Flood Risk Assessment in Manchester

A flood risk assessment in Manchester has to deal with four rivers and the culverts beneath the city. The Mersey runs along the southern edge past Didsbury and Chorlton, the Irk and the Medlock cross the north and east to reach the Irwell, and surface water spreads across streets the 2026 Strategic Flood Risk Assessment describes as highly urbanised. Our consultants prepare assessments for planning applications, purchases, lenders and insurers, together with sequential and exception tests and surface water drainage strategies.

On this page Manchester means the city's built-up area as the Office for National Statistics draws it: the city centre with Ancoats and the Northern Quarter (M1 and M4), Hulme and Moss Side, Cheetham Hill, Crumpsall and Blackley to the north, Ardwick, Gorton and Clayton to the east, and Fallowfield, Withington, Chorlton-cum-Hardy (M21) and Didsbury (M20) to the south, as far as the Mersey. Wythenshawe and Northenden, across the river, form a separate built-up area, although the same city council plans and drains them. Salford, on the far bank of the Irwell, is a separate city with its own council.

Over New Year 2025, 81.4 mm of rain fell in about 17 hours, the Mersey set new records at Didsbury and Northenden, and 60 homes and 29 businesses in Manchester flooded inside.

For a first conversation about a Manchester site, our flood risk consultants are on 01293 214444 and at enquiries@unda.co.uk.

Is Manchester at risk of flooding? The Mersey, the Irk, the Medlock and the brooks

Yes, and from more than one direction. Manchester City Council's June 2026 SFRA, prepared by JBA Consulting, names river and surface water flooding as the most significant sources in the city. The rivers that matter most are the Mersey, the Medlock and the Irk, with the Irwell along the Salford boundary and a group of south Manchester brooks, three of which (Chorlton, Platt and Gore Brooks) are classed as main rivers. The Environment Agency runs flood warning areas on each river and on the brooks inside the built-up area.

Away from the rivers, surface water decides many sites. Heavy rain on roofs and roads reaches the drains faster than the drains and sewers can take it, so a Manchester site can sit in Flood Zone 1 for rivers and still show high surface water risk on the Environment Agency's mapping. Groundwater, the canals and the reservoirs upstream add residual risks, set out further down.

Screen a Manchester postcode, from Crumpsall to Didsbury

Type a Manchester postcode into the live map below and it reports the planning flood zone and the Environment Agency's modelled risk from rivers, surface water and reservoirs; for anywhere else in England, use our full flood risk map. Used as a flood risk map of Manchester, it shows at a glance whether a street lies near the Mersey, the Irk or one of the brooks. The coral line marks the built-up area described above, so Wythenshawe and Northenden sit just beyond it.

Culverts, canals and sewers under the city centre

Much of Manchester's flood risk is out of sight. The industrial city put stretches of the Medlock, the Irk and Corn Brook underground, cut three canals through its centre and laid a dense sewer network over them, and any of these can flood a site that looks dry on a river map.

• Lost watercourses. The Greater Manchester SFRA records many unknown watercourses running through old culverts and tunnels beneath the city centre, with their condition and exact routes unknown. A culvert under or beside a plot can surcharge, collapse or block, and the draft Local Plan asks for further investigation wherever the council's asset register shows one beneath a site.
• Rochdale, Ashton and Bridgewater canals. The 2026 SFRA asks for a site-specific breach assessment for development next to any of the three. The older Manchester, Salford and Trafford SFRA drew a canal hazard zone across much of Ancoats, and the Bridgewater Canal takes inflow from the Medlock, which is why it overflowed at Deansgate in January 2025. Our note on canal breach risk explains what such an assessment involves.
• Sewer flooding. Chorlton ward has the highest recorded incidence of sewer flooding in the city, and the SFRA puts Chorlton-cum-Hardy, Withington, West Didsbury and Levenshulme at the highest risk. United Utilities owns the public sewers, and the SFRA advises developers to ask it directly about the sewers serving a site; our guide to sewer flooding covers the background.
• Groundwater. Groundwater rises close to the surface in parts of Withington and Fallowfield, Chorlton-cum-Hardy, Moss Side and Northenden, and in the city centre near the universities. That matters for basements and for any drainage that relies on soakage, and our ground conditions and soakaway checker gives a first read of the geology.
• Reservoirs. Six reservoirs lie inside the city and the SFRA counts 72 more beyond it that could send water into Manchester if one failed, a residual risk a site-specific assessment is expected to address.

