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The Coastal Gardener: In heavy rain, it's a slippery slope

December 31, 2010|By Ron Vanderhoff

Last week my little rain gauge mounted on my fence reached its capacity and overflowed. Next time I checked it, water was over the rim again. I have to buy a bigger rain gauge. But not far away a neighbor, an amateur meteorologist, has recorded almost 16 inches of rainfall so far this season. Because our yearly rainfall averages about 13 inches, and we're only a third of the way through the rainy season, we can safely say it is going to be a wet winter.

During the early morning following the wettest night of the storm, I managed to weave my way through flooded, rain-soaked streets. I carefully navigated alternate routes, avoided at least two road closures from flooding, and finally got to work.

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During my drive, as raindrops pelted at me, gutters raged and wipers worked hurriedly, I noticed several residential hillsides that had slipped, causing serious and expensive damage.

Wherever heavy erosion or slope failure is a possibility there are several considerations. Is the soil on the slope naturally formed or is it the result of artificial grading and construction? Is there a history of erosion on the slope? Does water move across the slope or is it moving down the slope? Are there erosion channels on the slope and are they small or large? Where does the water go that hits the slope? Where does the water go in the areas above and below the slope?

On landscaped slopes, soil is often artificially in place. In my experience, the placement of one soil type or texture on to the top of another soil type of texture is the most likely cause of slope failures. It's like trying to balance a cherry on the side of a mound of melting ice cream. We shouldn't be surprised to find the cherry in the bottom of the bowl, nor should we be surprised when these hillsides fail.

Slopes and hillsides with layered soils like these need trees and shrubs and other plants to stabilize them and to bind the different soil layers together.

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