MONITORING RAPID TOE RECESSION OF COHESIVE BLUFFS IN WESTERN LAKE MICHIGAN (2012 – 2020)

In 2013 water levels in the Laurentian Great Lakes began a multi-year rise, exceeding monthly records in both 2019 and 2020. Elevated lake levels increase wave runup heights, exacerbating nearshore flooding and erosion of coastal landforms including bluffs. Water levels in Lake Michigan rose quickly from late 2012-2015 at a rate of 0.36 m¹yr^-1, then rose more slowly from 2015-2018 at a rate of 0.09 m¹yr^-1, and then began rising quickly again from 2018-2020 at a rate of 0.38 m¹yr^-1. A fundamental question is whether absolute lake level, the rate of lake level rise, or some combination of both controls the retreat rates of coastal bluffs. While researchers have long hypothesized that elevated lake levels are correlated with accelerated bluff recession, observations of geomorphic bluff response are both temporally and spatially sparse, limiting robust quantification of the dependence on absolute elevation or rate of change in water level.

To address this problem, we present bluff toe and crest recession rates at annual and sub-decadal intervals from 2012-2020 along several kilometers of the western Lake Michigan coast. Bluff toes and crests were digitized using a combination of orthophotos, 1 m LiDAR, and structure from motion derived high resolution (10 cm) 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 but the rate of rise was lower, toe recession rates were greater than before. Between 2018 and 2020, when lake levels were at their highest and the rate of increase was large, toe recession was at a maximum.

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, but also indicate that the rate of rise is important. Such nonlinear response in geomorphic systems challenge efforts to forecast landscape change. Our findings suggest that the rate of bluff retreat scales with the rate of water level change past a threshold water level. This implies that, given sufficient time and sediment supply, restorative coastal processes may be able to rebuild protective beaches if water levels are elevated but stable.