Manchester flooding from Boxing Day 2015 to New Year 2025

Three floods in ten years hit different parts of the city: the Irwell and the Irk in 2015, the Mersey in 2021, and almost everything at once over New Year 2025.

1. 26 December 2015. Storm Eva lifted the Irwell to 5.67 m at Manchester Racecourse. It overtopped raised defences and banks at Cheetham, where 27 premises flooded inside, and the Irk flooded ten businesses at Hendham Vale to depths above 600 mm. Across Greater Manchester the Section 19 report counted over 2,200 properties flooded internally in eight districts.
2. 28 July 2019. The Medlock reached 3.37 m at Gurney Street in Ancoats, still the highest level that gauge has recorded.
3. 20 to 21 January 2021. During Storm Christoph the Mersey broke its records, the Environment Agency reported. The Environment Agency used the Didsbury flood basin, which protects around 3,000 properties, and its Flood Hub project page records about 2,000 properties evacuated along the river in south Manchester and neighbouring boroughs.
4. February 2022. The Didsbury and Sale Ees flood storage basins were used again.
5. 30 September 2024. The Irk at Collyhurst Weir peaked at 1.95 m, its highest level on record.
6. 31 December 2024 to 1 January 2025. Greater Manchester declared a major incident at 4am on New Year's Day. The council's flooding report counts 60 homes and 29 businesses flooded inside, and 62 more properties with flooding outside only. Mersey embankments breached into the Didsbury flood storage reservoir, 445 people were evacuated from the Britannia Hotel, a retaining wall on the Irk collapsed in Harpurhey and Crumpsall, and the Medlock and the Bridgewater Canal overflowed at Deansgate.
7. 2025 and 2026. The Environment Agency repaired 400 m of damaged embankment at the Didsbury sports ground and mended banks at Northenden Golf Club and behind the Britannia Hotel, and in September 2026 it asked residents to respond to a "case for change" for the Mersey through south Manchester.

The Environment Agency's gauges at Didsbury and Northenden on the Mersey, Collyhurst Weir on the Irk and London Road and Gurney Street on the Medlock report levels through the day, and our live river levels map puts them on one screen with the warnings in force, which covers flood monitoring across Greater Manchester in one place. The council's report on the 2025 flooding is a Section 19 investigation, the inquiry a lead local flood authority must make after significant flooding.

Flood protection on the Mersey at Didsbury, and what it does not change

Flood protection in south Manchester depends on embankments along the Mersey and a flood storage reservoir at Didsbury that fills when the river runs high. In 2021 the Environment Agency put the basin's capacity at more than 310 Olympic swimming pools and the properties it protects at around 3,000, and The Flood Hub credits debris-screen clearance on the brooks with protecting about 5,800 properties in south Manchester.

Those defences do not take land out of the flood zones. The Flood Map for Planning is drawn as if they were absent, and New Year 2025 showed why: the design standard was exceeded in places, banks were overtopped and some sections collapsed. The 2026 SFRA adds that no mapping yet shows where Manchester is at residual risk if a defence fails, so an assessment behind the Mersey embankments works out the residual flood risk for the site itself. The Greater Manchester SFRA lists the floodplain between Didsbury and Northenden, much of it open land, among its "areas of considerable fluvial risk".

The Environment Agency's River Mersey strategy for south Manchester, covering Didsbury, Northenden and Heaton Mersey along with the Stockport and Trafford reaches, expects to finish its second stage in 2027 if funding allows.

