ABSTRACT: Effects of Sea Level Rise on Barrier Islands, with Examples from Louisiana
Asbury Sallenger, Jr.
Conceptually, with sea level constant and no net losses or gains of sediment, a barrier island and shoreface profile would approach an equilibrium configuration. With a rise in sea level, the profile would adjust upward the same amount as the sea level rise, and adjust landward such that the amount of sediment eroded would be equal to the amount accreted. In an early model, the adjustment of the shorface profile was achieved by nearshore erosion and offshore deposition, a net offshore transport. However, a number of studies have shown that onshore transport through overwash and other processes is very important. To maintain an equilibrium configuration, the quantity of sand overwashed must be balanced by sediment eroded from the shoreface. Barrier islands that have an amp e supply of sand may approach an equilibrium configuration as required by the sea level rise models. However, a sand-starved island may not approach equilibrium. Recent field experiments on the Louisiana coast have examined the response of severely sand-starved barriers to rapid sea level rise (1 cm/yr relative to the land) and other processes. These islands are overwash platforms with little to no dune development and consist of a thin (1-2 m) wedge of sand overlying marsh. The shoreface is mud that in places is covered by a thin veneer of sand. Low berm elevations allow overwash even during moderate storms. Overwash builds the berm higher during storms, but eolian processes following overwash have been observed to plane the beach to roughly its prestorm elevation. Hence, the beach does not build vertically to limit overwash, and the sand wedge forming the beach rapidly migrates landward over the marsh. The muddy shoreface has the classic exponential shape that has been described for many sandy environments. However, unlike a sandy shoreface, the muddy shoreface does not appear to be evolving to a condition limiting erosion. Hence, both the muddy shoreface and the wedge of sand comprising the beach do not appear to approach an equilibrium configuration and would erode even if sea level were stable. Methods commonly used to assess the effect of sea level rise on barrier islands may not be appropriate for sand-starved barriers.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990