Flood Risk Assessment Cardiff

Flood Risk Assessment Services in Cardiff

Cardiff, as the capital of Wales, sits at the confluence of the River Taff and River Ely, with the River Rhymney forming its eastern boundary and Cardiff Bay and the Severn Estuary defining its southern edge. This confluence of major rivers and coastal influence creates a multifaceted flood risk environment that requires specialist assessment under Wales’s distinct regulatory framework.

Aegaea provides specialist flood risk assessment, hydraulic modelling, and drainage consultancy services across Cardiff. Our team has detailed knowledge of TAN15, Natural Resources Wales requirements, Cardiff Council’s local planning policies, and the mandatory SuDS Approval Body process that applies uniquely in Wales.

Understanding Cardiff’s Flood Risk

The River Taff

The River Taff is Cardiff’s dominant watercourse, flowing southward through a narrow valley from the Brecon Beacons before entering the city at Tongwynlais and passing through Llandaff, Pontcanna, the city centre, and Butetown to discharge into Cardiff Bay. The Taff catchment covers approximately 510 square kilometres, and the river’s steep upper catchment means it responds rapidly to heavy rainfall.

Cardiff has experienced severe flooding from the Taff throughout its history. The February 1979 flood was a defining event, causing widespread damage across the city centre and Riverside. More recently, Storm Dennis in February 2020 brought the Taff to near-record levels, causing flooding in Pontcanna, Llandaff, and Taff’s Well. The event was a stark reminder of the ongoing vulnerability of Cardiff’s riverside communities.

The Western and Eastern Flood Defence schemes along the Taff, completed in the 1980s and subsequently upgraded, provide protection to the city centre and Riverside against a 1 in 100 year event. However, protection standards fall short of the 1 in 200 year level, and climate change will progressively reduce the effective standard of protection. Development within the defended area must assess residual risk from defence overtopping and breach, and developments upstream and downstream of the defended reaches face undefended fluvial risk.

The River Ely

The River Ely flows through western Cardiff, passing through Ely, Caerau, Leckwith, and Penarth Road before reaching Cardiff Bay. The Ely’s catchment, covering approximately 145 square kilometres, is smaller and lower-lying than the Taff, but the river has a well-documented history of flooding.

The Ely Valley has experienced significant development pressure, particularly around the Leckwith and Cardiff Bay areas. The former Leckwith rail yard and surrounding brownfield sites have been the subject of major regeneration, and flood risk from the Ely is a key constraint for these proposals. The tidal influence extends upstream along the Ely, and combined fluvial-tidal analysis is required for development near the river mouth.

Surface water and ordinary watercourse flooding also affect the Ely catchment. The Caerau and Ely residential areas have experienced sewer flooding during intense rainfall, and the limited capacity of the drainage network in these older suburban areas is a recognised problem.

The River Rhymney

The River Rhymney forms Cardiff’s eastern boundary, separating the city from Newport. The lower Rhymney is tidal, and development on the Cardiff side of the river, including areas around Llanrumney, St Mellons, and the Rumney industrial area, must assess both fluvial and tidal flood risk.

The Rhymney catchment has experienced significant flooding events, and NRW flood maps show extensive flood risk areas along the river’s lower reaches. The interaction between river flows, tidal conditions, and the influence of the Severn Estuary’s extreme tidal range (the second highest in the world) creates a complex combined flood risk environment.

Coastal and Tidal Flood Risk

Cardiff’s position on the Severn Estuary exposes low-lying areas to tidal flood risk. The Severn Estuary has an extreme tidal range of up to 14 metres, and storm surges can significantly elevate water levels above predicted tidal peaks. The December 2013 tidal surge, which coincided with a high spring tide, caused flooding along the South Wales coast and tested Cardiff’s coastal defences.

Cardiff Bay Barrage, completed in 1999, isolates the inner bay from tidal influence and maintains a freshwater lake at a controlled level. Development around the bay perimeter must consider the residual risk of barrage overtopping or failure, the controlled water level in the bay, and surface water drainage to the impounded bay area. Sites south of the barrage, including the docks area and Penarth Marina, remain at risk from tidal flooding.

The Wentloog Levels, extending east of Cardiff toward Newport, are protected by sea defences and the Great Wall drainage system. These low-lying areas, parts of which fall within Cardiff’s administrative area, are at significant tidal and groundwater flood risk and require detailed assessment for any development proposals.

Surface Water Flooding

Surface water flooding affects widespread areas across Cardiff. The city’s topography, with ridgelines draining south toward the bay, creates natural surface water flow paths through the urban area. Intense rainfall events regularly cause localised flooding, particularly in areas with combined sewer systems and limited drainage capacity.

NRW’s surface water flood maps show significant risk areas across Cardiff, including parts of Roath, Cathays, Canton, and Splott. The Roath Brook and Nant Fawr, both relatively small watercourses flowing through residential areas, have caused flooding when their capacity is exceeded. Cardiff Council’s Local Flood Risk Management Strategy identifies these areas as priorities for surface water management.

