There are a lot of developed coastlines that have jetties and seawalls. Many of these were built years ago in an attempt to stem the erosion of the beach and to stabilize the coastline. Tides, seasonal winds and storms interact in ways that can move sand onto or off a beach. Barrier Islands, in particular, are especially succeptable to shifting shorelines caused by tidal movements of sand (see here to find out about changes to LBI).
Problems with this idea are well enough know that these types of public works are not constructed much anymore. With jetties a bad impact can be to trap sand in and around the jetty and in turn causing a place up or down the shoreline form the jetty to erode at a greater rate. So there may be a net plus for the jetty area but it can cause losses to the beach shoreline for an adjacent area.
A bad result often comes from seawalls that are made to protect property that is situated along the beach. Often this was done by placing a wall or barrier at the far back edge of the beach. Through time the beach erodes and often times the entire sandy beach is lost.
Shoreline erosion and rebuilding is natural, and occurs all the time. Putting barriers in place to protect beachfront homes and property is unnatural. The net effect of not letting nature takes its course, in other words not trying to stop sand movement, usually does not work well.
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