River Itchen Flooding & Flood Risk

River Itchen flooding comes in two forms that have little in common. Above Woodmill, on the northern edge of Southampton, the Itchen is a chalk stream: it rises over weeks once the aquifer under the Hampshire downs is full, and around Winchester in early 2014 it stayed high for three months. Below Woodmill it is a tidal estuary, where a spring tide with a Solent surge behind it can put water into Northam or St Denys in a few hours. This page is about the Hampshire Itchen, which runs about 47 km from springs near Hinton Ampner to Southampton Water. The River Itchen in Warwickshire, a tributary of the Leam, is a different river.

The highest flow gauged on the Itchen at Highbridge, near Eastleigh, is 22.5 cubic metres a second, on 12 February 2014. That is only four times the river's long-term average, yet the flow stayed above twice the average on 89 of the 94 days from 4 January to 7 April that year.

We assess flood risk and drainage for sites along the whole river, from the Cheriton springs to the Southampton wharves. Call 01293 214444 or email enquiries@unda.co.uk with the address and what is planned.

Why does the River Itchen flood slowly and stay high for weeks?

Because nearly all of its water has been through the chalk first. The National River Flow Archive gives the Itchen at Highbridge a base flow index of 0.96, meaning about 96% of the flow comes out of groundwater storage and very little runs straight off the land (NRFA station 42010). A storm barely moves the river. A wet autumn and winter fill the aquifer, the springs strengthen, and the flow climbs for weeks and falls just as slowly.

• A catchment bigger underground than on the map. The Archive puts the land draining to Highbridge at 360 km², and the groundwater catchment feeding the same gauge at about 490 km². Rain that falls outside the valley still reaches the river through the chalk.
• A flood that lasts a season. In the winter of 2000 to 2001 the daily flow at Highbridge stayed above twice its 5.6 m³/s average for 176 days in a row, from 6 November to 30 April (NRFA gauged daily flows). Land that floods here can stay wet from autumn to spring.
• Banks only a foot high. The Archive describes the banks at the gauge as 'very low, typically 20-30cm', and notes that weed growth often holds summer levels above winter ones. A modest rise in flow is enough to spread shallow water across the meadows, even though, in the Archive's words, 'flooding on any scale is rare'.
• Groundwater in cellars and lanes. When the water table reaches the surface it floods cellars, gardens and lanes well away from the channel. The Environment Agency runs five separate groundwater alert areas in the valley, among them Littleton, the Worthys, Easton and Chilland and Twyford and Hensting. Our guide to groundwater flooding explains how it differs from a river flood.
• Mills, sluices and carriers. Through Winchester and again below it the Itchen divides into mill streams, water-meadow carriers and the old Itchen Navigation. How the sluices are set decides which channel takes the water, and the Environment Agency credits a co-ordinated programme of flow management early in the 2014 flood with keeping water moving through the city (Environment Agency, October 2015).

When has the River Itchen flooded?

The two largest river floods in a gauged record that starts in 1958 came in the winters of 2000 to 2001 and 2013 to 2014, both after months of rain had filled the chalk. The tidal reach in Southampton has its own, separate record: the city council lists significant tidal flooding in 2005, 2008 and 2014, and the tide came over again in April 2024.

1. Winter 2000 to 2001. Flow at Highbridge peaked at 20.0 m³/s on 13 December 2000, a figure the Archive calls a 'remarkable peak flow confirmed by gauging', and Allbrook weir reached 0.66 m four days later, still its record. The Environment Agency's Test and Itchen catchment plan (December 2009) records up to 300 properties flooded across the two catchments that winter, with river flooding in Winchester and groundwater flooding in the chalk villages.
2. December 2013 to February 2014. Five of the river's nine level gauges set records that still stand: Bishopstoke on 24 December, Wharf Mill in Winchester on 7 January, and City Mill, Easton and Highbridge on 12 February, the day the flow peaked. In Winchester the Environment Agency, the councils, the emergency services and the military held the river with temporary barriers, sandbag walls, pumps and temporary dams upstream of the city (Environment Agency, October 2015). Winchester City Council counted 15 flooded properties in the town and the Agency 22 (cabinet report CAB2636, December 2014).
3. 8 to 9 April 2024. The tide overnight gave Southampton what the city council called 'the most significant flooding in over 20 years of records', roughly a 1 in 50 year tide (council report, December 2024). The river upstream was not in flood; this was the sea.

