Flood Risk Assessment in Cornwall

A flood risk assessment in Cornwall has to deal with short, steep rivers that rise within hours, Atlantic storms and spring tides along roughly 700 km of coast, and surface water in the 19 Critical Drainage Areas the county council works to. Our consultants write flood risk assessments in Cornwall for planning applications, for buyers and their lenders, and for insurers, as well as sequential and exception tests and surface water drainage strategies.

That takes in Kernow from Bude and Launceston on the Devon border to Newquay, Bodmin, Wadebridge, Padstow, St Austell, Mevagissey, Fowey, Falmouth, Penryn, Camborne, Redruth, Hayle, Liskeard, Saltash, Torpoint and Callington, with coastal villages such as Boscastle and Polzeath, the TR and Cornish PL postcode districts, EX23 around Bude, and the Isles of Scilly (TR21 to TR25). Penzance, St Ives, Helston, Truro and Looe each have a page of their own, linked further down.

On 17 November 2010 a single overnight storm flooded more than 400 properties in mid Cornwall, St Austell Market and the Eden Project among them, with about 100 in Mevagissey and 40 in Lostwithiel.

Tell us the address and what you plan to build or buy: call 01293 214444 or email enquiries@unda.co.uk.

Rivers, tides and storm runoff: where flooding in Cornwall starts

The county council's flood strategy sets out the scale: around one property in six in Cornwall stands where it could flood, about 5,000 from the tide, 12,000 from rivers and 20,000 from surface water, and more than 180 towns, villages and hamlets have ten or more properties at risk. Much of the county drains through steep, narrow valleys with small catchments, so heavy rain can reach a harbour or town centre with very little warning.

The warning areas come from the Environment Agency's register, which lists 59 flood warning areas and 16 alert areas under Cornwall. Flood monitoring in Cornwall depends on the river and tide gauges behind them, and current readings appear on our live river levels map. A Critical Drainage Area is a catchment the Environment Agency has notified to the council as having critical drainage problems.

Screening a site anywhere in Cornwall or on Scilly

Type a postcode to see the river and sea flood zones, surface water risk, reservoir extents and the nearest warning area; our flood risk map does the same for anywhere in England. The coral line follows mainland Cornwall and the Isles of Scilly together.

The screening shows modelled flood zones only. It cannot tell you whether a site falls inside a Critical Drainage Area or the Coastal Vulnerability Zone, because both come from Cornwall Council's own maps, so we check those separately for every Cornish address.

Cornwall's flooding history: what the council has recorded since 1993

Flooding in Cornwall has often come from one storm parked over a small catchment for a few hours, rather than a slow river rising over days. The council's 2011 flood chronology, its Section 19 investigations and its yearly flood reports give a run of dates showing how fast the county's small catchments respond.

1. June 1993. Storms over north-east Cornwall flooded Bude, Camelford and the Camel valley on 12 June; a downpour a few days later put water into 71 properties in Bude and 20 at Sladesbridge.
2. 16 August 2004. About 60 properties flooded at Boscastle and Crackington Haven was hit in the same storm. Our account of the Boscastle flood explains why the valley responded so fast.
3. 27 October 2004. High tides and storms battered the south coast from Mousehole to Saltash: 40 homes were under water in Flushing and 32 properties flooded in Fowey.
4. 17 November 2010. Intense rain on steep, saturated ground sent streams over their banks from St Austell Bay to Altarnun, with Lostwithiel, Mevagissey, Pentewan and St Blazey hit hardest.
5. 22 to 27 November 2012. Six days of rain took the Hayle, Gannel and Seaton rivers to their highest levels in more than 40 years of records, and about 260 properties flooded across the county.
6. 18 July 2017. Around 200 mm of rain in three hours, one of the most intense storms on record in the UK, flooded about 50 properties in Coverack and 11 in St Keverne and destroyed the road through the village.
7. July 2023 to June 2024. The council handled 5,247 flooding and blocked-drain reports and recorded 316 properties flooded inside, the most since 2013-14 and more than double its 14-year average of 147.
8. 23 October 2026. Storm Austen brought almost 400 reports to the council in a day, mostly fallen trees with much of the rest flooding, and coastal flood alerts followed along both coasts over the next three days.

Small streams, culverts and runoff from the slopes caused several of these floods, so an assessment in Cornwall looks hard at the ground above a site as well as any river or harbour beside it.

Do Critical Drainage Areas change the need for a flood risk assessment in Cornwall?

Yes. Inside a Critical Drainage Area the council expects new development to reduce flood risk downstream, not simply avoid adding to it, so even a Flood Zone 1 site there normally needs an assessment and a drainage strategy that proves the improvement. The Environment Agency's map sheets for the areas date from May 2015, with Saltash added in 2017.

The 19 areas cover Bodmin, Bude, Flexbury, Camborne-Pool-Illogan-Redruth, Falmouth and Penryn, Hayle, Helston, Launceston, Liskeard, Lostwithiel, Padstow, Penzance, Saltash, St Austell, St Blazey, St Ives, Wadebridge and two parts of Truro: the Kenwyn, Allen and Tregolls Road catchment, and the River Tinney.

