Environmental Impact Assessment: What Goes in the Flood Risk and Water Chapter

Major infrastructure and energy projects in England almost always trigger the requirement for an Environmental Impact Assessment. Within that assessment, the water environment chapter of the Environmental Statement is where flood risk, surface water quality, groundwater impacts, and water supply are evaluated. Getting this chapter right is critical — not only because it addresses a statutory requirement, but because the water environment is frequently where the most technically complex and politically sensitive issues arise.

This article sets out what the water environment chapter must cover, how it differs from a standalone flood risk assessment, and how to navigate the screening and scoping process effectively.

When Flood Risk Triggers EIA

The Town and Country Planning (Environmental Impact Assessment) Regulations 2017 set out the framework for determining when EIA is required. Two schedules are relevant.

Schedule 1 Developments

Schedule 1 lists development types that always require EIA, regardless of scale or location. These include thermal power stations above 300MW, chemical installations, motorways, and large dams. If a project falls within Schedule 1, an Environmental Statement is mandatory and the water environment chapter must be included.

Schedule 2 Developments

Schedule 2 lists development types that require EIA only where the development is likely to have significant environmental effects. The Regulations set screening thresholds — for example, industrial estate development exceeding 0.5 hectares, urban development projects exceeding 1 hectare in a sensitive area, or energy installations not included in Schedule 1.

For Schedule 2 developments, the local planning authority must issue a screening opinion. Flood risk is a material consideration in that screening. A development proposed in Flood Zone 3, or one that would significantly alter surface water drainage in a sensitive catchment, is more likely to be screened in.

Nationally Significant Infrastructure Projects

For projects that fall within the Nationally Significant Infrastructure Projects (NSIP) regime under the Planning Act 2008, EIA is typically required under the Infrastructure Planning (Environmental Impact Assessment) Regulations 2017. The examining authority will expect a comprehensive water environment chapter as part of the Environmental Statement accompanying the Development Consent Order (DCO) application.

What the Water Environment Chapter Must Cover

The water environment chapter sits within the broader Environmental Statement and must assess the full range of water-related impacts arising from the proposed development. The scope is significantly broader than a standalone FRA.

Baseline Conditions

The chapter must establish the existing baseline for the water environment. This includes:

• Fluvial and tidal flood risk: current flood zone classification, historical flood records, Environment Agency flood modelling data, and any available Strategic Flood Risk Assessment information
• Surface water flood risk: existing surface water flow paths, critical drainage areas, and pluvial flood mapping
• Groundwater conditions: groundwater levels, aquifer designations (Principal, Secondary A/B), groundwater vulnerability zones, and Source Protection Zones
• Surface water quality: Water Framework Directive (WFD) classifications for waterbodies within and downstream of the site, including ecological and chemical status
• Water supply: existing abstractions (licensed and unlicensed), water resource availability, and the sensitivity of supply infrastructure

The baseline must reflect current conditions and, where relevant, the future baseline (accounting for committed developments and climate change allowances).

Impact Assessment: Construction Phase

Construction-phase impacts often receive less attention than operational impacts, but they can be significant and are regularly raised by the Environment Agency and examining authorities. The chapter must assess:

• Temporary increases in flood risk from construction compounds, haul roads, and temporary works within the floodplain
• Sediment mobilisation and its effects on watercourse quality
• Dewatering impacts on groundwater levels and nearby abstractions
• Risks from construction-phase spills (fuel, concrete, chemicals) reaching watercourses
• Temporary drainage arrangements and their adequacy

Impact Assessment: Operational Phase

The operational phase assessment is the core of the chapter. It must evaluate:

• Flood risk: changes to flood risk on-site and off-site, including any changes to floodplain storage or conveyance. Where the development alters flood risk, hydraulic modelling is typically required to quantify the impact. This is where the standalone FRA feeds directly into the ES chapter
• Surface water drainage: changes to impermeable area and the adequacy of the proposed drainage strategy, including SuDS provision and attenuation
• Surface water quality: operational pollution risks (road runoff, industrial discharges, treated effluent) and their effects on WFD status
• Groundwater: permanent impacts on groundwater flow, levels, and quality — particularly relevant for developments involving below-ground structures, tunnels, or deep foundations
• Water supply: the development’s water demand and whether existing supply infrastructure has capacity

Cumulative Impacts

Cumulative impact assessment is a distinctive requirement of EIA that goes beyond what a standalone FRA addresses. The water environment chapter must consider the combined effects of the proposed development with other planned or committed developments in the same catchment.

For flood risk, this means assessing whether the cumulative loss of floodplain storage, or the cumulative increase in surface water runoff, from multiple developments could worsen flood risk downstream — even if the individual contribution of each development is minor. The assessment should draw on the local authority’s planning register, strategic housing land availability assessments, and any known NSIP proposals in the catchment.

How the ES Water Chapter Differs from a Standalone FRA

A common source of confusion is the relationship between the Environmental Statement water chapter and a standalone flood risk assessment. They are related but distinct documents, and both are usually required.

