Flood Risk Assessment in Helston

A flood risk assessment in Helston begins with the River Cober and with Loe Pool, the freshwater lake two kilometres downstream that the river drains into. When the pool cannot empty to the sea fast enough through Loe Bar, the Cober backs up into the lower town around St John's Road and Mill Lane. Our consultants prepare flood risk assessments in Helston for new homes and extensions, purchases, remortgages and insurance, together with sequential and exception tests and surface water drainage strategies.

On this page Helston means the town as the Office for National Statistics maps its built-up area: the old centre around Meneage Street, Coinagehall Street and Wendron Street, St John's and the Cober valley floor, Godolphin Road and the estates to the east, and the Water-Ma-Trout industrial estate to the north, all in the TR13 postcode district. Porthleven, Gweek, Breage, Lowertown, Trenear, Coverack Bridges and RNAS Culdrose sit outside that outline. They share the Cober catchment, Loe Bar or the same council, so they are covered here as well. Cornwall Council decides planning applications in the town and is its lead local flood authority.

In June 1993 water built up 600 mm deep behind garden walls in the St John's area of Helston until the walls gave way. About 50 houses flooded, some to 2 m, and firefighters broke doors and windows to let the water through.

To talk through a Helston site, call 01293 214444 or email enquiries@unda.co.uk.

Why does the Cober back up into Helston?

Water reaches the lower town by four routes, and the worst floods have come when two of them coincide. The Cober rises after heavy or prolonged rain on its catchment around Wendron. Two smaller channels beside it, the Mill Leat and the Town Leat, overtop or push water out through road gullies. Downstream, Loe Pool drains to Mount's Bay through a nineteenth-century tunnel under Loe Bar, helped since 2019 by a relief pipe, and when the pool is high the river has nowhere to go. The Environment Agency's account of the town describes each mechanism:

The Cober and the leats are watercourses the Environment Agency and Cornwall Council divide between them. The Environment Agency looks after the main river, and the council, as lead local flood authority, regulates ordinary watercourses; our note on main rivers and ordinary watercourses explains how that split affects consent for work near a channel.

Check a Helston postcode, from Water-Ma-Trout to St John's

Type a TR13 postcode into our flood risk map below to see the Environment Agency's mapping for rivers and the sea, surface water and reservoirs at that address. The coral line is Helston's built-up area. Coronation Park and the boating lake on the valley floor, Loe Pool and Porthleven all fall just outside it.

Reading the gauges at County Bridge and Loe Pool

The Environment Agency measures a Helston flood at two gauges. One records the Cober at Helston County Bridge, where the A394 Penzance Road crosses the river, and the other records the level of Loe Pool itself, two kilometres downstream where the valley meets the bar. Both records were set in the same week of December 2012:

The County Bridge record sits below the 2.38 m threshold, yet the agency still ties it to flooded properties at St John's. That gap shows why a Helston assessment cannot read one gauge in isolation: in 2012 river water came back up the leat below St John's bridge and out of road gullies before the river reached its published threshold. The agency runs two flood warning areas in the town, for the St John's Road area and for Coronation Park and St John's Industrial Park. Upstream there is a third from Wendron to Lowertown, and an alert area for the whole Cober from Trenear to the Loe valley. Our live river levels map shows the County Bridge reading against its normal range.

Floods recorded in Helston since 1945

The Environment Agency's list for the town runs through most of the last eighty years, and the council's critical drainage area sheet adds detail on the worst of them. Each entry below comes from one of those two documents:

1. 1945, 1979 and 1988. Blockage of the tunnel at Loe Bar combined with heavy rain to flood the town (Environment Agency).
2. 1989. The first Helston flood alleviation scheme: a second arch through St John's Road bridge, low-flow boards from St John's Road to the Porthleven Road, and channel improvement and de-silting from Zachary's Bridge down to Loe Pool.
3. June 1993. Intense rain on the small St John's catchment, where the Town Leat overflowed. About 50 properties flooded, some to 2 m, after garden walls holding back 600 mm of water collapsed (Cornwall critical drainage area sheet). Improvements to the Town Leat followed.
4. 1999. River and surface water flooding reached three homes and the Methodist Chapel.
5. 2000 onwards. The Environment Agency began bringing large mobile pumps to Loe Bar in storms to lift water from the pool to the sea.
6. 2003. The Cober flooded about 20 properties.
7. 2012. River water flowed back up the leat below St John's bridge and out of road gullies, affecting four properties. In December the County Bridge and Loe Pool gauges both set the records they still hold.
8. Early 2018. A storm caused a cliff collapse at Loe Bar that exposed the tunnel, which then blocked with sea-blown sand. The agency began over-pumping to stop the pool reaching dangerous levels (Water Projects Online).