• Less run-off than today. Every new development, brownfield included, has to cut the rate of rainwater it sends off site, using infiltration as far as the ground allows.
• Greenfield behaviour up to 1 in 10. Off-site discharge has to mimic greenfield performance, up to a maximum of the 1 in 10 year rate on most sheets; Truro's River Tinney sheet caps it at the 1 in 1 year rate.
• Storage for the 1 in 100 year storm. All surface water must be held safely on site up to the 1 in 100 year event plus climate change, which usually means more storage than elsewhere.
• Natural flood management on bigger schemes. Climate Emergency DPD Policy CC3 asks proposals of more than 50 homes, or 1,000 m² of non-residential floorspace, inside a drainage area to show measures such as tree planting or hedge restoration, or exceptionally to provide them off site.
• Extensions as well. Householder and minor extensions inside a drainage area, or in Flood Zones 2 and 3, use the council's own short-form assessment and guidance.

Our guide to what a critical drainage area means for planning covers the national rules behind the Cornish sheets.

What triggers a flood risk assessment in Cornwall at the planning stage?

National policy, in chapter 18 of the August 2026 NPPF, asks for one in Flood Zones 2 and 3, on sites of a hectare or more, and on land with critical drainage problems. Cornwall has mapped its drainage areas, so that last test is easy to apply, and the Local Plan and Climate Emergency DPD add requirements of their own:

• Flood Zone 2 or 3. Almost any development, from an extension to a new estate; new homes there usually face the sequential test as well.
• A hectare or more. Every site of that size, whatever its flood zone.
• Inside a Critical Drainage Area. Any development in one of the 19 areas, with householder schemes using the council's short form.
• On the coast. A site in the Coastal Vulnerability Zone needs a flood risk assessment for tide and wave risk and, under Policy CC1, the council's coastal vulnerability assessment, which is a different study.
• Ten homes or half a hectare. Local Plan Policy 26 asks for a long-term water management plan covering upkeep of the surface water system and improvements to culverts and drains around the site.
• Every application. Policy CC3 asks proposals to use the latest flood and surface water maps, climate change included, and to show in the assessment how existing overland flow routes across the site stay open or are diverted.

If you cannot tell which of these catches your site, send us the address and we will say, at no cost.

The Cornish coast: shoreline policy and candidate change areas

Cornwall's shoreline policy comes from the Rame Head to Hartland Point Shoreline Management Plan (SMP17), which divides the coast into 18 zones from Rame Head round Land's End to Hartland Point, with the Isles of Scilly as the eighteenth. Each stretch has a policy for now, the middle of the century and 2105: hold the line, managed realignment or no active intervention, as our guide to shoreline management plans explains.

• Coastal Vulnerability Zone. The Climate Emergency DPD draws a predicted 100-year erosion zone round the whole coast. New building there, replacements included, needs a coastal vulnerability assessment under Policy CC1, and soakaways within 5 m of the zone or drainage over a cliff face are refused unless that assessment shows the cliff stays stable.
• Candidate change management areas. Policy CC2 lists 17 places for a precautionary approach, among them Bude, Downderry and Seaton, Par Sands and Par Docks, Portmellon, Coverack, Praa Sands, Gwithian, Mawgan Porth, Portreath, Perranporth, Polzeath and Mullion Cove, with Newquay already designated through its neighbourhood plan.
• Strategies in progress. The council and the Environment Agency have consulted on a Mounts Bay coastal resilience strategy, Future Coast Bude is planning how that town adapts, and Making Space for Sand is studying dunes at 40 sites.

A coastal vulnerability assessment is a separate study, and not one we prepare. On the Cornish coast our work is the flood risk assessment, covering tide, surge and wave overtopping with sea level rise, and due diligence for buyers, lenders and businesses.

Drainage rules a Cornish surface water strategy has to meet

Cornwall Council reviews drainage as lead local flood authority through its own sustainable drainage team. Its requirements are spread across the Local Plan, the Climate Emergency DPD, the drainage area sheets and a set of technical notes, and since August 2026 national policy F8 also expects development to follow the 2025 National Standards for sustainable drainage.

Infiltration has to be proved by BRE 365 infiltration testing on site; our ground conditions and soakaway checker and infiltration rate calculator give a first view. Where the water table is in doubt, the council's advice can mean a season of groundwater monitoring before the scheme is accepted, so the monitoring needs to start before the layout is fixed. The full set of notes is on the council's SuDS for Cornwall guidance page.

Which plans and studies does a Cornwall application answer to?

Cornwall Council decides every mainland application as planning authority and lead local flood authority, working from the Local Plan adopted in 2016 and the Climate Emergency DPD adopted in February 2023. The Isles of Scilly have their own council and their own plan.

Policies F5 and F6 of the same national chapter set the sequential test and the exception test for sites in the higher flood zones, and our notes on the national framework go through each policy. Where the council's interim statement treats a Local Plan policy as out of date, the national tests carry more weight in the decision.

