UBC Theses and Dissertations
Modelling the thermal regime of a regulated coastal British Columbia river and assessing the potential of warming mitigation strategies West, David
The objective of this study was to (1) identify the dominant processes affecting stream temperature and (2) develop and apply a process-based stream temperature model to assess the potential for managing the thermal regime of a regulated river in coastal BC through bank re-forestation and changes to release regime. The study spanned June to September 2013, when temperatures are highest and multiple salmonid species begin their spawning runs. Following June 14, a steady discharge of approximately 2.7 m³/s was released from the reservoir through a low level outlet. The reservoir developed two-layer stratification and an internal seiche that caused oscillating withdrawal of epilimnetic and hypolimnetic water that was observed to propagate at least 14 km downstream using wavelet analysis. The stream temperature model was developed with field measurements of hydrological, meteorological, and geomorphic variables that govern stream heat fluxes. A Lagrangian model was employed to model a 14 km reach of Alouette River beginning at the dam. Using a 10-min time step, an accuracy of 0.54 °C RMSE was achieved over all predictions, and 0.50 °C RMSE for daily peaks. Water parcels arriving at the downstream extent near sunset gained most heat through direct and diffuse shortwave radiation, with the greatest cooling effect associated with evaporation, tributary inflows, and bed heat conduction. Parcels arriving near sunrise gained most heat through bed heat conduction, with the greatest cooling effect associated with longwave radiation and tributary inflows. Modelling a bank re-forestation scenario resulted in study reach temperature reductions of less than 0.5 °C for mean and daily peak temperatures. A lower reservoir outlet withdrawing from the cool hypolimnion resulted in mean daily peak reductions of 4.6 °C, while raising the outlet into the epilimnion resulted in mean daily peak increases of 0.5 °C. Increased reservoir discharge caused minor increases of mean and daily peak temperatures of less than 0.15 °C. When combined, increased discharge amplified the effect of modified release depth. The results of this study stress the importance of basing warming mitigation efforts on a detailed understanding of stream heat fluxes and reservoir stratification regime in regulated systems.
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