Construction phase drainage and silt control

The construction period is when a development is most likely to pollute a watercourse, and it is the period least covered by the drainage documents submitted with the application. An operational drainage strategy describes a finished site with landscaping established, roofs connected and flows attenuated. During the build, that site is bare earth, stockpiles and tracked mud, discharging to the same watercourse with none of the permanent treatment in place. Silt is the result, and silt is the commonest pollutant reported from construction sites.

Two things follow for anyone programming a scheme. The construction phase needs its own drainage design, and the permanent components need protecting from the construction phase.

Why the exposed period is the highest risk

Stripping topsoil removes the layer that held rainfall and bound the surface together. What is left generates runoff faster, carries fine particles, and delivers both to the lowest point on the site. A rainfall event that would have produced nothing measurable from a grass field produces a plume of suspended solids from a stripped platform.

The damage is ecological rather than dramatic. Fine sediment smothers gravel beds, clogs the interstitial spaces that invertebrates and fish eggs depend on, and reduces oxygen exchange. It also carries phosphorus and hydrocarbons bound to the particles, so a silt problem is frequently a nutrient and oil problem as well.

Regulators treat it seriously. Causing or knowingly permitting a water discharge activity without a permit, including the discharge of polluting matter to inland freshwaters, is an offence under the Environmental Permitting (England and Wales) Regulations 2016. Enforcement does not wait for the development to be finished.

Designing the temporary system

Treat the construction drainage as a designed system with a capacity, not as a set of measures bolted on when the inspector complains.

Start with a plan of the site at each main phase, showing where water will go once levels change. Earthworks reverse catchments. A platform cut into a slope collects water that used to run past it, and a haul road across a contour becomes a channel. Draw the temporary catchments and size the interception accordingly.

The usual components are perimeter cut-off ditches to keep clean upslope water out of the working area, in-catchment interception to collect dirty water, and settlement before discharge. Keeping clean water clean is the cheapest measure available and the one most often skipped. Every litre of upslope runoff you let into the working area is a litre you then have to treat.

Settlement needs residence time, and residence time needs volume. A shallow scrape at the site corner does not settle fine silt. Lagoons or tanks sized for the design event, with baffles or a long flow path, and with an emergency overflow that does not discharge straight to the watercourse, are the workable arrangement. Cohesive clay soils release very fine particles that will not settle by gravity in any reasonable time, and chemical dosing to assist settlement needs agreement with the Environment Agency before it is used.

Silt fences, traps and wheel washing

The site-level measures are simple and they fail in predictable ways.

• Silt fences work where they intercept sheet flow on a gentle gradient and are trenched in at the base. They fail where they are stapled to stakes on the surface, where flow concentrates, and where nobody empties the accumulated sediment.
• Check dams in temporary channels reduce velocity and drop coarse material. They need a stable spillway, or they simply wash out and add to the problem.
• Stone filter berms and geotextile-wrapped gully protection buy time at inlets, and they blind quickly in wet weather.
• Wheel wash facilities stop the road outside becoming the discharge point. Site them so vehicles travel a stabilised surface after washing, and collect the wash water rather than letting it run to the gutter.
• Stabilised haul routes and early placement of the permanent sub-base cut sediment generation at source, which beats treating it downstream.

Sequencing matters more than any individual item. Stripping the whole site in one operation because the earthworks contractor is on site is the single decision that creates most of the risk. Phase the stripping, stabilise or seed areas that will sit idle, and keep the exposed area proportionate to what is being built.

Consents during construction

Several consents can apply at once, and they come from different bodies.

An environmental permit from the Environment Agency is needed for a water discharge activity to surface water or groundwater unless an exemption applies. Dewatering an excavation can also require an abstraction licence where the volumes cross the licensing threshold, and that has been the position since dewatering was brought within the licensing regime. Works in, over, under or near a main river need a flood risk activity permit, and works to an ordinary watercourse need land drainage consent from the lead local flood authority rather than the Agency.

Discharging construction water to the public sewer needs the sewerage undertaker's consent and is usually charged. Discharging to a highway drain needs the highway authority's agreement.

Apply early. Permit determination periods are measured in months, and a contractor who discovers in week three that the intended discharge point needs a permit will either stop or do something worse.

Protecting the permanent drainage

This is where money is lost quietly. Infiltration components are the most vulnerable: a soakaway, an infiltration basin or permeable paving that receives construction runoff will silt up, and the loss of infiltration capacity is often permanent. Rebuilding is the only real remedy, and it happens after handover when nobody has budgeted for it.

Build infiltration devices last wherever the programme allows. Where they must be built early, keep them offline, bund them, and cover permeable surfaces or leave the final course until the site is stabilised. Do not use an attenuation basin as a settlement lagoon and then hand it over without desilting and reinstating the outlet control.

Tanks, pipes and flow controls need the same discipline. Hydrobrakes and orifice plates block with construction debris, and a blocked flow control is discovered at the first storm after occupation. Specify a jetting and inspection regime before handover and record it.

Set this out in the Drainage Strategy rather than leaving it to the contractor, because the strategy is the document the drainage officer reads and conditions against. Where the site is in a flood zone, the Flood Risk Assessment should also address the construction period, since a partially built site with temporary levels can behave very differently in a flood from either the existing or the finished condition.

Monitoring and who is responsible

Visual inspection of the discharge point, before, during and after rainfall, is the basic monitoring. Turbidity monitoring gives a number where a receiving water is sensitive or where there is a history of complaint.

An illustrative example. A 3 hectare site discharges to a small ordinary watercourse through a single outfall. The contractor inspects the outfall daily and after any rainfall over a stated depth, records turbidity readings in a log, and has a written trigger: above the stated reading, discharge stops and the settlement lagoon is bypassed to a holding tank. Three items, one log, and it is the difference between a warning and a prosecution.

Name the responsible person in the method statement rather than referring to the principal contractor as an entity. Pollution incidents happen at weekends, and a named person with authority to stop work is worth more than a procedure nobody owns.

Securing the measures through planning

Authorities normally secure construction phase drainage through a condition requiring a construction environmental management plan, or through a wider management plan covering traffic, noise, dust and water together. Draft the submission so the drainage element is specific: the temporary catchment plan, the settlement volume, the discharge point, the consents relied on, the monitoring regime and the responsible person.

A Construction Management Plan that says pollution will be controlled in accordance with good practice will be returned. One that names the discharge point and the design event will not. Where construction traffic is also a concern, the same submission usually sits alongside a Construction Traffic Management Plan, and the two should share one site layout so the wheel wash and the haul route agree with each other.

This is general guidance, not legal advice. Check the position with your local planning authority, the lead local flood authority, the Environment Agency and a qualified drainage engineer 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. For specialist technical reports, qualified professional input may be required.