Because the predominant direction of wave approach is from the southeast, beach drift at Rockaway and at other locations along the South Shore of Long Island is from east to west, causing an accumulation of sand on the east sides of the groins. Imagery from Google Earth on May 18, 2008.

For details about this phenomenon, see:
Wikipedia: Longshore drift
Grand Valley State University: Longshore Current and Beach Drift

As sea level rises, barrier beaches such as Rockaway and Fire Island tend to erode on the seaward side, and accrete on the opposite landward, or lagoonal, shoreline over the long term. This puts permanent structures at risk. But people like to live along the shoreline.


This image and the following quote are from: Geotimes: The Impending Coastal Crisis

Quote:

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In a natural system, barrier islands respond to storms and sea-level changes by a process called �barrier rollover.� In this centuries-long scenario, the front of the barrier island is cut back by waves and the sand is washed over the island to the back side. Thus, the island maintains its width, but moves landward with each storm. The landward bay shoreline will also move inland as sea level rises. Seen from space, the whole system looks the same after awhile, but all the elements are displaced landward. Where these barrier islands are developed and the natural sand supply is interrupted, however, the result of this process is that all the fixed structures on the barrier island, and on the bay shorelines as well, will eventually be destroyed unless we spend millions of dollars to replenish the beaches.

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In many cases, people need to live near the shoreline for economic reasons. This can expose them and their infrastructures to storms and tsunamis. See the following:
Google Earth Community: Sumatra-Andaman Earthquake+Tsunami overlays
Laem Pom, a community in Nam Khem, Thailand, where many people depended on fishing, and were strongly impacted by the Tsunami of December 26, 2004.
Google Earth Community: Burma's cyclone death toll soars: Update
Google Earth Community: Hurricane Katrina/Digitalglobe Imagery
Google Earth Blog: Hurricane Katrina - Summary of Google Earth Resources


Here is an old beach drift Java applet created by a student in 1999: The Effect of a Groin on a Longshore Sediment Transport System. Because nine years have passed since it was created, and it uses a very old version of Java, it could use some updating. Feel free to grab the source code, which is public domain. If you do any updating, please share your source code with me.


Image of "Duke" from Sun Microsystems in 1999