Before the latest works, the Environment Agency predicted that a 1 in 100 year flood would damage 115 properties in Helston, and that without action climate change would make the same flood a 1 in 2 year event by 2050. Those figures predate the defences. They also explain why the risk left behind the walls still matters in an application.

The 2019 Loe Bar pipe and the 2020 Cober defences

Between 2019 and December 2020 the Environment Agency completed a £5.4 million scheme in two phases, opened with the Mayor of Helston on 16 December 2020. It cut the annual chance of river flooding along the eastern bank of the Cober from 20% to 1% and protected 121 more properties (Environment Agency). The works were:

• A relief pipe under Loe Bar (2019). A new 185 m, 1.8 m diameter pipe below the bar, with repairs to the old tunnel. Its outlet doubles the previous discharge of 4 to 5 cubic metres a second and normally lies under 3 m or more of sand, with telemetry to warn of a blockage (Water Projects Online).
• A managed inlet. The inlet was altered so the agency can vary the pool level through the year. That protects the margins of Loe Pool, which is a Site of Special Scientific Interest.
• An embankment and a ramp at Mill Lane. A reinforced earth embankment along the left bank from Mill Lane to St John's Road Bridge, and a road ramp near the end of Mill Lane so flood water cannot run round the end of it.
• Flood walls to County Bridge. Walls along the left bank from St John's Road bridge to the Penzance Road at County Bridge.
• A flood gate at the Kingdom Hall bridge. Closed when a flood warning is issued, so it depends on the warning, on staff and on upkeep.

The defences protect the eastern bank of the Cober, and they were built for river floods. Water can still reach a site from the leats, from surface water held behind the walls or from a blocked outfall at Loe Bar. The agency's flood zones are drawn without the defences, too, so streets behind the walls still sit in Flood Zone 2 or 3 on the planning map. A Helston assessment therefore deals with residual flood risk behind the walls as well as the defended standard.

Loe Bar, Porthleven and the open sea

Helston has no seafront, but its flood risk depends on a shingle bar facing the Atlantic. The Cornwall and Isles of Scilly Shoreline Management Plan sets managed realignment for Loe Bar and Loe Pool for all three of its periods, up to 100 years. The Environment Agency manages the crest height of the bar to control the level of the pool and help manage flood risk in Helston. The plan records the bar moving landward by about a metre a year, and an agency study it cites expected 25 m to 74 m of movement over 50 years, so the outfall that drains the Cober sits in a beach that is rolling back towards the pool. Nothing about that is fixed.

At Porthleven, 1.5 km north-west of the bar, the policy is to hold the line, with a review before the final period. About 450 m of walls run east of the harbour, and the plan notes a risk of waves overtopping walls and defences there. The Environment Agency's coastal warning for Land's End to Lizard Point names Porthleven Harbour. On this frontage we prepare flood risk assessments and due diligence for buyers and lenders. Cornwall's Climate Emergency Development Plan Document also draws a coastal vulnerability zone round the whole coast, and Policy CC1 rules out soakaways within 5 m of it, and any discharge over a cliff face, unless the applicant shows the cliff will not be harmed. Our guide to shoreline management plans explains what each policy means for a site.

Surface water rules in a critical drainage area

Much of Helston lies in one of the 19 critical drainage areas the Environment Agency notified in Cornwall in May 2015, as the council's site allocations plan points out. The sheet for Helston explains why. Run-off from the streets above St John's drains towards the defences that keep the Cober out, and the drainage behind them is complex. Controlling that run-off is what keeps the standard of protection the St John's area has today. These are the council's standards:

The ground decides how much of this can soak away. The British Geological Survey maps Helston on the Mylor Slate Formation, a slate and siltstone, so an infiltration rate has to be proved on site rather than assumed. Our ground conditions and soakaway map gives a first read. The BRE 365 infiltration rate calculator works up trial pit results, and our note on when infiltration testing is required sets out the timing. More on the designation sits in our explainer on critical drainage areas.

Flood and drainage policies a Helston application must meet

A planning application in Helston is judged against national policy, the Cornwall Local Plan and two council documents that apply the plan to flooding and drainage. The site allocations plan also adds a specific concern about the Town Leat:

Cornwall's strategic flood risk assessment is published as an interactive map layer, not a report, so an assessment here leans on the Environment Agency's flood zones, the gauge record and the critical drainage area sheet. Where the Cober and Loe Pool interact, the flood level at a site can depend on both, and flood modelling is then the part of the assessment that settles it.

