Numerical simulation of storm surges in the Beaufort Sea and coastal flood hazards in the Hamlet of Tuktoyaktuk, Northwest Territories

The work described in this report was completed as part of the Coastal Flood Mitigation Canada project, which received funding from Defence Research and Development Canada's Centre for Security Science through the Canadian Safety and Security Program. The project included three coastal flood hazard and risk assessment case studies, one for each of Canada's Atlantic, Arctic, and Pacific coasts. These case studies were designed to explore and demonstrate good practices to inform the development of new guidelines for coastal flood hazard assessment in Canada. As part of the Arctic case study, a coastal flood hazard assessment was performed for the Hamlet of Tuktoyaktuk, Northwest Territories. The primary objective of the coastal flood hazard assessment was to develop estimates of storm surge-driven flood hazards in Tuktoyaktuk for a range of events with defined return periods or annual exceedance probabilies (AEPs), which provided key inputs to a risk assessment by Natural Resources Canada (NRCan). Several community and partner engagement workshops were conducted during the project to obtain community input on the project scope, identify community flood risk management priorities, communicate progress and findings to the community, and to obtain community input to the risk assessment by NRCan. Two numerical hydrodynamic models were developed to support the coastal hazard assessment: a regional-scale storm surge model with a domain corresponding to approximately the maximum extent of open water (i.e., minimum ice extent) in the Beaufort Sea; and a higher-resolution, community-scale, overland flood hazard model with a domain covering areas within Kugmallit Bay and the Hamlet of Tuktoyaktuk. Model elevations were based on a digital elevation model created using bathymetric data from the General Bathymetric Chart of the Oceans, multibeam surveys, and topographic data from NRCan’s High-Resolution Digital Elevation Model. The two-dimensional (2D) numerical models were forced with surface pressures and winds from the ERA5 global atmospheric reanalysis dataset (hereafter referred to as ERA5). Surface pressure data from the ERA5 reanalysis dataset was in close agreement with measured atmospheric pressures at weather stations near Tuktoyaktuk. The overall variability, mean and trends in ERA5 surface wind speeds were also consistent with observations but peak surface wind speeds were consistently underestimated by ERA5. With some adjustment of wind speeds to better match observed storm peak values, the ERA5 dataset provided useful input to the storm surge modelling. Fifty historical storm surge events captured by a tide gauge at Tuktoyaktuk between 1979 and 2019 were simulated at the regional and community scale to support model calibration and validation. The effects of sea ice on storm surges were also assessed by implementing a model parametrization that adjusted the momentum imparted by the wind to the sea surface depending on the ice concentration. Ice concentrations provided by the Canadian Ice Service were prescribed as input to the model, and it was found that if sea ice had not been present to attenuate storm surges, some historical winter storm events might have produced peak storm surges three times greater than what was observed. Examination of the tide gauge records in the Beaufort Sea revealed a lack of continuous, long-term records of water levels. Two of the most significant storm surge events in recent history were not captured by the tide gauge at Tuktoyaktuk. However, evidence of peak water levels produced by these two events was available from field surveys of driftwood line deposits conducted by Harper et al. (1988). The highest water level recorded by the Tuktoyaktuk tide gauge was +2.23 m above local Chart Datum (m CD) on October 4th, 1963. However, based on driftwood observations by Harper et al. (1988), storm surge events occurring on September 1st, 1944, and September 14th, 1970 (not captured by the gauge) produced peak water levels equal to +2.95 m CD. The inclusion (or exclusion) of these two events from extreme value statistical analyses significantly impacted estimates of return periods (or AEPs) and return levels associated with storm surge-driven extreme water levels. Accounting for tide gauge records alone, a return level of 2.1 m was estimated for the 100-year return period storm surge event (equivalent to 1% AEP). Incorporating the September 1st, 1944 and September 14th, 1970 events into the analysis increased the 1% AEP storm surge to 2.92 m. Tidal variation in the Tuktoyaktuk region is small, with the difference between the highest astronomical tide (HAT) and lowest astronomical tide (LAT) being 0.62 m and a mean higher high water (MHHW) level of +0.497 m CD. Flood extents, water depths, depth-velocity products, and momentum fluxes were mapped for storm surge events with return periods of 2, 5, 10, 20, 50 and 100 years (coinciding with 50%, 20%, 10%, 5%, 2%, and 1% AEPs) superimposed on a static MHHW tidal elevation. The effects of three sea-level rise scenarios on the expected flood hazard were examined, coinciding with global sea-level rises of 0.5 m, 1.0 m, and 2.0 m, which translated to local (relative) sea-level rises of 0.54 m, 1.05 m, and 2.15 m, respectively. These scenarios may be interpreted in different ways depending on emissions scenarios, and confidence limits for future sea-level projections. However, the first two scenarios align with the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) projections for the Representative Concentration Pathway (RCP) 8.5 median at 2080, and the RCP 8.5 upper 95th percentile at 2100. A 2.0-m global sea-level rise represents a low probability, high-end scenario for the year 2100, or sea-level rise at some time beyond 2100, depending on the climate scenario. Under present-day sea-level conditions, Tuktoyaktuk is exposed to wide-scale storm surge-driven flooding with potentially destructive momentum fluxes for return periods of 50 years and longer (AEPs ≤ 2%). As sea levels continue to rise, Tuktoyaktuk faces increasingly frequent exposure to storm surge-driven flood hazards, and more severe hazards.

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