DOCUMENTING LAKE MICHIGAN COASTAL BLUFF EROSION AND CORRESPONDING NEARSHORE CHANGES USING SFM AND SINGLE-BEAM SONAR

The North American Great Lakes experience long term (decadal-scale) water level fluctuations. This causes the shoreline to oscillate between wide, protective beaches during low lake levels and narrow beaches susceptible to erosion during high lake levels. The SW Lake Michigan shoreline is primarily backed (70%) by sediment bluffs 10-30 m high which feed sediment to fronting beaches and the nearshore system. Lake level and the resultant beach state have implications for bluff stability. The dynamics of bluff erosion during high lake levels is well documented and suggests that the rate of response of a bluff is proportional to its height, resulting in a lag between peak lake level and peak erosion. However, there are no high-resolution datasets investigating the dynamic feedbacks between nearshore and onshore systems, which are important for understanding the behavior of these shorelines over the range of lake level variation. This work investigates sediment delivery from a bluff to the beach, and subsequent distribution of the material in the nearshore through time using a combination of mapping techniques. The site chosen is a 1 km stretch of bluffs that reach ~30 m in height, which represents the tall bluffs along SW Lake Michigan. This site will allow investigation into whether taller bluffs are continuing to adjust to peak lake levels in 2020. Topographic surveys used Structure from Motion photogrammetry from a Phantom4 RTK drone, yielding 3 cm of precision in the final models. Offshore mapping implemented shore-perpendicular nearshore transects spaced at 20-30 m were measured using a Bathycat RC boat equipped with a single-beam echosounder and Emlid RTK GPS, recording at 10 hz. Contour maps were created from the transects using triangulated irregular network (TIN) interpolation. Difference maps between each topobathy survey pair were executed in ArcMap. To date, four surveys have been conducted, bracketing key seasonal changes and weather events. Preliminary results show significant variability in the magnitude of sediment mobilized from the bluff between survey periods, while the nearshore differences vary less in magnitude.