Mapping Chicago’s beach and nearshore sand distributions for effective management

Lake Michigan water levels fluctuate by as much as two metres over decadal time spans, altering how waves and currents interact with coastal sediments and lakefront infrastructure in ways not yet fully understood. Understanding sediment transport dynamics is foundational to effective coastal resilience planning. This article shares insights from an offshore sand assessment and beach topobathymetric monitoring study of the greater Chicago coast, a collaboration with the Illinois-Indiana Sea Grant and Illinois Coastal Management Program.

Semi-periodic fluctuations in Lake Michigan’s water level, measured in metres at the decadal scale, combine with storms to create a variety of coastal management problems along the greater Chicago lakefront. There are around two dozen engineered beaches along this ~40km-long stretch of coast, which provide recreational opportunities to a metropolitan population approaching ten million. Vegetated dune terrains are also found within this urban lakefront landscape. These beach areas are proven to be ecologically important as a bird nesting habitat, including for the threatened Great Lakes piping plover, and as a general migratory stopover site. Changes in lake level, which alter sediment transport dynamics along the coast, also factor into other management challenges. These include the shoaling of marine navigation and harbour approach channels and lakebed downcutting, which threatens lakefront infrastructure integrity. Despite the importance of understanding coastal sand transport patterns along heavily urbanized portions of the Great Lakes coast, little work has gone into mapping offshore sand distributions and monitoring geomorphic changes close to shore. Such efforts must be at the core of coastal resilience and other management endeavours.

The Illinois State Geological Survey’s Coastal Geology Group (ISGS-CGG) has been working closely with the Illinois Coastal Management Program, the Chicago Park District and the National Oceanic and Atmospheric Administration’s Illinois-Indiana Sea Grant Office since 2020 to improve geological maps and geomorphic models for the SW Lake Michigan coast. The aim is to couple an improved understanding of offshore sand occurrence to beach geomorphic response to lake-level variances and punctuated storm events. This requires onshore and offshore sampling to characterize materials, monitoring beach and nearshore elevations to quantify changes, and subsurface imaging for sand volumetric assessments.

Lake-bottom geology

To confidently map lake-bottom geology and quantify offshore sand volumes at a regional scale, the ISGS-CGG collected marine ‘chirper’ seismic reflection data, sediment cores and sediment grab samples in the summers of 2022 and 2023. Offshore geologic units differentiated based on this data were traced regionally using 2020 Lidar-derived DEMs and slope aspect models, which showed close agreement between (1) surface and subsurface geological sample information, (2) geophysical data resolving the ‘base of sand’ subsurface reflection, and (3) the relative smoothness of the lake bottom. In particular, the difference between sandy substrate and non-sandy substrate is manifested as a contrast between smooth and rough lake bottom ‘texture’. It is important to note that all data was acquired within a relatively short period (2020–2023), providing a ‘snapshot assessment’ of sand distribution across the Chicago littoral sand transport zone. Non-sandy nearshore substrates consist of outcrops of bedrock, many recognized by the marine biological community as ‘reef-style’ fish spawning sites, and undifferentiated sediments consisting mainly of muddy gravel-cobble deposits.

GIS-based integration of multiple corroborating surficial and subsurface geological datasets, acquired within a few years, has provided an unparalleled blueprint for regional, high-resolution delineation of nearshore sand bodies from Lidar-based bathymetric terrain models. Knowing offshore sand extent and thickness allows us to understand the role of infrastructure and bedrock morphology on alongshore sediment routing and sand sequestration patterns. This data also informs recent geomorphic patterns along the coast, including that within urban lakefront embayments containing engineered pocket beaches. Chicago’s urban beaches are shown to benefit to varying degrees from their coupling to the offshore sand resource.

Nearshore topobathymetric changes

United States federal agencies, particularly the United States Army Corps of Engineers and National Oceanic and Atmospheric Administration, strive to acquire topobathymetric Lidar data along the Illinois coast of Lake Michigan around twice per decade. This does not adequately capture nearshore and beach geomorphic changes at a sufficiently high resolution to provide coastal managers with useful information on developmental trends. While 2008, 2012 and 2020 Lidar datasets provided insights into geomorphic patterns and sand volumetric changes associated with decadal lake-level rise from 2013–2020, the absence of such federal information since 2020 has left beach managers in Chicago with little information on developments during beach re-exposure and recovery with lake-level fall. To increase the temporal data resolution, the ISGS-CGG started supplementing federal datasets with annual topobathymetric surveys at priority sites in Chicago. These efforts, underway since 2020, involve deployment of single-beam sonar across nearshore waters >0.5 metres in water depth, wading surveys between that coverage and the shoreline, and beach topographic assessments using small aerial drones. All geospatial data acquisition methods integrate real-time kinematic positioning technologies for cm-scale precision in horizontal and vertical dimensions.

