Landscape Design to Minimize Runoff
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
For flagstone, brick or paver patios, setting them in decomposed granite or sand instead of
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
* Green roofs
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.
slowly into the groundwater. A simple drywell is a pit filled with rubble, but 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!.
click here for a pdf of the Landscape to reduce Surface Runoff slideshow.
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.