Surface Catchment: Every Hard Surface Is a Water Source

What it is

Every hard surface is a roof. Driveways, paths, patios, compacted earth. They all shed water almost as well as tin.

A 50 m² driveway in an 800 mm rainfall zone yields about 32,000 litres a year. Add a 30 m² patio and 20 m of paths and you are past 50,000.

In most suburbs the impervious ground outside the house is bigger than the roof on top of it. That water is going to the storm drain right now.

Read the site in the rain

Walk the property mid-storm. Boots on, hood up.

Watch where water sheets, where it pools, where it leaves. The land is already routing it. Half the work is just noticing the path.

A driveway that tips toward the street is bleeding thousands of litres per storm. A 50 mm lip at the low end sends that flow into a bed instead. A patio downspout tied to stormwater can be cut and rerouted to a rain garden in an afternoon.

Streets and verges count too. Many cities now permit kerb cuts that pour street runoff into pollinator-friendly verge plantings and tree pits. Cooler streets, fewer floods, free irrigation for the canopy.

Move the water

Gravity does the work. You build the path.

Grading. A 1 to 2 percent slope sheds water cleanly. That is 10 to 20 mm of fall per metre. New hardscape should tip toward beds from day one, not toward the street.

Channels. A V-cut in concrete, cobbles set around a recessed line, a commercial channel drain with a grate. All do the same job: intercept sheet flow, deliver it somewhere useful.

French drains. Gravel trench, perforated pipe optional. Good along driveway edges where a surface channel would catch a tyre. They take water along the whole length and either convey or infiltrate. Wrap the pipe in geotextile or the fines will silt it up inside two seasons.

The point of every one of these is the discharge. Not the storm drain. A tree basin, a bed, the inlet of a swale.

Chain the elements

On a complex site, link them. Driveway channel into French drain into rain garden into swale.

Each step slows, filters, sinks. Together they can absorb a serious storm with nothing leaving the property.

Where to send it

Rain gardens. Shallow planted depressions sized at 10 to 15 percent of the catchment area. They drink most storms without overflowing and filter the water on the way down.

Tree basins. A ringed depression around a fruit tree, fed by a driveway channel. Berm 15 to 20 cm high, two metres across. The tree gets deep watering right at the root zone. In arid climates this alone can carry a fig or pomegranate through summer with no hose.

Swales on contour. Best for sloping ground where the driveway sits above the garden. Pour runoff into the uphill end and it spreads down the full length of trench.

On clay or a high water table, infiltration is slow. Use a gravel soakaway pit for temporary storage and slow release.

When it goes wrong

Surface water is not roof water. Driveways carry oil, tyre dust, brake pad grit. Streets add heavy metals and hydrocarbons.

Filter biologically. Rain gardens and swales with mulch, living soil, and deep roots strip 80 to 95 percent of suspended solids, 50 to 80 percent of metals, and 60 to 90 percent of hydrocarbons. Soil pores trap. Organic matter binds. Microbes break down what is left.

For edibles, be sensible. Send driveway flow to ornamentals, fruit trees, or non-food beds first. If it must reach vegetables, route it through a rain garden, then keep it away from raw leaf crops. Root crops and fruiting plants sit lower on the risk list.

The other failure mode is overflow. Every catchment chain needs a planned exit. Decide where the water goes in a 100 mm storm before you build, not during it. A rain garden with no overflow path will undercut its own berm in one big event. See overflow management.

Watch for ponding that lingers past 48 hours. Mosquitoes breed, roots drown, the system has failed. Either the infiltration rate is too slow for the catchment area, or fines have sealed the surface. Rake the top 50 mm, mix in coarse sand, and reset.