The standalone FRA is a focused technical document that assesses flood risk to and from the development, proposes mitigation, and demonstrates compliance with national planning policy (NPPF Chapter 14 and the PPG).

The ES water chapter is broader. It covers flood risk but also addresses surface water quality, groundwater, water supply, cumulative impacts, and cross-references with other ES chapters. It follows the EIA methodology — establishing a baseline, identifying receptors, assessing impact significance using a defined matrix, and proposing mitigation and monitoring.

In practice, the FRA informs the ES water chapter. The flood risk sections will draw heavily on the FRA’s analysis. But the FRA does not replace the ES chapter, and the ES chapter does not replace the FRA. Submitting one without the other is a common error that leads to requests for further information.

EIA Screening and Scoping for Flood Risk

Screening

When requesting a screening opinion for a Schedule 2 development, the flood risk and water environment should be addressed directly. Set out the flood zone classification, the scale of development, the sensitivity of the receiving watercourses, and whether the development is likely to have a significant effect on the water environment. If the site is in Flood Zone 3, or if it involves substantial changes to surface water drainage in a sensitive catchment, make this clear — it strengthens the case for a robust and proportionate assessment.

Scoping

The scoping report is where the scope of the water environment assessment is defined. A well-prepared scoping report will:

• Identify all relevant flood sources (fluvial, tidal, pluvial, groundwater, sewer, artificial)
• Set out the proposed assessment methodology, including the significance criteria
• Identify the need for hydraulic modelling and the proposed modelling approach
• Define the study area for each water-related topic
• Identify cumulative developments to be assessed
• Propose any topics to be scoped out, with clear justification

The Environment Agency is a key statutory consultee at scoping stage. Their scoping response will often set out specific requirements — for example, the need for breach modelling for developments behind tidal defences, or the need for detailed groundwater assessment in Source Protection Zones. Addressing these requirements early avoids costly rework later.

Coordination with Other ES Chapters

The water environment does not exist in isolation within the Environmental Statement. Effective coordination between chapters is essential for a coherent assessment.

Ecology

The ecology chapter and the water environment chapter are closely linked. WFD compliance, which is assessed in the water chapter, directly affects ecological receptors. Protected species associated with watercourses (water voles, otters, white-clawed crayfish) depend on water quality and hydrological conditions that are assessed in the water chapter. The two chapters must be consistent in their baseline descriptions and impact conclusions.

Transport and Highways

Highway drainage is a significant source of surface water runoff and potential pollution. The transport chapter will assess traffic volumes and movements; the water chapter must assess the quality and quantity of highway runoff from new or altered road infrastructure. Where the development includes new highway connections (section 278 or section 38 works), the drainage design must be reflected in both chapters.

Ground Conditions

Contaminated land can have a direct impact on water quality, particularly groundwater. If the ground conditions chapter identifies contamination, the water chapter must assess the risk of contaminant mobilisation — both during construction (disturbance of contaminated soils) and operation (long-term leaching). Source Protection Zones and WFD groundwater body status are relevant receptors.

DCO and NSIP Process Differences

For Nationally Significant Infrastructure Projects, the EIA process sits within the DCO examination rather than the standard planning application process. This introduces several additional requirements for the water environment chapter.

The examining authority will expect the applicant to engage with the Environment Agency early and to document areas of agreement and disagreement in a Statement of Common Ground (SoCG). The SoCG on water environment matters typically covers the agreed baseline, the modelling approach, the significance of identified impacts, and the adequacy of proposed mitigation. Reaching agreement on these matters before the examination hearings significantly reduces risk.

For essential infrastructure proposed in Flood Zone 3, the Exception Test applies. The ES water chapter must demonstrate that the development provides wider sustainability benefits that outweigh the flood risk, and that the development will be safe for its lifetime without increasing flood risk elsewhere.

Proportionality

Not every development requires the same depth of water environment assessment. The principle of proportionality — matching the depth of assessment to the significance of the impact — is embedded in EIA practice and endorsed by the Institute of Environmental Management and Assessment (IEMA) guidance.

A development on a greenfield site in Flood Zone 1, remote from sensitive watercourses, may justify a brief water chapter that scopes out detailed assessment. Conversely, a major energy project straddling a floodplain, upstream of a WFD-failing waterbody, demands comprehensive assessment with detailed hydraulic modelling and rigorous cumulative impact analysis.

The scoping stage is the right time to establish proportionality. Engage with the Environment Agency and the LPA (or examining authority) at scoping to agree what is required. This avoids both under-assessment (leading to requests for further information) and over-assessment (wasting time and cost).

How Aegaea Can Help

Aegaea delivers EIA water environment chapters for major infrastructure and energy projects across the UK. Our team combines flood risk expertise with environmental assessment experience, ensuring the water chapter is technically robust, proportionate, and fully coordinated with the wider Environmental Statement.

Whether you need a standalone flood risk assessment, detailed flood modelling, or a complete EIA water chapter, we can scope, prepare, and defend the assessment through the planning or DCO examination process. Get in touch to discuss your project.