The Isles of Scilly: a separate council and the five-metre line

The Council of the Isles of Scilly is a unitary authority in its own right, acting as planning authority and lead local flood authority for St Mary's, Tresco, St Martin's, St Agnes and Bryher. Its Local Plan, adopted on 25 March 2021, treats flooding from the sea as the main risk: land below the 5 m contour (above Ordnance Datum Newlyn) is shown on the policies map, and development there is avoided where possible or needs an assessment under Policy SS7 showing that damage would be limited and recovery quick.

The council commissioned a Level 1 Strategic Flood Risk Assessment for its Local Plan review in July 2026, with the contract running to the end of the year. Until it is published, an assessment on the islands draws on Environment Agency data, the 5 m line and the shoreline plan's Isles of Scilly zone. The islands share the South West regional flood committee and the Devon, Cornwall and Isles of Scilly resilience forum with the mainland.

Reports for Cornish sites, from the Tamar to Land's End

A flood risk assessment, a sequential or exception test and a drainage strategy answer different questions: is the site safe, should it be here at all, and where does the rain go. Each report below is written around Cornwall's rivers, coast and drainage rules.

• Every source at once. A site above Mevagissey can face a stream, a culvert, runoff from the valley side and a high tide together, and our flood risk assessment weighs each one and how they combine.
• For the planning application. Where Policy 26, a drainage area or the coastal zone applies, a flood risk assessment for planning gives Cornwall Council the evidence it asks for.
• Before exchange. Searches flag harbourside cottages and valley-floor houses across the county, and a flood risk survey for property purchase explains the risk to a buyer before they commit.
• Testing other sites. New homes in Flood Zones 2 or 3 normally need a sequential and exception test that compares reasonably available sites elsewhere.
• The drainage picture. A drainage strategy sets out how foul and surface water leave a site, from a single plot to an estate.
• Rain on the site. Inside a drainage area the council wants run-off cut below today's rate, and a surface water drainage strategy shows the storage, discharge rate and above-ground features that achieve it.
• After permission. When an approval comes with a drainage condition, discharge of drainage conditions supplies the detail the council needs before work starts.
• Repeat flooding. With 316 properties flooded inside in 2023-24, the most for a decade, a flood risk assessment for insurance gives a broker or underwriter evidence for the individual property.
• Coastal and commercial property. Along a coast with 17 candidate change areas from Bude to Mullion Cove, flood risk due diligence sets out the flood risk and shoreline policy for a harbourside business, holiday park or development site before purchase.
• Keeping open after a flood. St Austell Market and the Eden Project were among the premises flooded in November 2010, and a business continuity flood plan prepares a business for the next event.
• Water from below. Cornwall Council's groundwater monitoring advice, updated in November 2024, governs infiltration schemes, and a groundwater flood risk assessment looks at rising water where the ground or the records suggest it.
• Main rivers and estuaries. Sites beside the Camel, the Fowey or the tidal Fal can need levels the published maps do not give, which flood modelling provides.
• Getting out safely. Tidal and flash-flood sites, from Wadebridge to Bude Haven, often depend on warnings and a planned route out, set down in a flood warning and evacuation plan.
• Land for the next plan. The new Local Plan has to provide for 4,421 homes a year under the Government's standard method, and a feasibility study tests a site's flood constraints before it is promoted.
• Solar and wind. The council maps areas suitable for wind energy alongside its Climate Emergency DPD, and our renewable energy flood risk assessments cover panels, turbines, substations and access tracks.

Penzance, St Ives, Helston, Truro and Looe, and next door in Devon

Five Cornish towns have their own pages, which go further into their rivers, defences and local plans:

• Mount's Bay. Penzance, with Newlyn and the seafront along the bay.
• St Ives Bay. St Ives, where steep streets drop to the harbour.
• The Cober valley. Helston, on the River Cober above Loe Pool.
• The Fal. Truro, at the head of the tidal Truro River.
• South-east Cornwall. Looe, the harbour town the council calls the most frequently flooded in the UK.
• Two rivers. The River Tamar along the Devon border and the River Valency through Boscastle each have a page.
• Over the Tamar. Plymouth faces Saltash and Torpoint across the river, and the rest of the neighbouring county is on our Devon page. Every place we cover appears on our areas page.

Flood contacts from the Tamar to the Isles of Scilly

If lives are at risk, call 999; if the power goes off while the water is up, ring 105. Floodline, on 0345 988 1188, gives warnings and advice day and night, and you can sign up for flood warnings by phone, text or email.

Householders whose homes have flooded can ask the council about its property flood resilience grant scheme, which helps towards measures that keep water out of a home or limit the damage when it gets in.

Questions we are asked about flood risk assessment in Cornwall

Every report is prepared by our chartered consultants and signed off by a senior consultant, and Unda Consulting Limited is a CIWEM Business Partner. Call 01293 214444 or email enquiries@unda.co.uk.