CITY CLIMATE INSIGHTS
22 May 2026 - LocalRisk
Wales has 92,226 postcodes across 22 unitary authorities. Around 2,099 sit in the highest flood-risk band - about 2.3% of the total. Once surface-water (flash) flooding is counted alongside rivers, the risk concentrates in the South Wales Valleys - Merthyr Tydfil and Rhondda Cynon Taf - and the upland catchments of mid and west Wales.
Welsh flood risk ranked by council using Natural Resources Wales data. Merthyr Tydfil and Neath Port Talbot lead across all 22 Welsh councils.
Wales has 92,226 postcodes across 22 unitary authorities. Around 2,099 of them sit in the highest flood-risk band, with another 979 in the medium band. That works out at about 2.3% of Welsh postcodes flagged as higher flood risk.
Welsh flood-risk levels look lower than English and Scottish equivalents at council aggregate (England: roughly 15% of postcodes in the high band; Scotland: roughly 19%; Wales: 2.3%). The gap is partly methodological - the Environment Agency, Natural Resources Wales and SEPA use different mapping products and band thresholds, so the headline national comparison is not strictly like-for-like. The Welsh picture is best read on its own terms.
This article draws on Natural Resources Wales river, sea and surface-water flood mapping, integrated into LocalRisk's postcode-level dataset. Each postcode is assigned the most material of its river/sea and surface-water bands - the same approach the Environment Agency's NaFRA2 takes in England, where surface-water (flash) flooding is assessed alongside rivers and the sea. The figures below are accurate as of June 2026.
Once surface water is counted alongside rivers and the sea, the highest-risk councils are a mix of the steep-sided South Wales Valleys and the upland-river and coastal councils of the north and west.
| Council | Postcodes | High flood risk | | --- | --- | --- | | Merthyr Tydfil | 1,631 | 5.2% | | Neath Port Talbot | 3,662 | 4.7% | | Rhondda Cynon Taf | 5,431 | 4.6% | | Blaenau Gwent | 1,844 | 4.0% | | Gwynedd | 5,598 | 4.0% | | Powys | 6,125 | 3.3% | | Ceredigion | 3,080 | 3.3% |
_Source: Natural Resources Wales river, sea and surface-water flood mapping, analysed by LocalRisk, June 2026._
Merthyr Tydfil leads at 5.2%. The town sits at the head of the Taff valley, hemmed in by steep ground on both sides, where intense rainfall runs off quickly and concentrates along the narrow valley floor. Neath Port Talbot (4.7%) combines the Afan and Neath river valleys draining steep upland catchments into a narrow coastal plain, with tidal influence from Swansea Bay.
The defining feature of Welsh flood risk is the South Wales Valleys - and the mechanism is largely surface water, not main-river flooding. Rhondda Cynon Taf records 4.6% of postcodes in the high band (248 postcodes). On river and sea flooding alone, far fewer would flag - it is the surface-water dimension, water running off the steep valley sides faster than drains can carry it, that drives the figure.
This fits the lived record. Storm Dennis in February 2020 caused widespread flooding across the Taff and Rhondda valleys; the council's official post-event report recorded 1,498 properties flooded across Rhondda Cynon Taf, with 158 of those in Pontypridd. Much of that was surface water and flash flooding off saturated upland, overwhelming watercourses and drainage in the valley bottoms. Counting surface water alongside rivers brings the formal band ratings into line with what Valleys communities already know.
Merthyr Tydfil (5.2%), Blaenau Gwent (4.0%) and the wider Valleys network - Caerphilly, Torfaen, Bridgend - show the same pattern: steep catchments, fast run-off, and narrow valley floors where homes, roads and railways sit close to watercourses.
Gwynedd (4.0%), Powys (3.3%) and Ceredigion (3.3%) form the next tier. These are large, rural councils where flood risk spreads across many river valleys rather than concentrating in one urban centre.
Powys is geographically the largest council in Wales and contains the upper catchments of the Severn, Wye and Vyrnwy; the risk disperses across its many small market towns and riverside settlements. Gwynedd contains long river corridors draining Snowdonia - the Glaslyn, Dwyryd and Mawddach among them - as well as an extensive Cardigan Bay coastline at Barmouth and Harlech. Ceredigion includes Aberystwyth, where the Rheidol meets the sea, and a long stretch of Cardigan Bay coast where coastal flooding adds to river risk.
Wales gets roughly 1,400mm of annual rainfall on average, with upland areas receiving considerably more. England gets about 870mm. Welsh upland topography funnels that water quickly into rivers and down valley sides, and many small communities sit along the valley floors that drain it. A Welsh village can carry far more flood risk than a similar English village simply by sitting lower in a steep catchment.
Denbighshire (2.6%) and Conwy (2.4%) sit above the Welsh average. Historic flooding hotspots include Llanrwst (on the Conwy), Ruthin and parts of the Vale of Clwyd in Denbighshire, and St Asaph - where the River Elwy burst its banks in November 2012, flooding around 320 properties in the city in one of the most significant flood events in north Wales in recent decades.
Flintshire and Wrexham, the most easterly councils, sit at the bottom of the table - 0.0% and 0.9% respectively. Eastern North Wales borders the West Cheshire plain, with flatter terrain and slower-responding rivers than the mountainous interior.
