Quantifying Differences in Beach Volume Change Between 2-D and 3-D Survey Methods
Abstract
RTK-GPS surveys are the most widely used method of beach surveying for engineers and researchers, but new 3-D methods are gaining popularity because of their increased efficiency and accuracy in topographic and volumetric analysis. The utility of these new methods in comparison to traditional GPS surveys remains poorly understood. This study seeks to assess the differences in volume change calculations between 3-D terrestrial laser scanning and traditional 2-D GPS profile surveys. Onslow Beach in Eastern North Carolina was selected as the study area because of high along-beach variability in beach morphology over a 12 km distance. This allows for comparison of difference estimates between the two survey methods in a variety of beach settings (i.e. wide beaches and narrow beaches) impacted by similar physical processes. 3-D data was collected with a Riegl 3-D terrestrial laser scanner and used to make digital elevation models (DEMs) at seven focus sites along Onslow Beach for three time intervals: annual (May 2008 to May 2009); seasonal (May 2008 to Sept 2008); and storm (before and after a Nor’easter in September 2008). 3-D volume changes during these time intervals were calculated by subtracting the DEMs. 2-D data was generated by slicing the 3-D data at 25m, 50m, and 150 m intervals along each of the focus sites for each of the time intervals to simulate traditional RTK-GPS surveys. 2-D volume changes were calculated by differencing the profiles and translating the volume change along-beach. Differences in volume change estimates were assessed using the percent error between 2-D and 3-D data. 3-D data was the standard in percent error calculations based on the assumption that DEMs generated with millions of data points are closest to the actual morphology of the beach. When the entire profile is considered; dune to beach, which includes the foreshore, backshore, and foredune, the percent error between 2-D and 3-D data (25 m profile spacing) is highest at sites with low dunes and a narrow beach and lowest at sites with high dunes and a wide beach. When the dunes are removed, percent error between 2-D and 3-D data decreases, suggesting that along-beach and across-beach variations in dune morphology contribute greatly to differences between the two methods. Preliminary data suggest that along-beach variations in morphology (i.e. beach cusps) can lead to increased percent error between 2-D and 3-D volume change estimates. At most of the sites, percent error between 2-D and 3-D data increases as the profile spacing interval increases, suggesting that widely spaced transects do not accurately represent the true beach morphology and can enhance error in volume change calculations over 200%. The results of this study suggest that traditional RTK-GPS surveys are not detailed enough to accurately represent some beach morphologies and introduce significant error in volume change calculations, thus more detailed methods such as terrestrial laser scanning should be employed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMEP51D0579T
- Keywords:
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- 3094 MARINE GEOLOGY AND GEOPHYSICS / Instruments and techniques;
- 4217 OCEANOGRAPHY: GENERAL / Coastal processes