By Sharon Cohoon, Sunset senior garden writer

Roofs collect a lot of nitrogen from contaminants in the air.  And the more densely populated and smoggier the area, the more they collect.  When it rains that nitrogen-spiked water races off the roof, spills into the street, pours down the storm drains, and flows into the ocean. And more nitrogen is the last thing the ocean needs, says Doug Kent, a member of The Surfrider Foundation, a spokesman for them on designing Ocean Friendly Gardens, and a garden designer who specializes in the same.

The increase in red tides we've all noticed in Southern California is primarily due to nitrogen-rich urban runoff, he says.  The increased number of dolphins, sea lions, seals, and grey whales that have beached themselves along the West Coast is also attributed to run-off; they are poisoned by eating fish that have feed on toxic algae. People get sick, too, says Kent.  Respiratory-related problems such as asthma, bronchitis, and sinus infections skyrocket during red tides, he says.

The Costa Mesa, California house shown below illustrates how all that nitrogen ends up in the ocean.  The house has sloped roofs and no down spouts, and a big, wide driveway ready to funnel all that roof runoff right to the street.  Absolutely nothing to stop it racing to the street as fast as it can flow.  And there are a lot of homes with situations just like this, unfortunately. (Sorry about those black squiggles; the homeowner drew on the prints, and they're the only "befores" I have.)

Here's how Kent fixed the problem.  He opened up the middle section of the driveway--where you see the pavers alternating with ground covers--and installed a French drain underneath.  The French drain leads to a larger hole that is filled with gravel. This diversion allows plants to absorb the nitrogen in the rainwater before it percolates slowly into the soil.

The bio-catch basin itself behaves rather like an sunken outdoor living room.  It is "floored" with pavers interspersed with dymondia and creeping thyme.  The homeowners love it.  There is some psychological separation from the street, yet it feels neighborly.

For more ideas on rain harvesting, see Jim McCausland's story, Saving on a rainy day, on page 32 of Sunset's January issue or go here.