Flood Risk Assessment in Truro

A flood risk assessment in Truro deals with water arriving from three directions at once: the River Kenwyn and the River Allen, which meet beneath the city centre, and the tide, which rises up the Truro River from the Fal estuary. We prepare these assessments for planning applications, house purchases, mortgages and insurance, together with sequential and exception tests and surface water drainage strategies, for sites from Lemon Quay and Newham to Kenwyn, Highertown and Treliske.

On this page Truro means the city as the Office for National Statistics maps it, in the TR1 postcode district: the centre and the quays, Moresk and Trehaverne, Kenwyn, Hendra, Highertown, Gloweth, Treliske and Newham. Threemilestone and the Langarth land to the west, Malpas at the head of the estuary, and Shortlanesend, Idless and Tresillian further out (TR2, TR3 and TR4) sit outside that outline. They answer to the same council and drain into the same estuary, so they are covered here as well. Cornwall Council decides planning applications and is also the lead local flood authority; the rest of the county is on our Cornwall page, and other towns and cities are listed among the areas we cover.

Only about 2% of Truro's built-up area lies in Flood Zone 3, yet almost all of the city sits inside one of two Environment Agency critical drainage areas. A site well away from the rivers can still be asked to let less rain off the land than it does today.

Call 01293 214444 or email enquiries@unda.co.uk with a Truro address and one of our flood risk consultants will tell you what the site is likely to need.

Rivers, tide and runoff: what does a flood risk assessment in Truro have to weigh?

Truro sits in a bowl of steep valleys. The Kenwyn comes in from the north-west past New Mills and the Allen from the north past Idless, and the two join under the centre to become the tidal Truro River. When heavy rain on those valleys meets a high tide, river water backs up in the city and the street drains have nowhere to empty, which is the combination behind the city's worst floods.

Our check of the Environment Agency's Flood Map for Planning against the ONS outline puts about 2% of the built-up area in Flood Zone 3 and about 3% in Flood Zones 2 or 3. Almost all of it lies in the narrow valley floors where the centre was built. Those zones ignore the dams and the tidal gates, so a defended street still shows as Flood Zone 3; our guide to flood zones in England explains what Flood Zone 3 means for an application. The Kenwyn, the Allen and the Tinney are main rivers, which puts the Environment Agency rather than the council in charge of them; main rivers and ordinary watercourses are handled differently from the first consultation onwards.

Screen a TR1 address against the flood maps

Enter a Truro postcode below to see the Environment Agency's river and sea, surface water and reservoir layers for that spot; our flood risk map runs the same screen for any address in England. The coral outline is the ONS built-up area named Truro, running from Newham and the quays in the south-east to Treliske in the west and Kenwyn and Trehaverne in the north. Threemilestone, Malpas, Calenick, Idless, Shortlanesend and Tresillian are outside it.

Two dams and a set of tidal gates: how Truro is defended

The city flooded twice in 1988. A three-part system followed, designed in the early 1990s: storage dams on the Kenwyn and the Allen to hold back river peaks, and tidal gates on the Truro River to keep high tides out of the centre. All three were being renewed or tested in 2024 and 2025.

• New Mills dam on the Kenwyn. It holds back flood water upstream of St George's Road and River Street. In July 2025 the Environment Agency tested high-volume pumps for lowering the water behind it, followed by a desktop exercise in September.
• Idless dam on the Allen. It stores flood water above Truro before it reaches St Clement Street. Upgrades to the dam were due to be fully operational by December 2025, according to the Environment Agency's announcement on the tidal gates of 13 October 2025.
• The tidal gates. The old gates had reached the end of their working life. Work began in April 2024 and the new gates were lifted in by a 400-tonne crane in August 2024, timed to the spring tides. The Environment Agency says they protect 185 properties for the next 30 years. While the work was going on, surges ran higher than forecast and temporary stoplogs were used at Truro for the first time.
• Calenick on the Tinney. A flood alleviation scheme built by the council protects the hamlet, and the Environment Agency now maintains the Tinney there as a main river.
• Drainage storage at the Trafalgar roundabout. The second phase of works at the roundabout, begun in 2023, includes water storage tanks meant to ease the drainage problems that close Morlaix Avenue in heavy rain.

None of this removes the need to assess a site. The dams and gates lower the chance of flooding, but the flood zones are drawn without them. An assessment still has to show what happens if a gate fails to close, a dam spills or a storm beats the standard it was built to. Our guide to residual flood risk sets out how that is tested. The Environment Agency and Cornwall Council are also preparing a Truro flood risk strategy that looks at rising sea levels, higher river flows and heavier rainfall.

Warning areas and gauges: how Truro gets notice of a flood

The Environment Agency issues flood warnings for Truro in five small areas, each named by its streets. The gauges behind them show how far the 1988 flood still stands above anything since. The figures below come from the agency's flood-monitoring service.

