OCEANSIDE — Oceanside’s annual harbor dredging and the occasional regional sand replenishment projects are not enough to save the city’s eroding beaches, a new study shows. A proposal to build rock groins on the beach appears to be the best way to stop or at least slow the steady erosion that has been chewing away the city’s coastline since the 1940s, according to the study prepared for Oceanside by the Long Beach-based consulting firm GHD. “Sand from this (harbor dredging) program does little to really benefit the city beaches,” said Brian Leslie, a senior coastal scientist and project manager with GHD, in a June 30 community Zoom meeting.