SEDIMENT TRANSPORT AND TRAPPING IN THE LOWER DELAWARE RIVER AND ESTUARY: INITIAL RESULTS OF A REGIONAL SONAR STUDY

Patterns of modern sedimentation in the lower Delaware River and upper estuary are being investigated using a suite of new high-resolution sonar datasets. During 2001, approximately 300 miles of dual-frequency side-scan and CHIRP sonar trackline were collected within a 50-mile river reach to map bottom morphologies and to characterize sediment types with regard to hydrographic properties and anthropogenic disturbance. The new sonar and sedimentological data are being used to address issues related to contaminant burial and dredging, as well as to gain new insight into the most recent morphologic evolution of the sedimentary system. While sonar groundtruthing studies are in progress, noteworthy results have emerged. First, active bedload transport within the northern dredged channel is evinced by 0.5-3 m amplitude, ebb- and flood-oriented sand waves. These bedforms exhibit marked cross-channel morphologic variability, are composed of well sorted, medium-grained fluvial sands, and appear to reflect local sediment sources and measured maximal tidal current velocities of 1-1.5 m/s. Second, two discrete mud-trapping zones have been identified within the river-estuarine transitional region. These zones are located (1) within an anchorage near Marcus Hook (Pa) and (2) immediately south of the Delaware Memorial Bridge. Both zones are characterized by morphologically smooth bottoms that flank both sides of the main channel and appear to be a consequence of fluid-mud deposition. Where not obscured by biogenic gas, sonar records indicate that these acoustically transparent deposits are laterally extensive with thicknesses of up to 2 m. The origin of the trapping zones is presently under investigation, but flocculation and localized flow convergence are likely causal mechanisms. The significance of sediment transport and trapping with regard to the estuarine sediment budget will be discussed.