How Is the San Ysidro Debris Basin Designed to Work?

It looks like the San Ysidro Debris Basin is done. Can you ask an engineer what the point of the three structures is? —N.

During most storm flows, the goal is for everything to continue downstream. In a major flow or debris flow, however, large material will ideally stall in the basin, while smaller sediments continue onward. “In San Ysidro Creek, there used to be an earthen dam with a concrete cap on top and a 48-inch-wide pipe going through it,” explains Seth Shank, Regulatory Compliance Manager for Santa Barbara County Flood Control. “The thought was that it would capture anything over 48 inches wide. But we learned over the years that the culvert pipe would get overwhelmed and back up, and then finer stuff would become trapped.”

And in the 2018 debris flow, the county discovered that the broad, flat basin does more than the dam to slow down the debris coming from the steep, narrow channel upstream. “Once the velocity is reduced, the big material stays in the basin instead of carrying through,” says Shank.

The new concrete structure is designed to hold water back, the better to lower the speeds of what’s coming downstream, with an opening the width of the natural channel to allow for the passage of fish (the southern steelhead trout, in particular). As for the two metal debris racks, they might trap some rocky material, but they’re really meant to catch floating woody debris that could cause blockages downstream. They’re angled that way to avoid complete blockage, and so that if the top rack gets filled, the second one can catch more.

P.S. When I went to take the above photo, I noticed what look like stone steps on the San Ysidro Ranch side of the creek. “Those are actually drainage swales for runoff during rain events, with rock placed there to prevent erosion,” says Shank.

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