Nonlinear Sediment Transport Applied to Lake Michigan Coastal Bluffs during Freeze/Thaw Conditions

Steep coastal bluffs made of unconsolidated glacial deposits are a primary source of sediment for beaches in the North American Great Lakes (NAGL). Studies have quantified the average long-term (centuries) sediment supply from bluffs in the NAGL but there is currently no numerical method for estimating sediment transport. Previous work has shown that most of the sediment liberated annually from the bluffs occurs during freeze-thaw conditions in spring. Our work adopts a large-scale geomorphic transport law to calculate sediment yield from a stretch of coastal bluffs for comparison with observed sediment yield over a short period (6 months) when the bluffs were thawing. Using a small uncrewed aerial system (sUAS) and Structure-from-Motion techniques, elevation data was collected for 0.75 km of shoreline at a resolution of 10 cm/pixel during December and May of 2019. Field observations show a combination of large scale mass wasting, undercutting at the bluff toe, and slope wash. Between survey periods there was a net erosion of ~1700 m3. For the 6 months between surveys, the sediment flux rate was calculated to be 0.355 m3m-1a-1. The average net elevation change was measured to be -6.07 cm1m-2a-1. A 1-D Stefan Equation with a modified Berggren correction coefficient was used to estimate the maximum seasonal frost depth and maximum thaw depth into the bluffs. The maximum thaw depth was used as the mobile soil depth in a nonlinear diffusion-type sediment transport law. We adopted a value for the diffusion coefficient which resulted in rates of sediment transport consistent with observed sediment transport that occurred over the 6-month spacing between observations. Results show that sediment transport capacity increases with thaw depth. With these data, we find that the nonlinear diffusion equation is able to predict the rate of sediment transport on steep coastal bluffs undergoing freeze-thaw, allowing for estimations of sediment yield from the coastal bluffs to the nearshore.