Flood Risk Assessment in Hemel Hempstead

For a flood risk assessment in Hemel Hempstead, the storm drains matter as much as the rivers. The River Gade and the River Bulbourne meet in the valley at Two Waters, beside the Grand Union Canal, but the floods Hertfordshire County Council has investigated in the town came from storms that overwhelmed the drains in under two hours. We prepare flood risk assessments in Hemel Hempstead for planning, property purchase, mortgage and insurance, along with sequential and exception tests and surface water drainage strategies.

Most people just say Hemel. The new town designated in 1947 grew outwards from the Old Town and the Marlowes valley into neighbourhoods on the chalk slopes: Gadebridge, Warners End and Chaulden to the west, Boxmoor, Felden and Corner Hall to the south-west, Adeyfield, Highfield, Grovehill and Woodhall Farm to the north and east, and Bennetts End, Leverstock Green and the Maylands business area towards the M1. Apsley, Nash Mills and Two Waters sit on the valley floor in the south. Addresses carry HP1, HP2 or HP3 postcodes. Kings Langley, just down the Gade, is outside the town's built-up area but shares its river, its council and an Environment Agency warning area.

On 16 September 2016, 48 mm of rain fell on Apsley in an hour and three-quarters, a storm the county council's investigation puts at about a 1 in 100 year event. The same morning 30 properties flooded around Ryder Close, Rymill Close and London Road.

If you have a Hemel site in mind, speak to one of our flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

Storms on the drains: what Hemel's flood investigations recorded

Dacorum's 2024 Strategic Flood Risk Assessment records significant surface water flooding in Hemel Hempstead in recent years, and most of the flood incidents the county has logged in the borough are in the town. The county investigates under Section 19 of the Flood and Water Management Act when five or more homes flood inside in one event, and three Hemel investigations have been published.

1. 16 September 2016, Apsley. 48 mm fell in 1.75 hours in the early morning. Four commercial properties on London Road and a house on White Lion Street flooded internally when the highway drains could not take the water, and the SFRA adds that a blocked trash screen and waves from passing cars made it worse. The county's investigation records repeat flooding at the spot.
2. 16 September 2016, Ryder Close to Green Lane. The borough's flood history lists 30 properties flooded around Ryder Close, Rymill Close, Eastnor, London Road and Green Lane, some with water up to 18 inches deep inside, and the canal overtopped at Pix Farm near Bourne End. In the same month two homes on Fishery Road flooded from the Bulbourne and the Gade.
3. 23 November 2017, Adeyfield. Surface water closed the roads at Corner Hall and Longlands.
4. 12 August 2020, Boxmoor. 38.9 mm in thirty minutes, peaking at almost 107 mm an hour, flooded five homes on Kingsland Road, Fishery Road, Horsecroft Road and St Johns Road. The Boxmoor investigation puts the storm at a 0.38% annual chance, against drains built for about 5%, and notes earlier flooding there in January 2007 and again in October 2020.
5. 3 and 4 October 2020, Kings Copse. During Storm Alex five homes on Kingcup Avenue, Chervil Road and Blackberry Way flooded inside. The Kings Copse investigation traces the water to the overflow of a Thames Water balancing pond above the estate, which had also flooded new homes in 2014.

Four of the six Boxmoor properties sit where the national map shows very low surface water risk, which is why a Hemel assessment reads the county's incident record as well as the Environment Agency layers. The Dacorum Surface Water Management Plan of 2017 named Highfield as one of four hotspots in the borough, and Thames Water's sewer records put 17 incidents in each of the HP3 0 and HP3 9 postcode sectors, which cover the southern half of the town. Our note on Section 19 flood investigations explains what these reports can and cannot be used for.

Screening an HP1, HP2 or HP3 address

Enter a postcode below to see the river, surface water and reservoir layers at that point, or open our full flood risk map. The county's Flood Risk Management Viewer adds recorded incidents and areas where groundwater can come up.

