Numerical simulation of tides and storm surges at Chignecto Isthmus during post-tropical storm Fiona

A numerical modelling investigation of the combined effects of tides and storm surges on total water levels at Chignecto Isthmus was performed for post-tropical storm Fiona, which impacted Atlantic Canada in 2022, and two other hypothetical extreme scenarios. The analysis is intended to assist the provinces of Nova Scotia and New Brunswick in understanding the effects of tides, storm surges and sea-level rise (SLR) on extreme high-water levels along the Chignecto Isthmus. The study made use of a TELEMAC-2D hydrodynamic model of the Bay of Fundy and adjacent coastal floodplains, developed by the Ocean, Coastal and River Engineering Research Centre of the National Research Council Canada (NRC-OCRE), which was forced by tides, and atmospheric data (surface wind fields and pressures) from the European Centre for Medium-Range Weather Forecasts’ 5th generation reanalysis (ERA5). The model was calibrated and validated using water level records in the Cumberland Basin, measured by the TransCoastal Adaptations Centre for Nature-Based Solutions at Saint Mary’s University (TCA-SMU) and the Geological Survey of Canada during field campaigns from 2020-2022. Modelling scenarios were developed in consultation with a project Technical Advisory Committee, and included: • 2022 post-tropical storm Fiona; • A hypothetical future storm event (based on the 1976 Groundhog Day storm) coinciding with a high-tide event at the peak of the lunar nodal cycle (which will occur in 2033); • A hypothetical future storm event (based on the 1976 Groundhog Day storm) coinciding with a high-tide in 2100 based on relative SLR projections for the RCP8.5 emissions scenario; • Sensitivity testing to determine the effect of shifts in the timing of a storm event (based on the 1976 Groundhog Day storm) relative to high tide on resulting total water levels. findings from the analysis are summarized as follows: Effects of post-tropical storm Fiona: The storm surge from post-tropical storm Fiona in the Cumberland Basin represented a relatively small contribution to peak water levels along the Chignecto Isthmus, which were primarily dominated by the astronomical tide. Both negative and positive storm surges (up to +0.15 m) occurred in the Cumberland Basin during Fiona’s passage up the eastern seaboard. However, positive storm surges occurred close to the astronomical low tide, and therefore the resulting total water levels did not exceed those associated with astronomical high tides, or local dyke crest elevations. This illustrates the importance of the timing of storm passage through the region relative to astronomical tides on high-water levels, and the associated potential for overtopping of dykes. Storm track and wind direction: The study suggests that storms resulting in sustained southwesterly winds over the Cumberland Basin (i.e. with storm tracks slightly to the west of the basin) are likely to generate more significant storm surges on the Chignecto Isthmus due to the orientation of the longest fetches in the Bay of Fundy and the Cumberland Basin. A hypothetical storm directly impacting the Chignecto Isthmus, with winds directed towards the shore and coinciding with the highest astronomical tides, resulted in total water levels that reached and, in some places, exceeded the elevations of the existing dyke system. Sea-Level Rise: The highest astronomical tides do not exceed the elevations of the dykes under present-day (i.e. reference year of 2021) sea levels. However, a relative SLR of +0.63 m (corresponding to year 2100 projections for the RCP8.5 emissions scenario) would result in the highest astronomical tides exceeding dyke crest elevations, even without any storm surges or wave effects. Timing of storms: Timing of storm surges relative to tides, and tide-surge interactions, can greatly affect peak total water levels. The model showed that a storm with peak local windspeed coinciding at or near the time of high tide can significantly affect storm surge. For hypothetical atmospheric events similar to the Groundhog Day storm, total water levels increased by more than 1 meter at the time of the highest astronomical tide, when peak local winds peaked 2.5 hours before it. The findings highlight a crucial 4-to-6-hour window during each 12.4-hour tidal cycle where storm timing can significantly impact peak water levels, especially during spring tides. Outside this window, especially at low tide, the impact is much less. In the low-lying region of the Chignecto Isthmus, the timing of storm surges relative to astronomical tides is of critical importance in determining the potential for flooding. The Chignecto Isthmus, connecting New Brunswick and Nova Scotia, is to a certain extent protected from tide- and storm surge-driven flood hazards by coastal dykes. However, a combination of future projected SLR, increasing tidal ranges during the lunar nodal cycle (18.6 years), and extreme weather events will result in more frequent, and higher, peak water levels. This study quantifies the contributions of tides and storm surges to water levels during post-tropical storm Fiona, and several hypothetical storm events, highlighting the importance of storm track and timing relative to tides. Model results show that storm surges compounding with high tides within a critical 4-to-6-hour window of the 12.4-hour tidal cycle can result in significantly higher peak water levels, especially during spring tides that occur approximately every 15 days. Storm tracks, and the resulting wind directions, are also crucial in determining the impacts of storms on peak total water levels in the Cumberland Basin. In fact, the timing of storms relative to astronomical tides, and storm wind directions, are more important in determining peak water levels in the Cumberland Basin than storm intensity. By 2100, SLR projections for the RCP8.5 emissions scenario indicate that the highest astronomical tides will exceed current dyke elevations, underscoring the need for adaptation of flood risk management infrastructure and strategies. There were a number of study limitations and sources of modelling uncertainty, linked to the limited availability of bathymetric data, the absence of long-term water-level records, and the omission of subsurface drainage, groundwater flow, precipitation, and upland (freshwater) discharges from the model. Recommendations were provided to address data gaps and uncertainty, and to leverage the hydrodynamic model as a tool for future probabilistic flood risk assessments, and design of flood risk management strategies or infrastructure adaptation plans.

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