Flood Risk Assessment Devon

Flood Risk Assessment Services in Devon

Devon’s dramatic landscape, from the granite heights of Dartmoor and the moorlands of Exmoor to the steep-sided river valleys and 200 kilometres of coastline, creates one of England’s most varied and challenging flood risk environments. The county’s rivers respond rapidly to rainfall, its coastline is exposed to Atlantic storms, and its communities have experienced some of the most devastating flood events in recent British history. For developers working in Devon, understanding and addressing these risks is essential.

Aegaea provides specialist flood risk assessment, hydraulic modelling, and drainage consultancy services across Devon. Our team has detailed knowledge of the county’s rivers, coastal flood risk, the Environment Agency’s flood defence infrastructure, Devon County Council’s LLFA requirements, and the specific planning policies of each district council.

Devon’s Flood Risk Landscape

The Boscastle Flash Flood: A Defining Event

The Boscastle flash flood of 16 August 2004 remains one of the most dramatic flood events in modern British history. An extreme convective rainfall event dropped approximately 200 millimetres of rain in five hours over the steep catchments of the Valency and Jordan streams above the village. The resulting flash flood sent a wall of water through Boscastle’s narrow valley, destroying or severely damaging over 100 buildings, sweeping 84 cars into the harbour and sea, and causing an estimated 20 million pounds in damage.

The Boscastle event, while extreme, is not unique in Devon’s history. The county’s steep topography, combined with its exposure to moisture-laden Atlantic weather systems, creates the conditions for rapid-response flooding that can be devastating in its intensity. Similar, if smaller, events have occurred at Ottery St Mary in October 2008, at Clovelly in October 2014, and at multiple locations across the county during Storm Dennis in February 2020.

These events underscore the importance of thorough flood risk assessment for development in Devon’s valleys and coastal communities. Standard approaches to flood risk estimation may not adequately capture the extreme nature of Devon’s flash flood risk, and site-specific assessment using appropriate rainfall and flow estimation methods is essential.

The River Exe and Exeter

The River Exe is Devon’s principal river, rising on Exmoor and flowing approximately 80 kilometres southward through Tiverton and Exeter to the sea at Exmouth. The Exe catchment covers approximately 1,500 square kilometres, encompassing the eastern slopes of Dartmoor and the fertile farmland of Mid Devon.

Exeter has a long history of flooding from the Exe and its tributaries, particularly the River Creedy and the Alphin Brook. The most significant event in modern times was the October 1960 flood, when the Exe overtopped its banks and flooded much of the St Thomas, Haven Banks, and Countess Wear areas. The 1960 flood prompted the construction of the Exeter Flood Defence Scheme, which provides protection to the city against approximately a 1 in 100 year event through a combination of raised walls, embankments, and channel improvements.

The flood defence scheme has been tested by subsequent events, including the November 2012 floods when the Exe approached the defence design level. Climate change will progressively reduce the effective standard of protection, and the Environment Agency is planning improvements to the scheme to maintain protection levels into the future.

Development within Exeter’s defended area must assess residual risk from defence overtopping and breach. The EA’s breach modelling provides data on predicted flood depths and timing in the event of defence failure, informing finished floor level requirements and flood resilience design. Areas outside the defended zone, including parts of Topsham, Countess Wear, and the Exe Estuary, face undefended fluvial and tidal risk.

The Exe Estuary, between Exeter and Exmouth, is an important ecological site and presents combined fluvial and tidal flood risk. Development along the estuary requires careful assessment of the interaction between river flows and tidal conditions, as well as consideration of sea level rise over the development’s lifetime.

The River Dart

The River Dart drains the southern and eastern slopes of Dartmoor, flowing through a steep-sided valley to the sea at Dartmouth. The Dart catchment’s steep topography and impermeable granite geology generate rapid runoff, and the river can rise dramatically during heavy rainfall.

Totnes, the principal town on the Dart, has experienced repeated flooding. The river’s tidal reach extends to the town, and combined fluvial-tidal events present the greatest risk. The November 2012 floods caused significant damage in Totnes, and the town remains at significant risk despite some property-level protection measures.