Where the Gade, the Bulbourne and the canal meet at Two Waters

Both rivers are chalk streams and both are main rivers, so the Environment Agency regulates them. The Gade comes down from Great Gaddesden past Piccotts End and through Gadebridge Park and the Water Gardens. The Bulbourne arrives from Berkhamsted and Bourne End along the line of the canal. They join at Two Waters, and the 2024 SFRA puts the town's main river risk in the south, around Apsley, where the combined flow carries on towards Nash Mills and Kings Langley. Steep valley sides keep most of Flood Zone 3 close to the channels.

The Environment Agency runs four flood warning areas for the town's rivers and measures the Bulbourne at two points. Their published records show the September 1992 storm as the highest on both Bulbourne gauges, above anything seen in the wet winter of 2014. Current readings are on our live river levels map.

Downstream the Gade joins the Colne near Rickmansworth, and our River Colne page follows the water from there to the Thames at Staines.

Chalk groundwater, source protection and the Buncefield legacy

Hemel stands on the Chalk, a principal aquifer that supplies much of Hertfordshire's drinking water, mostly under a cap of clay-with-flints. Large parts of Dacorum lie in groundwater source protection zones, and the adopted Core Strategy's Policy CS31 says development must not harm them. On the valley floors the SFRA's groundwater map puts the water table at or within 25 mm of the surface in a 1 in 100 year event, so a basement or soakaway beside the Gade or Bulbourne needs ground evidence before anyone relies on it. Our guides to groundwater source protection zones and BRE 365 infiltration testing cover both questions, and the ground conditions checker gives a first read of the geology under a postcode.

Buncefield adds a second, less common constraint in the east of the town. When the Buncefield fuel depot exploded and burned on 11 December 2005, firefighters used about 68 million litres of water and 786,000 litres of foam concentrate containing PFOS. According to an Environment Agency paper published in 2013, most of that water was never recovered: it soaked into the ground and polluted the chalk aquifer below the site, a public supply borehole about 3 km away was closed, and contaminated groundwater was still being pumped out and treated at about £1 million a year. On Maylands or the Herts IQ land, a drainage strategy that relies on soakaways should show they will not spread contamination already in the ground.

Do I need a flood risk assessment in Hemel Hempstead while the new plan is examined?

Yes for any development in Flood Zones 2 and 3, and for a site of a hectare or more in Flood Zone 1. A smaller Flood Zone 1 site can need one too where the SFRA shows surface water risk, which in Hemel is common. Dacorum Borough Council decides applications against its 2013 Core Strategy, while the Local Plan to 2041 has been with inspectors since March 2025: hearings ran into March 2026 and the inspectors were still consulting on further documents in September 2026. Hertfordshire County Council reviews the surface water drainage of every major scheme as lead local flood authority.

Two SFRA figures set the flood levels in a Hemel report. For the Colne catchment, which includes both of the town's rivers, the higher central allowance for peak river flow in the 2080s is 35% and the central allowance 21%. The plan's 10 m strip beside a watercourse is wider than the 8 m that Watford, Three Rivers and Stevenage ask for, and on a narrow valley-floor plot it can decide the layout. Our explainer on NPPF climate change allowances shows how the percentages reach a floor level.

Two Waters, the town centre and North Hemel: where the allocations meet the water

Hemel Garden Communities, a joint programme of Dacorum and St Albans councils, plans up to 11,000 homes and about 10,000 jobs by 2050 to the north and east of the town, around 6,000 of the jobs at Herts IQ beside Maylands. Dacorum's submitted plan adds about 2,000 homes in the Two Waters opportunity area and about 1,500 in the town centre, which puts much of the new building in the valley.

• Riverside, Hm06. About 500 homes on 1.79 ha by the Gade in the town centre. The plan says no built development in Flood Zones 2 and 3, a 10 m ecological buffer to the river, and a check on the bridges up and downstream in case they block.
• Apsley Mills Retail Park, Hm10. About 500 homes from 2035/36 between the Gade and the canal. The assessment has to answer groundwater risk and include detailed emergency plans and evacuation routes.
• Civic Centre, Hm02. About 200 homes. The Level 2 SFRA would allow building in the Flood Zone 1 parts, with groundwater, surface water and safe escape in an extreme storm left to the site assessment.
• Station Gateway, Hm08, and the National Grid site, Hm09. Around 390 homes at the station, low river risk, with the surface water risk and, at Hm09, safe access to be settled at the application stage.
• North Hemel Hempstead, Hm01. At least 1,500 homes by 2041 and 5,000 in all, from 2032/33, on land beyond Grovehill and Woodhall Farm. River risk is low; the Level 2 study leaves present and future surface water risk to the site assessment.
• East Hemel and Herts IQ. The homes and employment land east of the town lie in St Albans district and are decided under that council's plan, not Dacorum's.

