A Nearshore Sediment Transport Model for the Northeast Gulf of Mexico Coast, U.S.A.

Abstract

The 400 km-long stretch of coast along the northeastern Gulf of Mexico from Dog Island, Florida, to Morgan Point, Alabama, exhibits highly variable coastal deposits ranging from (1) late Holocene beach-ridge plains located along the Apalachicola protuberance and much of the Alabama coast; (2) a number of late Holocene, overwash-dominated barrier islands and baymouth barriers interspersed along the entire stretch of coast; and (3) a late Pleistocene (Sangamon?) barrier complex located between Saint Andrew and Choctawhatchee Bays. The mere morphological diversity along this coast implies that the previously held monotonic, integrated, longshore transport model deemed representative of this area is overly simplistic.

A sediment transport model is presented for the entire stretch of coast based on evidence obtained from (1) numerical modeling of the nearshore wave field and sediment transport flux; and (2) sediment budgets calculated from historic hydrographic sheet and historic map comparisons. Numerical modeling of the fair-weather, wave-energy transformation across the low-gradient inner shelf adjacent to Apalachicola and the Alabama coast indicates that the critical threshold velocity for potential onshore transport is exceeded a significant distance offshore. Although the exact mechanism for sediment dispersal is not yet understood along this area, some or significant onshore transport of sediment apparently must have prevailed during the late Holocene given the occurrence and geographical juxtaposition of well developed beach-ridge plains. The intervening Pleistocene barrier complex has served as an important source of sediment for Santa Rosa Island to the west, and the Shell/Crooked Island complex to the east. There is no significant net communication of sediment with the coast west of Santa Rosa Island or east of the Shell/Crooked Island complex indicating two fixed depositional cell boundaries. Littoral disintegration increases significantly east and west of these cell boundaries because of the refractive effects of the lower gradient inner shelf on shoreward propagating waves. These findings have significant implications for the late Holocene development of this coast and the morphodynamics of other coasts that exhibit similar wave-morphosedimentary characteristics