Cardiff is the largest Welsh local authority by postcode count (7,685 postcodes) and one of the lowest-ranked on flood risk: 0.3% of postcodes in the high band. That puts the capital well below the Welsh average and far below the councils that lead the table.
Infrastructure, not geography, explains the low score. Cardiff sits on the Taff floodplain at the river's confluence with the Severn estuary, which has one of the highest tidal ranges in the world. Without defences, much of central Cardiff would face serious tidal-river flood risk. With them - the Cardiff Bay barrage (completed 1999), the Taff and Ely flood defences, and ongoing NRW channel maintenance - that risk is largely contained.
NRW publishes residual risk - what remains after defences are taken into account. Cardiff scores well on the bands for that reason. Newport, downstream on the Usk, scores 0.7% on the same basis: defences manage the Severn-estuary tidal risk down to a low residual probability.
The defended-area framing matters for two reasons. First, a low band in a defended area is contingent on those defences continuing to work as designed. Second, residual coastal-flood risk in the Severn estuary remains an active climate-adaptation question for Cardiff and Newport, even where the present-day classification is comfortable.
Pembrokeshire (2.0%) and Conwy (2.4%) are among the Welsh councils where coastal flooding contributes to the high-risk band, alongside river and surface water. The Pembrokeshire coast runs from low-lying estuarine settlements - Pembroke Dock, Milford Haven, Carmarthen Bay edges - to exposed Atlantic cliffs.
Anglesey records 1.2% high flood risk. Most of the island is elevated farmland with little coastal floodplain. The exception is the southern coast facing Caernarfon Bay, where Newborough and parts of the Menai Strait edge appear in the higher bands.
For anyone making a property decision in Wales, four points follow from this data.
First, regional generalisations only tell you so much. Two postcodes a few hundred metres apart can sit in different flood bands when one is on the valley floor and the other on rising ground. The LocalRisk postcode page returns the band for any UK postcode along with the underlying NRW data source.
Second, surface water matters as much as rivers in Wales. Much of the highest risk - especially in the Valleys - comes from intense rainfall running off steep ground, not from a named river bursting its banks. A property can sit well away from any river and still carry a high surface-water band.
Third, the "defended" classification matters. A property in central Cardiff, Newport, or downstream of a major NRW-maintained flood defence has a different risk profile to one in an undefended valley-floor location with the same NRW flood-zone classification.
Fourth, Flood Re covers eligible properties across Wales on the same terms as England. The scheme is open to residential properties built before 1 January 2009, in Welsh Council Tax bands A to I (Wales uses a nine-band system, A through I, rather than the English A through H). Buy-to-let landlord policies and commercial cover are excluded, in line with the rest of the UK.
For Welsh homeowners with a documented flood history, the Build Back Better scheme run by Flood Re allows up to £10,000 of property flood resilience measures to be reimbursed as part of a flood-related claim. The list of participating insurers covers most of the Welsh home insurance market, but eligibility and policy specifics vary - check directly with your insurer.
Natural Resources Wales publishes flood-zone data that broadly mirrors the Environment Agency's classification for England. Both agencies use the same underlying methodology of annual probability of flooding, adjusted for the presence of flood defences where they are accredited and maintained.
NRW's High band corresponds to an annual flood probability of 3.3% or greater - roughly a 1-in-30 chance per year or more frequent. Medium covers 1% to 3.3% probability, and Low covers 0.1% to 1%. Properties outside the mapped flood zones sit in NRW's Very Low classification (less than 0.1% annual probability).
NRW maps river/sea flooding and surface-water flooding separately. Surface-water flooding - sometimes called pluvial or flash flooding, where rainfall overwhelms drains and runs off the land before reaching a river - is a major driver of risk in the steep Welsh valleys. LocalRisk reports a single band per postcode reflecting the most material of its river/sea and surface-water risk. That mirrors the Environment Agency's NaFRA2 in England, which also combines these sources, and makes cross-UK comparison with EA and SEPA data possible.
LocalRisk's flood data covers England (Environment Agency), Wales (NRW) and Scotland (SEPA). Northern Ireland flood data from DfI Rivers is not yet integrated. NI postcodes show a default low band with a "data not available" notice instead of a true assessment. For NI properties, the Northern Ireland flood maps service published by DfI Rivers is the authoritative reference.
All flood-band data in this article comes from Natural Resources Wales river, sea and surface-water flood mapping (current as of NRW's most recent published release), aggregated by LocalRisk to council and postcode level. Each postcode is assigned the most material of its river/sea and surface-water bands. Total postcode counts come from the LocalRisk postcode index, which is updated from ONS Postcode Directory releases.
Climate context (Welsh annual rainfall averaging around 1,400mm, England around 870mm) is drawn from Met Office long-term station data. UKCP18 projections used elsewhere on LocalRisk indicate Welsh winter rainfall is likely to increase further under most modelled scenarios, while summer rainfall is likely to decrease - a shift that concentrates flood risk into more intense winter and flash-flood events.
To check the flood-risk band for any Welsh postcode, enter it on the LocalRisk homepage. The result includes the underlying NRW data source, comparison with the Welsh council average, and a breakdown of how the postcode compares with the rest of the UK.