Welsh Flood Risk by Postcode: Where the 1,062 Highest-Risk Postcodes Sit

Why Welsh flood risk looks different to England

Wales has 92,226 postcodes across 22 unitary authorities. Around 1,062 of them sit in the highest flood-risk band, with another 910 in the medium band. That works out at about 1.2% 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: 1.2%). 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 flood data integrated into LocalRisk's postcode-level dataset for every NRW-mapped flood zone in Wales. The figures below are accurate as of May 2026.

Where flood risk concentrates in Wales

Despite their association with dramatic flood events like Storm Dennis, the South Wales Valleys are not the highest-risk councils in the dataset. Neath Port Talbot, Gwynedd, Ceredigion and Powys lead the table - a mix of coastal, estuarine and upland river councils with large, fast-responding catchments.

| Council | Postcodes | High flood risk | | --- | --- | --- | | Neath Port Talbot | 3,662 | 3.0% | | Gwynedd | 5,598 | 2.5% | | Ceredigion | 3,080 | 2.2% | | Powys | 6,125 | 2.1% | | Denbighshire | 3,428 | 1.8% | | Conwy | 4,113 | 1.5% | | Carmarthenshire | 6,726 | 1.5% |

_Source: Natural Resources Wales flood zone data, analysed by LocalRisk, May 2026._

Neath Port Talbot leads at 3.0%. The Afan and Neath river valleys drain steep upland catchments into a narrow coastal plain, and the main towns - Port Talbot, Neath, Briton Ferry - sit on valley floors at the confluence of river and tidal influence from Swansea Bay.

Mid and west Wales: large catchments, dispersed risk

Gwynedd (2.5%), Ceredigion (2.2%) and Powys (2.1%) form the middle 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. At 2.1% high-risk, the risk disperses across the council's many small market towns and riverside settlements.

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 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 the catchment.

Ceredigion includes Aberystwyth, where the Rheidol meets the sea, and a long stretch of Cardigan Bay coast. Coastal flooding is more relevant here than in the inland valleys, particularly in lower-lying settlements near Aberystwyth and the Dyfi estuary.

Gwynedd at 2.5% is the highest-risk council in North Wales and second-highest overall. Despite being sparsely populated, it 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.

The South Wales Valleys: dramatic events, lower band ratings

The South Wales Valleys - Rhondda Cynon Taf, Merthyr Tydfil, Blaenau Gwent - sit in the lower half of the table despite their association with severe flooding. RCT records 0.8% high-risk (43 postcodes out of 5,431). Merthyr Tydfil is 1.2%. Blaenau Gwent is 0.3%.

This runs counter to the narrative from Storm Dennis in February 2020, which 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 as a whole, with 158 of those in Pontypridd. NRW's subsequent review identified the same upper-valley corridors as high-risk.

Two factors explain the lower band ratings. NRW's flood bands reflect residual risk after defences are taken into account, and significant flood defence infrastructure exists in the main Valley towns following previous events. The Dennis flooding also swept through many properties not in the formally mapped high-risk zone - the event was driven by exceptional peak flows that overwhelmed defences and affected areas outside the standard probability bands. The formal band reflects ongoing statistical risk, not rare but severe events.

North Wales: a varied picture

Denbighshire (1.8%) and Conwy (1.5%) 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 320 properties in the city in what became 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 0.4% respectively. Eastern North Wales borders the West Cheshire plain, with flatter terrain and slower-responding rivers than the mountainous interior.

The Cardiff paradox

Cardiff is the largest Welsh local authority by postcode count (7,685 postcodes) and one of the lowest-ranked on flood risk: 0.2% of postcodes in the high band, 1.2% in medium. 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 0.4% 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, it means a low band in a defended area is contingent on those defences continuing to work as designed. Second, it means residual coastal-flood risk in the Severn estuary remains an active climate-adaptation question for Cardiff and Newport councils, even where the present-day classification is comfortable.

Pembrokeshire and the Welsh coast

Pembrokeshire (1.1%) and Conwy (1.5%) are the Welsh councils where coastal flooding contributes most to the high-risk band, alongside river flooding. 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.

What this means for homeowners and renters

For anyone making a property decision in Wales, three 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 a rising bank. The LocalRisk postcode page returns the band for any UK postcode along with the underlying NRW data source.

Second, 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 upper-valley location with the same NRW flood-zone classification. The flood band reflects residual risk, but the underlying hazard does not disappear if defences are removed or fail.

Third, 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.

How NRW classifies flood risk

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 from rivers and the sea, 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).

Surface-water flooding - rainfall that overwhelms drains before reaching a river - is handled separately in NRW's mapping. Surface-water risk concentrates in the same urban valleys as river risk. Several coastal towns are also exposed because their drainage was built for older, less intense rainfall.

The Welsh data integrated into LocalRisk includes both NRW's river/sea flood mapping and surface-water flood mapping where available. LocalRisk reports a single band per postcode reflecting the most material risk. That simplification makes cross-UK comparison with EA and SEPA data possible.

A note on Northern Ireland

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.

Sources and methodology

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. 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. That shift concentrates flood risk into more intense winter events and increases summer drought risk in the same areas.

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.