Assessing Bank Erosion Hazards Along the St.Lawrence Fluvial Shoreline

Over the past few decades, changes in shoreline land use coupled with increasing river infrastructure have increased vulnerability for communities and ecosystems along the St. Lawrence. Exposure to geohazards is also evolving due to ongoing climate change, highlighting the need for flexible management strategies for riparian environments. In this perspective, GIS-based mapping allows the integration of a wide range of environmental data. However, these datasets are often incomplete and non-homogeneous over large geographic scales, which can be problematic for the implementation of regional land-use strategies.

Using the St. Lawrence Fluvial System (SLFS) (Quebec, Canada) as a case study, this analysis reports and describes a high-resolution approach to mapping the position, characteristics, and erosion susceptibility of natural and artificial stream banks from remote sensing, fieldwork, and local knowledge data. This approach allowed the identification of erosion-prone sites and the identification of dominant erosion processes as well as their spatial constraint along the SLFS. The proposed geospatial framework provides (1) a comprehensive picture of the river landscape that will enable effective implementation of future monitoring and process-based studies; and (2) a first step to support land use stakeholders in selecting appropriate measures to ensure greater resilience of riparian communities and ecosystems.

Understanding and Assessing Impacts

Coastal and riparian municipalities along the St. Lawrence River are exposed to water level fluctuations and shoreline erosion, the frequency and intensity of which are affected by climate change. These hazards are important components of the river system that control the evolution of shorelines and coastal ecosystems, but they pose a significant threat to the built environment and the safety of populations. The project is part of the “fluvial section” component of measure 2.6 of the Quebec government’s 2013-2020 Action Plan on Climate Change (PACC), which aims to support municipalities located along the St. Lawrence facing coastal erosion. In addition, the major floods experienced in various parts of the province during the spring of 2017 and 2019 have highlighted the importance of revising existing planning tools and collecting new data to improve the management of hydrometeorological hazards. This project relied on close collaboration with the organizations coordinating the Regional Round Tables (TCR) in the territory concerned, as well as with the Ouranos team involved in a joint project that is also part of this PACC measure.

Identifying Actions

Following the 2017 and 2019 events in the St. Lawrence River System (SLFS), many questions regarding future large-scale management policies have been raised to help municipalities address erosion risks. With ongoing climate change, the issue of maintaining or adapting geohazard management strategies has become even more critical to ensure the safety of residents while limiting financial costs. Currently, a high percentage of the decision-making process for erosion management is based on (1) economic considerations, (2) non-integrated local approaches to developing solutions, and (3) stakeholder fears. These flawed strategies generally result from the scarcity of knowledge and information at the regional scale, which limits objective dialogue between stakeholders.

From this perspective, the development of an accurate geospatial database of the riparian environment becomes necessary to establish a framework for SLFS-specific analysis and a baseline for future quantitative studies of erosion-prone sites. With such missing information along the SLFS, there was a need for a large-scale qualitative mapping approach to support decision making and integrated management by establishing (1) the conceptual basis for erosion risk; and (2) a standardized method of bank positioning and classification appropriate for this hydrosystem.

Implementation

This case study of the St. Lawrence River system proposes an adaptable geographic information system (GIS) approach developed to map riverbank positions and properties based on the integration of geospatial information from fieldwork, remote sensing tools, and local knowledge. This method, which can be adapted to any large river system, provides a detailed picture of the river landscape through high resolution (1:600 scale) and accuracy (<1 m) maps. It also identifies bank erosion hotspots and geomorphic processes at work, while defining their spatial distribution along the river. The development of such an accurate geospatial framework provides (1) an initial picture of the river landscape to enable effective implementation of future monitoring and process studies; and (2) a first step to support land-use stakeholders in selecting appropriate measures to ensure greater resilience and adequate long-term protection of riparian communities and ecosystems.

Examples of Erosion Index and classification on natural and artificial riverbanks

The study assessed and classified various St. Lawrence shorelines into different categories, each with a different erosion level. Source : Bernier, Chassiot & Lajeunesse

The image includes 6 photographs. 3 different natural riverbanks (with sand or rocks) and 3 different artificial riverbanks (with embankments or low walls) are represented.

Outcomes and Monitoring Progress

The research team has developed a geospatial information base that is essential for assessing the risks associated with shoreline erosion in the St. Lawrence River section. The user will be able to find (1) the mapping of the shoreline classification (types of banks and their artificialization), (2) the conditions of degradation of artificial structures and (3) the state of erosion of natural or artificial segments. The areas most vulnerable to erosion were also mapped and documented by qualitative image sheets. These sheets present the shoreline characteristics and the main natural (currents, ice, etc.) and human (wave action, water level management) geomorphological processes associated with shoreline erosion for these sensitive areas, in order to better represent the local dynamics. The geospatial data associated with the shoreline mapping, the qualitative sheets and the project report, which includes a description of the methodology and results, are available for download.

Next Steps

The project’s findings revealed that very little information on riverbank dynamics is currently available for the St. Lawrence River. This lack of information is a major obstacle to informed decision making in order to implement sustainable development strategies, but also to anticipate the role of climate change on erosion hazards. In order to identify the best adaptation solutions for riverside communities, it became essential to better evaluate the erosion mechanisms involved in the short, medium and long term on the St. Lawrence River.

To fill this major knowledge gap, a new four-year research project funded by the MELCC was launched in the summer of 2020. It aims to develop and test erosion monitoring methods at eight sites exposed to erosion in the St. Lawrence River. These sites were selected based on the results of the characterization, the needs expressed by the communities and in order to represent the greatest possible diversity of shoreline types.

This project is based on a seasonal morphosedimentary monitoring approach using drones and photogrammetric analysis that allows the production of topographic models with high geodetic accuracy. In addition, historical shoreline migration rates will be estimated using aerial photographic analysis, which will also allow for the observation of modifications related to shoreline habitat changes over the past century. All of the results will be integrated into new tools, namely technical portraits and interactive web platforms accessible to all.

Resources

Link to Full Case Study (in French only)

Additional Resources (in French only):