Salfords Stream Flooding & Flood Risk

Few watercourses in Surrey carry as much flood-warning machinery per kilometre as the Salfords Stream. Its network of channels — taking in the Redhill Brook and the Earlswood Brook — amounts to barely twelve kilometres of main watercourse, yet the Environment Agency runs three separate flood warning areas along it, covering Salfords, Whitebushes, South Earlswood, Redhill and Redhill Aerodrome, with a wider flood alert area reaching Merstham and South Nutfield. Flooding from the Salfords Stream is a recurring winter event here, not a distant possibility. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the stream’s catchment. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

No reading on the Salfords Stream has ever beaten Christmas Eve 2013, when the Salfords Bridge gauge hit 3.05 metres — well above the 2.4 metres the Environment Agency treats as the top of its typical range.

• Three warned communities on one small stream. The Environment Agency operates flood warning areas for the Salfords Stream at Salfords, the Redhill Brook at Redhill and the Earlswood Brook at Whitebushes and South Earlswood, inside a catchment-wide flood alert area.
• A designated main river. The warned reaches of the Salfords Stream, Redhill Brook and Earlswood Brook appear on the Environment Agency’s Statutory Main River Map, so the 8 metre byelaw and flood risk activity permits apply to work near the channels.
• Roughly 57 square kilometres of catchment. The stream system drains the country between Redhill, Nutfield and Horley — most of it slow-draining Weald Clay — into the River Mole near Sidlow.
• A short but telling gauge record. Levels at Salfords Bridge typically move between 0.18 and 2.40 metres; the record is 3.05 metres, set on 24 December 2013.
• Two regulators, split by reach. The Environment Agency regulates the main river channels; Surrey County Council, as Lead Local Flood Authority, oversees the ordinary watercourses, surface water and groundwater around them.

Check flood risk along the Salfords Stream

The map draws the Salfords Stream and its arms in coral, with the Environment Agency’s live Salfords Bridge gauge pinned on the channel — click it to see the latest level, the typical range and the record peak. Enter a postcode near the stream 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.

When did the Salfords Stream last flood?

The Salfords Stream system has flooded repeatedly in living memory: badly in September 1968, worst of all across the winter of 2013/14, and often enough since that the Environment Agency issued flood warnings for the stream in December 2023 and again in February and March 2024. Its flood record is winter-weighted and unusually well documented for so small a watercourse.

The benchmark is the winter of 2013/14. Exceptional rainfall through December and January — for the neighbouring Burstow catchment the investigation that followed put the 60-day winter rainfall at around a 1-in-103-year event — drove the whole Upper Mole system to record levels. On 23 and 24 December 2013 the Salfords Stream rose to 3.05 metres at Salfords Bridge, its highest level on record, flooding low-lying land and disrupting the A23 through Salfords. Surrey County Council’s Section 19 flood investigation, published in October 2015, logged around 60 incidents of internal property flooding across Reigate & Banstead that winter, concentrated in Horley & Salfords, Redhill and Reigate. If terms like “1-in-103-year event” feel slippery, our guide to flood return periods and annual probability explains what the odds really mean.

The Redhill Brook told the same story more dramatically. It responded within hours of the 23–24 December storms, peaking at 2.46 metres — roughly a 1-in-50-year event on the investigation’s estimate — and on 31 December 2013 it burst its banks and poured into a quarry beside the channel. The Environment Agency turned the accident into flood relief, cutting an emergency outfall and letting the quarry attenuate the brook’s flows through January and February before a new channel was dug in March 2014.

None of this started in 2013. The Great Flood of September 1968 — still counted among the worst inland floods the South East has seen — hit Redhill and Horley hard along with the rest of the Mole system. Salfords has lived with the stream far longer than that: the village name means “ford by the willows”, and the watercourse that once turned the mill wheel by the Mill House has risen most autumns and winters since anyone kept records.

Why does the Salfords Stream flood?

The Salfords Stream floods because most of its catchment cannot absorb heavy rain. The system drains roughly 57 square kilometres of predominantly Weald Clay country between Redhill and Horley, and clay sheds rainfall into the channels within hours rather than soaking it up. Add a heavily culverted urban tributary through Redhill town centre and ground that is already saturated by mid-winter, and the stream can climb from its typical range towards a warning level in a single wet day. This is fluvial flooding at small-catchment speed.

• Impermeable geology. The southern part of the borough, from Reigate down to the Mole, sits on the Wealden Group — clays the 2015 flood investigation describes as “typically very impermeable in nature, giving rise to a rapid river response to the rainfall events”.
• The neighbouring gauge proves the regime. On the River Mole at Kinnersley Manor, close to where the Salfords Stream joins, the National River Flow Archive records a baseflow index of about 0.4 for a catchment it calls “largely impervious (mostly Weald Clay)” — and that station’s highest daily mean flow, 65.7 cubic metres per second, came on 24 December 2013, the same day the Salfords Stream set its own record.
• A culverted urban tributary. The Redhill Brook is classed by the Environment Agency as a heavily modified water body. Culverted and urbanised through Redhill, it moves flood flows on faster than a natural channel would — the 2015 investigation noted its increased conveyance in flood conditions.
• One permeable exception. The Redhill Brook’s headwaters cross more permeable sands where the brook threads the gap in the North Downs ridge, and the water meadows and marshes upstream of Redhill add natural attenuation — one reason Redhill’s flooding is patchier than Salfords’.
• Three channels converge above Salfords. The upper Salfords Stream (4.3 km of channel), the 12.5 km Redhill Brook with its Nutfield Brook arm, and the Earlswood Brook all meet around the village, concentrating runoff through Salfords on its final run to the Mole. The figures come from the Environment Agency’s Catchment Data Explorer.

