THE INFLUENCE OF BACKBARRIER SEDIMENTATION ON TIDAL INLET MIGRATION: MASON INLET, NORTH CAROLINA

Mason Inlet, located in southeastern North Carolina, is a small southwesterly migrating tidal inlet. Migration rates of the downdrift shoulder from 1938 to 1974 were on average 6 m/y with several short-term reversals. However, since 1974 there have been no reversals and migration rates as high as 56 m/y have been measured. Absence of reversals and accelerated migration are thought to indicate a change in inlet behavior.

The hypothesis states that behavior change can be attributed to a reduction in tidal prism, which has curtailed the ability of the inlet to maintain itself; allowing waves and flood directed tidal currents to become the dominant processes. Observations of Mason Creek, the main access channel, shows there has been significant sedimentation over the past 27 years. Mason Creek has been reduced in surface area by 108,460 m² from 1971 to 1998. Additionally, the USACE has dredged between 15,300 m³and 76,400 m³ of sediment from the bisection of Mason Creek and the Atlantic Intracoastal Waterway, suggesting a persistent infilling of the backbarrier basin. Sediment which has infilled the basin may ultimately be derived from longshore transport, however, analysis of Mason Creek has shown that 16,725 m² of material was eroded from within the creek boundary.

Data from near bottom current meters moored within the main channels and backbarrier region illustrate distinct sediment transport pathways. Site one shows a strong flood component in the along-channel direction quantifying the observed infilling of Banks Channel. In addition, water level data collected within the backbarrier illustrate flood tide dominance. Sites two and three both show ebb dominance in the along-channel component. However, the main ebb channel at the inlet throat only encompasses 24% of the entire cross-sectional area with the remaining 76% above mean sea level. Literature review of shallow inlets show subaerial and shoaling channels to be dominated by tidal truncation and friction, which may produce asymmetric tides favorable for landward transport of sediment.