Surface Water Drainage for Rural Properties

The general hierarchy for dealing with surface water is infiltration into the ground first, discharge to a watercourse second, and connection to a surface water sewer as a last resort where the first two are not viable. Many rural Cumbrian sites have no surface water sewer nearby at all, which pushes the decision straight to infiltration versus watercourse.

A soakaway only works if the surrounding ground actually accepts water at a reasonable rate. Percolation testing — digging a trial pit, filling it with water and timing how quickly the level falls — establishes that rate before a soakaway is sized and built.

Building a soakaway in heavy clay does not solve a drainage problem; it just relocates the puddle underground, where it is harder to see and more expensive to fix. Where percolation is poor, a soakaway is simply not the right answer and another route should be found.

Where the rate a site is allowed to discharge water — to a watercourse or a sewer — is restricted, attenuation stores water temporarily and releases it slowly rather than all at once. This is done with storage crates, oversized attenuation pipes, or a pond or basin, sized for the roof and hard surface area draining to it.

Attenuation is increasingly expected on new development, particularly where a site adds hard surface area that did not drain that way before, and it is far easier to design in from the outset than retrofit.

Discharging surface water to a watercourse — a stream, beck or ditch — generally needs the agreement of whoever has responsibility for that watercourse, which may involve the relevant environmental regulator, the local authority as lead local flood authority, or a private riparian owner depending on the situation. This should be checked and confirmed in writing before a system is designed around it, not assumed.

Surface water — from roofs, drives, yards and hardstanding — should be kept entirely separate from foul drainage and, critically, from a septic tank or package treatment plant. Roof water entering a treatment system dilutes it, can overload its capacity, and reduces how well it treats the sewage it was actually designed for.

On working farms this separation matters even more: clean roof water kept out of a dirty water (yard washings, effluent) system saves a large amount of storage capacity that would otherwise be needed.

Land drainage — perforated pipe laid to lower a naturally wet water table, rather than to carry water from a roof or hard surface — is common on rural Cumbrian plots where fields have historically been drained for agricultural use. Existing land drains crossing a building plot are easy to sever accidentally during excavation, and doing so without reinstating them can make a previously dry area wet.

Where a site has a genuinely high water table or springs, new land drainage may be needed around a building or a track as part of the wider drainage strategy, separate from the surface water system serving the building itself.

A well-designed system still needs looking after. Most surface water drainage failures on established rural properties are maintenance failures — a silted soakaway, a blocked gulley or a collapsed field drain — rather than a design fault, which is worth remembering before assuming a whole system needs replacing.

Sustainable drainage — often referred to by the acronym SuDS — aims to manage surface water as close to where it falls as practical, and at a rate closer to how the undeveloped land behaved before, rather than piping it away as fast as possible. Permeable paving, swales (shallow, planted channels), and soakaways or infiltration basins are all sustainable drainage components as much as they are individual design choices.

On a rural plot, sustainable drainage principles often align naturally with what is already sensible — keeping hard surfaces to what is genuinely needed, directing yard water to a grassed area where percolation allows, and avoiding unnecessary extra impermeable area that then has to be piped and disposed of somewhere.

Some older rural properties have a single system carrying both foul and surface water together, a legacy of how they were originally built rather than current good practice. Where a property is being extended or its drainage upgraded, separating surface water out from a combined system is usually preferable, since it reduces the volume reaching a septic tank or treatment plant and reduces flood risk from an overloaded combined pipe.

Whether separation is required, and to what standard, is worth discussing with building control at an early stage, since it affects both the design and the sequence of the drainage works.

Surface water systems are sized against expected rainfall intensity for the area and the size of the catchment (roof, yard or hard surface) draining to them, generally following guidance referenced by building control and, on larger schemes, by the local authority as lead local flood authority. Undersizing a soakaway or attenuation system against optimistic assumptions is a common cause of a system that copes in light rain but floods in a genuine downpour.

Climate patterns are also shifting toward more intense rainfall events in parts of the country, which is one of the reasons attenuation and generous soakaway sizing are increasingly favoured over the more minimal provision that was once considered adequate.

As with foul drainage, distance and ground conditions along the route tend to matter more to the final cost than the headline size of the building or yard being served.

Beyond how a building's own surface water is dealt with, its position on the plot relative to natural water flow paths matters — building across a route that water has historically taken across a site in heavy rain tends to store up problems, however good the drainage design around the building itself.

Where a site is in or near an area with any known history of flooding, or is identified as being at risk on flood maps published by the relevant environmental agency, this is worth raising with the designer and the local planning authority early, since it can affect finished floor levels and the drainage strategy as a whole, not just the surface water design in isolation.

Permeable paving — block paving laid on a permeable sub-base rather than a sealed one, or a resin-bound surface — lets some rainfall infiltrate through the surface itself rather than all of it running off to a drain. On a rural drive or yard this can meaningfully reduce the volume a soakaway or attenuation system has to handle, though it still depends on the ground beneath accepting water, and it needs periodic maintenance to stop the surface clogging with fine silt over time.

On an individual rural property, the owner is normally responsible for maintaining their own soakaway, attenuation system and the pipework serving it, in the same way they are responsible for the septic tank or treatment plant. Where a shared system serves more than one property — a shared soakaway or attenuation pond on a small development, for example — responsibility should be set out clearly, ideally in a legal agreement, so maintenance does not become a dispute later.

Keeping a simple record of what was installed, where inspection chambers are, and what the design assumed is a modest piece of paperwork that saves considerable time if the system is ever queried, extended, or sold on with the property.