This paper is concerned with the application of fractal analysis to
understand the structure of water-worked gravel-bed river surfaces.
High resolution digital elevation models, acquired using digital
photogrammetric methods, allowed the application of two-dimensional
fractal methods. Previous gravel-bed river studies have been based upon
sampled profiles and hence one-dimensional fractal characterisation.
After basic testing that bed elevation increments are Gaussian, the
paper uses two-dimensional variogram surfaces to derive directionally
dependent estimates of fractal dimension. The results identify mixed
fractal behavior with two characteristic fractal bands, one associated
with the subgrain scale and one associated with the grain scale, The
subgrain scale characteristics were isotropic and sensitive to
decisions made during the data collection process. Thus, it was
difficult to differentiate whether these characteristics were real
facets of the surfaces studied. The second band was anisotropic and not
sensitive to data collection issues. Fractal dimensions were greater in
the downstream direction than in other directions suggesting that the
effects of water working are to alter the level of surface
organisation, by increasing surface irregularity and hence roughness.
This is an important observation as it means that water-worked surfaces
may have a distinct anisotropic signal, revealed when using a fractal
type analysis.