[1] This article describes the development and validation of a method
for representing the complex surface topography of gravel bed rivers in
high-resolution three-dimensional computational fluid dynamic models.
This is based on a regular structured grid and the application of a
porosity modification to the mass conservation equation in which fully
blocked cells are assigned a porosity of zero, fully unblocked cells
are assigned a porosity of one, and partly blocked cells are assigned a
porosity of between 0 and 1, according to the percentage of the cell
volume that is blocked. The model retains an equilibrium wall function
and an RNG-type two-equation turbulence model. The model is combined
with a 0.002 m resolution digital elevation model of a flume-based,
water-worked, gravel bed surface, acquired using two-media digital
photogrammetry and with surface elevations that are precise to +/-
0.001 m. The model is validated by comparison with velocity data
measured using a three-component acoustic Doppler velocimeter (ADV).
Model validation demonstrates a significantly improved level of
agreement than in previous studies, notably in relation to shear at the
bed, although the resolution of model predictions was significantly
higher than the ADV measurements, making model assessment in the
presence of strong shear especially difficult. A series of simulations
to assess model sensitivity to bed topographic and roughness
representation were undertaken. These demonstrated inherent limitations
in the prediction of 3-D flow fields in gravel bed rivers without
high-resolution topographic representation. They also showed that model
predictions of downstream flux were more sensitive to topographic
smoothing that to changes in the roughness parameterization, reflecting
the importance of both mass conservation (i.e., blockage) and momentum
conservation effects at the grain and bed form scale. Model predictions
allowed visualization of the structure of form-flow interactions at
high resolution. In particular, the most protruding bed particles
exerted a critical control on the turbulent kinetic energy maxima
typically observed at about 20% of the flow depth above the bed.