LiDAR-derived measurements of rapid coastal change along Wisconsin’s Lake Superior coast (2009–2019)

Abstract

Coastal landscapes are highly dynamic, and comprehensive measurements of morphological change are important for improved coastal management. Past assessments of morphological change along Wisconsin’s Lake Superior coast were limited in either their spatial coverage or their ability to incorporate three-dimensional changes due to their reliance on 2D transect-based or plan-form measurements. Here, we quantified erosion and deposition for 74 km of Wisconsin’s Lake Superior coastline using airborne LiDAR data collected in 2009 and 2019, spanning a transition from below-average to near-record water elevations. We used open-source methods to generate new 1-m resolution DEM’s and semi-automatically delineated bluff toes and crests. The study reach experienced 3.03 ± 0.59 $\times$ 10⁶ m³ of net erosion, corresponding to a normalized mean erosion rate of 3.9 m³ yr⁻¹ per m of coastline. The mean bluff toe retreated 6.8 ± 5.6 m over this time period (0.7 ± 0.4 m yr⁻¹), while the mean bluff crest retreated 2.9 ± 5.7 m over this time period (0.3 ± 0.4 m yr⁻¹), leading to widespread bluff steepening. Coastal erosion rates exhibited a strong lithological dependence, with sandier bluffs losing 7.0 ± 4.9 m³ yr⁻¹ per m of coastline while more clay-rich bluffs eroded at a rate of 3.2 ± 2.3 m³ yr⁻¹ per m of coastline. Aggradational reaches were limited to Wisconsin Point. Our findings indicate that Wisconsin’s Lake Superior coast is eroding at rates similar to other soft coastal cliff systems and there is substantial spatial variability in erosion rates. As of 2019, bluff slopes had increased and bluff crest retreat is expected to continue to achieve stable slope angles.