Flood Risk Assessment in Hassocks

A flood risk assessment in Hassocks usually turns on a small stream and the culverts it has to squeeze through. The Herring Stream rises from chalk springs below the Downs at Clayton, picks up the Keymer and Adastra streams inside the village and runs north under Keymer Road at Spitalford Bridge, where a blocked arch or a full culvert backs water into gardens, shops and roads. Our consultants assess Hassocks sites for planning applications, for buyers and their lenders, and for insurers, and we write the sequential and exception tests and surface water drainage strategies that often go with them.

On this page Hassocks means the civil parish: the village either side of the railway, Keymer to the east, Clayton at the foot of the Downs, and the new Ockley Park homes to the north, almost all within the BN6 8 and BN6 9 postcode sectors. Hurstpierpoint, about a mile and a half to the west, shares the council and sends some of its water to the Herring Stream by the Hurst Wickham Stream, so it is covered here too, though it sits outside the outline. Mid Sussex District Council decides most applications, the South Downs National Park Authority decides those on the parish's downland edge, and West Sussex County Council is the lead local flood authority. Our Mid Sussex District Council page sets out what the council asks for, our West Sussex page covers the county, and other places are listed among the areas we cover.

West Sussex County Council counts Hassocks among its flood wet spots, with 685 homes and businesses susceptible to flooding, and its 2016 study found that most of the risk comes from ordinary watercourses overtopping at a handful of bridges and culverts.

Email enquiries@unda.co.uk or ring 01293 214444 with a Hassocks or Keymer address, and one of our flood risk consultants will tell you which reports the site is likely to need.

The Herring Stream and the culverts that hold it back

Hassocks has no large river. Its flooding comes from the Herring Stream and the brooks that feed it, all draining north off the Downs to the eastern arm of the River Adur near Twineham. Through the village these are ordinary watercourses, looked after by the people whose land they cross; on the Environment Agency's main river map the Herring Stream only becomes a main river north of the village, beyond Friars Oak. The county's 2016 Surface Water Management Plan describes each one and the points where it chokes.

No Environment Agency flood warning area is drawn around Hassocks itself. The village falls inside the Agency's River Adur East Branch flood alert, which covers the Adur and its tributaries from Burgess Hill to Henfield, and our live river levels map shows the gauges downstream and any alert in force. The river itself, from its sources to Shoreham, has its own page on the River Adur.

Search a Hassocks, Keymer or Clayton postcode

The coral line is Hassocks civil parish as the Office for National Statistics publishes it, not the wider built-up area it shares with Hurstpierpoint. Enter a postcode to see the planning flood zone, the modelled risk from rivers and surface water, and any alert in force, or use our flood risk map for an address anywhere in England.

The planning flood map only draws river flooding where a stream drains more than about three square kilometres, so the smaller Hassocks brooks can be missing from it altogether. Mid Sussex fills that gap by treating high and medium surface water risk as if it were Flood Zone 3, and low surface water risk as Flood Zone 2. A garden beside the Keymer Stream can therefore look clear on one map and still need an assessment.

Why the chalk decides when Hassocks floods

Most of the village stands on Lower Greensand sands, with the Folkestone Formation and Gault clay in places, and the chalk of the Downs rises to the south. After a dry spell, rain sinks into that chalk and comes out slowly through the springs, so even a heavy storm may do little harm. After weeks of rain the chalk is full, and the same storm runs straight into the streams.

• The sponge effect. Mid Sussex's own report on the autumn 2000 floods put it simply: "The chalk downs acted as a sponge." Once the sponge was full, runoff to the north was very heavy even from minor rain.
• Wet ground, small storm. The Environment Agency rated the January 2003 rainfall at only about a 1 in 1.2 year event, yet the Lodge Lane culvert still overflowed, because the catchment was already saturated and the pipes were partly blocked.
• Water rising from below. The 2016 study judged flooding from groundwater rising beneath the village to be low risk, but recorded one cellar flooded to about 50 mm in the winter of 2013 to 2014, and groundwater was struck at shallow depth when the Damian Way pond was dug.
• Debris and blockages. A fallen tree above Spitalford Bridge made the 2000 flooding worse, and in February 2014 debris caught on a foul sewer crossing at Woodsland Road held the stream about half a metre higher upstream, by the district council's estimate.

For a site assessment that means the time of year matters as much as the rainfall figure, and that a culvert's condition can matter more than its size. Our guide to culverts, who owns them and who must keep them clear explains the rules behind the Hassocks pinch points.

Floods in Hassocks from 2000 to the Keymer Road works

The village record is one of repeated, local flooding at the same few places, rather than one great flood. The dates below come from the 2016 Surface Water Management Plan, the parish council and the county council.

1. October and November 2000. About 165 mm of rain fell in four and a half weeks, the worst of it on 29 October (54 mm in six hours). Forty-four homes and businesses flooded directly and about 300 more had water close by, including ten homes on Damian Way, Ockley Lane and Church Mead and shops on the High Street and Parklands Road.
2. January 2003. The Lodge Lane culvert overflowed and homes came close to flooding inside.
3. June 2012. During the county-wide storm, 54.2 mm fell in 12.5 hours and the Herring Stream gauge recorded its highest flow, 1.66 cubic metres a second.
4. 14 February 2014. After almost 250 mm of rain in six weeks, a further 23 mm flooded Lodge Lane, Keymer Road, Parklands Road, Downs View Road and the High Street.
5. 24 October 2020. Several businesses on Keymer Road flooded, and county officers told the parish council of about eight incidents on that road in two years.
6. June 2021. A late-evening downpour sent water at the Keymer Road and Parklands Road shops, and residents with brooms and buckets kept it out. The county moved the Parklands Road gullies from a four-yearly to a yearly clean, and the parish set up a flood working group.
7. July and August 2025. West Sussex County Council replaced failing highway drainage on Keymer Road, laying a new network between Keymer Road and Station Approach and diverting rainwater away from Spitalford Bridge, in a nine-week scheme.

