Morphotomology of Rhode Island Barrier Shores: A Method of Distinguishing Beach from Dune/Barrier Component Histories Within a 29 Yeah Record of Shore Zone Profile Data, with Special Reference to the Role of the Beach as a Buffer and Modulator of Erosional Coastline Retreat
Date of Award
Master of Science in Oceanography
29 years of shore profile data collected at weekly, bi-weekly, and monthly intervals from Rhode Island's south shore have been re-examined to determine the role the active beach component plays in the complex history of coastal change. Each of more than 3600 individual plotted shore profiles taken at 10 locations has been dissected into active beach and inactive backshore/dune/barrier components, their respective volumes calculated, and the configuration of each beach unit assigned to either low, intermediate, or high volume, reflective, dissipative, and hybrid states. New time series of beach and non-beach profile component volumes are produced, and data sorted by month are presented in scatter plots.
These new data revealed distinct seasonal and annually repeated patterns of beach growth and depletion, and associated seasonal shifts from reflective to dissipative beach shapes, an expression of true seasonality in meteorologic conditions and associated wave climate. Moreover, the influence active beach "health" -a function of volume and shape has on the susceptibility of backshore and dune/barrier erosional retreat has been demonstrated.
The stacked barrier/dune and overlying/fronting active beach profile volume time series has shown in addition to an annual beach cycle, a secondary multi-year signal of inactive backshore and foredune growth/depletion. Similar periodicities in total profile volume have been revealed through previously reported spectral analyses (Gibeaut, 1987). In addition, aerial photogrametry of shoreline and duneline change over periods of 3, 5, and 10 years or more has revealed the secondary signal as out of phase dune and waterline relocations which describe a barrier-wide shift in overall shoreline orientation, apparently as a consequence of multi-year net longshore drift changes.
Recognition of a distinct hierarchy of sediment reservoirs present; and the appearance of separate but dependent histories for each -active beach, inactive backshore, and dune/barrier- as resolved by the techniques developed, provide details above and beyond those resulting from previously employed spectral and eigen approaches. Those techniques tend to oversimplify the importance of interactions among components of the total shore profiles. The new approach and results obtained reveal the previously stressed multi-year cycles as secondary phenomena, at least as far as processes are concerned, and show the most obvious signal instead to be one of seasonal and annual beach component changes.
Moreover, limitations in the effectiveness of previously employed techniques used in calculating the retreat rate of the coast based upon high water line photogrametrics are pointed out; not the least of which is the sometimes misleading implications those results have generated for coastal zone management's implementation of construction setback policies. The suggestion is made here that documentation of coastal change and projections of future retreat, should be based upon identifying and measuring changes in the landward-most positions of the erosional barrier profile. This feature corresponds to the interface between barrier and fronting/capping transient beach and dune sediment bodies, and is by far a more stable feature than the high water line whose position changes dramatically with beach configuration.
A new and more precise description of the process-responses of shore zone components along the south coast of Rhode Island has resulted from the approaches taken in this study. The barrier-spit beaches are envisioned as being a part of a distinct hierarchy of sediment reservoirs built, maintained, and slowly translated landward by waves, wind, and tide. There, beaches act or respond as a primary buffer of storm wave effects, and modulate long-term retreat of the backing/underlying dune and barrier units.
The dunes are secondary sediment reservoirs and buffers. Their slowly accumulated aeolian and interbedded washover deposits are eroded only by the more severe storms of late fall and winter when the volume of the beach reservoir is at a minimum, and by infrequent extreme events. Where the shore fronts headlands, the beaches alone provide the buffering of storm wave erosion.
Graves, Scott Michael, "Morphotomology of Rhode Island Barrier Shores: A Method of Distinguishing Beach from Dune/Barrier Component Histories Within a 29 Yeah Record of Shore Zone Profile Data, with Special Reference to the Role of the Beach as a Buffer and Modulator of Erosional Coastline Retreat" (1990). Open Access Master's Theses. Paper 2124.