IMPACTS OF ELEVATED WATER LEVELS ON EROSION OF WISCONSIN’S LAKE MICHIGAN COASTAL BLUFFS

In 2013, water levels in the Laurentian Great Lakes began a multi-year rise and exceeded monthly records in both 2019 and 2020. Elevated lake levels increase wave runup heights, exacerbating nearshore flooding and erosion of coastal landforms including bluffs. While researchers have demonstrated that elevated lake levels are correlated with accelerated bluff recession, robust quantification of the relationship between bluff recession rates and variables such as water level, cumulative wave impact height, and incident wave energy remains elusive. Developing such relationships requires temporally and spatially dense observations of bluff erosion that are currently unavailable.

To address this information gap, we have collected repeat high resolution topographic data along six kilometers of the western Lake Michigan coast. We present bluff toe and crest recession rates along with volumetric erosion rates at sub-decadal intervals from 2012-2020. Bluff toes and crests were digitized using a combination of orthophotos, one meter LiDAR, and structure from motion derived high resolution (10 centimeter) topography. Between late 2012-2015, when lake levels rose rapidly to elevated levels, bluff toe recession was minimal. Between 2015-2018, when lake levels were higher, toe recession rates were greater than before. Between 2018 and 2020, when lake levels were at their highest, toe recession was at a maximum. Topographic information is available at a lower temporal resolution, but a similar trend of larger volumetric erosion rates during periods of higher water level is evident. In general, the extensive steepening of the monitored coastal bluffs has not yet translated to upslope crest recession.

These measurements of bluff response during a period of rising water levels corroborate prior research indicating that higher water levels are correlated with accelerated bluff recession. Such nonlinear response in geomorphic systems challenges efforts to forecast landscape change, but the observations presented here may be used to quantify correlative relationships and calibrate landscape evolution models. Our observations of widespread high-magnitude bluff toe retreat indicate that many of Wisconsin’s coastal bluffs are already committed to extensive crest recession due to the current high stand.