Sediment transport and deposition in flood-tidal deltas: Insight from the Texas Coast

Abstract

While tidal environments are ubiquitous around the planet, limited work on these systems, particularly those that are in microtidal settings, hinders our understanding of tidal paleo-environments. In this study, we analyze the modern sediment dynamics of the San Luis Pass area, a microtidal system located on the Texas Gulf Coast. The focus of this work is to characterize sediment distribution and establish a quantitative relationship between sedimentary structure geometry, water depth, sediment grain size, and current velocities. A total of 160 seafloor sediment samples were collected in the modern San Luis Pass tidal delta between July 2017 and August 2018; this time span coincided with the landfall of Hurricane Harvey in the area in August 2017. Ripple geometry was measured in 46 locations along the flood-tidal delta. There is a rapid transition in grain size, from proximal (fine sand) to distal (silt and very fine sand) sediments at about 4.3 km from the mouth of the tidal inlet. This change is likely due to a decrease in current velocities away from the inlet entrance, resulting in a change in sediment transport from a combined bedload/suspension to suspension. Post Hurricane Harvey sediments are finer grained compared to those collected before the hurricane landfall, most likely due to high runoff during the storm event. In addition, increased water depths were measured after the hurricane landfall, indicating a significant erosion in the San Luis Pass area. Current ripples were only present in the proximal (sandier) area. We found a positive correlation between ripple height/ripple wavelength and water depth. However, the correlation is much stronger for ripple wavelength, even when compared to other published ripple measurements. We, therefore, propose to use ripple wavelength as a more reliable means to estimate paleo-water depths instead of ripple height. Nevertheless, more measurements from modern environments are necessary to establish a more robust geometric relation from ripples in a variety of paleoenvironments.