Kahnawà:ke Shoreline Vulnerability Assessment

This report presents a shoreline vulnerability assessment study completed for a portion of the Kahnawà:ke shoreline on the St. Lawrence River in Quebec by Shoreplan Engineering Limited for use by the Kahnawà:ke Environment Protection Office (KEPO). The study examines climate change, erosion and flooding processes and risks, shoreline protection structures, and shoreline management planning. A two-day field review was conducted to assess and document conditions within the study area. Aerial surveying and aerial photography work were completed by a sub-contractor. A relative erosion risk rating was developed for each of the 44 shoreline reaches included in this study. The risk rating is intended to convey the relative level or extent of erosion that is expected over the coming years; it does not consider the consequences of that erosion. The rating for each reach should be viewed as relative to other reaches within the study area, but not to other shoreline on the St. Lawrence River. The most significant cause of erosion of the above water bank within the study area is due to wind wave action, particularly at high water levels. Ship waves contribute to that erosion, but to a lesser degree. River currents will also contribute to erosion, but to an even lesser degree, due to their relatively low speed at this wide section of the river. A review of published climate change projections showed predicted higher average temperatures, heavy rainfalls, droughts, and more destructive storms. Each of these has the potential to affect erosion processes along the Kahnawà:ke shoreline, but more frequent and more severe storms will cause the greatest increase in erosion to unprotected shoreline. Possible prevention and protection solutions were described for a number of reaches. The study concluded by noting that KEPO’s shoreline management plan should include adaptive measures to accommodate the impact of climate change.

Understanding and Assessing Impacts

The climate affects shoreline processes in a number of ways. This case study uses the climate change data from the projections published by Ville de Montreal in its Climate Change Adaptation Plan, which shows predicted higher average temperatures, heavy rainfalls, droughts, and more destructive storms. More frequent and more severe storms will cause the greatest increase in erosion to unprotected shoreline along the Kahnawà:ke shoreline. The potential impacts of higher average temperatures include milder winters with shorter periods of ice cover, more frequent freeze-thaw cycles, and potential changes to the plant species found along the shoreline. Shorter ice periods increase the risk of erosion by shorting the period that shore-fast ice protects the bank and potentially extending the shipping season. Freeze-thaw cycles weaken exposed clay soils, which in turn makes them more easily eroded. Shoreline plants strengthen the soil with their roots so changes to those species could impact erosion rates. Heavy rainfall produces surface runoff when there is insufficient time for the water to be absorbed by the land. Surface runoff frequently contributes to erosion at the crest of shoreline banks, and saturated banks are more prone to slope failure. During our field review we did not observe any erosion scarps that we thought were caused by surface runoff, so it is not confirmed as an issue along the Kahnawà:ke shoreline but has the potential to be. Changes to both water levels and storm frequency/duration are potentially the most important climate change parameters for the Kahnawà:ke shoreline. This case study undertakes an erosion hazard assessment. A relative erosion risk rating was developed for each of the 44 shoreline reaches. The rating is intended to convey the risk or likelihood of erosion occurring over the coming years. It is based on a qualitative assessment of erosion indicators.

Identifying Actions

A shoreline management plan provides a context through which management decisions regarding shoreline development are made. This case study describes key principles of shoreline management planning, which is intended to provide KEPO with the information they require to advance their own planning processes and is based off of the Province of Ontario’s framework for shoreline management planning on their Great Lakes Shoreline. The process contains six major components: i. Prevention ii. Protection iii. Emergency Response iv. Environment v. Public Information vi. Monitoring.

The way some of these components are considered has evolved through practice. In terms of the Kahnawà:ke shoreline the role of these components is as follows. Prevention is considered to be the implementation of controls, regulations, and land uses to avoid the risk of flooding or erosion. Protection is considered to be the implementation of capital works for new or existing development. It includes both structural methods such as constructing revetments or floodproofing a dwelling by sealing all openings below a given level, and non-structural methods such a shoreline vegetation or sand fill. The Emergency Response component includes reviewing existing flood/storm warning and forecasting measures and recommending improvements if necessary. The Environment component of a plan includes a preliminary assessment of both the short- and long-term potential effects to both terrestrial and aquatic ecosystems. The objective of the Public Information component is the dissemination of information about the plan and education of the public regarding shoreline management in general. A public information plan was not considered as part of this study. The Monitoring component reviews local conditions affecting shoreline management and identifies implications to the plan resulting from changes to those conditions. This component could include erosion rates, terrestrial and aquatic habitat, and development.

Implementation

Key principles of shoreline management planning were outlined in order to provide KEPO with the information they require to advance their own planning processes. A number of prevention and protection techniques were described including relocation, minimum setbacks and elevations, non-structural protection and structural protection. Structural protection included sloped revetments, vertical walls, breakwaters, and bioengineering alternatives. Possible prevention and protection solutions were described for a number of reaches. Solutions were based on our interpretation of the physical characteristics of the site and outlined what could be done to address flooding and erosion issues. We did not address the social or economic factors that must ultimately be part of the decision making process. KEPO’s shoreline management plan should include adaptive measures to accommodate the impact of climate change.

Outcomes and Monitoring Progress

The shoreline vulnerability study provides several options for remedial actions that can be taken to help mitigate the collective impacts of shoreline erosion, briefly outlined in the above ‘Implementation’ section. This assessment determined the most significant cause of erosion of the above water bank within the study area is due to wind wave action, particularly at high water levels. Ship waves contribute to that erosion, but to a lesser degree.
A monitoring program has not been implemented, but the case study details components of what the assessment should entail. This would be a long-term exercise and while the data collected will have limited use over the first years of the program, it could be quite valuable in the decades to come. A proper monitoring program would involve setting erosion monitoring stations at key locations where profile lines perpendicular to the shore can be extended both landward and offshore. A baseline point should be established so that all future measurements can be tied back to the baseline. Marking that point with a steel bar such as a piece of rebar would be advantageous. Redundant vertical benchmarks and horizontal control point should be established so that the baseline point can be-established if it is somehow lost or moved. Surveying a profile is recommended over simply measuring the distance to the top of the bank. Full profile recession data provides better information than linear bank recession, and allows the nearshore bottom downcutting rate to be determined. All surveying should be performed with a survey rod in contact with the ground and lakebed. As the objective is to establish long-term erosion rate data, the surveys do not need to be repeated frequently. Initially the profiles could be surveyed every two or three years, then the interval could be extended to five years.

Next Steps

The case study highlights the importance of KEPO’s shoreline management plan including adaptive measures to accommodate the impact of climate change.

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