HIGH-RESOLUTION, 2-DIMENSIONAL SPATIAL MODELING OF FLOW PROCESSES IN A MULTI-THREAD CHANNEL

This paper describes application and testing of a two-dimensional nume rical flow model in a multi-thread reach of a proglacial stream. The m odel solves the depth-averaged form of the Navier-Stokes equations for open channel flow, incorporating a two-equation turbulence closure, a n analytical correction for the effects of secondary circulation and a rigid lid approximation. The model requires as input the channel bed topography, the water surface, bed roughness and inflow discharge info rmation, and predicts the spatial distribution of depth-averaged veloc ity and eddy viscosity. The results have been subjected to intensive t esting using simple assessment of numerical performance (e.g. conserva tion of mass and momentum, numerical convergence), distributed sensiti vity analysis and comparison of model predictions with field measureme nts of velocity. The results are encouraging, particularly given some of the difficulties in obtaining accurate, distributed cross-stream an d downstream velocities. Distributed sensitivity analysis allowed more detailed consideration of the necessary development of the model. Thi s suggested that significant errors in the velocity predictions were l argely a result of uncertainty in the specification of both the magnit ude and the spatial variation of bed roughness. Secondly, the two-equa tion turbulence closure was observed to have little effect upon model predictions, except in the vicinity of the side walls. In applications of models of this type to irregular, coarse-bedded channels, improvem ents in the specification of the topographic boundary condition specif ication and bed roughness are likely to be more important than a sophi sticated turbulence closure scheme. Thirdly, although the secondary ci rculation correction was observed to reproduce some of the expected st reamwise transfer of momentum, the effects were seen to be relatively small. Given the intensity of secondary circulation defined in field c ontexts, the inability of the model to correct effectively for the mom entum transfer associated with secondary circulation processes related to topographic discordance and shear-generated turbulence suggests th at further work is required in this respect. (C) 1998 John Wiley & Son s, Ltd.