Landscape Design to Minimize Runoff

Stormwater runoff and wastage occurs when things that man builds keep rainwater from soaking into the ground at or near where it falls. Rooftops and paving are the main culprits—instead of rainwater soaking into the ground, it falls on the roof, spills into the gutter, down the downspout, across the impermeable driveway and sidewalk into the street, then the stormdrains, and ultimately into creeks, rivers and the ocean, all the while unavailable for local irrigation, and without ever nurturing the land.

Problems caused by runoff include:

Overall reduction in groundwater recharge
Long-term lowering of groundwater tables and loss of stream flow during dry weather
Increased erosion of stream banks
Water quality impacts caused by pollutants from rooftops, lawns, driveways and streets
Flooding—especially more frequent “flash” flooding.

There are several techniques useful for minimizing runoff where landscape design is concerned. Most focus on using permeable rather than impermeable hardscapes to allow water penetration, rather than diversion to drains. Harvesting rainwater is a separate approach.

Replacing impermeable asphalt or concrete with:

Newer permeable concrete
"Plantable" pavers for driveways and walkways
Packed crushed rock or packed decomposed granite
Gravel.

For flagstone, brick or paver patios, setting them in decomposed granite or sand instead of concrete.

Replacing the pavement at the bottom of your driveway with a French drain or grate. This allows water that falls on the bulk of your driveway to soak into the ground instead
of running into street drains.

In renovation or new construction, incorporating drainage as a design element:

Berms and Vegetated swales
Rain gardens
Drywells
Green roofs.

A berm is a slightly raised area, while a swale is shallow ditch with a mild slope. Berms can be used to slow or direct runoff on steep slopes, and swales planted with
grass or other plants can direct water to a rain garden or an appropriate area for absorbtion to occur, and since the swale itself slows waterflow and abosrbs water, less of the water that enters a vegetated swale will actually make it to the end.

A rain garden is a simply a collection of water-loving plants in a slight depression in the ground, that collects water and allows it to gradually permeate into the soil. Rain gardens are usually planted at the base of a slope or even at the outlet to a downspout—anywhere where water naturally flows or can be directed. Plants and a base of permeable soil enhanced with organic material allow the rain garden to quickly absorb even large amounts of water, often in just a few hours.

A drywell is an underground space that can accept runoff, and allows it to disperse slowly into the groundwater. A simple drywell is a pit filled with rubble but it will not hold much water. A buried perforated concrete or plastic tank will be full of air and can absorb its full volume when runoff hits. Drywells can be fed by roof drains or located at low spots, slope bases, or at the end of a swale.

Planted, or "green" roofs. If your roof needs replacing, consider installing a green roof, a roof with plantings over an impermeable membrane atop it. These reduce runoff and can also lower your heating and cooling bills—and they look cool!

Harvesting Rainwater

One possibility for both water conservation and runoff reduction is rainwater harvesting for irrigation use. A 1,000 square foot roof can produce more than 600 gallons of runoff for every 1" of rain that falls on it. This can be trickier in our climate, as we get very little rain when it is useful for irrigation (summer) and when we do get rain, there is usually no need for irrigation systems to be running. So the only really useful method involves directing downspouts into a collection system and from there into a fairly large tank for stockpiling rainwater at the end of the rainy season, and using it through the beginning of the dry season—not overly practical for most people. However, even if there is no good irrigation use for the rainwater, diverting the runoff from urban stormdrains, and using one of several simpler methods—drywells at downspouts, or diversion to planted areas—to allow the ground to absorb it is beneficial.

Certainly most home gardeners are not going to employ a host of these methods in regard stormwater runoff, but even one or two implementations—everyone doing a small bit—can make an overall difference over time.