Rainbow Trail Bridge Replacements Hydrologic and Hydraulic Analysis

Water Street led an assessment of flood flow levels to inform requirements for two bridge replacements along the Rainbow Trail for the Resort Municipality of Whistler.

The scope of this project included estimating the 100-year and 200-year flood flows and water levels at the locations of the bridge replacement projects. For the hydrologic analysis, the contributing catchment area to each bridge was established using LiDAR data and key hydrologic characteristics were determined. A regional analysis was then completed to identify hydrometric stations with similar hydrologic characteristics. For the comparable stations with sufficient data, a Bulletin 17-C statistical analysis was completed to establish return frequency design flows. From this analysis, a relationship between drainage area and design flow was established to estimate return design flows for the subject locations. Confidence intervals were used to establish appropriately conservative flow estimates and an upward adjustment was applied to account for climate change impacts.

After establishing the design flows, one-dimensional hydraulic modelling of design flows in HEC-RAS was completed using topographic survey data to establish high-water levels at the bridge crossings. Hydraulic parameters and boundary conditions were assigned based on site observations. Simplified two-dimensional modelling was also completed to verify results. The hydraulic modelling included sensitivity checks for potential hydraulic jumps that could impact flood levels at the bridge crossings. Construction for the new bridges is expected to occur in 2026.