The plan to 2041 is not adopted and its allocations may change at examination, so a promoter needs flood evidence that holds under either plan. Once it is adopted, an allocated site that reaches Flood Zone 2 or 3 does not normally need the sequential test again for the use it was allocated for; until then, a scheme in those zones should expect to run it.

Which report does a Hemel Hempstead proposal need?

Our page on the flood evidence a planning application needs sets out what goes into each, and our note on house extensions in a flood zone explains when standing advice is enough.

Reports for Hemel sites, from Gadebridge to Maylands

A house in Boxmoor, a block in the town centre and a warehouse on Maylands each raise a different question, and each report below answers one of them. A senior consultant signs off each one before it is issued; the consultants who write them are chartered, with Unda a CIWEM Business Partner.

• Planning applications in Dacorum. New homes and commercial schemes need a flood risk assessment for planning that answers CS31 now and NE9 once the new plan is adopted.
• Every source in one report. A flood risk assessment for a Hemel site weighs the Gade, the Bulbourne, the canal, storm runoff, the chalk water table and the sewers together.
• Before exchange. In Boxmoor, Apsley or the Kings Copse estate, a flood risk survey for property purchase tells a buyer and the lender what the 2016 and 2020 floods mean for that house.
• Homes on the valley floor. Unallocated housing in Flood Zone 2 or 3 near Two Waters needs a sequential and exception test report that sets the area of search and tests the alternatives.
• The county's review. Hertfordshire County Council reads the drainage on every major Hemel scheme, and a drainage strategy shows it where the water goes and at what rate.
• Storms like Apsley's. Storage sized for the 1 in 100 year storm plus climate change, BRE 365 results and the outfall all go into a surface water drainage strategy.
• Conditions on a Dacorum consent. When a permission leaves the drainage detail to a condition, the discharge of drainage conditions supplies what the county asks for to clear it.
• Chalk and Buncefield. Where the water table rises near the surface or the ground under Maylands may be contaminated, a groundwater flood risk assessment looks at levels, monitoring and source protection in detail.
• Levels the national map cannot give. Next to the 5-year walls at Two Waters or a bridge on the Gade that could block, flood modelling sets flood levels and depths for the site itself.
• Escape routes the SFRA asks for. The Level 2 study wants emergency plans for Apsley Mills, the Civic Centre and Riverside, and a flood warning and evacuation plan ties a scheme to the Gade or Bulbourne warning area.
• Buying on Maylands. Before a business buys or lends on a unit near the depot or the valley floor, flood risk due diligence sets out the flood and groundwater position.
• Homes that have flooded. After repeat flooding on Kingsland Road or White Lion Street, a flood risk assessment for insurance gives the insurer evidence about that one property.
• Land in Hemel Garden Communities. Before an option or a promotion agreement on land north or east of the town, a flood risk feasibility study checks the flow paths, the ground and the drainage outfall.

Rivers, drains, sewers and the canal: who answers for each in Hemel

Call 999 if flood water puts anyone in danger, and 105 to report a power cut. For anything else, the right body depends on where the water came from, and in Hemel that is more often a drain than a river.

A government decision in July 2026 to replace Hertfordshire's councils with four unitary authorities was paused in September 2026 for a national review, so for now Dacorum and the county keep their present roles. Our note on local government reorganisation covers what a change of authority means for a live application.

What Hemel buyers, landowners and agents ask us

Our consultants assess flood risk, run sequential and exception tests and write drainage strategies for sites across Hemel, from the Old Town to Nash Mills. To talk one through, call 01293 214444 or email enquiries@unda.co.uk. For the rest of the county see our Hertfordshire page; Hertford has a page of its own on the Lea, while in Buckinghamshire we cover Aylesbury and High Wycombe. Towns and counties beyond Hertfordshire are listed on our areas page.