Flood Risk Assessment in Hertfordshire

A flood risk assessment in Hertfordshire usually begins with the chalk. Rain soaks into it, feeds the Colne, Ver, Gade and Lea, and can come back up months later as groundwater, while on the clay and in the towns it runs straight off into sewers and streams. Most of the county drains south to the Thames; Hitchin, Letchworth Garden City, Baldock and Royston drain north to the Great Ouse. Our consultants write flood risk assessments for Hertfordshire sites that need one for planning, a purchase, a mortgage or insurance, as well as sequential and exception tests and surface water drainage strategies.

Herts covers ten district and borough councils, from Watford, St Albans and Hemel Hempstead in the south-west to Stevenage, Welwyn Garden City and Hatfield in the middle and Bishop's Stortford, Cheshunt and Hoddesdon in the east. Hertfordshire County Council reviews the drainage for all of them. In April 2028 four new unitary councils take over both roles, so a scheme that is slow to reach a decision may end up with a different authority.

Hertfordshire County Council's flood strategy puts 29,669 properties at high risk of surface water flooding and 62,878 at medium risk. Environment Agency figures in the same strategy put 1,709 homes at high risk from rivers.

To talk a Hertfordshire site through with one of our consultants, call 01293 214444 or email enquiries@unda.co.uk.

Thames or Great Ouse: where does a Hertfordshire site drain?

The Chilterns escarpment, running north-east from Tring towards Hitchin and Royston, is the watershed. South of it, the Colne and the Lea carry almost everything to the Thames, and a corner around Long Marston, north of Tring, drains to the Thame. North of it, the Hiz, the Purwell, the Ivel and the Rhee flow to the Great Ouse. Which side a site is on decides its Environment Agency area, its funding committee and, at Hitchin, its sewerage company.

The Environment Agency lists 64 flood warning and alert areas touching Hertfordshire: 59 run from its Hertfordshire and North London office, three from East Anglia and two from its Thames area. Our river pages cover the River Colne, the River Lea and the Great Ouse, and the gauges on all three feed our live river levels map, with current readings and any warning in force.

Screening a Hertfordshire postcode before you commission anything

Type a postcode into the map for the Environment Agency's flood zones, surface water and reservoir layers at that spot, or use our full flood risk map anywhere in England. Hertfordshire County Council's own Flood Risk Management Viewer adds what the national maps leave out: the county's flood incident record, ordinary watercourses, surface water hotspots and the areas where groundwater can reach the surface.

Neither map can see a floor level, a culvert that blocks or a winter water table in the chalk, which is why a Hertfordshire flood map is only the first screen. Our guide to flood zones in England explains what each zone means for planning.

Ten councils decide now, and four from April 2028

Each of the ten districts decides its own applications against its own plan and its own Strategic Flood Risk Assessment, and Hertfordshire County Council is the Lead Local Flood Authority for all of them. On 16 July 2026 the government confirmed that four unitary councils will replace the county and districts, with elections in May 2027 and the handover on 1 April 2028. Until that date the councils below stay in charge.

Some districts go further than national policy. Three Rivers allows no new development in the functional floodplain and wants floors 300 mm above the 1% flood level with climate change, as Hertsmere does; Three Rivers, Watford and Stevenage look for an undeveloped strip 8 m wide beside watercourses; and Welwyn Hatfield refuses surface water into a combined sewer. St Albans' new plan asks for surface water flow paths to stay open as green corridors. Our East Hertfordshire council page sets out that district's rules in full, and our note on local government reorganisation covers what the 2028 change means for live applications.

What Hertfordshire County Council looks for in a drainage strategy

Hertfordshire County Council has been a statutory consultee on surface water for every major application since 15 April 2015. Its summary guidance for developers, last updated in August 2021, is read with the drainage policies in its Local Flood Risk Management Strategy, and the 2025 national standards for sustainable drainage now sit over both.

