INSIGHTS FROM REPEATED TERRESTRIAL LASER-SCANNER SURVEYS OF CHANNEL-WALL EROSION ALONG THE SOUTH RIVER, VIRGINIA

This study presents our insights regarding the collection and analysis of repeated terrestrial laser-scanner surveys of 14 eroding channel walls along a 10 mile reach of the mercury-contaminated South River, Virginia. Terrestrial laser-scanner data, used to monitor bank retreat, were collected between 2006 and 2008, repeated between 2013 and 2014, and repeated again between 2018 and 2019. The initial project design included the potential for long-term study so that survey markers were embedded in concrete at each of the 14 survey sites. Approximately 7 years into the project, we were able to relocate original survey markers on all but 1 site where lateral bank erosion removed the marker. However, 13 years into the project during the 2018-2019 survey, the number of missing survey markers increased to 6, while another 3 were no longer useable due to extensive vegetation cover. Of the markers missing after 13 years, 2 were subject to bank failure and erosion, 1 was buried by floodplain accumulations, and 3 were removed by anthropogenic alterations to the landscape. Due to the loss of survey markers, precise positioning of our laser scanner was not possible. Despite the increase in spatial uncertainty, we applied a different approach to continue monitoring channel wall erosion. This included aligning our repeated laser-scanner data using a minimum of 10 natural and anthropogenic objects and features that were well represented in the scans and remained stationary between surveys. Our alignments using this method were completed with an RMS error below 0.10 meters. Therefore, our resolution of derivative erosion models was reduced from the original 5 cm to 10 cm grid models. Given that many of our survey sites have experienced one or more meters of lateral erosion since the initial survey, the increased error does not obfuscate the 13-year long erosion signal. Our results are significant in that despite the loss of installed survey markers, we are able to continue to align data with objects and features of opportunity, a promising approach for continued long-term use of laser scanners for this type of research.