Monitoring Bluff Erosion Rates Using Terrestrial Laser Scanning on Minnesota’s North Shore Streams
2016-05
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Monitoring Bluff Erosion Rates Using Terrestrial Laser Scanning on Minnesota’s North Shore Streams
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2016-05
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Abstract Portions of ten streams along Minnesota’s North Shore currently are listed on the Environmental Protection Agency’s 303(d) list as impaired for turbidity, due to excess sediment. Bluff erosion is a large contributor of fine sediment to Minnesota’s North Shore streams; however, inadequate data are available to quantify rates of bluff erosion relative to total suspended solid (TSS) loads. This bluff monitoring project is designed to quantify site-specific bluff erosion rates at ten sites and combine them into annual fine sediment loads attributed to bluff erosion. Bluff erosion rates are quantified using terrestrial laser scanning (TLS), which allows for the creation of high resolution digital elevation maps (DEMs) of bluff surfaces. DEMs can be used to quantify geomorphic change over time on riverine bluffs. In this study, we assessed multi-year bluff erosion rates on a series of bluffs on Amity Creek and Lester River. We compared previously scanned and analyzed data from 2011 and 2012 on eight bluffs along Amity Creek and Lester River with scans collected from these same bluffs in fall 2013, fall 2014, and spring and fall 2015. The second component of the study involved comparing erosion rates from natural bluffs with bluffs that have undergone stabilization work. Two bluffs on Amity Creek were stabilized in 2009 and these bluffs have been scanned at least once a year since 2012. One more site on Amity Creek and one on the Knife River were stabilized in 2014. These two bluffs were scanned in fall 2014, spring 2015 and fall 2015. Retreat rates from stabilized bluffs were compared with natural bluffs to analyze the initial effectiveness of these stabilization efforts. This study used a Faro Focus 3D phase-shift laser scanner to collect TLS point cloud datasets. Post-processing of the data involved using Faro Scene, Topographic Point Cloud Analysis Toolkit (ToPCAT) and ArcGIS software, which included using the Geomorphic Change Detection (GCD) Add-in. Data produced were used to calculate average retreat rates between successive scanning campaigns of each bluff and to determine an overall average retreat rate over the entire study period. Geomorphic change was measured on eight of the study sites over a four-year period and change was detected in seven to nine periods for these bluffs. The time periods between scans ranged from one month to one year. Geomorphic change was differentiated on these eight bluffs over a three-year period, November 2012 to November 2015, to obtain an average retreat rate. The average annual retreat rate, determined from a weighted average based on bluff area, is -0.08 m/yr. Since most of the study sites are in Amity Creek watershed, we extrapolated this erosion rate to bluffs throughout the watershed to determine a total mass of sediment that is being contributed to the stream by bluff erosion. It is estimated that bluffs contributed 564,000 kg of fine sediment to Amity Creek annually, which when compared to total suspended solid (TSS) load estimates in Amity Creek from previous years, is anywhere from 40-558% of the total. Results show that high flow events, such as the 500-year flood that occurred on June 19-20, 2012, and resulted in 7.25 inches of precipitation, have the greatest impact on bluff erosion. However, detectable geomorphic change was measured even when there were no high flow events, showing that other processes, such as freeze-thaw cycles influence bluff erosion. This study also looked at four stabilized bluffs, where sediment reduction was attempted to be quantified. This was not possible on two of the stabilized bluffs. However, on the two other bluffs, stabilized in 2014 and scanned over a 14-month period, results show that stabilization efforts have worked to slow erosion rates and retain eroded sediment that does occur. Due to these results and the evidence that bluffs are contributing a large portion of the total TSS to Amity Creek and therefore likely other watersheds in the North Shore region, it is clear that controlling bluff erosion is an effective measure to improving water quality in these turbidity impaired streams.
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University of Minnesota M.S. thesis.May 2016. Major: Natural Resources Science and Management. Advisor: Joe Magner. 1 computer file (PDF); vii, 107 pages.
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Hall, Leah. (2016). Monitoring Bluff Erosion Rates Using Terrestrial Laser Scanning on Minnesota’s North Shore Streams. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/181794.
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