Tidally varying bottom stress owing to waves and currents at a coastal inlet

Anna Wargula, Applied Ocean Physics & Engineering
Advisor: Britt Raubenheimer, Applied Ocean Physics & Engineering

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Project Goals

I request travel funds to visit Dr. Maitane Olabarrieta (U. Florida), who will teach me to use her model, which has been shown to predict flows at New River Inlet (NRI), NC (Spydell et al. 2015), and who will provide model setup files and grids. Using the model predictions and our observations, I will address my hypothesis that tidal variations in bottom stress, owing to tidal changes in waves and currents, are the same order of magnitude as tidal variations in horizontal advection near the inlet mouth (final thesis chapter, expected defense Nov 2016). The simulations will enhance my work by enabling me to estimate advection with higher spatial resolution than I could with observations alone.
 
Background: Bottom stress affects circulation and sediment transport in the coastal ocean by generating mixing near the bed and dissipating flows. Waves and flows affect the bottom stress, with the relative importance depending on their magnitudes and directions and water depth (Ganju and Sherwood, 2010). At an inlet, tidal changes in currents and water depth modulate wave heights and breaking, leading to a temporally varying bottom stress (Kang and Di Iorio, 2006). Numerical simulations suggest that wave-driven bottom stresses affect the tidal amplitude and phase and may reduce currents on the ebb shoal (Olabarrieta et al., 2014). Improved understanding of the temporal variation of bottom stresses in regions with waves and currents is important to developing accurate models for circulation and morphological evolution, and for estimating nutrient and pollutant transport and coastal storm hazards.
Observations of waves and flows were collected in May 2012 at NRI (Fig 1), which is 1000 m wide at the mouth and has a shallow semi-circular ebb shoal (~800 m radius, 1-2 m deep). Inside the inlet, there is a 5-m deep channel to the southwest and a shallow 2-m deep channel to the northeast. The inlet is well mixed. Along-inlet tidal currents were depth-uniform, ranging +/- 1.5 m/s in the SW channel. During the flood, water funneled slowly into the mouth and accelerated as the inlet width narrowed (Stommel and Farmer, 1952). During the ebb, water exited the mouth in two jets, one in each channel. Offshore significant wave heights ranged from 0.5 to 2.5 m. Wave-breaking was primarily depth-limited, with most wave energy dissipating on the offshore edge of the ebb shoal at low tide (peak ebb), and increasing energy reaching the inlet mouth with increasing tide level (Chen et al., 2015). Thus, flow accelerations (advection, which can be a source or sink of momentum), and waves and currents were tidally modulated on the ebb shoal (e.g., at the locations where Reynolds stresses were measured).
Analysis: I will examine the effects of tidally varying waves and currents on bottom stress by estimating the dominant terms in the simplified, depth-integrated momentum balance for a shallow, narrow, barotropic inlet (Wargula et al., 2014 and references therein)
 
Thesis work/timeline: The funding provided here will be a critical component to my thesis work and will provide experience with numerical modeling during the last year of my PhD (expected defense Nov 2016). Using observations from NRI, I showed that wave forcing can be significant in the subtidal along-inlet momentum balance and that breaking waves during storms enhance flows into the inlet (Wargula et al., 2014). This fall, I expect to complete analysis of the tidal and subtidal cross-inlet momentum balances inside the inlet, where I have found that wind modifies cross-inlet flow structure. For my final chapter, I will examine tidal fluctuations in bottom stress in the tidal along-inlet momentum balance using observations and modeling. I will use the WHOI cluster to conduct the model simulations. My recently purchased laptop contains sufficient CPU and disk space for the proposed work. The recent opportunity to add a modeling component in this analysis was unanticipated. My advisor has agreed to match the funding provided here so that I can stay at UFL a full 10 days this Nov 2015.
 
References:
Chen, J-.L., T-.J. Hsu, F. Shi, B. Raubenheimer, & S. Elgar, 2015, J Geophys Res-Oceans, 120.
Ganju, N. K. & C. R. Sherwood, 2010, Ocean Model, 33, 299-313.
Kang, K.R. & D. Di Iorio, 2006, Estuar Coast Shelf S, 66, 395-408.
Olabarrieta, M., W. R. Geyer, & N. Kumar, 2014, J Geophys Res-Oceans, 119.
Spydell, M. S., F. Feddersen, M. Olabarrieta, J. Chen, et al., 2015, J Geophys Res-Oceans, 120.
Stommel, H. & H. G. Farmer, 1952, Ref. 52–88, Woods Hole, Mass.
Trowbridge, J. H., W. R. Geyer, M. M. Bowen, et al., 1999, J Phys Oceanogr, 29, 3056-3072.
Wargula, A., B. Raubenheimer, & S. Elgar, 2014, J Geophys Res-Oceans, 119.