Stripping a site of vegetation cuts infiltration from hundreds of millimetres an hour to almost nothing, turning rainfall into a flashy, sediment-laden discharge. Construction-phase drainage needs its own permit — the approved permanent SuDS scheme does not cover it. This guide covers silt traps, oil separators and flow attenuation.
Why construction sites must control surface water runoff
During construction, the hydrological response of a site changes abruptly. Vegetation and topsoil are stripped, exposing subsoil and hardcore. Infiltration capacity drops from 100–500 mm/h on grassland to below 1 mm/h on compacted clay or haul roads. The result is a flashy hydrograph: short lag times, high peak flows, and massive sediment loads.
A one-hectare site in the UK can generate 50–150 m³ of runoff from a 10 mm rainfall event — water that, if untreated, carries suspended solids at 1,000–50,000 mg/L, hydrocarbons from vehicles, and cementitious alkalinity from concrete washings. Discharging that to a watercourse without control is a criminal offence under s. 85 of the Water Resources Act 1991 and carries unlimited fines.
The regulatory framework: NPPF, Schedule 3 and discharge consents
National Planning Policy Framework
The NPPF, updated in December 2024, requires that major developments — ten or more dwellings, or half a hectare or more of non-residential floorspace — incorporate sustainable drainage systems that mimic natural drainage, unless there is clear evidence that this would be inappropriate.
While the NPPF primarily governs permanent drainage, construction-phase drainage must also be sustainable in the sense that it prevents pollution and manages flow rates. The Environment Agency expects construction-phase drainage to be designed with the same pollution-prevention mindset as permanent SuDS.
Flood and Water Management Act 2010, Schedule 3
Schedule 3 of the FWMA 2010, commenced in England in 2024, creates a SuDS Approval Body (SAB) in each lead local flood authority. For major developments the SAB must approve the drainage system before construction begins.
The practical implication is that the construction-phase drainage plan forms part of the SAB application. It must show how runoff is intercepted, treated and discharged during each construction phase; the sediment and pollution control measures; flow rates that do not exceed greenfield rates (or pre-development rates on brownfield); and the adoption and maintenance arrangements.
Discharge consents for construction-phase runoff
Construction-phase runoff is not exempt from environmental permitting. The discharge route determines the permit.
| Discharge route | Permit / consent | Trigger |
|---|---|---|
| Watercourse | Standard rules permit (SR2015 No. 4, surface water discharge) or bespoke | Any discharge from site drainage |
| Combined sewer | Surface water discharge agreement with the sewerage undertaker | Connection to public sewer |
| Ground infiltration | Groundwater activity permit | Soakaway or infiltration basin |
| Highway drain | Highway authority consent plus the underlying permit | Discharge to adopted highway |
Designing SuDS for construction-phase runoff
The construction-phase drainage train handles higher sediment loads and more variable flows than the permanent system. The design philosophy is to treat close to source, slow the flow, and protect the receptor.
| Stage | Feature | Purpose | Design parameter |
|---|---|---|---|
| 1. Source control | Wheel wash, hardstanding bunding | Keep mud and oil out of the drainage | 100% vehicle coverage |
| 2. Pre-treatment | Silt fence, check dam, swale | Remove coarse sediment | Swale velocity limit |
| 3. Primary treatment | Silt trap / settlement pond | Remove settleable solids | HRT >24 h; overflow rate <0.5 m³/m²·h |
| 4. Secondary treatment | Filter drain or bioretention | Remove fines and hydrocarbons | 50 mm graded sand media |
| 5. Flow control | Orifice plate or vortex control | Limit discharge to greenfield Qbar | Sized on head and target flow |
| 6. Discharge | Watercourse, sewer or soakaway | Compliant outflow | Meet permit limits |
Where the solids are too fine to settle, the same options apply as anywhere else — see DAF vs sedimentation for the selection logic, and media filtration design for the polishing stage.
Silt trap sizing
The silt trap is the workhorse of construction drainage. Size it for the one-in-one-year, one-hour storm as a minimum, with capacity for sediment accumulation between clean-outs.
- Runoff coefficient: C = 0.9.
- Peak flow: Q = C × i × A = 0.18 m³/s.
- First-flush volume: the first 10 mm of rainfall is the most sediment-laden: V = 0.010 m × 20,000 m² = 200 m³.
- Provide: 250 m³ (25% extra for sediment storage) at 1.5 m depth.
- Surface area: 250 / 1.5 = 167 m² — about 12 m × 14 m.
- Clean-out: every two weeks, or when 30% of the volume is sediment.
Flow control and greenfield rates
The NPPF and the SAB require that post-development runoff rates do not exceed greenfield rates. During construction the same principle applies: the site must not increase flood risk downstream.
For a small site, a workable rule is to limit discharge to 2 L/s per hectare for the one-year event. The two-hectare site above must therefore discharge at below 4 L/s (0.004 m³/s) — a 98% reduction from the uncontrolled peak of 0.18 m³/s. That requires attenuation storage.
