Lane, S.N. and Richards, K.S. (1998)

High resolution, two-dimensional spatial modelling of flow processes in a multi-thread channel

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This paper describes application and testing of a two-dimensional numerical flow model in a multi-thread reach of a proglacial stream. The model solves the depth-averaged form of the Navier±Stokes equations for open channel flow, incorporating a two-equation turbulence closure, an analytical correction for the e€ects of secondary circulation and a rigid lid approximation. The model requires as input the channel bed topography, the water surface, bed roughness and in flow discharge information, and predicts the spatial distribution of depth-averaged velocity and eddy viscosity. The results have been subjected to intensive testing using simple assessment of numerical performance (e.g. conservation of mass and momentum, numerical convergence), distributed sensitivity analysis and comparison of model predictions with field measurements of velocity. The results are encouraging, particularly given some of the diculties in obtaining accurate, distributed cross-stream and downstream velocities. Distributed sensitivity analysis allowed more detailed consideration of the necessary development of the model. This suggested that significant errors in the velocity predictions were largely a result of uncertainty in the specification of both the magnitude and the spatial variation of bed roughness. Secondly, the two-equation turbulence closure was observed to have little e€ffect upon model predictions, except in the vicinity of the side walls. In applications of models of this type to irregular, coarse-bedded channels, improvements in the specification of the topographic boundary condition speci®cation and bed roughness are likely to be more important than a sophisticated turbulence closure scheme. Thirdly, although the secondary circulation correction was observed to reproduce some of the expected streamwise transfer of momentum, the e€ects were seen to be relatively small. Given the intensity of secondary circulation defined in field contexts, the inability of the model to correct eff€ectively for the momentum transfer associated with secondary circulation processes related to topographic discordance and shear-generated turbulence suggests that further work is required in this respect.