B
Bioretention
A shallow vegetated basin with an engineered filter media that treats and, where conditions allow, infiltrates surface water. Also known as a rain garden.
BRE 365 Digest
BRE Digest 365: the UK's standard procedure for field-testing soil infiltration rates and sizing soakaways.
BNG Biodiversity Net Gain
The statutory 10% uplift in biodiversity units required of most new development in England since February 2024.
C
Catchment
The area of land that drains to a single point. In urban drainage, subdivided into impermeable, permeable and receiving-water sub-catchments.
CC uplift Climate Change
Percentage increase applied to design rainfall to account for climate change, published by the Environment Agency by management catchment and time horizon.
CIRIA C753
The SuDS Manual: the industry design reference covering site assessment through to operation and maintenance, published by CIRIA.
Conveyance
The pipes, swales and channels that move surface water from catchment to storage or discharge point.
Cover level
Finished ground level at a manhole cover, used alongside invert level to size shaft depth and check against hydraulic grade line.
E
Exceedance
The overland flow path used when rainfall exceeds the design event. NSTS S6 requires an exceedance route for the 1-in-100-year + CC storm.
Embodied carbon
The carbon emitted in producing and installing drainage materials (concrete, HDPE, aggregate, geotextile), increasingly included in scheme appraisals.
F
FEH Flood Estimation Handbook
UK rainfall and flood frequency method. Produces design rainfall depths for storm durations and return periods.
Filter strip
A gently-sloped vegetated area that treats runoff from a contributing area by filtration, sedimentation and infiltration.
Forebay
A small settlement pool upstream of a pond or wetland that intercepts coarse sediment and extends the O&M interval of the main feature.
FRA Flood Risk Assessment
A report demonstrating that a development is safe from flooding and does not increase flood risk elsewhere, required for most planning submissions.
I
IH124 Institute of Hydrology 124
Classic UK method for estimating greenfield runoff rates (QBAR), still widely accepted by statutory consultees.
Impermeable area
The portion of a catchment from which runoff is effectively 100%: roofs, paving, tarmac.
Infiltration rate
The rate at which water soaks into the ground, typically measured by a BRE 365 soakaway test and expressed in m/s or mm/hr.
Interception
Capture and attenuation of the first 5 mm of rainfall, required by NSTS S1 for every development.
Invert level
The lowest internal level of a pipe or manhole, used to calculate gradients and cover depth.
M
Management train
The chain of source, site and regional controls that together deliver interception, treatment and attenuation, the central design concept in CIRIA C753.
Manning's n
A roughness coefficient used in open-channel flow equations: swales typically 0.05–0.15, smooth pipe 0.010–0.013.
S
S1–S7
The seven Defra Non-Statutory Technical Standards: runoff destination, peak rate, volume, water quality, amenity, exceedance, and construction/operation.
SAB SuDS Approving Body
The statutory approver of SuDS schemes under Schedule 3 of the Flood & Water Management Act. Operational in Wales; England awaits full commencement.
Source control
Any SuDS component that manages runoff close to where it falls: green roofs, permeable paving, soakaways, rainwater harvesting.
Surcharge
When a pipe runs full under pressure, the hydraulic grade line rises above the pipe soffit. Beyond design capacity this triggers exceedance.
Swale
A broad, shallow vegetated channel that conveys, treats and, where ground conditions allow, infiltrates surface water.
W
WFD Water Framework Directive
EU-derived regime (retained in UK law) that requires no deterioration of waterbody status, screened for any scheme discharging to a watercourse.
Wetland
A SuDS component with permanent shallow water and emergent planting, providing high treatment and ecological value.