ABSTRACT: Synergism of Riverine and Winter Storm-Related Sediment Transport Processes in Louisiana's Coastal Wetlands

Joann Mossa, Harry H. Roberts

The roles of various mechanisms that supply sediments from major sources, including rivers and the nearshore shelf, to coastal Louisiana are not well understood or quantified, temporally or spatially. Recent studies reveal that an important association between riverine sediment input and the cyclic passage of winter storms results in a periodic supply of suspended sediments to coastal marshlands. The fact that these two mechanisms coincide maximizes the availability of particulate matter for counteracting coastal land loss.

Overbank sedimentation is one mechanism that supplies sediment from rivers to coastal wetlands. It occurs when stages exceed bankfull, most commonly in winter and spring. The timing of riverine sedimentation events is also related to suspended sediment concentrations and loads, which are also consistently greater during the winter and early spring months. During high discharge years, more sediment is available in winter than in other seasons, because at such times the sediment concentration and load maxima generally precede discharge maxima by several months. This period of maximum suspended sediment availability is coincident with the most severe winter storm activity, which elevates water levels near the coast and enhances suspended sediment transport to wetlands.

Cold fronts also serve as a mechanism for suspended sediment transport, and qualitative observations suggest that during a typical cold-front passage onshore transport of suspended sediments is more likely. Pre-frontal stages of winter cold-front passages along the Louisiana coast are characterized by prolonged periods of high wave action from the southerly quadrants, water level setup along the coast, and strong alongshore as well as onshore transport. At these times, suspended sediments from riverine input and the nearshore shelf are mobilized by the combination of riverine and cold front-related processes. Post-frontal conditions bring dry, cold winds from the northerly quadrants; the winds cause rapid water-level setdown along the coast, a significant reduction in wave energy, and stranding as well as drying of newly deposited sediments in the wetlands.

Identification of these optimum periods of suspended sediment activity can maximize the performance of wetland restoration efforts, because mineral matter and nutrients are critical for marsh vitality. The timing of riverine sediment supply shows that optimum conditions for use of sediment diversions occur in the winter and early spring. The collective effects of riverine and frontal mechanisms of sediment transport indicate that marsh management projects may be enhanced if structures are managed to allow sediments to enter project areas during pre-frontal phases during these seasons.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990