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Space-time variability of coastal morphology: results from vídeo remote sensing

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Cássia Pianca Barroso
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Instituto Oceanográfico (IO/DIDC)
Defense date:
Examining board members:
Eduardo Siegle; Javier Alcántara Carrió; Pedro de Souza Pereira; Moyses Gonsalez Tessler; Elirio Ernestino Toldo Junior
Advisor: Eduardo Siegle; Robert Alan Holman

The ability to predict changes of the coastal morphology has been restricted by the lack of observational data in a sufficient spatial and temporal coverage. With the advent of remote sensing, the low spatial and temporal resolution could be overcome, especially with the development of video cameras to study nearshore environments. The goals of this thesis are, using remote sensing techniques, to (1) develop a robust method for extracting shoreline locations; (2) analyze a unique 16 and 26-year record of daily to hourly video images; (3) characterize the space-time scales of shoreline variability at a representative site at Duck, NC; (4) test this method at a reflective and cuspy beach at Massaguaçu Beach located at Brazilian coast; (5) describe recent observations of meso-scales morphology associated with tidal inlets using an innovative optical method and document rates and patterns of migration of these features at New River Inlet, NC. A model was developed, called ASLIM (Augmented ShoreLine Intensity Maxima) to extract the shoreline positions based on fitting the band of high light intensity in time exposure images to a local Gaussian fit with a subsequent Kalman filter to reduce noise and uncertainty. The ASLIM model showed good agreement with survey data (correlation coefficient of 0.85, significant at 95\\% confidence level). Wave forcing was characterized in terms of the significant wave height and the cross-shore and longshore components of wave energy flux. 66\\% of the shoreline variability was explained by periods longer than the annual cycle, despite the fact that wave forcing is dominated by shorter periods. The first EOF mode of shoreline variability contained 49\\% of the variance and represented the cross-shore movement (landward- seaward) of the shoreline. The second mode (26\\% of the variance) is associated with alternating accretion signals on either side of the pier, while the next two higher EOFs (7\\% and 5.6\\%) describe the local pier effects. The pier was found to have a significant influence on shoreline behavior that extends out to 500 meters, nearly twice the length scales assumed by previous studies. The pier restricts seasonal longshore transport from the south (summer) and north (winter) sides, resulting in a seasonally-reversing sediment accumulation on the up-drift side. Erosion signals on the down-drift side of the pier were found propagate away from the pier at 1200 m/year. A shoreline erosion trend that was found only on the north side of the pier may be related to the trend found in the alongshore transport, that it is increasing toward the north and is being blocked by the pier. The ASLIM method was also tested at Massaguaçu Beach and showed to be a valuable tool to investigate shoreline variability processes. Our observations, at New Rivet Inlet (NC), revealed a complex set of swash bars and meso-scale sand banks that migrated in a coherent clockwise pattern with movement in offshore regions being away from the inlet mouth while nearshore migration was back toward the inlet. To quantify migration rates and patterns objectively based on sequences of time exposure images, a Lagged Least Squares Algorithm (LLSA) was developed that found the vector migration rate for which the suite of lagged images were most similar, computed on a tile-by-tile basis. The mean migration rate was found to be 1.53 m/day (standard deviation of 0.76 m/day). 72\\% percent of estimated rates were greater than 1.0 m/day, 31\\% percent were larger than 2.0 m/day, and the maximum rate round was 3.5 m/day, averaged over 23 days. Alongshore averages of cross-shore migration rates showed a node at 110 m from the shoreline that separates migration away from the inlet (offshore) from migration toward the inlet near the shore. The circular pattern of migration appeared to be consistent with expected residual flow on an ebb delta. In conclusion, our results showed that the use of video cameras are a useful tool for providing information about the dynamics of coastal morphologies with a high temporal and spatial resolution, from short to long-term. (AU)

FAPESP's process: 10/13083-3 - Extracting coastal variables from video images: short to medium-term coastal processes
Grantee:Cássia Pianca Barroso
Support Opportunities: Scholarships in Brazil - Doctorate