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2.4.2 Transport Sub-Model The transport sub-model calculated the advection of water temperature, surface ice, and suspended frazil ice. Frazil ice production was assumed to begin through the introduction of seed crystals at the water surface. The concentration of the frazil ice was calculated by balancing the heat loss from the water surface and latent heat released from the growing frazil. The frazil ice was assumed to rise to the water surface with a known velocity. At the water surface, the frazil ice formed into floes that were transported downstream at a rate determined by the flow velocity. The heat loss from the water surface was calculated as a linear function of the difference between the water temperature and the air temperature. The transport sub-model used the Preissman-Holly advection scheme (Gunge et al. 1980). This scheme has been shown to minimize numerical diffusion. 2.4.3 Ice Cover Progression Sub-Model The ice cover progression sub-model calculated the rate at which stationary ice covers formed. A stationary ice cover was assumed to initially form at a pre-selected bridging location when the concentration of surface ice reached a pre-selected value. The ice cover then progressed upstream at a rate determined by the rate of arrival of the surface ice and the thickness of the ice cover. The ice cover was allowed to thicken through heat transfer to the atmosphere from the ice surface and through the deposition of frazil ice underneath the ice cover. The ice cover could also melt out through heat transfer from the water flowing beneath it. When the ice cover lost a certain percentage of its thickness, it was assumed to break up and be transported in the downstream direction. The river cross sections and channel bed and ice cover roughnesses developed for the unsteady flow model were used to describe the channel geometry in the transport model. The initial ice cover bridging location was set at the Ouray Bridge and bridging was assumed to begin when a surface ice concentration of 50% was reached. The initial stationary ice cover thickness for the bridging location was based on the observations of ice cover thickness during the 1997 field measurements. The frazil ice rise velocity was set at 0.03 cm/sec, initial floe thickness was set at 3 cm, and the model used a time step interval of 2 hours. The channel and ice cover roughnesses used in the UNET simulation described in section 2.4.1 were used in the ice progression simulation. -13-