You can follow the Kenwyn and harbour gauges on our live river levels map, and anyone living or working in the warning areas can sign up for flood warnings. On a planning application, a building inside a warning area needs a flood plan built around that warning: who is signed up, when people leave or move upstairs, and which way they go.

Truro's floods, from the record of 1988 to the Trafalgar roundabout in 2024

Truro has flooded throughout its history, but the dates below are the ones with a primary record, from the Environment Agency's gauges, Cornwall Council's flood investigations and local reports.

1. 27 January 1988. Heavy rain on the Kenwyn and Allen met a spring tide on the Fal. The Kenwyn gauge reached 2.117 m, still its highest reading, and the centre flooded. Truro Civic Society's recollections of the 1988 floods describe staff trapped on the first floor of a travel agency and rescued by fire crews in dinghies.
2. October 1988. The city flooded a second time in the same year. The dams and tidal gates that followed were designed in the early 1990s.
3. 7 December 2000. The Allen reached its highest level on record at the Idless dam gauge, 1.253 m.
4. 22 March 2007. The harbour gauge recorded its highest tide since 1998, 3.363 m.
5. 17 August 2012. Two bands of rain crossed Cornwall and surface water running off the highway flooded businesses in Bridge Street, according to the council's 2012 flood investigation reports.
6. 14 to 16 December 2012. Spring tides, low pressure and south-easterly gales brought coastal flooding along the south coast, and the tidal gates at Truro were shut.
7. 23 December 2013. At Calenick the Tinney burst its banks along Mill Lane, flooding the road for about 150 m to a depth of up to 1 m. Five properties flooded, and the sewage pumping station failed three times between 23 and 27 December, the council's 2013 investigation records.
8. 7 October 2024. A short, intense downpour near high tide made Morlaix Avenue at the Trafalgar roundabout impassable and closed the St Austell Street junction; the council blamed leaves and debris washed onto the drains.

Two kinds of flood run through those dates: river floods made worse by the tide, and short storms that swamp the drains in the valley bottoms while the rivers are low. A Truro assessment has to test a site against both.

Policy 26, CC3 and Policy E2: the rules a Truro application answers to

Three layers of planning policy apply to a site in Truro: national policy, the Cornwall Local Plan with its Climate Emergency Development Plan Document, and the Truro and Kenwyn Neighbourhood Plan. The critical drainage area rules sit on top, and are covered in the next section.

Cornwall Council is consulting on the scope of a new Cornwall Local Plan 2050 until 16 November 2026, with adoption expected in 2029. Until then the 2016 plan and the 2023 document are the ones applications are judged against. The Truro and Kenwyn plan also asks schemes to show how they improve the area's flood resilience. Our guide to climate change allowances explains which figures apply to which kind of building.

Truro's critical drainage areas and their discharge limits

Truro is covered by two of Cornwall's critical drainage areas, notified by the Environment Agency in May 2015. In both, new development has to cut the rate at which rain leaves the site below today's rate. That applies to brownfield land as much as to fields.

• Kenwyn, Allen and Tregolls Road. The Environment Agency's sheet names Truro as a priority community in the county's flood strategy, and warns that unchecked growth could make the centre flood more often. It also records a history of flooding at the Tregolls Road culvert. Off-site discharges should mimic greenfield behaviour up to a maximum of the 1 in 10 year rate, with all surface water managed on site up to the 1 in 100 year event plus climate change.
• River Tinney. On the Tinney sheet the limit is tighter: greenfield behaviour up to a maximum of the 1 in 1 year rate, a standard the council agreed for the catchment above Calenick.
• Water quality. In both areas SuDS have to include treatment that protects the Fal Special Area of Conservation and its shellfishery, so runoff cannot go untreated into the nearest stream.
• Infiltration first. The drainage hierarchy starts with getting water into the ground as far as practicable. A soakaway or infiltration basin therefore has to be tested on site, not assumed, and our guide on when infiltration testing is needed covers the timing.
• Larger schemes. Above 50 homes or 1,000 m2 of non-residential floor space inside a critical drainage area, Policy CC3 adds natural flood management, such as tree planting or hedge restoration, on site or exceptionally elsewhere.

Cornwall Council's guidance also expects householder and minor extension applications inside a critical drainage area to come with a flood risk assessment, for which it publishes a short form. Our explainer on critical drainage areas covers what that means in practice.

Pydar, Langarth and the waterfront: where Truro is changing

Most of the city's new building is happening in two places: on cleared land beside the Allen in the centre, and on farmland west of the city towards Threemilestone. The waterfront at Newham and the port is a third, smaller area of change.

