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MMWD Blog: Think Blue Marin

Welcome to our blog! Written by staff at MMWD, “Think Blue Marin” explores all things water in south and central Marin—water supplies, conservation, new projects, watershed management, and more.

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Oct 28

Choosing Permeable

Posted on October 28, 2016 at 8:36 AM by Ann Vallee

by Charlene Burgi

Several months ago a stonemason completed the face of my fireplace with Three River rock. There was a fair amount of stone left over from that job, so I asked the stonemason if he could install a patio in the backyard using the remaining rock.

Shortly after my query, he submitted an estimate for the job that included concrete curbing, a 4-inch concrete base pad, and mortared joints. In putting the estimate together, he was no doubt considering the frost heave here in Lassen. He wanted to prevent the stones from moving as the earth undulates due to freezing and thawing. 

But my vision for the back garden was to maintain the excellent drainage from the permeable soils we have here. What this stonemason didn't consider is my home is built on rock cap—perhaps from Mt. Lassen's lava flows thousands of years ago. This rock cap, which is found several inches below the soil, curtails the frost heave. In sum, a permeable patio is a viable and beneficial option. 

You might ask what is permeable material, and why choose it for landscaping patios, walkways and even driveways? Permeable material is open to the soil. In other words, it is porous and allows water to flow through. This material can include rock, mulch, loamy soil, and even some types of asphalt that absorb surface water and carry it to the ground underneath, rather than forcing water to runoff or puddle on roads or sidewalks.

As to the why: Misdirected surface water and poor drainage can play havoc with landscaping. By allowing rainwater to work its way back into our soil via a porous medium, we can eliminate potential problems. For example, if a site is not properly graded and an impermeable material such as concrete is poured without thought to surface water, that water may end up under your house!

Drain pipes can carry water away from collection points, but permeable materials go beyond that to help evenly distribute rainwater throughout the landscaping. Just as bioswales on a hillside slow down sheeting rain, permeable surfaces provide an opportunity for water to spread out and sink into the ground. This recharges groundwater supplies instead of funneling runoff directly into to storm drains.

Finally, when water falls on impermeable surfaces—whether from rain or errant irrgation—it can collect pollutants and carry them to those same storm drains, local creeks, and the Bay. This polluted runoff harms our waterway ecosystems. In contrast, permeable surfaces help to filter contaminants. 

Misty on new patio
 Misty on the new patio
With all these benefits, permeable walkways, patios, and driveways are well worth investigating. Ask your landscape professional what options are best for your garden. As for the installation of remaining stone here ... much to the consternation of the stonemason, he installed the patio as directed by yours truly. Now I only need wait for spring to intersperse vibrant blue, low-water-using Turkish speedwell (Veronica liwanensis) into the areas where mortar was to be used. 

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