GEOMORPHIC-BASED BARRIER ISLAND TRANSGRESSION MANAGEMENT, CHANDELEUR ISLANDS, LOUISIANA

Increased hurricane intensity and frequency in the northern Gulf of Mexico during the past decade has forced the Chandeleur Islands into a mode of rapid dissection and transgressive submergence. Based on extrapolated historical land loss and shoreface retreat rates, the islands will be completely converted to a system of submerged shoals within ~25 yrs. This 80-km long barrier arc plays a crucial role in: regulating conditions for a 12,000 km² estuary, providing unique habitat for endangered species, mitigating offshore wave attack on mainland marsh, reducing storm surge, and affording protection to metropolitan New Orleans and the Mississippi Gulf coast.

The Chandeleurs were produced by marine reworking of seaward portions of abandoned Mississippi River deltas; a process dominated by lateral spit accretion downdrift from a central deltaic headland sediment source. Lateral transport along the Chandeleurs has produced up to ~10-m thick spit platform deposits and a series of relict recurved spits that today are overlain by backbarrier marsh. During shoreface retreat (~15 m/yr for 1855-2008), sands from the subsurface relict spit deposits are liberated in the nearshore providing a local sand source to the active littoral system. During the past 125 yrs ~300 x 10⁶ m³ of sand has accumulated in deepwater sinks at the flanks of the barrier island arc; twice as much as deposited in the backbarrier. In contrast to popular transgressive barrier island models, no new backbarrier marshes have formed during the last century, so that the loss of sediment to flanking sinks has resulted in island area reduction from 44.5 km² in 1855 to 4.7 km² in 2005.

We propose a barrier island management strategy that aims to replicate the natural processes of island development by: 1) reintroducing sand at updrift backbarrier feeder sites, 2) using shoreface retreat to liberate placed sand into the littoral system for lateral distribution, and 3) establishing salt marshes upon backbarrier sand placement sites. This comprehensive plan derives from extensive studies on long term geomorphic evolution and short term changes, driven primarily by rapid relative sea level rise and hurricanes, to provide the barrier system the means to be sustainable for generations.