The stream’s hydrology also feeds the Mole’s. A wet week that floods Salfords is usually a wet week that floods the wider Mole valley, because the same clay is shedding the same rain a few kilometres in every direction — which is why the flood story here reads so like the ones in Crawley, Horley and Dorking downstream.

Which areas does the Salfords Stream put at risk?

The Salfords Stream system puts five communities at documented flood risk: Salfords village along the A23 corridor, Whitebushes and South Earlswood on the Earlswood Brook, Redhill town centre and Redhill Aerodrome on the Redhill Brook, South Nutfield near the Nutfield Brook confluence, and the low ground towards Sidlow where the stream meets the River Mole. The Environment Agency’s warning and alert areas — the official register of who gets warned when levels rise — map directly onto that list.

Salfords itself is the most exposed — the flood warning area wraps the channel through the settled heart of the village, and land beside the stream falls within Flood Zones 2 and 3. Our guide to flood zones 1, 2 and 3 in England explains what those designations mean for a site. For the wider catchment either side of this one, see our pages on flood risk assessment in Crawley, at the top of the Mole system, and across Surrey.

Is the Salfords Stream a main river?

Yes — through every reach the Environment Agency formally warns. The Statutory Main River Map shows the Salfords Stream, the Redhill Brook and the Earlswood Brook as designated main rivers through Salfords, Redhill, Earlswood and Whitebushes, which places them under Environment Agency regulation. The field ditches and minor side channels feeding them remain ordinary watercourses, overseen by Surrey County Council as Lead Local Flood Authority and by the riparian owners whose land they cross. The distinction decides who you deal with and which rules bite.

• The 8 metre byelaw. Works within 8 metres of the bank of a main river — fences, sheds and landscaping included, not just buildings — generally need Environment Agency consent.
• Flood risk activity permits. Structures, culverts, outfalls or bank works in, over or near the channel need a flood risk activity permit under the Environmental Permitting Regulations 2016 — and permits cannot be granted retrospectively, a trap our note on retrospective applications and flood risk covers in detail.
• Planning consequences. Development beside the stream routinely needs a site-specific flood risk assessment, and the Environment Agency is a statutory consultee where the flood zones are engaged.

Who manages flood risk in the Salfords Stream catchment?

The stream sits in the Mole management catchment of the Thames river basin district, so the strategic framework comes from the Environment Agency’s Thames flood risk management plan, while Surrey County Council — the Lead Local Flood Authority created by the Flood and Water Management Act 2010 — is the statutory consultee on surface water drainage for major development and the author of the Section 19 investigation into the 2013/14 floods. Reigate & Banstead Borough Council determines planning applications along the stream, against its own strategic flood risk assessment. In practice a single development near the stream can involve all three bodies at once, which is one of the things we manage for our clients.

How Unda helps with flood risk on the Salfords Stream

Unda’s consultants know how the Salfords Stream behaves in flood: a flashy, clay-catchment watercourse whose levels move quickly, whose warnings arrive in clusters through wet winters, and whose main river status shapes what can be built near it. We have prepared flood risk assessments and drainage strategies for sites across the Mole catchment, from the Upper Mole around Crawley and Horley to the towns downstream, and we deal with the Environment Agency and Surrey County Council on our clients’ behalf. If you are weighing up a site anywhere near the stream, talk to one of our experienced flood risk consultants early — it is cheaper to design around the flood risk than to argue with it later.

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 answer different questions: is the site safe, should the development be here, and where does the surface water go. Unda prepares each of them for sites near the Salfords Stream.

• Flood risk assessments for planning. Site-specific reports showing a development near the stream is safe for its lifetime and will not increase flood risk elsewhere — see our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent, building-specific advice when a conveyancing search, lender or insurer flags the stream, through our flood risk survey for property purchase and flood risk assessment for insurance.
• BREEAM assessments. A flood risk assessment for BREEAM, structured around the Pol 03 flood and surface water management credits.
• Sequential and exception tests. The locational case — whether the development should be on this site at all — through our sequential and exception test reports.
• Drainage strategies and SuDS. Surface-water designs that manage runoff on clay that infiltrates poorly — see our sustainable drainage strategy service and, at the detailed stage, discharge of drainage conditions.
• Flood modelling and groundwater. Site-specific flood modelling where national mapping is not detailed enough beside the channel, and groundwater assessment — a live question on the more permeable ground towards Redhill — within the wider flood risk assessment. Everything is framed by the NPPF and its planning practice guidance.

While you scope a project, our free tools help: the live river levels map, the flood risk map, the geology and ground conditions checker, and for drainage design on the clay, the drainage calculators and BRE 365 infiltration calculator.

Live levels and flood contacts for the Salfords Stream

• Live river level. The Environment Agency publishes the Salfords Stream level at Salfords Bridge every 15 minutes.
• Flood warnings. Sign up free on GOV.UK, or call Floodline on 0345 988 1188 (24 hours a day).
• Surface water, groundwater and ordinary watercourses. Surrey County Council, as Lead Local Flood Authority — report flooding on 0345 600 9009.
• Sewer flooding. Thames Water, as the sewerage undertaker for the catchment.
• Sandbags. Reigate & Banstead Borough Council on 01737 276 000.
• In an emergency. Call 999 if there is danger to life, and 105 for power cuts.

Salfords Stream flooding: your questions answered