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Using terrestrial laser scanning to investigate changes in beach morphology during swell events

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Item Summary

Title: Using terrestrial laser scanning to investigate changes in beach morphology during swell events
Authors: Barnes, Austin T.
Keywords: shoreline change
beach morphology
Terrestrial Laser Scanning (TLS)
Issue Date: Dec 2013
Publisher: [Honolulu] : [University of Hawaii at Manoa], [December 2013]
Abstract: Developing a better understanding of beach response to a variety of forcing mechanisms is necessary in efforts to predict and mitigate the effects of both short-term events and long-term trends on shoreline change. The evolution of beach morphology has been studied on time scales of days to weeks but rarely repeatedly during large swell events due to the limitations posed by traditional surveying methods. With the advent of Terrestrial Laser Scanning (TLS), studying the beach response to individual storm events and even wave events has become possible.
TLS was used to quantify morphological changes at Waikiki Beach, a high traffic and economically important beach, on Oahu, Hawaii's south shore. TLS scans were generated approximately every 30 minutes for a 3-day period starting at the peak of an August/September 2011 swell event that was the largest south swell on record since September 2005 (Caldwell, 2007). A digital elevation model (DEM) of Waikiki was produced to calculate relative subaerial cumulative beach volume change over the course of the study period and three sections of Waikiki were used for analysis. These three sections accreted between ~11 and 29 m3/m of sediment on the subaerial beach during the study period, with most accretion occurring during the falling tide following the highest tide of the day. Little or no cumulative beach volume change was observed preceding these accretionary phases. Pressure sensors deployed at the study site were used to obtain estimates of the incident and nearshore wave conditions in approximately 6.9 and 1.5 meters water depth respectively. The observed nearshore swell wave height was tidally modulated, and we speculate that the water level associated with the highest tide of the day allows wave energy to reach the shore and deposit sediment onto the subaerial beach.
A one-dimensional numerical model, XBeach, was implemented to compare with the observations and the model is shown to inaccurately predict the subaerial beach response. The model is forced with observed incident wave and tidal conditions and predicts erosion of the subaerial beach between ~5 and 10 m3/m instead of the observed accretion. The model results from XBeach reveal a disparity between observations of accretion and the modeled erosion of Waikiki's subaerial beach. In addition, the one-dimensional XBeach model fails to accurately predict nearshore infragravity wave conditions and subaerial beach response in Waikiki during swell events of this magnitude.
This thesis demonstrates the utility of TLS in studies of morphologic change on beaches with an adequate vantage point for studying the evolution of beach morphology, especially on time scales previously too short for traditional survey methods. It also reveals accretionary characteristics of subaerial beach response in Waikiki to this large swell event and tidal levels, as well as limitations of the one-dimensional XBeach model in predicting subaerial cumulative beach volume response in Waikiki.
Description: M.S. University of Hawaii at Manoa 2013.
Includes bibliographical references.
Appears in Collections:M.S. - Ocean and Resources Engineering

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