Marin Master Gardeners
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Marin Master Gardeners

Marin IJ Articles

Permeable surfaces let rainwater soak naturally into the earth

04/24/2010
Marie Narlock

What happens to all the rain that ripples off our roofs, sheets down our driveways and pours off paved sidewalk curbs? Where does it go and why does it matter?

For years, landscape architects and environmentalists alike have recommended that rainwater - or any excess water - be whisked away from our homes as expediently as possible. Consequently, most of our homes come equipped with drainage systems that include rooftop gutters and impervious concrete driveways and patios that divert thousands of gallons of rainwater into PVC pipes. From there it sloshes its way through our streets, dumps into storm drains and creeks, and gushes into the bay.

It turns out there are numerous problems with this familiar scenario. Most obvious is that, like energy, water is a valuable commodity that should be conserved and retained. For this reason, many gardeners are "catching" rain in barrels or ponds for use during the dry months.

In addition, by channeling our rainwater into the bay rather than letting it soak naturally back into the earth, we have depleted our groundwater. Groundwater is the water that flows beneath the earth's surface and is typically discharged through springs and seeps. It is also frequently accessed through wells for agriculture and for human consumption. In fact, in the United States approximately a third of all public water supplies and 95 percent of all rural domestic supplies use groundwater. Reduced groundwater necessitates expensive, deeper wells and also negatively affects our quantity of surface water (streams, ponds, lakes), since the two are inextricably linked.

Finally, by encouraging runoff we have also accelerated erosion and increased the temperature of our waterways. Most disturbing, we have inadvertently allowed loads of toxins to enter our creeks, streams and bay. Like a magnet, flowing water grabs whatever debris is in its path. This debris comes in many forms - fertilizers, pesticides, oils, grease, cigarette butts, soaps, paint, chlorine, dog poop. This cacophony of pollutants creates health issues for many living creatures, including humans. The contaminants that don't make it to the bay tend to settle at the bottom of our lakes and reservoirs. This in turn reduces the capacity of these waterways and ultimately requires expensive and time-consuming dredging.

To alleviate these problems, today's landscape architects and environmentalists design drainage systems that include strategically located berms and swales as well as water-permeable surfaces that allow rainwater to slowly infiltrate the ground rather than being diverted to pipes. Allowing water to soak into the earth recharges our groundwater, slows erosion and reduces pollution. The goal is threefold: create systems in which rainwater can seep in, spread out and sink down.

Berms and swales allow rainwater to be channeled and captured on site. By digging out trenches of varying lengths and depths and "berming up" the edges, rainwater can be moved from high points to low points where it can slow, spread and sink. Some gardeners choose to celebrate the beauty of these earthly indentations by creating seasonal ponds. Placing rocks and boulders around the edges to hold the soil is not only functional, but beautiful.

Any surface that allows water to seep through it or around it is permeable. Some obvious examples are bare soil, mulched beds and gravel. But stone, pavers and concrete can be permeable, too, so long as there are crevices where water can percolate.

There has been an explosion of permeable materials on the market, including porous cement concrete, porous asphalt concrete, and pavers that are specially designed to allow water to flow through. Stop by your local landscape supplier for samples. Some carry high price tags, but don't be daunted. Old broken up concrete and bricks, floor tiles, pieces of wood - so long as these are laid where water can soak in between, you've created a permeable surface.

Permeable surfaces are becoming more popular in municipal settings. The Environmental Protection Agency is conducting a 10-year study in which a 43,000-square-foot parking lot will be replaced with three types of permeable pavement. At University of California at Davis researchers are comparing the strength and percolation of gravel pavers, porous cement concrete and other materials to replace conventional asphalt in parking lots.

Many municipalities are slicing sections of concrete out of median strips and installing native plants that happily soak in winter rains and, as a bonus, filter out pollutants.

Next time you have a hardscape project in your backyard - a path, patio or driveway, for instance - consider a material that allows water to soak through. Installing a permeable surface is not only beautiful, but it also helps keep our waterways clear, healthy and replenished.

The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.

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