In particular, we now understand the effect of rock jetties in blocking longshore transfer of sand. After a hurricane breached Assateague Island, plans for development were dropped and a national seashore was created. The inlet from the ocean to the bay between the island and the mainland was valued for recreational fishing boats, and that inlet has been maintained by jetties that block sand from drifting southward.
Keeping the channel clear has blocked the normal flow of sand. It has created a wide beach north of the inlet, and extensive erosion south of it. The jetty on the northern (upstream) part of the inlet intercepts the sand that would have drifted south, and blocked the channel naturally. South of the inlet, the ocean currents have continued to move sand further south. Without natural replenishment, the barrier island just south of the inlet has shrunk and migrated westward substantially.
That Assateague sand flows to Virginia, ending up at Tom's Cove at Chincoteague. You can see the shift in the shape
of the sandbar "hook" as natural processes have extended the southern end of the island.
Overwash during storms is a normal process, absorbing the storm energy and redistributing the sand inland. Tree stumps from forests that once grew on the bay side of barrier islands are occasionally exposed on the ocean side, showing the migration of the beaches to the west after years of big storms. All the "armor plating" of sea walls can only slow the process and - as shown at Atlantic City, New Jersey - cause the waves to expend their energy by moving sand further downstream (south) rather than inland (west).
Ocean City and Virginia Beach require constant beach replenishment, or "nourishment." They pump sand from offshore to the beach in front of their breakwaters, temporarily restoring a wide recreational beach until the next series of storms. The economy of these communities depends upon beach visitors, so maybe (maybe...) it's a good idea to use Federal and State tax dollars to fight the natural pattern and benefit a local community's economy.
The Nature Conservancy helped Virginia avoid this expense along the Eastern Shore. It purchased the marshes and barrier islands, choosing to perpetuate the natural setting. By minimizing the "built environment" of hotels and recreational facilities on the edge of the beach, The Nature Conservancy minimized the rational for building sea walls and pumping sand. Not only do the birds and crabs appreciate the preservation... so should the taxpayers.
The law was modified eight years later with the Coastal Barrier Improvement Act of 1990, eliminating the requirement that undeveloped coastal barrier consist of "unconsolidated sedimentary materials" and the exclusion of areas that theoretically were already protected before the 1982 law because they were "included within the boundaries of an area established under Federal, State, or local law, or held by a qualified organization as defined in section 170(h)(3) of the Internal Revenue Code of 1954, primarily for wildlife refuge, sanctuary, recreational, or natural resource conservation purposes." The amendments tripled the barrier island acreage where Federal subsidies were limited.
The Department of the Interior was directed to map the exact boundaries of the Coastal Barrier Resources System. The maps are based on water levels and topography, since floodwaters don't stop at property lines or political boundaries, and most landowners and mortgage lenders use the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps to see which parcels are not longer eligible for Federal funding.