Physical Modelling in Fluvial Geomorphology

PHYSICAL MODELLING IN FLUVIAL GEOMORPHOLOGY 237
Figure 9.7 Model:prototype time scales for different processes in a 1:20 FSM (λ t) y, (λ t) s, (λ t)u,
(λ t) m,and (λt) ug, are the time scales for vertical erosion/accretion, sediment transport, downstream
displacement of individual sediment grains, flow velocity, grain motion during saltation and particle
fall velocity respectively (see text for more details). Some processes such as vertical erosion are
much faster in the model than in the prototype whilst others, such as fall velocity and grain motion
during saltation, are slower
This issue may have important ramifications for the modelling of alluvial architecture. For
example, since vertical erosion and deposition are much faster than horizontal accretion,
there may be distortion in the size of scours and overbank splays preserved in aggrading
river models. Initial FSM work on braided river aggradation (Ashworth et al. 1994;
Peakall 1995) suggests that the influence of multiple time scales is limited, but detailed
experiments are still required to resolve this issue. The question of time scaling within
hydraulic models and the period required for 'equilibrium' to be reached in the initial
stages of experiments is clearly a subject that warrants further attention, especially if such
models are to be used to investigate long-term alluvial channel behaviour.
Hydrograph scaling
A complementary approach to modelling time within physical models is through use of
the geomorphological concept of event magnitude-frequency. Wolman and Miller (1960)
suggested that there is a good correlation between the 'dominant' discharge (i.e. that which
does most 'geomorphological work' in terms of sediment transport) and the bankfull
discharge. Most physical models use a constant discharge which approximates to bankfull
(e.g. Leopold and Wolman 1957; Ashmore 1982). The sequential simulation of a