Date of Award


Degree Type


Degree Name

Master of Science in Geological Oceanography



First Advisor

Robert L. McMaster


A pattern of mesoscale, seaward extending fine sand lobes and shoreward extending coarse sand troughs often form a rhythmically interfingering, recurrent transverse bar morphology of low relief on the subtidal shoreface of southwestern Rhode Island barrier beaches. The development of these shoreface features, and of the encompassing larger scale beach and nearshore morphology, is intensively investigated in a year-long program of regular beach and shoreface profiling at an East Beach site along this coast. Although previous investigators had inferred a storm-related origin for these mesoscale bedforms, this study documents their development by shoreface erosion during periods of fair-weather incident waves, onshore transport and beachface accretion. Conversely the lobes and troughs become filled-in and buried by shoreface deposits during periods of storm-induced beach erosion and associated seaward sediment transport and deposition. These features may therefore represent a class of rhythmic shoreface features that generally form in conjunction with beachface accretion. In this sense they may be comparable to beachface cusps, and similarly have multiple origins.

Although the accumulated data on these lobes and troughs do not indicate the specific processes involved in their formation, several previously recognized mechanisms are suggested that may be important in their development. These potential mechanisms include: 1) edge wave/incident wave interactions, 2) intersecting incident wave interactions, 3) wave refraction by bottom topography, 4) macroturbulent eddys in wind-generated coastal flows, or 5) the propagation of bottom irregularities by incident wave. Of these mechanisms, stationary edge waves are particularly well-suited to explain the rhythmic spacing and fixed patterns of recurrence of these features at this site.

These shoreface features develop within an annual pattern of storm-cycle dominated beach morphodynamics which also exhibits a significant seasonal trend. In addition to a dependence on sea level and incident storm intensity, the extent of beach erosion and the occurrence of shoreline and dune retreat in this cycle are thought to be strongly dependent on the beachface morphology prior to major incident storms.



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