|Citation||Hay & Company Consultants Inc. 2004. Comox Lake and Puntledge River temperature modelling study. Prepared for Fisheries and Oceans Canada.|
|Abstract/Description or Keywords||The Puntledge River flows from Comox Lake to Comox Harbour on the east coast of Vancouver Island,
BC, and provides important spawning and rearing habitat for salmon. A storage dam at the outlet of the
lake and a diversion dam on the river are used to control lake levels and river flows. Fisheries and
Oceans Canada (DFO) is considering cold lake water withdrawal from Comox Lake as an option to
reduce water temperature in the Puntledge River in the summer months, since high temperatures
experienced in the river are a negative stressor for fish. This report presents a modelling study of lake and
river temperatures. A prediction system, consisting of three numerical models, has been developed for
the simulations of water level, velocity and temperature in both Comox Lake and the Puntledge River.
Circulation within Comox Lake was simulated using Hay & Company’s proprietary three-dimensional
hydrodynamic model H3D. The model is able to capture the important processes of the development of
thermal stratification over the course of the year as it evolves from the uniform conditions of winter,
through the stratified conditions of summer, to the turnover during fall. The model is driven by heat
exchange at the lake surface, wind forcing and river inflows. The outflow from the lake was modelled in
two ways. For the existing condition, outflow was through shallow submerged gates in the Comox Dam;
the deep-water withdrawal was modelled as an extraction from a depth of 35 m. Withdrawing the colder
deep water results in a warming of the lake water down to about 40 m depth over the summer and fall,
with a maximum temperature difference of over 5 degrees compared to the surface withdrawal
simulation. The outflow from the lake model for the two runs was used to create two series of inflow
temperatures for the Puntledge River model.
The river temperature model is an implementation of H3D, which simulates changes in river temperatures
due to meteorological forcing and mixing of tributary waters. A major modification for the Puntledge
River implementation was the provision for heat flux into and out of the rock bed of the river. Another
major change was ‘flattening’ the river. If the entire 125 m elevation drop over the river’s length were to
be modelled, the grid would be computationally expensive. Instead, the one-dimensional model MIKE 11
was used to generate the mean geodetic water surface elevation, which was then used to calculate the
gravity force in the three-dimensional model. This accounts for the true surface slope while conducting
calculations on a flattened grid with no elevation drop. The river bathymetry was generated from a
topographic survey of the Puntledge River conducted in 2002. Water levels from the same survey were
used to calibrate the river model.
Water temperatures have been collected at seven sites on the Puntledge River since 1999. The
temperatures were used as input temperatures for the river model as well as to validate the river model.
Two simulations were performed, one with the existing surface water supply, and the second with the
deep lake withdrawal, to evaluate the impact of the cold water withdrawal on river temperatures.
Supplying water from the deep withdrawal in the lake results in much cooler river temperatures. The average temperatures along the river, based on a reach-by-reach comparison, decreased by as much as 5.5
degrees in the hottest month of August, compared to the surface supply. The maximum river water
temperature in August was about 16 degrees with the deep withdrawal, compared to a maximum of over
20 degrees with the current surface withdrawal. These cooler temperatures would be beneficial for fish
migrating up the Puntledge River during the warm months.
|Regional Watershed||Vancouver Island North|
|Sub-watershed if known||Comox Lake, Puntledge River|