To keep the beach sand from washing down the gentle slope and onto the nearby lake floor with each incoming wave, you know what works good?
Line the lower edge of your sandy beach with a row of dark gray ....
www.quikrete.com/productlines/riprap.asp ...... set half-way down into the old sand to create an esthetically natural looking lower border that keeps the sand from washing downward into the lake. Is very dark gray and looks like a line of dark gray granite stuck in the sand. It blends.
Price: about $4.50/60-lb bag, packaged in a biodegradable paper bag.