THE EVOLUTION OF HEWE'S POINT, CHANDELEUR ISLANDS, LA: A RARE NATURAL RESOURCE

Hewe’s Point, a prograding sand spit at the northern terminus of the Chandeleur Islands, constitutes the largest sub-aqueous accumulation of sand in the Louisiana barrier islands. These sand bodies are increasingly sought for restoration and mitigation projects along the coastal Gulf of Mexico. The existence of Hewe’s Point is related to a unique arrangement of transport conditions and accommodation space. The Chandeleur Islands is a transgressive barrier-island system, formed during Holocene sea-level rise through the erosion and reworking of the deltaic deposits of the abandoned St. Bernard delta complex. Over the last 2000 years, waves and currents concentrated the coarser material into shoals, which evolved into islands through littoral and aeolian processes. As a result, the sandy deposits of the island platform rest unconformably on muddier pro-delta and delta-front sediments. The oblique orientation of the islands relative to the prevailing southeasterly wave climate drives a northward sediment transport that deposits material along the northern flank of the island chain at Hewe’s Point. This longshore-transport mode is the dominant distribution system, although frequent storms also significantly alter island configuration and accelerate sediment transport. The edge of the barrier-island platform is situated along the flank of the underlying delta lobe, creating accommodation space immediately adjacent to the platform. As sediment is transported northward, it progrades into deeper water where littoral energy diminishes, allowing for the rapid accumulation of sand seaward of the spit. During transport, the sand is sorted, resulting in average sand content that is higher (97%) and coarser (0.15 mm) than that on the rest of the island platform (where average sand content is 85% and grainsize is 0.11 mm). Historical analysis of sea-floor change indicates that 150 x 10⁶ m³ of sediment has been deposited in the area of Hewe’s Point over the past 125 years, creating a set of prograding clinoforms approximately 27 km² in area and up to 9 m thick.