A multi-scale assessment for managing coastal geomorphic changes in southwestern Lake Michigan

TL;DR

This study develops a multi-scale framework to assess coastal geomorphic change (CGC) along a 125 km segment of the southwestern Lake Michigan coast, analyzing bluff crest, bluff toe, and shoreline positions across county, reach, and 100 m transect scales for long-term ($1937$–$2020$) and short-term ($1995$–$2020$) periods. It combines digitization of historical aerial imagery with Digital Shoreline Analysis System (DSAS) recession-rate calculations and analyzes wave forcing via a Wave Impact Height metric, identifying scale-dependent erosion patterns and hotspots influenced by coastal defenses. The results show persistent bluff erosion but accelerating shoreline retreat, with notable variability among reaches and strong geography at hotspot sites, and demonstrate higher uniformity at the reach scale than at the county scale. The work provides a public coastal-change dataset (Wisconsin Shoreline Inventory and Oblique Photo Viewer) and a multifactor clustering approach to refine reach delineations for evidence-based coastal management.

Abstract

Understanding coastal geomorphic change is essential for advancing the United Nations Sustainable Development Goals (SDGs) through a multi-scale coastal management framework. In particular, characterization of coastal geomorphic change across multiple spatial and temporal scales can provide essential insights and context-specific knowledge that can inform and empower local communities. In this study, we present a multi-scale assessment of coastal geomorphic change in southwestern Lake Michigan in the Laurentian Great Lakes. Three spatial scales: county, reach, and transect and two temporal scales: long-term and short-term were examined using nine sets of historical aerial imagery spanning 1937 to 2020. The site-averaged long-term (1937-2020) change rates for the bluff crest, bluff toe, and shoreline were -0.22, -0.17, and -0.16 m/year, respectively. In the short term (1995-2020), the corresponding rates were -0.22, -0.15, and -0.32 m/year, indicating an increasing shoreline erosion in recent years. The coastal geomorphic changes at county, reach, and transect scales were further characterized, showing regional and localized distributions of coastal erosion in our study sites. The mechanisms driving coastal change,particularly wave impacts, were also examined to assess their correlation with coastal geomorphic change across different spatial scales. The results indicate that wave impacts influence coastal environments at certain scales more strongly than at others. Several erosion "hotspots" were examined to identify local factors contributing to severe site-specific erosion. Lastly, the spatial uniformity of coastal geomorphology was examined between the county and reach scales. Overall, the findings suggest that multi-scale analyses provide a valuable insight for effective management of coastal geomorphology.

Figures (6)

• Figure 1: Study sites. (a) the location of study site in USA. (b) The southeastern region of Wisconsin on the western coastline of Lake Michigan. (c) Regional map of four counties and 13 reaches, denoted as R1, R2, ..., R13. (d) - (h) are the oblique photos for different geology in study sites: sandy shore without bluffs, low bluff (height<10 m), intermediate low bluff (height between 10 and 20 m), intermediate high bluff (height between 20 and 30 m), and high bluff (height>30 m). Oblique images are accessed from the Wisconsin Shoreline Inventory and Oblique Photo Viewer (http://floodatlas.org/asfpm/oblique_viewer/)
• Figure 2: Schematic of geomorphic feature digitization and bluff crest recession rate calculation along 10 meter transects.
• Figure 3: Coastal geomorphic change in local scales. (a) long-term bluff crest, (b) long-term bluff toe, (c) long-term shoreline, (d) short-term bluff crest, (e) short-term bluff toe, (f) short-term shoreline.
• Figure 4: Bluff toe recession rate and cumulative wave impact height for local (a), reach (b) and county (c) scales for the period of 1995-2020. Heavy and mild erosion are distinguished as values greater/less than 0.2 m/year, respectively, and large/small (CWIH) are differentiated by values greater/less than 0.5 m/day.
• Figure 5: Bluff toe recession rate and cumulative wave impact height for local (a), reach (b) and county (c) scales for the period of 1995-2020. Heavy and mild erosion are distinguished as values greater/less than 0.2 m/year, respectively, and large/small (CWIH) are differentiated by values greater/less than 0.5 m/day.