GPR INTEGRATION WITH TOPOGRAPHIC MONITORING DATA AS COASTAL RECONSTRUCTIVE TOOL: EXAMPLES FROM NATURAL AND URBAN SHORELINE REGIONS IN ILLINOIS

The Illinois State Geological Survey has been assessing beach-topographic changes at select sites across the state since 2017. This presentation addresses the utility of integrating data from repeat topographic assessments with 200-250 MHz GPR imagery (from 2021-2022). The combined dataset provides insights into coastal response to the ~1 m-rise in Lake Michigan’s decadal water level from 2017 through 2020. Data examples are provided from (1) A natural shoreline section at Illinois Beach State Park, where geomorphic shoreline studies (e.g., of berm-creation dynamics) offer process-based blueprints for interpreting late Holocene depositional units; and (2) Urban beaches of Chicago, whose morphodynamics during lake-level rise are influenced by embayment characteristics (e.g., aspect) and beach-management activities (e.g., grooming).

High decadal water-level conditions in the Great Lakes are associated with increased potentials for shoreline recession and overwash. Along a high sand-supply portion of Illinois Beach (with ~15 km of sandy shoreline in the up-drift direction as sand source), the 2017-2020 rise in lake level created an extensive overwash berm over multiple storm events. Annual topographic data extractions capture the progressive landward shift in extent of overwash accretion (up the former beach face), recognized in subsurface imagery as landward-sloping units onlapping the former paleotopographic surface. Similar depositional architectures are resolved throughout the ridge plain, where late Holocene reconstruction efforts are underway using such process-based blueprints.

GPR data acquired from Chicago beaches provide not only useful estimates of sand volumes, but also provide insights into the role of antecedent topographic conditions on beach response to lake-level rise. Paleotopographic surfaces predating accretion with elevated water levels are recognized throughout GPR imagery acquired during the lake-level highstand. Data capture architectural differences between ungroomed beach areas, with cross-shore undulatory topographic variances, and groomed beaches, with more uniform lakeward slopes. GPR thus enhances models of beach geomorphology that may inform management decisions during the beach-recovery phase (in anticipation of the next lake-level rise).