ANTHROPOGENIC INFLUENCES ON THE MORPHOLOGY OF THE TIDAL DELAWARE RIVER AND ESTUARY

Changes in the tidal regime of the Delaware Estuary over the past century have been attributed to dredging practices, yet changes in the hydraulic geometry of the Estuary have yet to be rigorously quantified. To this end, digital terrain models (DTMs) created from bathymetric datasets for 1877 and 1987 (normalized to a common datum) have been used to determine historical bathymetric and shoreline changes along a 116 kilometer reach (Philadelphia, Pennsylvania–Bombay Hook, Delaware).  This comprehensive investigation has resolved significant changes to hydraulic geometry variables (depth, width, area) during the period of interest.  Results indicate that an increase in depth has occurred throughout 74 % of the study area by an average of 1.5 meters. Mean width has increased by over 10 meters on average, with two particular spatial trends: 1) width increases in the lower estuary due to “natural” erosion, and 2) width decreases in the upper estuary due primarily to shoreline development.  The cross-sectional area of the estuary has increased by roughly 4500 square meters on average, and the overall wetted volume has increased by 5x10⁸ cubic meters.  The increase in channel cross sectional area, created largely through dredging of the main shipping channel, implies that a substantial increase in tidal discharge has taken place to maintain continuity in the system.  This conclusion is supported by increases in tidal range from 1890 to present as recorded by gauges in the upper estuary.  Ongoing work will attempt to quantify the relative effects of natural versus anthropogenic changes in the morphology of the estuarine floor, specifically whether localized dredging and shoreline hardening have led to far-field changes in deposition and erosion patterns.