http://coastalchange.ucsd.edu/images/ljshores_winter.jpg

                      This link shows the processes involved in beach accumulation and retreat caused by accumulation and retreat.  As does the image above show erosion and its forces at work with impact of seawalls and lack thereof. 

                                                                                 http://www.hawaii.gov/dlnr/exhibits/clp/CoastalErosion2.html