• Pydar, beside the River Allen. The 4.5 ha site is now clear of buildings. In 2026 the council's company Treveth submitted a fresh outline application (PA26/03332) for up to 320 homes, 400 student bed spaces and 16,500 m2 of other floor space, with a riverside green corridor shaped by the Allen. Neighbourhood Plan Policy EJ2(b) asks for SuDS that keep as much water above ground as possible.
• Langarth Garden Village. Planning permission was granted in April 2022 (PA20/09631) for homes for between 8,000 and 10,000 people, with ponds, wetlands and swales in the drainage. The site is in the Truro critical drainage area, and the council's compulsory purchase order was confirmed on 6 December 2024, according to its Langarth page.
• Newham and the Port of Truro. Policies EJ3 and EJ4 protect the Newham waterfront and the port for marine-related uses, next to the Fal and Helford Special Area of Conservation and the Malpas Estuary SSSI. Newham Road and the wharves are in the tidal warning area.
• Sewer capacity. A 2020 briefing on Langarth recorded that the sewers in the city centre and the Kenwyn valley were at capacity. It listed upgrades at the Newham treatment works and a review of the Calenick pumping station to serve the growth.

Smaller schemes, from a flat over a shop on Boscawen Street to an extension in Highertown, face the same critical drainage area rules on a smaller scale. Our guide to house extensions in a flood zone covers the planning side of a small scheme.

Which Truro sites need a flood risk assessment, and which need more?

The answer depends on the flood zone, the size of the scheme and the critical drainage area, and in Truro the last of these catches more sites than the first. The table below gives the usual position.

If a site sits between two rows, send us the address and we will tell you which one fits. Our guide on when an FRA is needed sets out the national triggers in more detail.

Reports for Truro's quays, valleys and new neighbourhoods

A buyer on River Street, a retailer on Lemon Quay and a housebuilder at Langarth need different reports. All of them are prepared by our chartered consultants and signed off by a senior consultant.

• Every source on one plot. A flood risk assessment for a Truro site deals with the Kenwyn or the Allen, the tide, runoff off the valley sides, the sewers and what happens if a dam or the tidal gates do not perform.
• Applications in the river valleys. Cornwall Council expects a flood risk assessment for planning with any application in Flood Zone 2 or 3, and inside the critical drainage area it can ask for one in Flood Zone 1 as well.
• Buying near the quays. When a search flags a house or flat in one of the warning areas, a flood risk survey for property purchase checks its floor and threshold levels against the river and tidal flood levels.
• New homes in the centre. Housing in Flood Zone 2 or 3 near Victoria Square or the Allen has to pass the sequential and exception test before the council can approve it, whatever the defences.
• The council's drainage team. On a major Truro scheme the lead local flood authority reads the drainage strategy first: where runoff goes, how much is stored and whether it meets the critical drainage area limits.
• Below greenfield rates. A surface water drainage strategy for a Truro site sizes storage so discharges stay within the 1 in 10 year limit, or the 1 in 1 year limit in the Tinney catchment.
• Conditions on Langarth and Pydar phases. Large permissions carry drainage conditions, and discharge of drainage conditions supplies the calculations and maintenance details the council asks for.
• Where river meets tide. Truro's floods come from the Kenwyn and Allen meeting a high tide behind dams and gates, which national mapping does not model in detail, so flood modelling sets levels where a scheme needs them.
• Warnings in five areas. Homes and businesses in the River Street, Lemon Quay and Calenick warning areas need to know what to do when a warning comes. A flood warning and evacuation plan sets that out for the building.
• Premiums after 1988. River Street and Victoria Square flooded in 1988 and sit in a warning area today. Owners there who face high premiums or refusals can use a flood risk assessment for insurance to show insurers how the property actually stands.
• Waterside transactions. Before buying or lending on a wharf, a shop on the quays or land at Newham, flood risk due diligence puts the tidal and river risk in front of the deal.
• Shops and offices in the warning areas. Retailers on Boscawen Street and businesses at Newham can plan for closures and recovery with a business continuity flood risk assessment.
• Commercial floor space at Pydar. Non-residential buildings seeking BREEAM credits need a BREEAM flood risk assessment to cover the Pol 03 issue.
• Ground conditions under Policy CC3. Cornwall's Policy CC3 asks for ground and groundwater conditions to be assessed, and a groundwater flood risk assessment covers that where a site has basements or relies on infiltration.
• Sewers at capacity. With the centre and Kenwyn valley sewers recorded as full, a foul drainage strategy shows South West Water how a scheme's flows can be taken.
• Before buying land. For edge-of-city land near Langarth or Threemilestone, a flood risk feasibility study tests the flood and drainage constraints before an offer is made.

Dams, drains and sewers: who answers for each in Truro

Several bodies share the job of managing flood risk in Truro, and the right one depends on where the water is coming from. In an emergency, ring 999 first.

For planning matters, the council's flood risk pages and its SuDS for Cornwall guidance hold the critical drainage area sheets and the householder guidance, and its Local Flood Risk Management Strategy sets out who does what. Our page on the lead local flood authority explains the council's role, and why LLFAs object to drainage proposals.

Questions about flood risk in Truro, from the tide to the drains

Send us the address of a flat on the quays or a plot at Langarth and our consultants at Unda Consulting Limited will tell you which reports it needs. Call 01293 214444 or email enquiries@unda.co.uk.