Tech that you can use when landscaping your house: sustainable drainage and terrain grading done the estuary way

Sustainable drainage systems, usually shortened to SuDS, are the set of techniques that slow rain down, spread it out, soak it in and clean it before it reaches a pipe or a watercourse. In England the planning system has leaned on them since 2015 and Wales has made them mandatory for most new development since 2019, with approval bodies in each council. The technology behind them is modest, affordable and mostly misunderstood. Here is how we would use it on a house plot or a municipal yard, borrowing from the kit we carry on an estuary survey.

Laser-level and LiDAR site contouring

You cannot grade what you have not measured. A garden that looks flat is usually falling by 1 in 80 in a direction you have not noticed, and that direction is where the water will go after you have laid the patio. The first tool is a rotating laser level with a receiver staff. A good one costs less than a weekend of mini-digger hire, reads to 2 millimetres over 30 metres, and lets one person take 40 spot levels in an hour. Mark the levels on a sketch plan with a tape and a few fixed reference points, and you have a contour map good enough for a drainage design.

For anything bigger than a plot, or for a slope that has already failed once, we use the same approach we use for an intertidal survey: airborne LiDAR for the shape and ground truth for the trust. The Environment Agency and Natural Resources Wales both publish 1 metre and 50 centimetre LiDAR digital terrain models for most of England and Wales under an open licence. Load the tile for your postcode into a free GIS package, draw a few cross-sections, and you have the catchment above your site, which is what actually decides how much water arrives at your boundary in a storm.

Two cautions. LiDAR is a surface model, so it sees hedges and sheds as ground unless you choose the bare-earth product. And it was flown on one day some years ago. If a neighbour has since raised a drive, your model is out of date by exactly the thickness of their tarmac.

Permeable sub-base hydrological calculators

A permeable paved drive is a storage tank with a hard lid. The paving blocks let water through their joints, and the open-graded aggregate beneath holds it in its voids, typically 30 to 35 percent of the volume, until it soaks into the subgrade or drains through a perforated pipe. The question every calculator is answering is simple: for the design storm, is the void volume larger than the rain that falls on the drive minus whatever infiltrates during the storm?

The free calculators published by the block-paving manufacturers do this arithmetic and pick a sub-base depth for you. They are honest tools. What they cannot do is measure your infiltration rate, and that single number swings the design more than any other. Dig a 300 millimetre test pit, fill it, time the fall, and repeat it three times. If the third fill takes more than 24 hours to drain, your clay is telling you to plan for a pipe to a soakaway or a watercourse, with a flow control at the outlet so that you do not simply move the flood to the house downhill.

Rain-garden retention sizing

A rain garden is a shallow planted basin that takes roof or drive runoff, ponds it for a few hours and lets it soak away through a prepared soil mix. On a house plot it is the single most effective SuDS feature per pound spent. Sizing it is the same impoundment problem we solve for a tidal lagoon, only in reverse: you know the volume coming in and you need to choose an area and a depth to hold it.

• Catchment. Measure the plan area of every roof and hard surface that drains to the garden. A typical semi-detached roof half is 40 to 50 square metres.
• Design rain. For the UK, 25 to 30 millimetres over an hour is a sensible garden design storm. Multiply by the catchment: a 50 square metre roof gives 1.5 cubic metres.
• Ponding depth. Keep it to 150 millimetres so the plants survive and the children do not need a lifeguard.
• Area. Volume over depth, so 1.5 cubic metres over 0.15 metres is 10 square metres, before counting the storage in the soil mix below. In practice a rain garden of 15 to 20 percent of the catchment area works on most British soils.

Build the inlet with a stone apron so the water does not cut a gully on arrival, and give the basin an overflow at the top of the ponding depth that leads somewhere safe. Every impoundment needs a spillway. The ones that fail are the ones that were never given one.

Preventing surface runoff erosion

Erosion is not a strength problem, it is a velocity problem. Water at 0.3 metres per second will carry silt. At 0.6 it moves sand. At 1.0 it starts to lift gravel and take the topsoil with it. On the Severn we see the same thresholds in the tide, scaled up with a lot more water. On a sloping garden the trick is to keep the water below the speed at which it can pick anything up, and that is done with terrain grading rather than with product.

Grade slopes to no steeper than 1 in 4 where they will be planted, and break any run longer than 10 metres with a swale, a shallow grassed channel running across the slope at a fall of 1 in 100. Swales hold the water back, spread it sideways and let it soak. Where a drive or path crosses the slope, put a slot drain at the top of the run, not the bottom, so the water never gets the length to accelerate. Coir matting pinned over bare slopes for the first season gives the roots time to do the job the mat was doing. And resist the urge to lay a plastic membrane under a gravel slope. It turns the gravel into a river bed.

A short kit list

1. Rotating laser level and receiver staff, or a borrowed dumpy level if you are patient.
2. The open LiDAR terrain tile for your area and a free GIS package to read it.
3. A spade, a stopwatch and a bucket for the infiltration test. This is the most important item.
4. A sub-base calculator from a paving manufacturer, cross-checked against the formula above.
5. A roll of coir matting and a box of biodegradable pegs.
6. A copy of your council's SuDS approval guidance, if the work touches more than 100 square metres of hard surface.

None of this requires an engineer. All of it rewards thinking like one. If you would like to see where the same thinking leads when the water is 14 metres deep and arrives on schedule twice a day, the main feasibility desk is one click away.