Smaller river peaks followed in January 2003 (16.2 m³/s at Highbridge), February 2020 (16.0) and January 2024 (16.1). Winchester's temporary line of 2014 has since been made permanent: a wall along Water Lane, then defences at North Walls, and in October 2021 the Durngate scheme, which the city council says completed an unbroken 950 m of defence from Wales Street to Gordon Road (Winchester City Council). Hampshire County Council, as lead local flood authority, investigates significant floods under section 19 of the Flood and Water Management Act 2010.

River Itchen gauges and the live map

The coral line follows the Itchen from its head near Hinton Ampner through Alresford, Winchester and Eastleigh to Southampton Water, with the Environment Agency's gauges pinned along it. Click a pin for the latest reading, or enter a postcode to see the flood zones around it. Every gauge in England is on our live river levels map.

Durngate and Stoke Lock give height above sea level, not above a local zero, which is why their figures look so much larger. The ranges are narrow: at City Mill the record is half a metre above the top of the normal range. On a river like this a few centimetres on the gauge can mean a long way across the meadows. The flow gauges show the same steadiness, with a long-term mean of 4.4 m³/s at Easton and 5.6 m³/s at Highbridge (NRFA 42016, NRFA 42010). At Riverside Park the Agency says property flooding is possible above 2.89 m, a level the gauge has not reached since it opened in 1999.

What changes below Woodmill, where the Itchen turns tidal?

Below Woodmill Bridge the Itchen is an arm of the sea for its last 7 to 8 km, past St Denys, Bitterne Park, Northam and Woolston to Southampton Water. River flow matters little here. The risk is a high spring tide lifted by a surge in the Solent, and it grows as sea level rises: Southampton City Council puts more than 150 homes and more than 200 businesses at tidal risk on the west bank today, and over 1,100 homes and 900 businesses by 2115 (RIFAS, Southampton City Council).

• No raised defence on the west bank. The River Itchen Flood Alleviation Scheme was to be a 3.7 km defence from Mount Pleasant Industrial Estate, north of Northam Bridge, to just south of the Itchen Bridge. Its estimated cost rose from £29.8m in early 2022 to about £96.7m, leaving a gap of some £50m (council report, December 2024). After the Environment Agency withdrew its funding, the council announced on 12 May 2025 that it had temporarily halted the project.
• What that means for a site. Land in Northam, St Mary's, Chapel or St Denys is assessed against the undefended tide, with sea level rise added for the lifetime of the building. Our articles on tidal flooding and sea level rise set out how those levels are derived.
• Where river and tide meet. Mansbridge and Woodmill sit at the tidal limit, where a high tide and a full river can coincide. The Environment Agency keeps a river warning area for this short reach, separate from the estuary one further down.
• The rest of the city. The docks, the Test frontage and the council's planning policies are covered on our page about a flood risk assessment in Southampton.

Who is at risk along the River Itchen, source to sea?

From Cheriton to Bishopstoke the risk is a chalk river and the groundwater that feeds it; from Mansbridge to Woolston it is the tide. The Environment Agency divides the river into four river warning areas, one tidal warning area and four alert areas, with separate warnings on the Cheriton Stream, the Alre and the Candover Stream. The table runs from source to sea.

The Agency's 2009 catchment plan counted 599 properties in Winchester and 281 on the lower Itchen around Bishopstoke and Eastleigh with a 1% chance of flooding in any year from the river or groundwater (Test and Itchen CFMP). To see which flood zone an address falls in, use our flood risk map.

Is the River Itchen a Main River, and what does that mean beside it?

Yes. The Environment Agency's Statutory Main River Map shows the Itchen as Main River for its whole length, from its head near Hinton Ampner to Southampton Water (checked 10 November 2026), so the Agency's Solent and South Downs area regulates the channel. Works in, over or near it may need an environmental permit from the Agency: within 8 metres of the bank on the non-tidal river above Woodmill, and within 16 metres on the tidal river below it or of a tidal flood defence (GOV.UK). Our guide to Main Rivers and ordinary watercourses explains the difference.