Welsh Regulatory Framework

TAN15: Technical Advice Note 15

TAN15 is the Welsh Government’s planning policy guidance on development, flooding, and coastal erosion. The updated TAN15, which came into effect in June 2023 following a transitional period, fundamentally changed the approach to flood risk in the Welsh planning system.

Under TAN15, flood risk areas are categorised into zones based on the NRW Flood Map for Planning. Development proposals within flood risk zones are subject to a justification test and must be accompanied by a Flood Consequences Assessment (FCA). The FCA is Wales’s equivalent of a Flood Risk Assessment, though the terminology and specific requirements differ from those in England.

TAN15 introduces the concept of “tolerable” flood consequences, which must be demonstrated through the FCA. The criteria for tolerability depend on the vulnerability classification of the development and the flood zone in which it is located. Residential development is classified as “highly vulnerable” and faces the most restrictive requirements.

Key differences from England’s approach include the use of the term Flood Consequences Assessment rather than Flood Risk Assessment, specific tolerability criteria for flood depth, velocity, speed of onset, and duration, a different flood zone categorisation system aligned with NRW’s flood maps, and the requirement for a justification test for development in flood risk zones.

Natural Resources Wales

NRW performs a combined role in Wales, bringing together the functions of the former Environment Agency Wales, Countryside Council for Wales, and Forestry Commission Wales. For flood risk purposes, NRW maintains flood risk maps, provides flood warnings, manages main river flood defences, and acts as a statutory consultee on planning applications with flood risk implications.

NRW’s consultation responses on Cardiff planning applications are a critical input to the planning process. Where NRW objects to a planning application on flood risk grounds, Cardiff Council will typically refuse the application unless the objection can be resolved. Aegaea regularly engages with NRW on behalf of clients to resolve flood risk concerns and facilitate positive consultation responses.

Schedule 3 SuDS Approval Body

Wales is the first nation in the UK to implement mandatory SuDS through the SuDS Approval Body (SAB) process. Under Schedule 3 of the Flood and Water Management Act 2010, which commenced in Wales in January 2019, all new developments of more than one dwelling or with a construction area of 100 square metres or more require SAB approval before construction can begin.

SAB approval is a separate consent from planning permission and is administered by Cardiff Council. The SAB assesses drainage designs against national standards for SuDS in Wales, which set requirements for surface water runoff destination, hydraulic control, water quality treatment, amenity, and biodiversity.

The SAB process adds a layer of complexity to Cardiff developments that does not exist in England. Developers must obtain both planning permission and SAB approval, and the two processes run in parallel. Aegaea supports clients through both the planning FCA process and the SAB approval process, ensuring that drainage strategies satisfy both requirements.

Key Development Areas in Cardiff

Cardiff Bay and Atlantic Wharf

The Cardiff Bay area, including the Atlantic Wharf regeneration zone, represents one of Wales’s most significant development opportunities. Flood risk from the impounded bay, surface water, and groundwater must be addressed, along with the residual risk associated with the Cardiff Bay Barrage. The council’s masterplan for the area sets out specific requirements for flood risk and drainage.

City Centre and Capitol Quarter

Cardiff city centre has seen intensive development in recent years, including the Capitol Quarter mixed-use scheme and the Central Square development around Cardiff Central station. Surface water management in the constrained city centre requires innovative SuDS solutions, and developments must demonstrate compliance with both NRW flood risk requirements and SAB drainage standards.

Northern Expansion Areas

The major housing allocations to the north of Cardiff, including sites around Pontprennau, Lisvane, and the proposed Plasdwr development of up to 7,000 homes in the north-west of the city, require comprehensive flood risk and drainage assessment. While these sites are generally outside the main river floodplains, surface water management is critical to avoid increasing downstream flood risk in the Taff and Ely catchments.

Splott and Tremorfa

The eastern industrial and regeneration areas around Splott and Tremorfa face complex flood risk challenges including tidal risk from the Severn Estuary, fluvial risk from the Rhymney, and surface water flooding. The area’s industrial legacy means that contaminated land assessment is frequently required alongside flood risk work.

Aegaea’s Cardiff Services

Aegaea provides comprehensive flood risk and drainage services across Cardiff, including Flood Consequences Assessments under TAN15 for planning applications, hydraulic modelling of the Taff, Ely, Rhymney, and smaller watercourses, coastal and tidal flood risk assessment, SuDS design and SAB approval applications, NRW pre-application consultation and objection resolution, drainage strategies meeting both planning and SAB requirements, and expert witness services for planning appeals and inquiries.

Our detailed understanding of the Welsh regulatory framework, including TAN15, the SAB process, and NRW’s consultation requirements, ensures that clients receive efficient, compliant assessments that support successful planning and drainage approvals. We work with developers, architects, and planning consultants across Cardiff and South Wales.

Contact Aegaea

If you are planning a development in Cardiff and need a Flood Consequences Assessment, drainage strategy, or SuDS design, contact Aegaea for a no-obligation discussion. We can advise on TAN15 requirements, SAB approval, and the likely scope of work for your site.