Sorry to hear about the loss of those near you.
My condolences.
Engineering of hills/topography, vegetation, root absorption, saturation, and regional zoning planning might have prevented this, long ago.
Northern California coastal forests | Ecoregions | WWF
"Habitat Loss
Less than 4 percent of the original extent of virgin redwood forests remain, and only 2.5 percent of this is protected. Unfortunately, protected lower elevation groves can be threatened by severe flooding and sedimentation caused by logging in surrounding watersheds. Several large groves of old growth redwoods on the Eel, Klamath, and Van Duzen Rivers were lost due to severe floods in the 1950's exacerbated by extensive and ecologically-damaging logging in surrounding watersheds. Most of the coastal grasslands have been converted to agriculture or rangelands. All vegetation types have been reduced due to urbanization."