Cumulative Impacts to Bluff Recession in the vicinity of Coastal Structures under Fluctuating Water Levels in Lake Michigan

Abstract

Cumulative impacts of coastal recession in the United States impact critical infrastructure, threaten the safety of 127 million lives, the security of over $9 trillion in goods and services, and 80,000 acres of wetlands. Bluff recession on Wisconsin's Lake Michigan coast is driven by natural and anthropogenic (e.g. manmade) processes that vary both in space and time. These factors include changes in water levels, waves, sediment budget and coastal structure development. This research focuses on bluff recession in Southeastern Wisconsin under lowest to highest Lake Michigan water levels. Oblique and aerial imagery are used with LiDAR data and field measurements in Geographic Information Systems (GIS) to analyze bluff variation. Bluff recession is quantified using digitized bluff features and the Digital Shoreline Analysis System (DSAS). Comprehensive field measurements and historical data from USACE and NOAA are used with existing methodology to characterize water levels and wave impacts. Results show that higher regional bluff recession occurs under higher water levels and larger wave impacts. Local recession is elevated at the flanks of parallel coastal structures, even during periods of minimal regional recession. Crucially, nearshore deepening is observed in lakebed bathymetry over time near structures where abrupt recession occurs. Together, factors driving bluff recession influence an increasingly eroded and hardened coast. This research offers a framework for characterizing relationships between bluff recession, lake hydrodynamics, and proximity to coastal structures over time. The presented cumulative impacts and their causes may help develop more resilient and sustainable coastal protection strategies.