Subtidal contributions to hydro- and morpho-dynamics at a wave dominated inlet system

Observations of currents and water pressure obtained at 7 locations within Oregon Inlet, NC, over a 40-day period in the spring of 2019 showed subtidal water level oscillations on the sound side of the inlet with magnitudes often exceeding typical tidal ranges. These oscillations, which are correlated with regional wind patterns, induce cross-shore gradients in sea surface slope from the back bay to the ocean that are strongly coupled to inlet current modulations at subtidal frequencies. Owing to their longer temporal scale, the sub-tidal gradients often combine with typical tidal flows to accelerate one phase while retarding or entirely reversing the other. Concurrent observations of bedform migration on the ebb-tidal delta, obtained with x-band radar, suggest these current modifications at sub-tidal frequencies may be important to sediment transport patterns in the inlet and on the ebb-tidal delta. The role of subtidal oscillations on large scale morphologic evolution is further examined through an idealized numerical model that couples hydrodynamics, waves, and sediment transport through the Delft3D modelling suite implemented at Oregon Inlet over observed bathymetry. The nature of the morphologic development of the inlet is discussed in terms of the presence or absence of subtidal oscillations. This work was supported by a DOD SMART Fellowship, UNH, the USACE CIRP program and the USACE Field Research Facility in Duck, NC.