CHRONIC SILTING WITHIN A HUMAN-CREATED OXBOW ALONG A HISTORIC HARBOR FRONT (SAMPIT RIVER, GEORGETOWN, SC)

The historic Georgetown inner harbor happened to be turned into an artificial oxbow-like river loop in 1949, though a wealth of traditional businesses and heavy industry are traditionally located here. Since this Sampit River modification chronic, massive silting has plagued the inner harbor, which, thus, represents a fine experiment of mud accumulation dynamics in a complex estuarine waterway system. The goal of this study was to understand the silting dynamics inside and around the inner harbor, including the harbor mud composition and its possible geohazardous potential.

A combination of sidescan sonar surveying, hydrodynamic monitoring, suspended and settled sediment sampling, and recovering four long sediment cores was performed throughout and around the inner harbor since 2018. Physical and geochemical properties of each sediment core were examined including grain-size distribution, total organic content, dry bulk density, 1-cm XRF element distribution, X-ray imaging, lead and cesium dating, and heavy metal concentrations.

The harbor mud of gel-like consistency is composed of 52 % of cohesive material (< 10 μm) with at least 15 % TOC. The mud that enters the inner harbor in suspension settles rapidly due to estuarine flocculation. This process causes 2 % coarser material (20-40 μm) to settle at the two harbor entrances, forming ‘sills’ here, while 2 % finer material (<10 μm) is deposited in the backside of the harbor loop.

Deposition occurs rapidly during slack tide until the accommodation space available has been filled up to about 0.5 m, which seems to be the depth controlled by wave action. The re-filling after dredging happens over a few lunar tidal cycles, i.e., within a few months only, significantly exceeding the so far assumed silting rate by a magnitude.

Arsenic (19.3 to 23.8 ppm), chromium (77.4 to 105.6 ppm), copper (26.8 to 40.2 ppm), nickel (28.8 to 36.6 ppm), and zinc (90.7 to 180.3 ppm), were found to be above Effects Range Low (ERL) concentrations. Potential sources identified, based on the temporal and lateral distribution point to contributions by various local industrial sources, shipyards and marinas, local business as well as tourism.