Silt management and pre-treatment before infiltration

Most infiltration features that stop working were built correctly. They clog, because suds pre treatment silt control was either absent or designed for the finished development rather than for the construction period. Fine sediment blinds the infiltration surface, the observed rate falls far below the tested rate, and a system that satisfied the drainage strategy on paper backs up in its third winter.

The fix is structural rather than operational. Catch sediment upstream, in features that can be cleaned without excavating the thing you are protecting, and protect the infiltration surface until the site is finished.

Why infiltration features fail

Three mechanisms, in order of frequency.

Construction phase sediment. Bare ground generates orders of magnitude more suspended solids than the same site does once it is surfaced and planted. If a soakaway, trench or basin is built early and takes construction runoff, it is loaded with silt before a single dwelling is occupied.

Absent pre-treatment in the completed system. Road and parking runoff carries fines, hydrocarbons and metals. Without an interception stage, all of it arrives at the infiltration surface.

Maintenance that never happened, usually because the feature could not be reached, or because nobody knew whose job it was.

A fourth cause is less common and more serious: infiltration attempted through ground where the design assumptions were wrong, whether because the test was done in the wrong place, at the wrong depth, or in the wrong season.

Treatment stages and the train approach

The principle is that no single feature does all the work. A sequence of stages each removes part of the load, and each is easier to maintain than the one downstream of it.

A workable train for a small residential scheme:

1. Source control, such as permeable paving with a sub-base that acts as its own filter, or a green roof, dealing with rainfall where it lands.
2. Conveyance with treatment, meaning a grass swale or a filter strip rather than a pipe, which strips sediment along its length.
3. Interception, meaning a catchpit at each gully, a silt trap, or a proprietary separator where the hazard justifies it.
4. A forebay or settlement stage immediately upstream of the infiltration feature.
5. The infiltration feature itself.

The industry manual for sustainable drainage sets out an index-based approach that matches the pollution hazard of the contributing surface against the mitigation the proposed train provides. Use it and show the working, because an officer or a lead local flood authority can check it in minutes, and a strategy that simply asserts adequate treatment cannot be checked at all.

Roofs are a low hazard surface and generally need little. A lorry yard is a high hazard surface and needs a great deal, and on such a site infiltration may not be appropriate at all.

Forebays, filter strips and catchpits

Each stage has a job and a failure mode.

A forebay is a shallow settlement area separated from the main feature by a low berm or a check, with hard access so a machine can clear it from the bank. Its failure mode is inaccessibility: a forebay that can only be reached across a grass slope will not be cleaned, so detail the access and the standing area for the arisings.

A filter strip is a gently graded vegetated surface across which sheet flow passes before entering a swale or basin. Its failure mode is channelling, where flow concentrates into a rill and bypasses most of the strip. Prevent that with a level spreader at the top and a shallow uniform cross-fall.

Catchpits and silt traps are the simplest stage and the most neglected. Their failure mode is simply not being emptied, which turns them from a treatment stage into a source of resuspended sediment during the next storm.

Permeable paving deserves a specific note. It is both a source control and a treatment stage, and its performance depends on sweeping rather than on anything buried. Where it is used, the maintenance schedule has to name the sweeping regime, and the construction sequence has to keep it clear of site traffic and of stockpiled material.

Construction phase silt and the damage it does

This is the part that belongs in the planning submission and usually is not there.

Set out a construction phase drainage plan showing how runoff from the working area will be managed and where sediment will be intercepted before it reaches any permanent feature. The measures are well established: temporary settlement lagoons or tanks, silt fences on contour, stabilised haul routes and wheel wash, minimising the area of bare ground exposed at any one time, and seeding or covering stockpiles.

Then sequence the permanent works to protect the infiltration surfaces:

• Build infiltration features last where the programme allows.
• Where they must be built early, leave the infiltration surface blinded with a protective layer that is removed and the surface scarified at completion.
• Keep construction runoff out of them entirely, by physical isolation rather than by instruction.
• Record what was done, with dated photographs, because the person who inherits the system will otherwise assume nothing was.

This material sits naturally in the Construction Management Plan, and it needs to be consistent with the permanent drainage design rather than written separately by a contractor who has not read it.

Dewatering discharge is a separate matter with its own regulatory route, and discharging silty water to a watercourse or to ground can need an environmental permit.

Groundwater and where infiltration is not appropriate

Two constraints rule infiltration out on a proportion of sites, and both should be settled before the drainage strategy is drafted.

Groundwater level, because an infiltration device needs unsaturated ground beneath it to work. A separation between the base of the device and the highest expected groundwater level is standard practice, commonly a metre, and establishing the highest level means monitoring rather than a single reading. Our note on evidencing groundwater flooding in a flood risk assessment covers how that evidence is built.

Contamination, because infiltrating clean water through contaminated ground mobilises the contamination and drives it towards groundwater. On a site with a history of industrial use, or within a source protection zone, the regulator's position may be that infiltration is not acceptable, and that answer changes the whole drainage strategy.

Test properly. Infiltration testing to the recognised method, at the location and depth of the proposed feature, repeated enough times to give a reliable rate, with the result reported alongside the test pit locations and the groundwater observations. A single test in a convenient corner of the site is not a design basis.

Inspection, cleaning and who is responsible

The maintenance schedule has to name three things for every stage: the operation, the frequency, and the trigger.

Frequency is a starting point and the trigger is what matters. A forebay cleared when sediment reaches a stated depth is maintainable. A forebay cleared every five years is either cleared too often or, more usually, not often enough.

Then name the responsible body. Adoption by the sewerage undertaker where the design meets its published requirements, adoption by the local authority as part of open space, or a private management company funded by a service charge. Each has different design consequences, and the decision has to be made before the geometry and access are fixed rather than after.

Inspection after the first heavy storm following completion is worth building into the schedule explicitly. That is when construction phase damage shows up, and it is the point at which it can still be attributed to someone.

What the strategy should specify

For a planning submission, the Drainage Strategy should carry all of this:

1. Infiltration test results with locations, depths, method and groundwater observations.
2. A pollution hazard assessment for each contributing surface, with the mitigation train shown against it.
3. A drawing of the treatment train, with every stage labelled and its access route shown.
4. Details of the forebay, catchpits and any proprietary device, at a scale that shows the invert levels.
5. A construction phase drainage and sediment control plan, cross-referenced to the construction management plan.
6. A maintenance schedule with operations, frequencies, triggers and the responsible body named.

Where the site also has a flood risk dimension, the residual risk if the infiltration system underperforms belongs in the Flood Risk Assessment rather than being left unstated.

This is general guidance, not legal advice. Check the position with your lead local flood authority, your local planning authority or a qualified drainage engineer before relying on it.

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