Sedimentological Signatures of Dredge and Fill Deposition in Boca Ciega Bay

Dredge-and-fill, the excavation of sediment at one location and deposition at another, is a process that can significantly alter natural coastal sediment patterns. This research investigates the sedimentological signatures of dredge-and-fill deposition in Boca Ciega Bay associated with the construction (1961) and rebuilding (2021) of the Pinellas Bayway Bridge. Twelve sediment cores were collected along transects on the western and eastern sides of the bridge, along with three cores from a control site at Indian Key. Each core was analyzed for grain-size distribution, texture, percent carbonate (CaCO₃), and total organic matter (TOM) to identify spatial and stratigraphic changes associated with anthropogenic sediment input. 

New data from three sediment cores show distinct stratigraphic units corresponding to pre-depositional, dredge-deposition, and post-depositional conditions. Cores from both the western and eastern sides of the bridge yield sediment compositional patterns similar to previously analyzed cores. Notably, units contain sharp increases in gravel concentrations (>15%) and decreases in sand concentrations. These units are absent from the control site and suggest the introduction of channel-derived sediments associated with higher-energy environments. The depth and thickness of these suspected dredge deposition units vary among the nine cores analyzed thus far. 

Together, these findings demonstrate that grain-size shifts, particularly elevated gravel and decreased sand concentrations, provide a sedimentological indicator of dredge-and-fill deposition and can help assess spatial extent and physical impact on coastal environments. 

Next steps include age dating sediment units to better determine the timing and duration of dredge deposition and the reestablishment of local sedimentation.