmas Eve 2013. Around 250 properties flooded across the valley: about 84 in Godalming, 92 in Guildford, and 74 around Byfleet and Weybridge. Some of that flooding came straight from the river, and some from the surface water, groundwater and overwhelmed drains that a wet winter brings with it.

The Wey has been in flood alert or warning many times since, and the pattern is consistent: it is the long, wet winters that fill the chalk and saturate the ground, rather than a single summer downpour, that bring the river’s worst flooding. These are the events a flood risk assessment for a nearby site has to reckon with.

Why does the River Wey flood?

The Wey floods because of the size and shape of its catchment and the way that catchment stores and releases water. The river has two arms: a north branch rising from a chalk spring near Alton in Hampshire, and a south branch rising below Black Down near Haslemere in West Sussex, the highest ground in the region. The two meet at Tilford in Surrey and the combined river then runs east and north through Godalming and Guildford before joining the Thames at Weybridge. Draining 904 square kilometres in total, it is a classic mixed catchment: chalk and greensand that soak up rain and release it slowly as groundwater, clay tributaries that shed it fast, and increasingly urban valley floors that speed everything up.

Much of the river’s everyday flow comes from the chalk aquifer, which is why the Wey keeps running through dry summers when flashier rivers fall low. That same store of groundwater is what makes its winter flooding so stubborn: once the chalk is full after months of rain, extra rainfall runs straight off, the tributaries pile in, and the river stays high for days rather than hours. The Tillingbourne, which rises on the slopes of Leith Hill and joins south of Guildford, along with Cranleigh Waters, the River Ock and the Hoe Stream, can each deliver a sharp pulse of water to the main river at just the wrong moment.

• Two branches and a large catchment. The north (Alton) and south (Haslemere) branches meet at Tilford, draining 904 square kilometres of Hampshire, West Sussex and Surrey over a 140-kilometre course to the Thames.
• Groundwater keeps it full. Baseflow from the chalk sustains the river in dry spells and, once the aquifer is charged in winter, leaves little capacity to absorb further rain.
• Tributaries concentrate the flow. The Tillingbourne, Cranleigh Waters, the River Ock and the Hoe Stream all add water to the main channel, and their peaks can coincide with the Wey’s own.
• Gauged the length of the river. Environment Agency gauges at Farnham, Tilford, Godalming, Guildford, Old Woking, Byfleet and Weybridge all appear on the map above, updating in real time.

The peak flow ever measured on the Wey came at Weybridge on 29 December 1979, when the river carried 74.8 cubic metres of water a second — more than ten times its long-term average, in records held by the Environment Agency and the National River Flow Archive. Figures like these, tied to specific gauges and dates, are the evidence a robust flood risk assessment is built on. For the underlying mechanism, our guide to fluvial (river) flooding explains how out-of-bank flooding is assessed for planning.

Which areas does the River Wey put at risk?

Flooding from the Wey is concentrated on the valley floors, and the Environment Agency runs a formal warning service for the communities most exposed. Two catchment-scale flood alert areas — the Upper River Wey, covering Alton, Farnham, Frensham, Tilford, Godalming and Guildford, and the Lower River Wey, covering Old Woking, Byfleet and Weybridge — sit above a set of more precise flood warning areas where the Agency can warn individual properties. The table below summarises the main settlements at risk, from the headwaters down to the Thames.

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 Godalming, Guildford and Weybridge, and across the wider county on our Surrey and Runnymede pages. Our guide to flood zones 1, 2 and 3 explains what each means for a scheme.

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

Through its populated reaches, yes. The Environment Agency has designated the Wey a Main River, which means the Agency, rather than the local council, regulates 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, and a site in the river’s floodplain will normally need a flood risk assessment to support a planning application. The many small brooks and headwater ditches that feed the Wey are ordinary watercourses instead, regulated by the county council as Lead Local Flood Authority and by the riparian owners whose land they cross. Getting that classification right at the outset saves time and avoids surprises later.

• The 8 metre byelaw margin. Building, excavation or structures near the Wey usually require a Flood Risk Activity Permit, sought alongside — not instead of — planning permission.
• 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.
• Groundwater is part of the picture. On the valley floor and the chalk, a high winter water table can be the deciding constraint — see our guide to groundwater flooding.
• Know which watercourse you are on. The main river is Environment Agency territory; its tributaries are ordinary watercourses under the Lead Local Flood Authority. We confirm this for every site.

Managing flood risk on the River Wey: policy and contacts

Several bodies share responsibility for flood risk on the Wey, and it helps to know who does what. The river sits within the Thames River Basin District, and the wider strategy for the catchment is set out in the Environment Agency’s Thames flood risk management plan. The Agency regulates the Main River, publishes the flood map for planning and issues flood warnings; Surrey County Council, together with Hampshire and West Sussex on the headwaters, is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses; and the borough councils along the river — Waverley, Guildford, Woking, Elmbridge and Runnymede — decide planning applications, each with its own strategic flood risk assessment. The Environment Agency is progressing flood alleviation schemes at Godalming, Guildford, and Byfleet and Weybridge, summarised in its overview of reducing flood risk on the River Wey; the story of the largest is set out in our note on the Guildford flood alleviation scheme. Climate change is factored into every modern assessment through the Environment Agency’s climate change allowances.

• Live river levels. Check the river’s gauges on the Environment Agency service — River Wey at Guildford and the Upper River Wey flood alert — or use our own live river levels map.
• Screen a location. Try our free flood risk map, and for drainage design the drainage calculators, the BRE 365 infiltration calculator and our ground conditions and soakaway map.
• Flood warnings. Sign up for Environment Agency flood warnings and call Floodline on 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

Flood risk and drainage services for sites near the River Wey

A development near the Wey 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 River Wey 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.
• Drainage strategies and SuDS. Surface-water designs that meet the discharge limits set by the Lead Local Flood Authority, through our sustainable drainage strategy service.
• 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 is in play.

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. Everything we do is framed by the NPPF and its planning practice guidance.

How Unda helps with flood risk on the River Wey

Unda understand how a river like the Wey behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. We know how the river’s groundwater-fed regime keeps it high through a wet winter, where its floodplain bites hardest — the pinch point through Guildford, the broad low ground around Byfleet and Weybridge, the valley floor at Godalming — and what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the valley.

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 River Wey, talk to one of our experienced flood risk consultants about what your application will need.

River Wey flood risk: your questions answered