Policy for a flood risk assessment in Manchester: EN14, Places for Everyone and the new Local Plan

Manchester City Council decides applications as the planning authority and, as lead local flood authority, is the statutory consultee on drainage for major development; the Environment Agency advises on sites in Flood Zones 2 and 3 and within 20 m of a main river. The local policy sits in three layers, one of them about to change.

Above all of these sits chapter 18 of the August 2026 NPPF: policy F4 sets when an assessment is needed, F5 and F6 the sequential and exception tests, and F8 expects every development to follow the 2025 National Standards for sustainable drainage.

On the sequential test, the 2026 SFRA spells out what Manchester expects. Allocations already tested in the plan, most changes of use, householder work and small non-residential extensions are exempt, but splitting a house into flats is not minor development and is tested. A landowner's wish to build on one site is not a reason to ignore alternatives. For sites at risk only from surface water, a proportionate approach applies where the layout keeps occupiers safe, as our note on the surface water exemption explains.

Critical drainage areas and runoff rates in Manchester

Drainage carries as much weight as river risk in Manchester. Heavy rain already overwhelms the sewers in places, the council no longer takes surface water into them as of right, and the city has mapped critical drainage areas where surface water already causes trouble. The rules the council applies come from the Greater Manchester SFRA and Places for Everyone, and the draft Local Plan repeats the main ones.

• Greenfield rates first. Places for Everyone expects every development to match greenfield runoff unless that is shown to be impracticable. On a greenfield site that means no more than today's runoff, up to the 1 in 100 year storm with climate change.
• Brownfield sites. Where greenfield rates cannot be met, the critical drainage area rules ask for runoff 50% below the existing rate, and the draft Local Plan applies the same 50% cut to brownfield land across the city.
• Inside a critical drainage area. An assessment for any development over 0.5 ha, laid out so the site does not flood in a 1 in 30 year event and no building floods in a 1 in 100 year event with climate change.
• Connecting to the sewer. The council states that new development will no longer be allowed to connect automatically to surface water or combined public sewers, so infiltration and watercourses come first.
• Into a brook or ditch. Connecting surface water to an ordinary watercourse, or altering one, needs the council's consent, explained in our guide to ordinary watercourse consent.
• The pro-forma. Greater Manchester councils use the North West SuDS pro-forma, and our page on the SuDS pro-forma shows what it asks for.

In practice a cleared mill site in Ancoats has to reach greenfield rates or, failing that, halve its outflow, which on a tight plot means storage below a courtyard or on the roof, while a new home in Withington has to prove the ground drains before a soakaway is drawn. Infiltration tests to BRE 365 settle that, and our BRE 365 infiltration rate calculator turns the readings into a rate.

Manchester schemes that need a flood risk assessment, from towers to basements

Do you need one? Yes for any scheme in Flood Zone 2 or 3, for any site of a hectare or more, for land the mapping shows at risk from another source, and, under the critical drainage area rules, for sites over 0.5 ha in those areas. These are the Manchester cases where it matters.

• Towers and flats near the Irk or the Medlock. City-centre and Victoria North sites in Flood Zone 2 or 3 need a full assessment with floor levels, safe access and the sequential test for new homes.
• Houses near the Mersey or a brook. Plots in Didsbury, West Didsbury, Fallowfield or Chorlton often fall in a warning area or Flood Zone 3, so even an extension needs a proportionate assessment.
• Next to a canal. Ancoats, New Islington and Castlefield sites beside the Rochdale, Ashton or Bridgewater canal need the canal breach and overtopping risk assessed.
• Basements. A basement in a groundwater hotspot or under a surface water flow route needs both checked, as our note on basement and cellar conversions explains.
• Conversions to homes. Turning offices or a mill into flats in Flood Zone 2 or 3 is a change to a more vulnerable use and needs an assessment even without building work.
• Small infill in Flood Zone 1. No assessment where the site is under a hectare, outside a critical drainage area and clear of other sources, though drainage evidence is still expected.

Reports for Manchester sites, from Victoria North to West Didsbury

A Manchester application can need three separate reports: one on the site's safety, one on whether the scheme belongs there, and one on what happens to the rain. Every consultant who writes them is chartered, Unda holds CIWEM Business Partner status, and nothing is issued until a senior consultant has signed it off.