Repeat topobathymetric data coverage of the urban coastal environment, at different lake-level positions, provides the basis for investigating patterns of sand redistribution and the impacts thereon of winter ice and storms. The recent period of lake-level rise, from a historical low in 2013 to a level >1.5 metres above in 2020, was associated with coastal inundation, beach shoreline retreat and sediment accretion along backshore regions. This general geomorphic trajectory, under rising water-level conditions, was ubiquitous to all Chicago area beaches. However, some notable differences in geomorphic response were noted. These included different degrees and directions of beach rotation, as related to influences of the surrounding infrastructure, embayment orientation and beach length. Changes across the inundated portions of coastal embayments/beaches and the nearshore, mapped by federal and ISGS-CGG datasets, are more highly variable. Patterns of change here are not easily recognized by coastal managers, but they are important to the continued geomorphic development of beaches. Differences in sand supply and geomorphic changes across nearshore regions have had an impact on beach recovery dynamics with the 2020–2024 lake-level fall. Beaches within coastal embayments that gained sand during the 2013–2020 lake-level rise are recovering more quickly, as seen at Montrose Beach, Chicago’s largest. Several new dune ridges have formed here since 2020, given sand influx during lake-level rise and rapid exposure of shallowly inundated beach terrains with lake-level fall. This has not been the case with other beaches, particularly those that lost sand during the lake-level rise.

Guiding coastal resilience

The availability of federal Lidar-based topobathymetric elevation models, along with targeted nearshore and beach topobathymetric monitoring and regional sand assessment data, are paving the way for an improved understanding of Great Lakes coastal morphodynamics along the urban lakefront. ISGS-CGG studies to date have revealed that: (1) sand supply, which itself is time-variable, can be drastically different from beach to beach due to sand trapping against natural and/or engineered structures; (2) lake-level rise, especially if at the metre scale over sub-decadal time frames, forces a regional cross-shore and alongshore rearrangement of sand deposits by way of storm-induced profile adjustments; and (3) individual beach recovery dynamics during lake-level fall and terrain re-exposure are influenced by prior geomorphic changes across the nearshore, time-varying patterns of sand supply and beach management activities.

Understanding offshore sand distribution and alongshore transport patterns can help inform beach managers in Chicago and elsewhere along the Great Lakes on likely near-future trajectories of beach geomorphic change with anticipated future lake-level fluctuations. The shape of the urban embayment and its groins and other shoreline structures are to be considered as well. While all beach shorelines along the urban lakefront overwash and retreat when lake levels rise, beach-specific geomorphic trends across the nearshore influence the beach recovery dynamics during subsequent lake-level fall, when inundated terrains become exposed. This is when informed management action can be taken to improve coastal resilience in anticipation of the next period of lake-level rise. Historical patterns of beach geomorphic change and knowledge of offshore sand distributions serve as guides to inform decision makers on the useful placement of sand fences and re-establishment of vegetation and rebuilt dune terrains, for example. They also point to where future problems can be anticipated for a given lake-level change trajectory.

Conclusion

This regional assessment of lake margin geology and geomorphology is the first of its kind along the Chicago coast of south-west Lake Michigan, a storm-dominated margin undergoing metre-scale lake-level fluctuations that modify sand transport dynamics in ways not fully understood. The more than 20 beaches along this stretch of coast are connected, to varying degrees, to the alongshore sand transport engine and respond differently to lake-level changes and storms. Precision mapping of coastal sand distributions using Lidar, sampling and subsurface geophysical imaging methods provides a sand supply context for beach geomorphic behaviour, as captured in topobathymetric monitoring datasets. The data-integrative approach provides useful information on how urban lakefront embayments are coupled to offshore sand resources and how lake-level changes drive patterns of sand redistribution along a fragmented urban littoral zone. This information can help guide future coastal management decision-making.