Physical Modelling in Fluvial Geomorphology

242 SCIENTIFIC NATURE OF GEOMORPHOLOGY
Figure 9.11 Chute cut-off in an established braided channel. (a) Point bar A begins to develop. (b)
After a period of growth and increasing sinuosity, point bar A is cut off leaving portions of the
original channel abandoned and developing a new point bar on the opposite bank. (c) Point bar at A
is converted to a medial bar by a second chute cut-off. The elapsed time between successive
photographs is 1 hour. Flow is from right to left. Interpretation and diagram after Ashmore (1991b,
p. 331), reproduced with the permission of the author and the NRC Research Press
distribution and had flow Reynolds and Froude numbers that were often transitional and
supercritical, respectively (see Table 9.1 and the presence of oblique diagonal standing
waves in many of Ashmore's photographs). However, these experiments clearly illustrate
the full power and potential of an FSM and suggest that it is not always necessary to scale
all parameters strictly to produce realistic predictive models.
DISTORTED SCALE MODELLING (MOVABLE-BED MODELLING)
For the study of large geomorphological scales, fine prototype sediments or precise
modelling of sediment transport and deposition, models may have to be distorted.
Geometrically distorted models have a small vertical:horizontal scale ratio in order to
model large prototypes, whilst maintaining adequate model flow depth. Perhaps the most
spectacular example of a distorted geometric model is that of the entire Mississippi basin
by the US Corps of Engineers at a horizontal scale of 1:2000 and vertical scale of 1:100,