• Runoff no worse. Peak discharge must not rise for any storm up to the 1 in 100 year event with climate change. Greenfield sites hold to greenfield rates, and brownfield sites are expected to aim for them.
• Dry for the 1 in 30. Nothing should flood on site up to the 1 in 30 year storm, with storage for the 1 in 100 year storm plus climate change, above ground where practicable.
• Tested ground. Shallow soakaways are tested to BRE 365 and deep bores by falling head tests. Where testing cannot happen before approval, the strategy assumes a worst-case rate and shows an above-ground alternative.
• Groundwater as a source. The county wants groundwater assessed alongside rivers and surface water, which matters on the chalk valley floors.
• A checklist for each stage. Outline, full, reserved matters and discharge of conditions each have their own list, and an outline scheme already needs infiltration evidence and a levelled topographical survey.
• Ordinary watercourses. Work in or over a ditch or stream needs the county's consent under section 23 of the Land Drainage Act 1991, as well as planning permission.

District policy sits on top. Watford will not accept deep bore soakaways in an inner source protection zone unless a hydrogeological risk assessment shows nothing else works, and Dacorum's CS31 protects the same zones. Our notes on groundwater source protection zones and on why drainage strategies draw an objection cover the commonest problems at the consultation stage.

Stevenage, the new town built around flood storage

Stevenage has little river floodplain. Its local plan says Flood Zones 2 and 3 run only along the Stevenage Brook and Aston End Brook in the south of the town, and that groundwater flooding is not a significant problem there. What it does have is a set of flood storage reservoirs that hold storm runoff from the town for a while before letting it go, and the plan protects eleven of them as functional floodplain.

• FP2, for Flood Zone 1. Every major site needs a surface water flood risk assessment estimating the runoff, showing how it is managed now, and proving there is no increase in its rate or volume.
• FP3, for Flood Zones 2 and 3. A natural buffer at least 8 m wide from the top of the bank along the whole watercourse, a sequential approach within the site, and culverted watercourses opened up where possible.
• FP4, the reservoirs. Aston Valley, Bragbury End, Broad Oak, Burymead, Camps Hill Park, Elder Way, Fairlands Valley Lakes, Meadway, Ridlins Wood, Sainsburys (Coreys Mill) and Wychdell are all designated as flood storage and Flood Zone 3b.
• Recorded flooding. Bragbury End, which gives its name to one of the reservoirs, is also on Hertfordshire County Council's list of completed flood investigations.

A Stevenage scheme in Flood Zone 1 can still turn on flood evidence: the surface water assessment FP2 asks for on any major site or, beside a reservoir, the storage the land already provides. In Stevenage, flooding mostly comes down to drainage, because Flood Zones 2 and 3 are not extensive.

Hitchin and the north: Purwell, Hiz and the Great Ouse

Hitchin flooding in late September 2024 came off the small brooks that feed the river. It began on 22 September after long, heavy rain overwhelmed those watercourses, and the North Herts council minutes record 20 properties flooded internally and 27 externally. The Environment Agency's warning stayed in force from 27 September to 5 October.

• Its own warning area. The Environment Agency's warning for the Ash Brook, Ippollitts Brook and River Purwell covers Hitchin, Ashbrook, Little Wymondley and Graveley. It is run from the East Anglia area, not Hertfordshire and North London.
• What went wrong. Hertfordshire County Council's draft flood investigation, published in May 2025, points to the rainfall, the Purwell overtopping, high groundwater and blockages in the Purwell and in Anglian Water's sewers.
• A drainage board. The Bedfordshire and River Ivel Internal Drainage Board works in the Ivel catchment and joined the clearance of the Purwell after the flood; a site draining to its watercourses may need its consent too.
• Another sewer company. Anglian Water, not Thames Water, runs the public sewers in Hitchin, so sewer capacity questions go to a different company from the rest of the county.
• North Herts policy. NE7 and NE8 of the 2022 Local Plan set the flood risk and sustainable drainage tests for Hitchin, Letchworth Garden City, Baldock and Royston.

