by Ty Fischer, Riparian Health Restoration Intern

This is part 2 of a series on the Great Lakes. To read the first part, click here.

With water level fluctuations operating on daily, seasonal, and annual cycles, and with frequent disturbances from high winds, large waves, seiches, and storms, the Laurentian Great Lakes are truly dynamic freshwater systems. As a result, they display a diverse range of coastline types ranging from steep coastal bluffs to vast sandy beaches that are highly susceptible to erosion. To address these issues, typical slope stabilization strategies must be adapted to make these areas resistant and resilient in the face of this constant change. In this article, part 2 of a series on the dynamic coastlines of the Laurentian Great Lakes, we will discuss the mechanisms by which erosion can occur on these coastlines, look into why hardened shoreline structures are an inadequate and potentially ecologically harmful option for managing such issues, and provide an introduction into the best options to implement instead.

When approaching shoreline stabilization projects on the Great Lakes coastlines, it is important to first understand the two on-shore geological factors affecting the speed and mechanism by which erosion occurs. The first of these factors is surface water runoff and groundwater seepage. Strong inflows of surface water runoff can carry sediment with it and accelerate the rate of erosion, so properly managing surface water can help safeguard the stability of the slope. You can manage surface water runoff by grading or re-grading the slope, re-sloping the land away from the edge of the bank or bluff, collecting runoff in a sewer or a private drain pipe that runs down to the water, and/or planting ground cover plants (Chase et al., 2012). Groundwater – particularly perched groundwater, where water pools higher than the main water level – can be even more influential than surface water runoff as it can often trigger large, deeply gouging landslides. It can be a more complicated thing to address since it is found below ground, but you can still help manage the influence of groundwater on erosion by correcting problems with slope seepage from septic systems or by intercepting perched groundwater that is flowing towards the coastal slope (Chase et al., 2012).

Another influential on-shore geological factor is the soil type. Different soil properties directly influence the overall structure of the shoreline and therefore dictate the way by which erosion may occur in a given area. For example, clay can remain in the shape of very steep slopes (called bluffs) when dry but can fail rapidly and catastrophically in the form of a landslide in times of intense rainfall, rapid snowmelt, or wave or current induced erosion of the slope (Chase et al., 2012). Bedrock can likewise remain in the shape of very steep slopes but can fail as a result of water seeping into the cracks, freezing, expanding, and finally cracking the bedrock. Sand and gravel are eroded easily since water can infiltrate and reduce the frictional resistance between particles, but it can maintain a more gentle slope and will take the form of beaches as a result (Chase et al., 2012). Therefore, understanding the soil composition of a given coastline can provide a look into its future regarding how it may be affected by the changing water levels, high winds, and large waves of the Great Lakes.

To manage erosion problems as a result of these factors, installing so-called “hardened” shoreline structures such as retaining walls has become common practice in the region (Wensink, Tiegs, 2016). Shoreline owners think they provide a strong barrier against the equally strong influences of these hydrological systems, even though in reality, such structures actually have an even shorter lifespan on these dynamic coastlines. This is due to a process in the Great Lakes called ‘lakeshore erosion’, or ‘lakebed downcutting’, where the sediment in the nearshore lakebed area closest to the shoreline is gradually swept away. This allows even larger waves to reach the toe (the base) of the slope (Chase et al., 2012). This phenomenon irreversibly undermines the foundations of shore protection structures and subjects them to increasingly destructive wave energy, shortening their life significantly (Baird, 2017). Moreover, even while these structures are still functional, they create a host of negative effects on the local ecosystem. This is because on naturalized shorelines, the water line advances and recedes cyclically to form a gradual transition zone between aquatic and terrestrial areas; hardened shorelines instead create a hard cut-off that can affect biogeochemical and ecological processes, alter invertebrate communities, change species distributions, decrease moisture levels on shoreline areas, and reduce retention and storage and retention of vitally important organic matter such as wrack (Wensink, Tiegs, 2016). In summary, hardened shoreline structures are temporary, ineffective, and potentially ecologically destructive compared to alternative options.

Even though hardened structures on the shoreline are not ideal, there is a place for engineered landscaping or hardscaping projects further up the slope, especially in cases of extreme erosion or where it presents a substantial risk to shoreline properties. For example, it is possible to regrade the slope to lessen the influence of gravity and/or fill all or a section of it with gravel to stabilize against deep slips. Such options also require that the toe of the slopes are stable so that sediment losses in the nearshore areas are limited (Keillor, Elizabeth, 2003). They can even be combined with planting projects to provide a double layer of defence against erosive processes and maintain riparian habitat. Overall, compared to hardened shoreline structures, engineered stabilization projects can still maintain most of the gradual transition between the land and water. This makes them a far better option both for protecting properties and local wildlife populations. However, given the complexity of such projects, they are often expensive and mandate consultation with regulatory agencies, coastal engineers, and geologists (Keillor, Elizabeth, 2003).

Shoreline naturalization, on the other hand, is on its own one of the most cost-effective and successful measures to reduce the influence of erosion, and you often do not need special permission to carry out such work on your property. In the final blog post in this series, we will detail the process for how to approach shoreline naturalization projects in both the dynamic coastal bluff and beach areas on the Great Lakes.

 

References

Baird, W.F. (2017). Considerations for Shoreline Protection Structures. Prepared for the Ausable Bayfield Conservation Authority. Accessed from: https://www.abca.ca/assets/files/Appendix_B_to_SMP_2019_Considerations_for_Shoreline_Protection_Structures_RE%281%29.pdf
Chase, R., Clark, G., Edil, T., Kehew, A., Keillor, P., Mickelson, D. (2012). Stabilizing coastal slopes on the Great Lakes. University of Wisconsin Sea Grant Institute. Accessed from: file:///C:/Users/Asus/Downloads/StabilizingCoastalSlopes.pdf
Keillor, P., Elizabeth, W. (2003). Living on the Coast: Protecting Investments in Shore Property on the Great Lakes. URL : https://repository.library.noaa.gov/view/noaa/45713
Wensink, S. M., & Tiegs, S. D. (2016). Shoreline hardening alters freshwater shoreline ecosystems. Freshwater Science, 35(3), 764–777. https://doi.org/10.1086/687279

 

This blog post is part of a Climate Change toolkit, generously funded by The Catherine and Maxwell Meighen Foundation. Access the full toolkit here.