A spatial and temporal assessment of dune-beach change: Fire Island, New York

Erika Emily Lentz, University of Rhode Island

Abstract

This series of studies assesses spatial and temporal changes to the dune/beach system at Fire Island, New York. This dissertation presents three chapters in manuscript format. The first manuscript provides a comprehensive literature review of existing sediment budget estimates at Fire Island to better understand sediment supply influences on behavior and evolution of the island. The manuscript finds agreement among all existing sediment budgets for Fire Island which estimate that approximately 200,000 m3/yr more sediment is leaving the system to the west than is entering the system to the east. This amount of loss is in spite of a relative lack of overall landward migration along the western reach of the island. The lack of migration coupled with the loss of sediment from the western reach indicates that a sediment source not quantified in the existing estimates must be present and contributing sufficient sediment for this part of the island to maintain position. A number of possible sediment sources are explored, with the most likely source identified as a series of shoreface-attached sand ridges found west of Watch Hill. These ridges are composed of texturally and mineralogically compatible beach sand of sufficient quantity to serve as a viable source to western Fire Island. ^ The second manuscript assesses morphologic change over the last decade at two Fire Island study sites to better understand specific subaerial expressions of east-west differences and responses to anthropogenic modifications. The sites are situated in eastern and western reaches of the island where antecedent geology and long-term landward island migration patterns are known to differ. High resolution topography from 13 surveys spanning a decade is used to measure elevation change, volume differences, shoreline, dune crestline, and dune toe positional change rates, as well as beach width and profile slope. Results show that western beaches are wider, dunes are lower, with more gradual cross shore profile slopes than areas further east. The differences in profile configurations from east to west are attributed to differences in antecedent geology and the offshore geologic framework. These results are compounded by impacts from a western beach replenishment project completed during the study period. A more dissipative cross shore morphology to the west may be better able to attenuate storm wave energy than the eastern portion of the island; the addition of replenishment material to the western dunes and beaches may therefore only add to a profile morphologically predisposed to a dissipative state. ^ The final manuscript assesses long-term morphologic change over 10, 30, and 40 year intervals along the entire dune/beach system at Fire Island to determine the relative importance of storm events, modifications, and geologic controls on long-term change. High resolution surfaces from 1969, 1999, and 2009, are supplemented by more temporally dense datasets in several sites along the coast. Topography is used to determine elevation change, net movement of dune crestlines and shorelines, and to isolate areas of persistent elevation loss and gain over 10 and 30 year intervals, both along-island and in morphologically defined zones of coast. In addition, spatial patterns of change, correlation among features, and significance were assessed to better understand behavior and response to natural events and human modifications to the coast. Results show that a series of storm events in the early 1990s had long-term impacts on the system, and that post-storm recovery of the dune crestline along the island is ongoing. Although a number of replenishment projects have been conducted over the last half century, and particularly over the last decade, clear evidence of morphologic change attributable to these modification impacts was not evident to extract in the long-term signal. Zonal patterns of change and correlation are closely linked with existing morphology; differences in morphology and landward migration patterns from east to west are distinctive and likely related to morphologic controls such as antecedent and offshore framework geology. ^ The three manuscripts demonstrate that although anthropogenic modifications have helped to shape the coast at Fire Island, more distinctive signatures in morphology and coastal change are attributable to geologic controls and storm events. Future attempts to control or manage the system, particularly with climate-driven increasing storm intensities, may be expected to offer little in the way of protection to island residents or resources. (Abstract shortened by UMI.)^

Subject Area

Geology|Geomorphology

Recommended Citation

Erika Emily Lentz, "A spatial and temporal assessment of dune-beach change: Fire Island, New York" (2011). Dissertations and Master's Theses (Campus Access). Paper AAI3445008.
http://digitalcommons.uri.edu/dissertations/AAI3445008

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