Mid Sussex built the Damian Way balancing pond after 2000 and replaced the trash screen at the Orion car park in 2016. The 2016 Surface Water Management Plan found that, if the culverts were left to block, flood damage over a century would exceed £9 million and 33 properties would face a 1 in 20 chance of flooding each year; with the culverts kept clear, that falls to nine. It costed storage upstream of Lodge Lane and on the Herring Stream at roughly £615,000 to £925,000 to build, and the parish has since backed smaller natural flood management, such as debris dams to slow the flow. Our article on leaky dams looks at what that kind of work can and cannot achieve.

Flood risk assessment in Hassocks: the plans an application is judged against

Most of Hassocks is decided by Mid Sussex District Council under its District Plan adopted in March 2018, read with national policy and the made Hassocks Neighbourhood Plan. The council's own flood risk and drainage team checks minor schemes as well as major ones.

A replacement District Plan is under examination. Its main modifications, consulted on from July to September 2026, would add 250 homes on land west of Ockley Lane, with a criterion to plan the layout and drainage around the surface water flow paths and watercourses on the site, alongside 25 homes on land rear of 2 Hurst Road. Hearings resume in November 2026; until the plan is adopted, the 2018 policies apply. The council's flood risk and drainage guidance lists what it expects with an application.

Building on greensand and clay: where the rainwater can go

Almost every road gully in Hassocks drains straight into the nearest stream, which is why runoff from new roofs and drives matters so much here. The Neighbourhood Plan asks schemes to soak water away close to where it falls, and the Lower Greensand sands under much of the village may take it, while the Gault clay and the wetter valley floors near the streams are less likely to. Soakage tests at the right depth settle which; the BRE 365 infiltration rate calculator turns the results into a first figure.

Where infiltration fails, Mid Sussex expects runoff held on site and released no faster than the greenfield rate to a watercourse, then to a surface water sewer. A garden that backs onto one of the Hassocks streams also brings riparian duties: the parish council reckons at least 450 properties have them and has sent each a booklet, and our guide to riparian ownership explains what keeping the channel clear involves.

Reports for Keymer extensions, Ockley Lane fields and Clayton cottages

A buyer on Lodge Lane, a builder near Ockley Lane and an insurer pricing a High Street shop need different answers. Will the site stay dry? Is this the right place to build? Where does the rainwater go? A flood risk assessment, a sequential test and a drainage strategy answer them in turn.

• Applications to Mid Sussex. Homes and extensions near the Herring Stream or in a surface water outline need a flood risk assessment for planning that answers Policy DP41 and the district's checklist.
• Streams, culverts and rain together. A flood risk assessment for a Hassocks site weighs the stream beside it, the culvert downstream, surface water and the state of the chalk, then sets floor levels and a dry way out.
• Buying on Parklands Road or Lodge Lane. Streets that flooded in 2000 and 2014 can still look low risk on a search, and a flood risk survey for property purchase checks one building's floors, doors and nearest culvert before exchange.
• New homes in a flood zone. Where new homes are proposed on land the Herring Stream or a surface water outline reaches, a sequential and exception test shows why no lower-risk site in the area of search will do and how the homes stay safe.
• Drainage at application stage. Mid Sussex wants a basic scheme before it decides, and a drainage strategy sets out the route the water takes and the hierarchy behind it.
• Runoff that ends up in a stream. Because road and roof water here reaches the streams within minutes, a surface water drainage strategy shows how a scheme holds it back to the greenfield rate.
• Conditions on a consent. Where Mid Sussex or the National Park attaches a drainage condition, discharge of drainage conditions supplies the soakage results, calculations and maintenance plan it asks for.
• Springs and wet ground. Chalk springs feed every Hassocks stream and groundwater has been met at shallow depth by Damian Way, so a groundwater flood risk assessment helps with cellars, low floors and soakaways.
• Levels where the maps stop. The 2014 Herring Stream model was built to draw flood outlines, not to test a site, and flood modelling gives levels beside a brook or culvert the national maps leave out.
• Fields on the village edge. With 250 homes proposed west of Ockley Lane and smaller sites coming forward, a flood risk feasibility study tests a field's watercourses and flow paths before money is committed.
• Cover for homes and shops that have flooded. Forty-four properties flooded in 2000 and Keymer Road shops again in 2020, and a flood risk assessment for insurance gives an insurer the evidence on one building.

Who to contact when a Hassocks stream overtops

In a flood that threatens life, call 999; for a power cut, call 105. Floodline, on 0345 988 1188, gives warnings and advice at any hour, and anyone living near the streams can sign up for flood warnings, including the River Adur East Branch alert.

Hassocks and Keymer: what owners and applicants ask

Every Hassocks report is checked and signed off by a senior consultant before it is issued. Unda Consulting Limited is a CIWEM Business Partner, and the consultants who write our reports are chartered. Email enquiries@unda.co.uk or call 01293 214444 about a site anywhere from Clayton to Ockley Park. Nearby, we also write about Haywards Heath to the north, Brighton and Hove over the Downs and Lewes to the east.