Lower Dorset Stour Flood Risk Management

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Our role
Client challenges
Innovative approach
Outcomes & benefits
Looking ahead

Our role
Client challenges
Innovative approach
Outcomes & benefits
Looking ahead

Markets

Water
Services

Engineering & design
Location

Dorset, UK
Client

Environment Agency

Clare Hodgson

Associate Director, Flood Risk, Epsom, UK contact form

The Lower Dorset Stour catchment has always been shaped by water. But climate change is accelerating that relationship in ways that place communities, infrastructure and landscapes under increasing pressure. The towns of Wimborne and Christchurch currently benefit from extensive flood defences, yet rising river flows and more extreme weather mean that even these established protections will not be enough on their own in the decades ahead.

Working with the Environment Agency, AtkinsRéalis developed the Lower Dorset Stour Flood Risk Management Strategy to help chart a long‑term approach to managing fluvial flood risk across the catchment over the next 100 years. The strategy brings together traditional engineered solutions with Natural Flood Management (NFM), land‑use adaptation and flexible decision‑making, recognising that future resilience will depend on working with natural processes with bold decision making for communities at greatest risk.

The result is a long‑term, evidence‑led strategy that balances flood protection, environmental recovery and community resilience — providing a clear framework for investment and adaptation as the impacts of climate change continue to unfold.

Client challenges

Vulnerable communities in the Lower Dorset Stour catchment face increasing flood risk driven by climate change and ageing defence infrastructure. Despite long lengths of existing linear defences protecting major urban areas such as Wimborne and Christchurch, the AtkinsRéalis projections show that by 2070, more than 1,700 properties could be at risk of flooding, even with defences maintained. In a ‘Do Nothing’ scenario, this risk would increase significantly as assets deteriorate.

Key challenges included:

Vulnerable Park home communities experiencing deep and frequent flooding
Extensive existing defences requiring sustained maintenance to retain protection standards
Growing flood risk to urban areas under future climate change scenarios
Strong stakeholder interest in nature‑based solutions, but a lack of evidence demonstrating their effectiveness at catchment scale

Our role

AtkinsRéalis provided the Environment Agency with specialist expertise in option identification, appraisal and strategy development, supporting the preparation of a technically robust and sustainable flood risk management strategy. Our work included:

Establishing a comprehensive baseline understanding of flood risk across the catchment
Identifying, appraising and selecting flood risk management options
Developing short‑term interventions with sufficient assessment to support future business case development
Applying an Adaptation Pathway Approach to enable flexible, long‑term decision‑making
Delivering supporting assessments, including: Strategic Environmental Assessment (SEA), Social Assessment and Habitats Regulations Assessment (HRA)

The Strategy was developed during a period of national transition in flood risk policy and guidance. The work undertaken helped inform and shape new national strategy guidance, replacing outdated processes from 2012 and providing wider benefits for future flood strategies across England.

Innovative approach

AtkinsRéalis applied innovative digital tools and modelling techniques to rapidly assess the feasibility and effectiveness of nature‑based solutions:

A new 2D HEC‑RAS hydraulic model was developed to rapidly test the impacts of NFM interventions in the lower catchment, efficiently assessing the impact of large-scale changes to channel conveyance and floodplain inundation.
The modelling demonstrated that while NFM could reduce flood peaks and delay downstream flooding, impacts in the lower catchment were constrained by existing floodplain connectivity and large flood volumes.
Our in‑house NFM Studio tool was used to identify NFM opportunities across the Crane and Moors tributary of the Lower Stour, with volumetric calculations resulting in prioritisation of the top 10% of land parcels which could deliver the greatest potential flood risk benefits.
NFM studio tool outputs were integrated into hydraulic modelling, showing potentially significant reductions in peak flows and flood extents, and supporting NFM as a recommended component of the strategy in the Crane and Moors catchments.

The modelling outputs also provided powerful visual tools to support stakeholder engagement and inform wider environmental initiatives in the catchment.

Lower Dorset Stour flood risk management strategy in numbers

100 Years

Appraisal period

£ 1.1 M

Net carbon economic benefit

£ 280 M

Present Value economic benefits

> 2000

Properties protected from flooding by 2070

Outcomes & benefits

Over the 100 year appraisal period, the preferred Strategy options:

Deliver £280 million in calculated Present Value (PV) economic benefits, of which £200 million is achieved through ongoing maintenance of existing defences
Cost an estimated £25 million, delivering a benefit–cost ratio of 11.1
Provide significant wider environmental and landscape benefits from working with natural processes
Reduce flood risk for 182 residential and 36 non‑residential properties in the present day
This increases to 1,995 residential and 310 non‑residential properties by 2070 compared to a ‘Do Nothing’ scenario
The two identified short-term interventions deliver £1.1 million of net carbon economic benefit.

Industry‑leading proposals for fluvial floodplain land‑use adaptation will support Local Authority planning policy and stakeholder engagement, enabling more sustainable long‑term management of the most vulnerable communities in the catchment.

Working on this strategy, with AtkinsRéalis, has been a real opportunity to test different flood risk management options beyond the traditional engineering solutions. Considering the future through adaptation of rivers and floodplains—from large scale river restoration through to relocation of vulnerable properties—has helped shape our view to deliver a more sustainable flood risk strategy.

Michael Holm

Flood & Coastal Risk Management Advisor, Environment Agency

Looking ahead

Implementation of the Lower Dorset Stour Flood Risk Management Strategy will significantly reduce the long‑term impacts of flooding across the Lower Stour, Crane and Moors catchments. By combining adaptive pathways, nature‑based solutions and targeted engineered defences, the Strategy establishes a resilient, flexible framework to protect communities and landscapes well beyond the next century.