Minimizing assumptions and overcoming challenges of watershed modeling in the Pacific Northwest Region

The municipalities of North Vancouver and West Vancouver reside on the North Shore Mountains across the Burrard inlet from Vancouver and are within the Pacific Northwest climactic region. The Pacific Northwest is one of the most unique areas in the world in terms of climate and topography. Annual rainfall along the British Columbian coast is comparable to the equatorial tropical regions and the coast has mountains with peak elevations as high as 4,000 metres. Due to the high amounts of precipitation that varies with elevation, stormwater modelling along the coast of British Columbia and the North Shore Mountains has its unique challenges and limitations. 

Recently, Kerr Wood Leidal (KWL) has developed two stormwater models in PCSWMM for the municipalities of North Vancouver and West Vancouver.  Each municipality will use the results of these models and the models themselves to assist with planning future development and watercourse management. The City / District of North Vancouver and the municipality of West Vancouver are part of the same mountain chain (North Shore Mountains) and the two municipalities geographically border one another. These watersheds are susceptible to varying precipitation intensities and volume from the southern oceanfront to their northern mountains. The developed stormwater models by KWL had to use multiple rain gauges at different elevations as rainfall inputs to account for differing precipitation as elevation changes. Calibration had to be performed at multiple flow monitoring stations with differing total impervious areas. A combination of continuous calibration in largely undeveloped catchments and single event calibration in developed area was found to minimalize error. No matter how similar or close two forested undeveloped catchments appeared, calibrated parameters could not be transferred from one catchment to another without further calibration. It was found that applying calibrated parameters from one forested area to another adjacent forested area in a different watershed resulted in vast differences when modelled flows and recorded flows were compared.  

To limit assumptions and maximize efficiency certain data collection methods, data quality, build processes and limitations have to be considered while or prior to developing a mixed land use model. Some of the challenges of modelling along the British Columbian coast and the lessons learned by the KWL staff will be summarized.