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 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
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 difficulties 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 effect 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 specification 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 effects were seen to be relatively small. Given the
intensity of secondary circulation defined in field contexts, the
inability of the model to correct effectively 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.