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Napa County issued a code violation letter to DDYM Inc. – a company associated with the Del Dottos of Del Dotto wineries fame – because of a mudslide from a hillside vineyard onto Yount Mill Road.

The order-to-correct code violation citation is dated Jan. 10. That is after a Jan. 6 slide sent mud oozing over the narrow, lightly traveled road near Yountville Hill in the middle of Napa Valley. Then a second, Jan. 16 slide closed a section of the road a quarter-mile east of Highway 29.

Napa County requires an erosion control plan to plant hillside vineyards. The plan for the vineyard at 1201 Yount Mill Road on Yountville Hill specified that soil disking and ripping in preparation for planting couldn’t exceed two feet on slopes greater than 5 percent.

But, the notice said, the plan’s engineer of record came to the slide and told county officials that soil had been excavated to a depth of four feet or greater. The county has yet to confirm this is the case with an engineering study.

The erosion control plan had a reason for prohibiting disking and ripping deeper than two feet on the steeper slopes.

“It kind of degrades the soil structure,” county Engineering Manager Patrick Ryan said. “What that means is you’re losing the cohesion of the native soil structure. When it does get wet, you’re lubricating those voids between those soil particles.”

The site had been prepared for the winter rains and was inspected by the Napa County Resource Conservation District, Ryan said. Such erosion-controlling steps as planting a cover crop had been taken.

But, while the vineyard passed the winterization inspection, the county was unaware of how the soil beneath had been prepared, he said.

Napa County placed a concrete K-rail along the side of the hill after the first slide to protect the road and adjacent creek. The K-rail proved insufficient to hold back the second slide, which Ryan said happened when the east and west flanks failed during heavy rains on Jan. 16.

About a half-foot of mud remained covering a section of the narrow road on Thursday. A cyclist on Yount Mill Road rode up to it, got off his bike and took a stab at trying to go around it, then gave up.

“I’ve been out of touch with the roads lately,” he said as he rode back the way he had come.

The owners have implemented aspects of an emergency remediation plan, Ryan said. They are covering the slide area with plastic to reduce the amount of water absorbed by the remaining soil. They will install a French drain above the slide.

A long-term fix could involve installing retaining walls and soil buttresses. The one-acre, young vineyard on the hillside is gone, Ryan said, with 80 percent or more lost to the slides and the remainder since pulled out.

Some of the slide debris went into the creek on the north side of Yount Mill Road, Ryan said. The San Francisco Bay Area Regional Water Quality Control Board and the state Department of Fish and Wildlife have been working with the county on the post-slide remediation steps.

Napa County has just begun the enforcement case and has yet to discuss a fine, though that’s always a possibility, Ryan said. The county could seek to recover costs for the road cleanup.

David Del Dotto could not be reached for comment on Wednesday and Thursday. The Piazza Del Dotto Winery & Caves is near to the slide site.

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Napa County Reporter

Barry Eberling covers Napa County government, transportation, the environment and general assignments. He has worked for the Napa Valley Register since fall 2014 and previously worked 27 years for the Daily Republic of Fairfield.