longshore bar

Visible at low tide as a sand bar, or a ridge of beach sediment sharing the general contour of the shoreline itself, a longshore bar is actually a feature sculpted by the push and pull of years of waves. It only seems to share the general contour of the shoreline because the break point of the waves (a point determined by depth) tends to be equidistant from shore—which itself has of course been rounded off or leveled by waves—lending any bar that shares its contour a generally level, or straight, appearance. However, this is just a function of the break point: the bottom current necessary to feed the shoreward current of a wave pulls silt in with it, then, the moment the wave breaks, deposits it, in the same place, time after time. That deposit—that steady accretion of silt—is the longshore bar. Due to the redistribution of silt, there will often be a trough between the longshore bar and the beach itself. Seaward of the longshore bar are other longshore bars at the break point of larger waves. A distinct but related action is longshore drift, which results when the waves strike the beach at an angle instead of straight on. Instead of forming a stable longshore bar, beach sediment is pulled deeper and deeper into the breaker zone—“passed back,” effectively. If groins (small jetties extending from shore) aren’t erected to trap this sediment and redistribute the effects of longshore drift downcurrent, the erosion can radically resculpt the shoreline.

Stephen Graham Jones