NUMERICAL MODELLING OF LAKE MICHIGAN SHORELINE EROSION: TWO RIVERS, WISCONSIN

Lake Michigan experiences quasi-decadal fluctuations between high and low water levels, resulting in variable impacts to navigation and tourism, as well as changes in shoreline position and coastal habitat extent. Recent high water levels are a concern for homeowners and policy-makers due to significant shoreline recession. Nearshore waves, currents and bathymetry are major factors influencing local sediment transport and shoreline behavior, but rigorous models of these processes are largely absent for Lake Michigan and frequent “real-time” surveying can be cost prohibitive for small communities. As such, there have been few attempts to quantitatively estimate potential rates and extents of wave-driven erosion and nearshore sediment transport at sandy coastal systems in the Great Lakes, particularly related to fluctuations in lake level. In this study we numerically model nearshore sediment transport and resulting changes in sandy beach and nearshore morphology in order to estimate future coastal geomorphic change in response to storms and varying lake levels.

Bathymetric surveying was conducted during summer of 2019 for 0.5 km of shoreline near Two Rivers, Wisconsin. The survey extended lakeward from the active shoreface to the approximate depth of closure (20 m isobath). High-resolution topographic surveying was done contemporaneously with bathymetric surveys, as well as immediately before and after the first high-magnitude storm. Coupled hydrodynamic and sediment transport models (Delft3d FLOW & WAVE) were used to propagate offshore wave conditions obtained from the USACE Wave Information Studies into the nearshore zone using wind and water level data from a nearby NOAA station. Two separate sediment transport models (Delft3d MOR and XBeach) were used simulate the morphologic response to modelled nearshore wave conditions. Hindcast simulations were used in preliminary calibration of the model and accuracy assessment of predicted subaerial shoreface changes. More exhaustive model calibration will be performed pending repeat bathymetric surveys planned for May 2020. Following satisfactory hindcast simulation of morphologic evolution, input data will be modified to simulate the response of the nearshore environment to cyclical fluctuations in Lake Michigan water levels.