Quick reference: the core terms in sustainable drainage are SuDS (the overall approach), infiltration and attenuation (the two strategies), the soakaway and geocellular system (the structures), and the percolation test and void ratio (the design inputs). Definitions below.
Last updated: 16 June 2026 · NGS Engineering Team
- Attenuation
- Temporarily storing stormwater and releasing it at a controlled rate into the drainage network. Used where the ground cannot infiltrate water quickly enough, or where discharge to a network is required. See infiltration vs attenuation.
- Drain-down time
- The time a storage or infiltration system takes to empty after a storm. A key check in soakaway design — the system must recover before the next event.
- Geocellular system
- A modular underground structure of high-void plastic cells used to store stormwater for infiltration or attenuation, replacing traditional gravel-filled storage. See the geocellular soakaway design guide.
- IDF curve
- Intensity–Duration–Frequency curve — rainfall data relating storm intensity to its duration and return period. Used to choose the design storm for a drainage system.
- Infiltration
- Allowing stored stormwater to soak into the surrounding soil — the principle behind a soakaway. Requires adequate soil permeability and a low groundwater table.
- Passive irrigation
- Using stored stormwater beneath planted areas to supply moisture to plant roots, reducing irrigation demand — a use of geocellular systems in landscape design.
- Percolation test
- An on-site test that measures the soil's infiltration rate, used to confirm whether a soakaway is viable and to size it. The BRE Digest 365 method is the common reference.
- Return period
- The average interval between storms of a given severity (for example, a 1-in-10-year storm). Used with IDF data to set the design rainfall.
- Sabkha
- Salt-flat ground found in parts of Arabia, often with high groundwater and poor bearing capacity. Sabkha conditions usually rule out infiltration and favour attenuation with controlled discharge.
- Soakaway
- An underground structure that stores stormwater and lets it infiltrate into the ground. Modern soakaways use geocellular cells instead of gravel. See the design guide.
- SuDS (Sustainable Urban Drainage Systems)
- An approach that manages rainfall near where it falls — through infiltration, storage and controlled release — rather than piping it straight to a network. See the pillar guide.
- Void ratio
- The proportion of a geocellular unit that is open space available to store water. Modern units exceed 95 percent, far more than gravel.
About this glossary: definitions are written by NGS for general guidance and reflect common usage in sustainable drainage practice. For governing requirements, see the
standards & references page.