- Inflow volume: 0.5 × 0.18 × 3,600 = 324 m³.
- Outflow over the same hour: 0.004 × 3,600 = 14.4 m³.
- Storage required: 324 − 14.4 = 310 m³.
That 310 m³ is in addition to the silt trap. In practice the two are combined, with a sediment forebay taking the first 30% of the volume and a clean-water zone discharging through the flow-control structure.
Sediment and hydrocarbon control
Oil separator requirements
Any site with vehicle movement, fuel storage or machinery maintenance must intercept hydrocarbons before discharge. The pollution prevention guidance on oil separators distinguishes a full retention separator, for sites with high oil risk such as fuel storage and refuelling areas, from a bypass separator for sites with low oil risk such as general vehicle movement.
The nominal size is the flow in litres per second the separator can treat. For the 0.18 m³/s (180 L/s) peak above, a full retention separator is NS 180; a bypass unit is NS 90, treating half the flow and bypassing the remainder. A Class I separator, treating to 5 mg/L oil, is required for discharge to sensitive watercourses; a Class II unit at 100 mg/L is acceptable for discharge to foul sewer with consent. The selection and sizing logic is covered in oil and grease separators.
Silt fences and check dams
Silt fences are temporary geotextile barriers, typically 0.6–1.0 m high, installed along contours. They are not primary treatment — they capture only coarse sand and visible debris. Practical limits: a maximum contributing area of 0.5 ha per 30 m of fence, a maximum slope length behind the fence of 30 m, 150 mm minimum embedment, and daily inspection during rainfall with immediate repair if undermined.
Check dams are small rock or timber barriers in temporary drainage channels that slow flow, promote settling and dissipate energy. Spacing is set so that the toe of one dam is at the crest level of the next.
Hydrocarbon interception at source
Beyond separators, bunded storage at 110% of the largest container volume, drip trays under all static machinery and refuelling points, spill kits within 10 m of any hydrocarbon storage, and a written emergency procedure for spills above 200 litres or any spill reaching a watercourse are all expected as standard practice.
Where runoff is being reused rather than discharged — increasingly common on large sites — the treated water still needs disinfection before any contact use. See UV disinfection design and our guide to industrial rainwater harvesting.
Adoption and maintenance
Under Schedule 3 the SAB must be satisfied that the permanent SuDS will be maintained for the lifetime of the development. Three routes exist: adoption by the sewerage undertaker, which requires compliance with the Design and Construction Guidance; adoption by the local authority through a Section 106 agreement; or private maintenance by the developer or a management company, which requires a maintenance manual and a funding mechanism such as a service charge.
Construction-phase features are not adopted. The developer must either remove them and restore the ground, or design them from the outset to be incorporated into the permanent system.
A SuDS maintenance manual should include as-built drawings of every drainage feature; design volumes, flow rates and treatment capacities; inspection frequencies (silt trap monthly, oil separator six-monthly); sediment removal trigger levels, typically when 30% of storage volume is lost; vegetation management; and emergency contacts with spill response procedures.
Frequently asked questions
Do I need a discharge consent for clean roof runoff during construction?
If the runoff is genuinely clean — no contact with construction materials, no sediment, pH 6–9 — it may be exempt as uncontaminated rainwater. On an active site, roof runoff is rarely clean; it picks up dust, bird droppings and atmospheric deposition. The EA default position is that all construction-site runoff requires a permit or consent unless proven otherwise.
How often must I clean a silt trap during construction?
Monthly as a minimum, and after every rainfall event above 10 mm. A two-hectare site can deposit 5–10 m³ of sediment in a single storm. Once the trap reaches 30% sediment by volume it loses effectiveness. Desilt by excavator when dry or suction tanker when wet, and test the sediment for contamination — if hazardous, it needs licensed disposal.
Can I use a portable treatment plant for construction runoff?
Yes. Containerised coagulation, flotation and filtration plant is routinely hired for short-term duties. It needs the same permit as fixed plant, and the application must include the plant specification, chemical storage details and emergency shutdown procedure. Mobile plant still has to meet the same emission limits.
What is the difference between a silt trap and a silt fence?
A silt trap is a volumetric feature — pond, tank or chamber — that stores the flow and lets sediment settle by gravity. A silt fence is a barrier that filters sheet runoff through geotextile; it has no storage volume and fails in concentrated channel flow. Fences are for sheet runoff on slopes, not for drainage channels.
Who enforces SuDS compliance during construction?
Three bodies, and any of them may inspect. The Environment Agency covers pollution control, discharge permits and water quality. The lead local flood authority covers SuDS approval, flood risk and surface water management. The local planning authority covers planning conditions and Section 106 agreements. The EA can issue stop notices for unpermitted discharges.
How do I calculate greenfield runoff for a small site?
For sites under 5 ha, use the IH 124 method or the FEH catchment descriptor approach. As a rule of thumb, Qbar is roughly 2 L/s per hectare for lowland England and 3–4 L/s per hectare for upland and western areas. For design work, use the FEH Web Service or drainage software rather than the rule of thumb.