Business parks, infill and conservation areas: building in Helston

The site allocations plan gives Helston 11,178 residents at the 2011 census. The Local Plan expects 1,200 homes in the town between 2010 and 2030. Completions and permissions already covered most of that, so the site allocations plan made no new housing allocations and new homes come from windfall, infill and conversion sites. Its Helston policies deal with employment land instead:

• Helston Business Park (He-E3). The council's main priority for employment growth, an edge-of-centre site that is already serviced. Commercial buildings here need drainage that meets the critical drainage area standards from the outset.
• Tresprison Industrial Park (He-E2). An extension is allocated for small industrial units, with the same discharge limits applying.
• Water-Ma-Trout Industrial Estate (He-E1). Safeguarded as a strategically important employment site. Redevelopment on the estate still has to meet the critical drainage area standards, brownfield or not.
• St John's. Workshops and homes on the valley floor sit in the warning areas and behind the 2020 walls, so an extension or change of use there needs floor levels and access worked out against the defended and the undefended case.
• The conservation areas. Helston's centre and Godolphin Road are conservation areas. Where a historic floor cannot be raised, the assessment proposes resistance and resilience that suit the fabric.

Extensions are the commonest small application near the Cober. Our guide to building an extension in a flood zone explains what the council will ask to see.

Helston proposals and the reports they call for

Because much of Helston is a critical drainage area, the answer is yes for many more sites than the flood zone map alone would suggest. This is how common Helston proposals line up with the reports the council will expect:

The sequential test asks whether the development could go on a site at lower risk. It is a separate report from the assessment, and the council applies it before the exception test. The NPPF guidance we publish covers the national rules in more depth.

Our reports for sites from Water-Ma-Trout to Loe Pool

Each of the reports below answers a different question about a Helston site, and each is scoped to the Cober, the leats, Loe Pool and the critical drainage area:

• Applications near the Cober. A flood risk assessment for planning shows the council that homes or extensions at St John's or Mill Lane are safe behind the 2020 walls and that Policy 26 and CC3 are met.
• Every source together. A flood risk assessment reads the river, the leats, Loe Pool, surface water and failure of the defences or the Loe Bar outfall as one system.
• Buying in the lower town. A flood risk survey for property purchase looks at one building's floor levels and its history in 1993, 2003 and 2012 before you exchange.
• Building on land that floods. New homes in Flood Zones 2 and 3 have to pass the sequential and exception test before the council weighs the rest of the scheme.
• Drainage evidence for the council. A drainage strategy sets out how a site meets the critical drainage area standards and the 2025 National Standards.
• Run-off from roofs and yards. A surface water drainage strategy tests the slate for infiltration and sizes storage to the 1 in 100 year plus climate change event.
• Conditions after approval. Where permission comes with a drainage condition, the discharge of drainage conditions is handled with the council's SuDS team.
• Levels where the river meets the pool. Where national mapping cannot show how the Cober and Loe Pool combine, flood modelling sets the flood level and overtopping depth for the site.
• Leaving before the gate shuts. With two warning areas on the Cober in Helston and a gate at the Kingdom Hall bridge that closes on a warning, a flood warning and evacuation plan sets out when people leave a building and which way they go.
• Homes with a flood history. After 1993, 2003 and 2012, a flood risk assessment for insurance gives an insurer evidence about one Helston address and the defences in front of it.
• Firms at St John's Industrial Park. The park shares a warning area with Coronation Park, so a business continuity flood risk assessment helps a tenant plan stock, power and access.
• New commercial space. Offices and units at Helston Business Park seeking a BREEAM rating need a BREEAM flood risk assessment for the Pol 03 credit.

Flood contacts for Helston: the council, the agency, the water company and the forum

If water is entering homes or anyone is in danger, call 999. For a power cut during a flood, call 105. The rest of the bodies split by source:

Ask Floodline for the St John's Road and Coronation Park warning areas by name when you register an address in the lower town.

Helston flood risk questions, answered

Contact one of our experienced flood risk consultants on 01293 214444 or at enquiries@unda.co.uk. For the county picture see our page on flood risk assessment in Cornwall; along the coast and inland, the pages for Penzance and Truro cover their own rivers and harbours, and every town we write about is listed under our area pages.