Monitoring event-scale stream bluff erosion with repeat terrestrial laser scanning: Amity Creek, Duluth, MN
2014-04
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Monitoring event-scale stream bluff erosion with repeat terrestrial laser scanning: Amity Creek, Duluth, MN
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2014-04
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Terrestrial laser scanning (TLS) technology provides high-resolution topographic data that can be used to detect geomorphic change in natural environments. In this study, we utilize successive terrestrial laser scans to investigate the relationship between peak flow rates and stream bluff erosion in the Amity Creek watershed in Duluth, Minnesota, USA. We selected eight study bluffs to conduct terrestrial laser scans following all significant flow events over a one-year time period. The study employed a Faro Focus 3D phase-shift laser to collect TLS point cloud data. Post-processing of the TLS point cloud data sets involved: (1) digital removal of vegetation and objects other than the erosional surface of interest; (2) decimation of the point cloud in PC Tools and extraction of zmin values to produce a data set manageable in GIS; (3) creation of a bare earth digital elevation model (DEM) for each set of scans using ArcMap; and (4) utilization of Geomorphic Change Detection (GCD) software to generate DEMs of Difference (DoDs) from previous terrestrial laser scans. GCD data were used to calculate volumes of erosion and deposition and average retreat distances for time periods over which change was detected. Analysis of DoD color change images allowed for interpretation of the dominant processes responsible for bluff erosion. Sediment samples were collected for bulk density and grain size analyses to determine bluff characteristics and convert erosional volumes to mass; mass was then compared with Total Suspended Solids (TSS) loads from previous years. Suspended Sediment Concentration (SSC) samples were collected in areas where bluffs were located to determine spatial variation in concentrations during precipitation events. There were four time periods from November 2011 to November 2012 in which change was measured at our study bluffs: winter and the transition into spring melt; two major precipitation events in late spring and early summer, and low base flow conditions continuing into late fall. These distinct time periods provided an opportunity to observe the impact of high flow events and other processes on bluff erosion throughout the watershed. Freeze-thaw cycling and saturated conditions in late winter and early spring were found to influence erosion of study bluffs; retreat distances averaged -0.22 m for sites measured during that time period, where negative values indicate erosion. Approximately 5 inches of intermittent rain over a six-day period from May 23-28 resulted in slumping at many bluffs and one major failure. The 500-year flood that occurred following 7.25 inches of precipitation on June 19-20 induced severe erosion; most sites experienced significant fluvial scour along the toe of the bluff. Very little change was measured in our study bluffs during the period of low base flow, spanning from early summer (post-flood) into late fall. Change detection results and stream gage flow data were utilized to examine the effects of peak flows, total flow volume, and duration above bankfull discharge on bluff erosion. Results from our TLS analyses show a strong correlation between high flows and erosion of stream bluffs in the Amity Creek watershed. Average bluff retreat distances vary significantly over the time periods in which change was measured, ranging from -0.41 m during the June flood, to essentially no change when base flow was low. Using our change detection measurements, we calculated an annual average retreat rate of -0.50 m/yr. By applying this rate to a watershed-scale bluff inventory, we estimated the total volume of sediment eroded from unvegetated bluffs in the watershed between November 2011 and November 2012. Results show that bluffs are likely the primary source of fine sediment contributing to the creek's turbidity impairment. Load calculations from our bluff erosion work were compared to Minnesota Pollution Control Agency (MPCA) and Natural Resources Research Institute (NRRI) estimates from previous years, and were found to conceivably represent 100% of the total sediment load even with the 500-year event removed. Erosion during the June flood accounted for approximately 80% of the sediment load from bluff erosion between November 2011 and November 2012. TLS results from this study show that major events are responsible for inducing considerable bluff erosion and moving the vast majority of sediment through the system. Additionally, our data demonstrate that management of turbidity-plagued watersheds along the North Shore and other regions of the state must include a focus on bluffs.
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University of Minnesota M.S. thesis. April 2014. Major:Geological Sciences. Advisor: Karen B. Gran. 1 computer file (PDF); vii, 118 pages, appendices p. 93-118.
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