• Sites with more than one source. A flood risk assessment for a Manchester site reads the river, the culvert under the street, the sewers and the groundwater together, because Flood Zone 1 on the river map says nothing about the other three.
• Applications to Manchester City Council. Schemes in Flood Zone 2 or 3, over a hectare, or over 0.5 ha in a critical drainage area need a flood risk assessment for planning that answers EN14, JP-S4 and the evidence in the 2026 SFRA.
• Buying in Didsbury, Chorlton or Ancoats. Homes near the Mersey embankments, the brooks or a canal attract questions from lenders, and a flood risk survey for property purchase reports on the one building before exchange.
• New homes in the flood zones. Flats in the Irk or Medlock valleys and conversions in Flood Zone 3 need the sequential and exception test, with an area of search drawn to the 2026 SFRA's guidance.
• Cutting brownfield runoff. A drainage strategy shows how a cleared site reaches greenfield rates or the 50% cut, and where the water goes once the sewer is no longer the first choice.
• Critical drainage areas. Inside the Didsbury or the Levenshulme and Fallowfield critical drainage area, a surface water drainage strategy proves no flooding at 1 in 30 years and none to buildings at 1 in 100 with climate change.
• After the permission. Where a Manchester approval comes with a drainage condition, discharge of drainage conditions supplies the calculations, drawings and maintenance plan the lead local flood authority checks.
• Culverts and breaches. The 2026 SFRA has no residual risk mapping and many culvert routes under the centre are unknown, so some sites need flood modelling of a culvert, a canal breach or a Mersey embankment.
• Getting out in time. In January 2025 the Britannia Hotel at Didsbury was evacuated beside a breached embankment, and a flood warning and evacuation plan sets out how occupants leave once a Mersey or Irk warning is issued.
• Cover after 2025. With 60 Manchester homes flooded inside that New Year and repeated sewer flooding in Chorlton, a flood risk assessment for insurance gives an insurer evidence on the building itself instead of a postcode average.
• New offices and commercial space. Commercial schemes in the city centre growth location, where the draft plan asks for SuDS and climate resilience, can use a BREEAM flood risk assessment for the Pol 03 credits.
• Buying land in the Irk valley. Victoria North aims for 15,000 homes on land threaded by the Irk, and flood risk due diligence sets out the culverts, warning areas and zones before a purchase or loan.
• Keeping premises open. Twenty-nine Manchester businesses flooded inside in 2025 and 27 premises at Cheetham in 2015, so a business continuity flood risk assessment plans for access, stock and power.
• Testing a layout early. Holt Town and Victoria North come with "sponge city" and nature-based drainage aims in the draft plan, and a flood risk feasibility study tests a layout against them before an offer.
• Water below the floor. Where the SFRA shows groundwater near the surface, in Withington, Fallowfield, Chorlton and Moss Side, a groundwater flood risk assessment checks basements and soakaways.

Reporting flooding in Manchester, and who deals with it

Ring 999 if floodwater threatens anyone, and 105 if the power fails. For everything else, the right body in Manchester depends on where the water came from.

The city's schemes sit within the Greater Manchester flood partnership, which reports to the North West committee and tracks the Mersey embankment repairs and the south Manchester strategy. Upstream on the Mersey, Stockport set its own record at Brinksway on the same New Year's Day, and the Goyt, which joins it there, runs down from Whaley Bridge. Downstream the Mersey continues to Warrington. On Boxing Day 2015 the Roch, which joins the Irwell above Salford, flooded the centre of Rochdale. Our other area pages are listed under the places we cover.

Flood risk questions we hear about Manchester sites

Unda Consulting Limited works on sites all over Manchester, from Cheetham Hill and Ancoats to Hulme, Chorlton and Didsbury, on flood risk assessments, sequential and exception tests and drainage strategies. Ring 01293 214444 or write to enquiries@unda.co.uk about yours.