Garden drainage: diagnosis, design and construction

Drainage manages excess water without drying out the ground. The design starts by identifying source, route and destination.

Look at levels, damp patches, erosion, roof discharges and how the ground behaves after rain.

The soil profile shows the layers that hold back infiltration.

Ground modelling, channels, ditches and paving all direct water. Wherever possible, the solution slows it down and encourages infiltration in suitable areas.

Water is never directed onto neighbouring properties.

Drains collect water when there is a gradient and an outlet. Pipes laid with no fall or nowhere to discharge work badly.

The filter material and the geotextile are chosen to prevent clogging.

Drainage should not dry out the whole profile. Species and emitters are matched to the conditions.

In planting holes, avoid creating a sealed-off container.

Sub-bases and falls keep water away from walls and buildings. Permeable surfaces only work if the layers beneath also allow infiltration.

Chambers, gratings and outlets need access and cleaning. The design records where they are.

If puddles appear, check the levels and look for a blockage first. The answer is not to add drains without understanding the system.

Wherever the soil and the location allow, part of the water can be infiltrated or held temporarily. Basins, planted swales and rain gardens reduce the immediate discharge and can be worked into the design.

The capacity needs to be tested. Compacted or saturated ground cannot go on taking water from roofs and paving indefinitely.

A permeable surface only works when the layers beneath and the soil can also store, infiltrate or carry water away. Laying free-draining material over an impermeable base simply moves the ponding elsewhere.

Maintenance is just as important. Sediment and organic matter can clog the surface and reduce performance.

Water behind a wall increases pressure. Waterproofing, a drain and an outlet are part of the structural solution.

Swimming pools, plant rooms and buildings need to be protected from flow paths. Drains should not discharge in a way that causes erosion, nor pass water on to neighbouring properties.

Gratings, chambers, channels and outlets need cleaning. After storms, check for sediment, obstructions or settlement.

When a puddle appears, do not immediately add another grating. First check the level, the pipework, the outlet and how the soil is behaving. The fix should deal with the cause, not just move the water.

Gratings, ditches and pipework are sized according to the contributing area, the falls and the rainfall intensity adopted in the design.

Before trenches and paving are closed up, the system is tested. Check that water reaches the intended chamber, that the pipework holds its fall and that the discharge does not cause erosion.

Drainage should remove the excess without turning the soil into a permanently dry medium. Trees and shrubs still need a reserve of water.

The position of the drains, the soil texture and the irrigation programme are thought through together. A moisture-tolerant species may suit a moist area; it should not be used to hide a leak or a planting hole with no outlet.

• Drainage at planting
• Ground modelling

1. Principles of hydrology and landscape drainage.
2. European Soil Data Centre.
3. Diaplant’s method for drainage design and construction.