UNMANNED AERIAL SYSTEMS 3D MODELING OF SHRINKING SHORELINES AND THE LEAKING LAKEBED, MOUNTAIN LAKE, GILES COUNTRY, VIRGINIA

Mountain Lake, in Giles County, Virginia, is one of only two naturally formed lakes in the state. Sitting at an elevation of 3,875 feet above sea level, it is a unique natural feature in the unglaciated southern Appalachians. Geophysical investigations by Radford University researchers suggest that the lake formed as a result of colluvial damming of an ancient water gap in the ridge of a breached plunging anticline some 6,000 to 7,000 years ago. Beginning in about 2002, water levels decreased significantly during fall months and recovered only partially during the summer months. In 2008, the lake went completely dry and then nearly so again in 2011, and most recently during the fall of 2016.

Through the use of overlapping images taken from unmanned aerial vehicles (UAVs) processed with structure from motion (SFM) software for analyzing thousands of key points spread across dozens of stereopairs of images, it is now possible to create digital 3D models of the lake bed, side banks, erosional and depositional sedimentary features, and the colluvial dam. It is also possible to stitch the images into high resolution orthophoto mosaics that show changes to the shoreline and lake features over time, along with the locations of leaks from the lake into the natural dam. Finally, the SFM generation of 3D point clouds and triangular mesh models makes it possible to print physical representations of the landscape, to scale, to aid others in the visualization of the lake bed and surrounding environment.

The authors utilized all of these techniques to evaluate changes in lake features over a period of several weeks in 2016, during which time water levels were dropping, until reaching a minimum of a small stream eroding channels across the dry lakebed and disappearing at conduits, flowing into the natural dam, and finally water level rising again, drowning newly formed deltaic deposits. The results support the landslide hypotheses for the formation of the lake and suggest solutions that might bring back the full lake if implemented.