As we build along the shoreline, walls, jetties, and other projects block the natural movement of sand along the shore. Where some area cliffs and beaches could historically rely on having new sand naturally deposited along the shore, the natural pattern has been so interrupted that as the old sand keeps moving along, the new sand is never redeposited and shoreline erosion whether from beaches or cliffs is increased. Well, a lot of it is behind upstream dams. A bunch more is being trapped in lagoons by other blockages such as roads and projects built on fill. The rest of it is trapped by manmade coastal blockages such as breakwaters which interfere with the natural movement and deposition of sand along the coast.
Erosion is nothing new for any coastline.
What's new are the blockages combined with the propensity for folks to build close to the edges of coastal bluffs - and for local cities to permit them to do so. This combines with the lack of sand redeposition to lead to some sad outcomes and unpleasant choices. There can be no happy endings for people whose houses or condos are being undermined by the relentless tides of the sea. And it's a terrible thing that the Navy is unable or unwilling to help due, evidently, to its own past practices. Recently the California Coastal Commission voted to allow eroding sea caves in Solana Beach to be plugged underneath a set of condos above in a desperate attempt to slow the erosion process.