Paper No. 196-7
Presentation Time: 10:00 AM
STRATIGRAPHIC FRAMEWORK AND GEOMORPHIC EVOLUTION OF A TRANSGRESSIVE TIDAL INLET-FLOOD TIDAL DELTA SYSTEM: RACCOON PASS, LA
Raccoon Pass is a transgressive, wave-dominated tidal inlet located between East Timbalier Island and the Miss Lena - Caminada-Moreau headland. During the past century, the inlet has experienced significant changes in morphology, hydrodynamics, and distribution of sedimentary deposits due to lessening sand availability, an ongoing transgression of the barrier arc, and anthropogenic forcings. During the past century, there was a sharp decrease in the longshore contribution of sediment transport to the Timbalier Islands due to the damming of Bayou Lafourche and the construction of the Belle Pass jetties in the early 20th century. This reduction in sediment supply resulted in a landward migration of the flanking barrier, widening of the tidal inlets, and a shutdown of sediment bypassing the inlets. In 2012, the West Belle Pass Barrier Headland Restoration Project altered this regime by introducing approximately 2.8 million cubic yards of sand to Miss Lena Island. Recent grab-samples and vibracores throughout the inlet complex document numerous 0.25-2-m thick subsurface sand accumulations located landward of the inlet in Timbalier Bay. Satellite imagery and grain size trends document the westward movement of sand from Miss Lena Island and growth of a 1-km wide spit platform trending into Raccoon Pass. Along a tidal inlet-to-bay transect, surface sediments exhibit sand and shell lag in the inlet transitioning to sandy-silt and sand in the vicinity of the flood tidal delta and finally to a shelly mud in the bay. These trends and vibracore data within Timbalier Bay also confirm a broadening and landward extension of the flood-tidal delta shoal. Model simulations and hydrodynamic field data indicate that the inlet complex is characterized by a flood-dominated spit platform and an ebb-dominated main inlet channel, suggesting that sand bypassing may be re-initiated. The sand nourishment project has provided a natural laboratory in which to study barrier arc development in the early stage of Louisiana barrier evolution (sensu; Penland 1987; barrier model) when sediment supply is abundant and barrier processes are dominated by spit progradation, storm breaching and inlet development, and aggradation of the flood tidal deltas.