Silt and runoff control during construction

The riskiest period in the life of a drainage design is the eighteen months before it exists. A finished scheme has attenuation, treatment stages and a controlled discharge. A site stripped to subsoil in February has none of that, and it sheds muddy water at a rate the permanent design was never meant to handle. Construction phase runoff control is the part of the drainage story that planning conditions cover thinly and that regulators pursue hardest.

It is also the part where the consequences are criminal rather than administrative. Causing or knowingly permitting polluting matter to enter controlled waters is an offence under the Water Resources Act 1991, and silt counts as polluting matter.

Why the construction phase is the highest risk period

Three things change at once when a site is stripped.

The surface stops absorbing water. Bare subsoil compacted by tracked plant can shed more water than the hardstanding that preceded it, and the runoff carries fines in suspension.

The drainage network is incomplete. Outfalls exist before attenuation, or attenuation exists before the treatment train, so there is a period where water reaches the receiving water with no control at all.

The receptors are unchanged. The watercourse, the sewer, the neighbouring land and the road gullies are all still there, and the first heavy rainfall after stripping is when the complaints arrive.

Silt is the dominant pollutant, and it is more damaging than its appearance suggests. Suspended solids smother gravel beds, remove invertebrate habitat and affect fish spawning, and the effect persists after the water clears. Concrete and cement washings are the other common offender, because high pH runoff is acutely toxic to fish at concentrations that look like nothing on site.

Temporary drainage and settlement measures

Design the temporary system as a system rather than as a set of products. The sequence that works runs: keep clean water out, slow dirty water down, settle it, then discharge it under control.

• Cut-off ditches and bunds along the upslope boundary, to divert clean run-on away from the working area before it becomes dirty.
• Phased stripping, so that the area of exposed ground at any time is limited and stabilised ground is reinstated quickly.
• Filter berms, silt fences and check dams across flow paths to reduce velocity and drop the coarser fraction.
• Settlement tanks or lagoons sized for the contributing area and the expected rainfall, with a design residence time rather than a guess.
• A controlled outlet, with the discharge point agreed in advance and accessible for sampling.
• Stabilisation of long term stockpiles and completed earthworks by seeding, matting or covering.

Settlement alone will not remove fine clay fractions, which stay in suspension for days. Where the ground is clay rich, the honest answer is that settlement has to be supplemented, and chemical dosing is a regulated activity requiring the agreement of the regulator rather than something to be arranged on site.

Wheel washing, haul routes and silt traps

Mud on the public highway is the most visible failure and the one that generates enforcement from two directions, since it is both a highways and an environmental problem.

A wheel wash is effective only if the vehicle reaches it clean enough and leaves on a hard surface. Design the exit sequence: a stoned haul route, a shaker grid or wash, then thirty metres of hardstanding before the gate, with the wash water contained and recirculated rather than draining to the kerb.

Road sweeping is a management measure, not a design one, and a condition requiring it should specify a frequency and a trigger. Gully protection at the site entrance prevents the sweeper's work entering the surface water sewer.

These items belong in the construction management plan alongside access routes and hours, because the highway authority and the environmental protection team read the same document and ask about different parts of it.

Dewatering and where the water goes

Dewatering an excavation produces a continuous discharge of water carrying fines, and it is the activity most likely to need a permission in its own right.

Two regimes can apply. Abstraction, because removing groundwater to lower the water table has been a licensable activity since 2018 above a daily volume threshold, so a long term dewatering operation may need an abstraction licence. And discharge, because putting that water into a watercourse or a surface water sewer needs either an environmental permit or the agreement of the sewerage undertaker.

Plan the route before the excavation starts. Discharge to foul sewer is usually refused on volume grounds. Discharge to a watercourse needs treatment and a permit. Tankering is expensive and sometimes the only answer. Where groundwater is close to the surface, the ground investigation should have flagged it, and evidencing groundwater flooding in a flood risk assessment is the same evidence base read for a different purpose.

If the discharge goes to or near an ordinary watercourse, remember the separate consent regime for works affecting it, covered in land drainage consent for works to an ordinary watercourse.

Permits and consents for temporary discharge

Keep a register of what you need and who grants it:

Apply early. Permit determination periods are measured in months, and starting an excavation on the assumption that a discharge consent will arrive is a common way to end up tankering at short notice.

Monitoring and incident response

Written measures that nobody checks are worthless. Three practical requirements for the plan.

A daily visual check of the discharge point and the receiving water, recorded, with photographs when conditions change. Visible turbidity downstream of an outfall is the first sign of failure and it is free to spot.

Simple quantitative monitoring where the receptor is sensitive: pH at a minimum, since concrete washings are the fastest way to kill a fish population, and suspended solids where a permit requires it.

An incident response procedure naming the person responsible, the containment equipment held on site, the isolation points in the temporary drainage, and the duty to report a pollution incident to the environment regulator. Spill kits in the site office are not a response plan.

Rainfall triggers are worth building in. A stated response to a forecast of heavy rain, covering stockpile covering, outfall inspection and a halt to earthworks, turns a reactive site into a managed one.

Where this sits in the submitted documents

Construction phase water management rarely gets a document of its own, and it should not need one. It belongs in three places.

The flood risk assessment should state how surface water will be managed during construction, including the interim discharge arrangements and any temporary attenuation, rather than assuming the permanent design applies from day one.

The drainage strategy should show the phasing of the permanent system and identify the period during which temporary measures carry the load. Lead local flood authorities increasingly ask for this explicitly.

The construction environmental management plan, usually secured by condition, carries the detail: the measures, the monitoring, the responsibilities and the incident procedure. Where a waste plan is also conditioned, the site waste management plan should align with it on the handling of contaminated arisings and washout residues.

One honest observation. Conditions on this subject are often drafted loosely, which gives a well-organised contractor latitude and gives a careless one enough rope to cause a prosecution. The regulator's interest does not depend on how the condition was worded.

This is general guidance, not legal advice. Check permit requirements and the position with your local planning authority, the environment regulator or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.