Additions coming to one of Napa’s two city dams are meant to keep the city’s water pure, reducing contamination risks from debris washed into reservoirs after the October wildfires that blackened huge swaths of local woodlands.
The containment system will be installed at Milliken Reservoir northeast of the city, the smaller of Napa’s two local water supplies after its larger source at Lake Hennessey. The U.S. Department of Agriculture will cover three-fourths of the estimated $336,000 cost and Napa the rest, under the federal disaster declaration Napa County received shortly after the Oct. 8 eruption of three major fires that consumed tens of thousands of acres and killed 44 people across the North Bay.
The City Council on Tuesday approved a contract with Worthington Products Inc., the Ohio-based maker of Tuffboom devices that act as floating chains to stop waterborne debris from accumulating against – and damaging – dams. Contract terms call for Worthington to complete its work within 10 days of signing.
Since the wildfires and the damage they did to Napa County forests, the city Water Division has kept an eye both on the amount of silt and sediment entering Milliken Reservoir as well as the entry of logs, tree trunks and other larger detritus, according to Water Division manager Joy Eldredge.
Wood, leaves and dirt entering reservoirs increase the carbon levels in water. When such organic materials mingle with chlorine used to disinfect the water and make it safe to drink, they produce by-products known as trihalomethanes that, in very high concentrations over decades, can raise the risk of certain cancers, city officials have previously said.
Buoyant barriers are intended not only to keep out plant matter from Milliken Reservoir, but to lessen the chance of immediate damage to the dam, Eldredge wrote to the council in a memorandum. Large objects floating toward the dam can chip or spall the dam face, water intake and other concrete structures.
The main risk from heavy objects striking the dam would follow periods of high-intensity rain, although Eldredge noted most of this winter’s Napa County rainfall has come at a slower per-hour rate less likely to shake loose large-scale debris.
Communities including Napa, Yountville and Calistoga have eyed the possible effects of last fall’s fires on water supplies, including increased erosion and ash from flames that reached the shores of various reservoirs.
The aftermath of a previous firestorm showed how water quality can be threatened long after flames are quenched. In early 2017 – nearly a year and a half after the 2015 Valley Fire that devastated parts of Lake County – powerful winter storms washed ash from the scorched Putah Creek watershed into Lake Berryessa.
The contaminants shut down treatment plants serving the Berryessa Estates and Berryessa Highlands communities for two weeks, although both were able to subsist on stored water. “Once it came, it was goo – ash is a very, very fine material,” Phillip Miller, engineer for the two water systems, said at the time.