• Check which channel the site is on. The valley floor carries mill streams and carriers beside the river. Some are Main River too; others are ordinary watercourses, where consent for works comes from Hampshire County Council, or from Southampton City Council inside the city.
• The Itchen Navigation alongside. The Navigation was cut in the late 17th and early 18th centuries to bring barges up to Winchester and fell out of use in the mid-19th. It still carries water beside the Itchen between Winchester and Woodmill, in places a field away from the river and at a different level, so a site can front one and flood from the other.
• A protected chalk stream. The Itchen is a Special Area of Conservation and a Site of Special Scientific Interest from its source to Swaythling. Works that touch the channel or its banks bring in Natural England as well as the Environment Agency, and new homes in the catchment have to show nutrient neutrality.
• If the bank is yours. Whoever owns land beside the Itchen or one of its carriers has duties to keep the flow clear. They are set out in our article on riparian ownership.

What a site in the Itchen valley or on its estuary may need from us

Three different reports come up on this river, often together. One assesses whether the site is safe from the Itchen, the chalk beneath it and the tide; one tests whether the development could go on land at lower risk; one shows where the rain that falls on the site will go. The work each involves depends on the reach.

• Any reach, every source. A flood risk assessment for land here weighs the river, the groundwater under it and, below Woodmill, the tide.
• Applications to Winchester, Eastleigh or Southampton. A flood risk assessment for planning sets floor levels and a safe route out against the Agency's modelled levels for that reach, as chapter 18 of the National Planning Policy Framework expects.
• Buying near the water. A flood risk survey for property purchase reads a house in Twyford, Bishopstoke or St Denys against the 2014 record, the defences built since and the tide.
• Land in Flood Zones 2 and 3. Where a scheme sits on the valley floor or the estuary frontage, a sequential and exception test shows whether lower-risk land was reasonably available and, if not, how the site is made safe.
• Rain on chalk. A drainage strategy for an Itchen site has to respect a protected river downstream, which limits both how much water leaves the site and how clean it is.
• Soakaways over a winter water table. Chalk drains freely in August and can be saturated to within a metre of the surface in February, so a surface water drainage strategy here depends on winter groundwater levels as much as on infiltration tests.
• After permission. Where a consent comes with a drainage condition, such as groundwater monitoring through a winter before work starts, we prepare the evidence for the discharge of drainage conditions.
• Channels the national model leaves out. Winchester's culverts and sluices, the carriers below it and the meeting of river and tide at Woodmill are where flood modelling gives levels for the site itself.
• Five groundwater alert areas. For basements, deep foundations and soakaways in the chalk villages, a groundwater flood risk assessment sets out how high the water table has come and for how long.
• An estuary with no raised defence. A flood warning and evacuation plan ties a building in Northam, Millbank or Crosshouse to the Itchen estuary warning area and a route to dry ground.
• Homes that flooded in 2014. Owners in Winchester and Twyford who are asked about that winter by an insurer can answer with a flood risk assessment for insurance based on the property's own levels.
• Businesses on the west bank. With more than 200 businesses at tidal risk there, a business continuity flood assessment plans for a tide that shuts the yard, the access road or the power.
• Wharves and industrial estates. Before a purchase or a loan on commercial land between Mount Pleasant and the Itchen Bridge, flood risk due diligence sets out the exposure now that the defence scheme is on hold.

Who manages flood risk on the River Itchen?

The Environment Agency looks after the river and the tide, two lead local flood authorities look after everything else, and Southern Water runs the sewers. In an emergency where life is at risk call 999, and call 105 if the power fails.

• Warnings. The Environment Agency's Solent and South Downs area issues the alerts and warnings for the Itchen. Floodline is on 0345 988 1188, and anyone can sign up for flood warnings.
• Surface water, groundwater and smaller watercourses. Hampshire County Council is the lead local flood authority for the Winchester and Eastleigh districts, and Southampton City Council for the city.
• Plans. The river sits in the South East river basin district, under the Agency's flood risk management plan for 2021 to 2027. The estuary frontage also falls under the North Solent Shoreline Management Plan.
• Funding. The Southern Regional Flood and Coastal Committee approves the programme of flood schemes and the local levy that helps to pay for them.
• Local advice. Winchester City Council publishes flooding advice for the city, and the Hampshire and Isle of Wight Local Resilience Forum covers preparing for a flood across the county.

The Itchen is one of the watercourses in our guide to river flooding in England.

River Itchen flooding: your questions

Tell us where the site is and what you have in mind, and we will say what it needs. Ring 01293 214444 or write to enquiries@unda.co.uk. Every report for an Itchen site, whether it is a barn conversion at Itchen Abbas or a warehouse in Northam, is prepared by Unda Consulting Limited, a CIWEM Business Partner.