The Dart’s catchment includes significant areas of Dartmoor, where the granite geology and thin soils generate almost instantaneous runoff. Development in the Dart catchment must account for this rapid hydrological response and the short warning times that characterise flood events.

The River Teign

The River Teign flows from Dartmoor through Newton Abbot to the sea at Teignmouth. Newton Abbot, at the tidal limit of the Teign, faces combined fluvial and tidal flood risk. The town experienced significant flooding in November 2012, and the Environment Agency has developed the Newton Abbot Flood Alleviation Scheme to provide improved protection.

The Teign catchment, like the Dart, includes steep Dartmoor tributaries that respond rapidly to rainfall. The Bovey, Lemon, and other tributaries contribute to flood risk in Newton Abbot and surrounding settlements. Development proposals in the Teign catchment require assessment of the rapid-response characteristics of these tributaries.

Coastal Flood Risk

Devon’s coastline extends for approximately 200 kilometres (excluding estuaries), from the Bristol Channel coast in the north to the English Channel coast in the south. The coastline includes a wide range of settings, from the towering cliffs of North Devon and the exposed headlands of South Devon to the low-lying estuaries of the Exe, Teign, and Dart.

The North Devon coast, including Lynmouth, Ilfracombe, and Combe Martin, is exposed to the full force of Atlantic storms. The Lynmouth flood disaster of August 1952, when an extreme rainfall event on Exmoor generated a devastating flash flood that killed 34 people, remains one of the deadliest flood events in British history. While the Lynmouth event was primarily fluvial, the coastal exposure of North Devon amplifies the impact of storm events.

The South Devon coast, including the seaside towns of Dawlish, Teignmouth, Sidmouth, and Exmouth, faces coastal flood risk from storm surges and wave overtopping. The February 2014 storms, which destroyed the railway sea wall at Dawlish and isolated Cornwall from the rail network, demonstrated the power of coastal storms in this area. Development along the South Devon coast must assess coastal flood risk including wave action, overtopping, and the long-term implications of sea level rise and coastal erosion.

Surface Water and Groundwater Flooding

Surface water flooding is a significant risk across Devon, driven by the county’s high rainfall, steep topography, and varied geology. Intense rainfall events can generate rapid surface water runoff that overwhelms drainage systems, particularly in the county’s towns and villages where urban development has reduced the available infiltration and storage capacity.

Devon County Council’s Surface Water Management Plan identifies areas at greatest surface water risk across the county, including locations in Exeter, Newton Abbot, Barnstaple, Torquay, and Exmouth. The Ottery St Mary flash floods of October 2008, caused by intense surface water runoff and small watercourse flooding, exemplify the damage that surface water events can cause in Devon’s communities.

Groundwater flooding affects areas of Devon underlain by permeable geology, including the greensand and chalk of East Devon and the limestone areas. Rising groundwater can cause flooding of basements and below-ground services, and can reduce the effectiveness of infiltration-based SuDS. Development proposals in groundwater-susceptible areas require assessment of seasonal groundwater levels and their implications for drainage design.

Planning and Regulatory Framework

NPPF and District Planning Policies

Devon’s eight district and borough councils (plus the unitary authorities of Plymouth and Torbay) each maintain their own local plan and planning policies. Flood risk requirements reflect the specific challenges of each area, from Exeter’s Exe floodplain to the coastal flood risk of Teignbridge and the flash flood vulnerability of North Devon.

Key districts with significant flood risk policies include Exeter City Council, which has detailed policies on development in the Exe floodplain; Teignbridge, which addresses Teign floodplain and coastal risk; East Devon, which has policies on coastal flooding and the Otter catchment; and North Devon, which addresses rapid-response flood risk in the steep valley catchments.

Devon County Council as LLFA

Devon County Council is the Lead Local Flood Authority, responsible for managing local flood risk from surface water, groundwater, and ordinary watercourses. The county council is a statutory consultee on surface water drainage for major planning applications and has published detailed SuDS guidance.

