The sand piled along the seawall makes a ramp to assist damaging waves and surge over the seawall, and it also defeats the curvature of the lower wall. The curve converts lateral velocity to upward velocity and prolongs the wall’s structural integrity under storm stresses, except now the waves hit the wall at 90 degrees head on where the sand has piled up.
I see. Thanks for helping me understand it. And thanks for that great graphic at post #1161. I'm embarrassed to admit that before yesterday I didn't even know where Galveston Island is. I thought it was much farther south, nearer Corpus Christi.