Little Wymondley, just outside Hitchin, has its own county flood investigation. The rivers here end in the Great Ouse basin, and our Bedfordshire page picks up the Ivel downstream.

Chalk groundwater: Kimpton, Lilley Bottom and Flamstead

Under most of Hertfordshire lies the Chalk aquifer, which also supplies much of Affinity Water's drinking water. After a long wet winter the water table rises in the dry valleys and springs break out where no stream normally runs. The county's strategy links the worst groundwater episodes to the months of rain in 2000 to 2001 and 2013 to 2014, and calls groundwater a small part of the overall risk, though where it happens it is slow to clear.

• Two standing alert areas. The Environment Agency issues groundwater flood alerts for Kimpton and Lilley Bottom, and for Flamstead, both in chalk valleys north of Harpenden.
• 2001 and 2024. Lilley Bottom flooded in 2001. In spring 2024, after rainfall at 157% of average from October to April, the borehole there stood about 0.7 m below that 2001 level, and the alert covered Kimpton, Lye Hill, Whiteway, Claggy Bottom and Lilley Bottom.
• Perched water. Clay over the chalk holds water near the surface and creates spring lines, so a site can be wet without being on a valley floor.
• Soakaways and protected water. High groundwater can rule out infiltration, and source protection zones around the public supply wells limit how deep a soakaway may go.

The months of monitoring that groundwater can need are the main reason to start early. Our guides to groundwater flooding, groundwater monitoring for planning and BRE 365 infiltration testing explain the steps, and our ground conditions checker gives a first read of the geology under a postcode.

Floods in Hertfordshire on the county's own record

Flooding in Hertfordshire follows two patterns. The county's investigations at Welwyn Garden City and Bovingdon both traced the damage to short, intense storms the drains could not take, while river and groundwater floods follow long wet winters. Hertfordshire County Council logged more than 800 new flood incidents between 2011 and its 2019 strategy and has published investigations from Bushey Heath to Little Wymondley. Its triggers for an investigation include five or more properties flooded internally in one event, a business flooded, or a road or railway shut for more than 10 hours.

1. Winter 2000 to 2001. Months of rain raised the chalk groundwater, with significant and widespread flooding at Lilley Bottom in 2001.
2. December 2013 to September 2014. Surface water events in December 2013 and February, July and September 2014; February 2014 brought the county's most notable river flood in recent years, and groundwater was high again.
3. 23 June 2016, Welwyn Garden City. About 41 mm fell in two and a quarter hours at Mill Green. The county's investigation recorded 47 homes flooded internally and 11 externally, with the surface water sewers at or near capacity.
4. 16 September 2016, Bovingdon. A storm peaking at 34.9 mm an hour before 7am flooded 23 properties, 14 inside, as surface water overloaded the drains and got into the foul sewer.
5. Spring 2024, Kimpton and Lilley Bottom. The Environment Agency issued a groundwater flood alert as the Lilley Bottom borehole came within about 0.7 m of its 2001 level.
6. September 2024, Hitchin. About 220% of the town's average monthly rainfall fell in 10 days, according to the county's draft report, and the Purwell burst its banks.

The national flood zones show almost none of this. These entries come from the county's Section 19 investigations, published on its flood investigations page, and from its Local Flood Risk Management Strategy for 2019 to 2029, the North Herts council minutes of January 2025 and the Environment Agency's groundwater reports.

When is a flood risk assessment in Hertfordshire required for a scheme?

Under chapter 18 of the NPPF, policy F4 requires an assessment in Flood Zones 2 and 3, on sites of 1 hectare or more in Flood Zone 1, and wherever the Environment Agency's flood map for planning or the district's SFRA shows risk from another source, surface water included. In Hertfordshire several district policies catch more than that.

Where a site sits in Flood Zone 2 or 3, the sequential test asks whether a lower-risk site is reasonably available, and the area of search often covers the whole district. Our explainer on surface water and planning covers the Flood Zone 1 cases.

