Glossary

The temporary storage of surface water runoff during a rainfall event, releasing it at a controlled rate to reduce peak discharge and downstream flood risk.

A mandatory requirement under the Environment Act 2021 for development in England to deliver a minimum 10% increase in biodiversity value compared to the pre-development baseline.

A standardised soakaway test method published by the Building Research Establishment that measures the rate at which water infiltrates into the ground, used to design soakaways and assess drainage hierarchy compliance.

Percentage uplifts applied to peak river flow, rainfall intensity, sea level rise, and offshore wind speed projections to account for the future impacts of climate change on flood risk.

A representation of the relationships between contamination sources, migration pathways, and sensitive receptors at a site, forming the basis of contaminated land risk assessment.

Land that has been polluted by substances at concentrations that pose an unacceptable risk to human health, controlled waters, or the environment, typically requiring assessment and potential remediation before development.

A 2D hydraulic modelling technique that applies rainfall directly onto a terrain model to simulate surface water flood risk, capturing overland flow paths and ponding without relying on predefined catchment boundaries.

The government body responsible for managing flood risk from main rivers and the sea in England, maintaining flood defences, and providing flood warnings and flood risk data.

A two-part test required when development in a high flood risk area cannot pass the Sequential Test. It must demonstrate wider sustainability benefits and safety for the lifetime of the development.

The final level of the ground floor of a building, set above the design flood level plus freeboard to protect against flooding for the development's lifetime.

A hydraulic modelling software package (formerly ISIS) developed by Jacobs, widely used in the UK for 1D river modelling and integrated 1D-2D flood simulation.

A site-specific assessment of all sources of flood risk to and from a development site, required to support planning applications in areas at risk of flooding.

Land assessed as having less than a 0.1% (1 in 1,000) annual probability of river or sea flooding. This is the lowest-risk flood zone.

Land assessed as having between a 0.1% and 1% annual probability of river flooding, or between 0.1% and 0.5% annual probability of sea flooding.

Land assessed as having a 1% or greater annual probability of river flooding, or a 0.5% or greater annual probability of sea flooding -- the high-probability flood zone.

The functional floodplain -- land where water must flow or be stored in times of flood, typically defined as the 5% (1 in 20) annual probability flood extent.

An additional height allowance added above the predicted design flood level when setting finished floor levels, providing a safety margin against uncertainties in flood level estimation.

Modular plastic crate systems installed below ground to provide surface water attenuation storage, offering high void ratios in a compact footprint.

The rate of surface water runoff from a site in its undeveloped (greenfield) condition. New development is typically required to limit post-development discharge to this rate.

A widely used hydraulic modelling software developed by the US Army Corps of Engineers, capable of 1D and 2D river flow simulation, sediment transport, and water quality analysis.

The use of computational software to simulate water flow in rivers, floodplains, drainage networks, and coastal zones, producing flood extent, depth, velocity, and hazard outputs.

The process by which surface water soaks into the ground. Infiltration-based SuDS techniques discharge runoff directly to the soil, eliminating the need for an outfall to a watercourse or sewer.

The local authority (county or unitary council) responsible for managing local flood risk from surface water, groundwater, and ordinary watercourses under the Flood and Water Management Act 2010.

The government's overarching planning policy document for England, setting out how the planning system should balance development needs with environmental protection, including flood risk management.

The government's adviser on the natural environment in England, responsible for designating protected sites and advising on nutrient neutrality, biodiversity net gain, and environmental impact.

A requirement for new development within specified catchments to demonstrate that it will not increase nutrient (nitrogen and phosphorus) loading to designated habitats, in order to comply with the Habitats Regulations.

A field test that measures the rate at which water drains through soil, used to design drainage fields for foul water treatment systems and to assess ground permeability for surface water infiltration.

A paving system designed to allow surface water to pass through the surface and into an underlying storage layer, providing source-control SuDS for trafficked areas.

A preliminary contaminated land assessment that reviews historical mapping, environmental data, and site conditions to identify potential contamination risks and determine whether intrusive investigation is needed.

An intrusive ground investigation that collects soil, groundwater, and gas samples to characterise contamination identified in a Phase 1 desk study and inform remediation decisions.

Online government guidance that supplements the NPPF, providing detailed technical advice on applying flood risk policy to planning decisions, including vulnerability classifications and the Sequential and Exception Tests.

Scotland's environmental regulator, responsible for flood risk management including flood maps, flood warning, and advising on planning applications in flood risk areas.

A legal agreement under the Water Industry Act 1991 for the adoption of new sewers and drainage infrastructure by the local sewerage undertaker (water company).

A legal agreement under the Highways Act 1980 between a developer and the local highway authority, enabling the developer to carry out works on the existing public highway at their own expense.

A legal agreement under the Highways Act 1980 for the adoption of new estate roads by the local highway authority, ensuring they are built to adoptable standards.

A planning requirement to steer development to areas of lowest flood risk. Applicants must demonstrate that no reasonably available alternative sites exist in areas of lower flood risk.

A subsurface drainage feature that collects surface water runoff and allows it to percolate into the surrounding soil, used where ground conditions permit infiltration.

A council-commissioned study that assesses flood risk across an entire local authority area, informing the allocation of development sites in the Local Plan and providing evidence for site-level Flood Risk Assessments.

A hierarchical approach to surface water drainage that uses a sequence of SuDS techniques -- from source control through site control to regional control -- to manage runoff progressively.

Drainage systems designed to manage surface water runoff in a way that mimics natural processes, reducing flood risk, improving water quality, and enhancing biodiversity and amenity.

A shallow, vegetated channel designed to convey, treat, and attenuate surface water runoff as part of a sustainable drainage system.

An industry-standard 2D/1D hydraulic modelling software package used to simulate flood behaviour in rivers, floodplains, estuaries, and urban environments.

The categorisation of development types by their sensitivity to flood risk, as defined in Planning Practice Guidance. The classification determines which uses are appropriate in each flood zone.