Alongshore Advective Acceleration in the Surf Zone
Overview: This project focuses on the alongshore flow variations and advective accelerations owing to surfzone bathymetric variations. Feedbacks between alongshore flow variations and surfzone morphological changes may be critical to the evolution of surfzone morphology on beaches with alongshore uniform incident waves. Determining the magnitude of the alongshore advective accelerations and flow variability resulting from surfzone features is important to developing accurate models for coastal evolution and transport of pollution. Field evidence that the advective acceleration contributes significantly to the momentum balance also may lead to new theories and models for shoreline change. Intellectual Merit: Analytical and numerical studies suggest that alongshore advective acceleration may be important to surfzone rip currents, alongshore flow meanders, dispersion, and sediment transport and morphological evolution. Recent numerical model simulations indicate that alongshore advective acceleration can be large relative to other terms in the depth- and time-averaged surfzone momentum balance on beaches with alongshore bathymetric variations as small as ten centimeters. However, field measurements have shown that alongshore flows are predicted well with simplified equations that neglect advective accelerations. Furthermore, except near strong rip currents, field observations show relatively small alongshore variations in alongshore surfzone flows. The simplified equations may reproduce observed velocities accurately because the advective acceleration term is balanced by an alongshore pressure gradient term, or because advective acceleration is accounted for by the parameterized bottom friction coefficient. It also is possible that the observation-based estimates of alongshore flow gradients are biased low because flow maxima and minima are not resolved owing to the relatively large spatial separations between in situ sensors. Alternatively, the numerical models, which are sensitive to the friction coefficient, may overpredict the advective acceleration. Densely distributed observations of wave forcing, pressure gradients, and surfzone flows will be obtained to calibrate numerical model simulations. The field measurements will be used to estimate advective acceleration and the field-tested model will be used to explore the causes of the advective accelerations in the presence of alongshore inhomogeneous surfzone bathymetry. In particular, the observations and simulations will address the following questions: Is alongshore advective acceleration important? Are model predictions of alongshore flow gradients and advective acceleration accurate? And what causes alongshore advective acceleration? By combining field observations designed to measure flow variations at the spatial scales of small surfzone bathymetric variations with numerical model simulations, the investigators will be able to address these questions about the importance and causes of alongshore advective accelerations on beaches with near uniform incident waves. Broader Impacts: The American Shore and Beach Preservation Association (ASBPA, http://www.asbpa.org) is concerned that Americans interested in coastal engineering are not able to obtain field experience. Thus, a partnership with ASBPA and interested engineering firms will be established to provide field training to undergraduates, recent graduates, and young professionals embarking on coastal careers. In particular, two programs will be offered: 2- to 3-week-long fieldwork-focused internships for young professionals currently employed in coastal consulting and engineering firms, and 6-month-long fellowships for students interested in coastal engineering careers. More than a dozen companies already have expressed interest in the internships, and have offered to support their employees to participate in the proposed program. The student fellowships would be similar to the Swashzone Fellowships previously conducted by the lead investigator, Raubenheimer, which have led to several students receiving positions in coastal engineering firms. The interns and students will work with the investigators to obtain field observations, and to improve models for nearshore processes, which in turn will improve coastal management, help protect coastal communities, and sustain coastal resources.