Coastline recession along the ocean and Great Lakes coasts of the United States has caused millions of dollars in damage to structures and property and threatens to produce significant future damage. The house shown to the right is located on the Wisconsin shoreline of Lake Superior. The house is not being constructed in this picture. It has been moved away from the bluff edge, 45 meters from where the house was built because it was threatened by bluff recession.
Setbacks are employed to protect new coastal developments (i.e. homes, cabins, roads, etc.) for a given time period (e.g. 60 years) based on historical recession rates (Platt, 1994). However, spatial variation and magnitude of future coastline recession depend on numerous natural and human factors. These factors may not be equivalent between the historical record used to obtain recession rates and the future period of concern. Thus, historical recession rates may not be representative of future variability in recession.
Establishing appropriate setbacks is challenging because coastline recession is dependent upon a complex interrelationship of numerous factors including wave climate, water level, wave runup, wave action at the bluff toe, shoreline orientation and fetch, downcutting, shoreline structures, beach morphology, bluff morphology, weathering, rain erosion, ground water levels and seepage, thaw failure, creep, lake-ice action, and lithology. Depending on the coastal setting, these factors will play varying roles in contributing to overall recession, and spatial variability in these factors are likely to result in spatial variability in recession.
This study investigates a wave-bluff interaction index, termed Wave Impact Height (WIH), and correlates cumulative hindcasts of WIH with bluff recession along the Wisconsin shoreline of Lake Superior. The Lake Superior shoreline of Wisconsin (see map in Study Area) provides a unique environment to study bluff recession because the shoreline is relatively undeveloped. However, the pressures of new development necessitate an understanding of the factors affecting recession for shoreline management.
28 sites are investigated. At each site the bluffs and beaches were characterized, measuring the profiles of each and noting material types. Wave runup was measured at 21 of the sites under different wave conditions. The wave runup records were analyzed to determine the most appropriate method for predicting wave runup in the study area.
Using the appropriate wave runup prediction method, WIH is hindcast for the years 1966-1998 at each bluff site using historical wave data, assuming the measured beach profiles were representative of historical beach profiles. Cumulative WIH (CWIH) is correlated with recession rates to investigate the utility of this index for predicting spatial variability of bluff recession rates.