Sediment dynamics and transport regimes in a narrow microtidal estuary

Estuarine sediment dynamics are a consequence of various forcings (barotropic, estuarine circulation, and fluvial) that vary in space and time. Here, we present a study examining sediment dynamics in a narrow microtidal estuary, the Lower Passaic River in New Jersey, USA. The analysis incorporates measurements of suspended sediments, morphological change, sediment erodibility, and a numerical hydrodynamic model. The former two datasets are used to develop an understanding of sediment dynamics over the full range of hydrologic conditions, and the latter two datasets are used to interpret the behavior of the system. Subsequently, a conceptual picture is developed, one that classifies the morphological status of the system at any given time into three regimes dependent on river flow—regime I includes conditions when the system is importing sediments, regime II includes conditions when the system is exporting sediments by flushing a thin easily erodible surficial stratum termed the fluff layer, and regime III includes conditions when the system is exporting sediments by scouring more consolidated strata underneath the fluff layer. Regime III is relevant for the long-term morphodynamic equilibrium of the estuary by providing a mechanism that erodes and exports sediment accumulated under regime I conditions. Consequently, sediment dynamics depend not only on short time-scale processes such as the instantaneous river flow rate, but also on the time-history of river flow, and the long-term morphological progression of the system. These regimes represent a conceptualization of estuarine sediment transport dynamics and can be useful in the development of effective estuarine sediment management strategies.