MORPHOLOGIC CONTROL OF RESIDUAL SEDIMENT FLUX IN THE DELAWARE RIVER ESTUARY

The geometry of funnel-shaped estuaries has a major influence on tidal hydrodynamics, sediment transport and patterns of morphologic change on the long term. Regime-type morphodynamic models specify that the direction of tidal current asymmetry in estuaries and (by extension) the potential for net import or export of sediment can be predicted from simple volumetric and tidal parameters. For much of the Delaware River Estuary, these parameters suggest a flood-dominant, sectionally averaged tidal current and net landward sediment movement. In 2005-2006 we obtained data necessary to test this prediction. Measurements of currents and suspended-sediment concentration recorded by sensors deployed for six months, as well as data collected during shipboard tidal cycle surveys, were used to compute residual (tide filtered) mass transport at locations of dissimilar cross-sectional geometry and mean circulation. The study area extended from the upper estuary (3-km width), landward of the null zone, to the lower estuary (12 km) where density-driven circulation is vigorous. Time- and depth-integrated sediment fluxes were decomposed to assess the relative importance of flux mechanisms along and across the estuary.

Results of the tidal and seasonal observations are in broad agreement; spatial variations in residual currents along with sediment lag effects complicate use of simple regime models to predict the trajectory of net transport. In the upper estuary, residual sediment flux over the axial channel is ebb-directed, in contrast to model predictions. This seaward flux is driven by significant non-tidal drift (10-40 cm/s), which is mostly compensatory flow for Stokes transport on flood tide. In the lower estuary, flood-directed residual fluxes produced by tidal pumping and gravitational circulation maintain high levels of turbidity in the middle estuary, but the relative importance of these mechanisms varies considerably between the channel and flanking subtidal shoals.