Recent technological advances in remote sensing have enabled
investigation of the morphodynamics and hydrodynamics of large rivers.
However, measuring topography and flow in these very large rivers is
time consuming and thus often constrains the spatial resolution and
reach-length scales that can be monitored. Similar constraints exist for
computational fluid dynamics (CFD) studies of large rivers, requiring
maximization of mesh-or grid-cell dimensions and implying a reduction in
the representation of bedform-roughness elements that are of the order
of a model grid cell or less, even if they are represented in available
topographic data. These ``subgrid'' elements must be parameterized, and
this paper applies and considers the impact of roughness-length
treatments that include the effect of bed roughness due to
``unmeasured'' topography. CFD predictions were found to be sensitive to
the roughness-length specification. Model optimization was based on
acoustic Doppler current profiler measurements and estimates of the
water surface slope for a variety of roughness lengths. This proved
difficult as the metrics used to assess optimal model performance
diverged due to the effects of large bedforms that are not well
parameterized in roughness-length treatments. However, the general
spatial flow patterns are effectively predicted by the model. Changes in
roughness length were shown to have a major impact upon flow routing at
the channel scale. The results also indicate an absence of secondary
flow circulation cells in the reached studied, and suggest simpler
two-dimensional models may have great utility in the investigation of
flow within large rivers. Citation: Sandbach, S. D. et al. (2012),
Application of a roughness-length representation to parameterize energy
loss in 3-D numerical simulations of large rivers, Water Resour. Res.,
48, W12501, doi: 10.1029/2011WR011284.