Our reports for Hertfordshire sites, from Tring to Bishop's Stortford

Which report a site needs depends on the question being asked: whether it is safe, whether it should be there, or where its water goes. Every report is checked and signed off by a senior consultant before it goes out; our consultants are chartered and Unda is a CIWEM Business Partner. Each of these is available for sites across Hertfordshire:

• Every source together. Chalk rivers, surface water on the clay, groundwater and sewers can all reach one plot, and a flood risk assessment weighs them in a single report.
• Planning applications. From a Watford town centre flat to a St Albans allocation, a flood risk assessment for planning answers the district's flood policy and chapter 18 of the NPPF.
• Buying a home. Homes in the Colne, Ver, Stort and Purwell warning areas can be flagged by a search or a lender, and a flood risk survey for property purchase explains the flag before exchange.
• Sites in Flood Zones 2 and 3. Where a scheme must show no lower-risk site is available in the district, a sequential and exception test report sets the area of search and tests the alternatives.
• Drainage for planning. The county council reviews the drainage on every major scheme in Hertfordshire, and a drainage strategy shows where the water will go before it does.
• Holding back runoff. Greenfield rates, storage for the 1 in 100 year storm and BRE 365 results on the chalk all go into a surface water drainage strategy.
• After permission. Where a Hertfordshire consent leaves the detail to a condition, the discharge of drainage conditions supplies the calculations and maintenance plan the council needs to clear it.
• Insurance. With 62,878 properties at medium surface water risk in the county strategy, an owner facing a higher premium after a flood such as Welwyn Garden City's can use a flood risk assessment for insurance.
• Groundwater. Where the chalk is near the surface, as around Kimpton, Lilley Bottom and Flamstead, a groundwater flood risk assessment looks at levels and monitoring in more depth.
• Rivers without detailed maps. On the Ver, the Gade or a smaller brook where the national map is coarse, flood modelling sets levels for the site itself.
• Living with a warning. For homes in the Colne at Watford or Stort at Bishop's Stortford warning areas, a flood warning and evacuation plan sets out what occupants do when one is issued.
• Foul sewers. Bovingdon's 2016 flood got into the foul sewer and Welwyn Hatfield bars surface water from combined sewers, so a foul drainage strategy keeps the two systems apart.
• Land being promoted. With 15,000 homes planned in St Albans and new plans in preparation in Dacorum and Welwyn Hatfield, a flood risk feasibility study tests a site before money is spent on it.
• National policy. Our NPPF flood risk guidance explains how chapter 18 and its policies F4 to F8 apply alongside the district plans in the table above.

Hertford, Ware and the counties around Hertfordshire

Hertford and Ware have pages of their own, as does Hemel Hempstead, and the counties on every side have theirs:

• The Lea towns. Hertford, the county town, and Ware, just downstream on the Lee, each have a page on their rivers, records and East Hertfordshire sites, and Waltham Cross, where the Lee leaves the county in Broxbourne, has its own.
• To the north and west. Bedfordshire shares the Ivel and the Lea's source, and Buckinghamshire the Chess and the Chilterns chalk.
• To the east and south. Essex shares the Stort at Harlow, Cambridgeshire the Rhee and the Cam, and Middlesex the lower Colne and Lea.
• Everywhere else. Every place we cover is on our areas list.

Reporting a flood in Hertfordshire, and who answers

If anyone is in danger from floodwater, call 999. For a power cut, call 105. Otherwise, who to report to depends on where the water came from.

The county's ordinary watercourses page explains the consent needed for work on a stream or ditch, and our guide to ordinary watercourse consent goes through an application.

Questions Hertfordshire applicants and buyers put to us

Our consultants assess flood risk, run sequential and exception tests and write drainage strategies for sites anywhere in Hertfordshire, from Tring and Berkhamsted to Royston and Sawbridgeworth. To discuss one, call 01293 214444 or email enquiries@unda.co.uk.