Serpentine & Nicomekl Rivers Climate Change Floodplain Review

CLIENT:  City of Surrey, British Columbia

Recent studies commissioned by the provincial government on sea level rise due to climate change suggest a significant impact on the Serpentine and Nicomekl River basins in terms of floodplain extents, adequacy of existing dikes, and functionality of spillways, sea dams, and other structures.  Projected ground subsidence exacerbates the effects. 

NHC was retained by the City of Surrey to assess the flood level impacts caused by a combination of climate change, ground subsidence, and land development in the basins with the objective of developing appropriate future design standards for flood protection. A key component of the work was to develop a robust, scientifically sound approach for defining the probability of occurrence of extreme flood events due to the combination of heavy precipitation, high tides, plus ocean storm surge.

A continuous simulation approach was adopted where approximately 50 years of simulations were conducted on the Serpentine/Nicomekl drainage system. A continuous time series of historic local hourly rainfall data was assembled and used as input to a hydrologic model of the watersheds to produce a time series of simulated historic runoff. Similarly, a historic ocean level time series was generated for Boundary Bay based on ocean modeling. The runoff and ocean time series formed boundary conditions for a simplified hydraulic model which was used to simulate water level time series at different locations in the basins. The water levels were then put through frequency analyses to estimate localized 200-year flood levels. By adjusting the ocean levels and runoff for future conditions and re-running the hydraulic model and subsequently the frequency analyses, the 200-year flood levels for future years (2100) could be estimated. 

The approach provides a statistically defensible approach for developing future design levels and avoids having to pair a particular return period design flood with a certain frequency ocean level. The project forms a pilot study for ocean communities in BC and elsewhere.