This sort of follows on the previous post, but focuses on something quite different. Singley Flat is a steep series of marine terraces marking rapid (and episodic) uplift of this section of coast just south of Cape Mendocino. But weak rocks (perhaps aided by frequent shaking?) leads to unraveling of the steep hillslopes. Scars and debris-filled stream channels can be found up and down this coast in aerial photos.
Here, an area of rapid erosion high on the hill appears to be creating a constant (well, not quite) stream of dirt and gravel that's building an alluvial fan across the terrace, which happens to be where the road is. The public works folks have tried to create a channel for the material and an oversized bridge, so that the material can pass below the roadway on its way to the beach.
I suppose this is a significant local supply of sediment for this section of beach - though figuring out the behavior of this beach given the complexities of offshore rocks, rapid (and recent - 1992) uplift, large storms, and this episodic sediment supply, would be challenging!
A foot note: Much of what I write here is based on general observations and a smattering of internet or book work. One of my standard go-to resources for California is Gary Griggs' (and Patsch and Savoy), 2005, Living with the Changing California Coast, and I would recommend it as a starting point for anyone exploring the state's shorelines.