Back-passing beach sand can help protect shorelines
A sand “cell” consists of three important parts: sand on the dunes, sand on the beach, and sand that is in the shallow water just offshore. Over time, some offshore sand will drift away laterally and can become captured in areas like a boat harbor or around other shoreline buildings or developments. This sand bottleneck compounds over time and can cause undue havoc to the public. In a boat harbor, sand will simply continue to pile up making navigation impossible. In other locations the bottlenecked sand can actually cover roadways or damage structures.
Back-passing is the the term used when moving that bottlenecked sand back to its original source within its sand cell. More than cosmetic, back-passing is done so Mother Nature has the needed sand to form dunes that protect our shorelines. In storms she moves sand offshore, which widens the beach and adds protection to shore developments, and in nice weather Mother Nature returns shallow offshore sand to the beach where the trade winds use it to rebuild the dunes.
Over time, some back-passed sand will again end up back at the bottleneck sites and that is what makes the continuation of these back-passing cycles necessary.
Sand is vital to our beaches and every cubic yard of it is important to save, even by relocation or back-passing when necessary. UH Sea Grant is involved in all of the sand back-passing projects on Maui and is the science behind the scenes.
Lis and Bob Richardson