Devon’s varied geology provides diverse ground conditions for drainage design. The sandstone and greensand of East Devon may support infiltration-based SuDS, while the clay soils of the Culm Measures in Mid and North Devon typically do not. Dartmoor’s granite and the Devonian slates and shales of South Devon also present limited infiltration potential. Site-specific infiltration testing is essential to determine the appropriate drainage approach.

The county council’s SuDS requirements include restricting discharge rates to greenfield rates, providing appropriate treatment through the SuDS management train, demonstrating that infiltration has been considered and tested, and securing long-term maintenance arrangements. The council encourages SuDS designs that deliver biodiversity and amenity benefits alongside their drainage function.

Environment Agency Requirements

The Environment Agency manages flood risk from main rivers and the sea across Devon. The EA holds modelling data for the Exe, Dart, Teign, Taw, and other main rivers, and provides flood risk data to support site-specific assessments. For coastal developments, the EA may require specific assessments of wave action, overtopping, and the interaction between coastal and fluvial processes in estuaries.

Devon’s rapid-response catchments present specific challenges for flood risk assessment. The EA’s guidance on rapid-response catchments acknowledges that standard hydrological methods may underestimate peak flows in steep catchments, and site-specific assessment using rainfall-runoff modelling may be required. Aegaea has experience applying appropriate methods for Devon’s steep valley catchments.

Key Development Areas in Devon

Exeter and the Exe Valley

Exeter is Devon’s principal development centre, with significant housing and commercial growth planned. Development sites along the Exe corridor, including the South West Exeter urban extension and the Marsh Barton regeneration area, face fluvial and tidal flood risk. Surface water management for the city’s growth areas is critical to avoid increasing downstream risk in the Exe catchment.

Newton Abbot and Teignbridge

Newton Abbot’s position at the tidal limit of the Teign creates complex combined fluvial-tidal flood risk. The town’s growth areas, including the Houghton Barton and Wolborough allocations, require comprehensive flood risk and drainage assessment. The Newton Abbot Flood Alleviation Scheme will change the flood risk context for development in the town, but sites must be assessed against current and future risk conditions.

Barnstaple and North Devon

Barnstaple, at the tidal limit of the River Taw, faces combined fluvial and tidal flood risk. The North Devon area includes numerous steep-sided valley catchments where flash flood risk is a defining concern. Development proposals must account for the rapid-response characteristics of these catchments and demonstrate safe access and egress during flood events.

Torbay Coastal Regeneration

The Torbay unitary authority area, including Torquay, Paignton, and Brixham, faces coastal flood risk and has experienced significant surface water flooding. Regeneration proposals along the seafront and in the town centres require coastal flood risk assessment and innovative surface water management in constrained urban settings.

Cranbrook and East Devon Growth

Cranbrook, Devon’s first new town in several hundred years, has been built on greenfield land east of Exeter. The development has required comprehensive flood risk and drainage strategies, including strategic SuDS to manage the impact of urbanisation on the Clyst and Exe catchments. Ongoing phases of development continue to require flood risk and drainage consultancy.

Aegaea’s Devon Services

Aegaea provides comprehensive flood risk and drainage services across Devon, including desktop and detailed flood risk assessments for planning applications, hydraulic modelling of the Exe, Dart, Teign, and other watercourses, coastal and estuarine flood risk assessment, rapid-response catchment flood risk analysis, infiltration testing (BRE365 and borehole permeability tests), surface water drainage strategies and SuDS design, Sequential Test and Exception Test assessments, Environment Agency and LLFA pre-application consultation, and expert witness services for planning appeals.

Our understanding of Devon’s distinctive flood risk characteristics, from the extreme flash flood potential of its steep valleys to the coastal exposure of its Channel and Atlantic coastlines, means we deliver assessments that capture the full range of risk. We work with developers, landowners, architects, and planning consultants across Devon.

Contact Aegaea

If you are planning a development in Devon and need a flood risk assessment, drainage strategy, or SuDS design, contact Aegaea. Our team can advise on the likely scope of work and help you navigate Devon’s flood risk requirements from the earliest stages of your project.