PhDA resurgence of interest in the concept of equilibrium in the aeolian
saltation system has been witnessed in the 1990's. Throughout the
aeolian field of research, i. e. wind tunnel, field and numerical models,
many highly successful individual investigations have been
conducted. Despite these data, however, the timing and location of the
mass flux equilibrium have not been quantified.
This research investigates the simultaneous downwind spatial and
temporal developments of the aeolian saltation system. Experiments
were conducted in the laboratory and the field. By unification of the
spatial and temporal dimensions in both environments one of the
major limitations of contemporary aeolian science, the inability to
relate data from different experimental environments, is addressed.
In the wind tunnel the development of the saltation system was
measured over a streamwise length of 8m. Sediment transport was
measured at 1m intervals by the downwind deployment of seven
Aarhus sand traps. In the field the development of the saltation system
was monitored over distances of 10m and 20m. Mass flux was measured
by the downwind deployment of five 'total load' sand traps. In both
environments temporal wind velocity and mass flux data were
collected simultaneously at a single site. Spatial profile velocity data
were later obtained by a streamwise traverse along the experimental
area.
The downwind spatial development of the saltation system, from a
point of initiation, in the laboratory and the field is manifest by an
overshoot in mass flux and shear velocity. It is shown that in both
environments mass flux increases with distance to a maximum at 4m
downwind. This result is in remarkable agreement with existing data
of a comparable scale. In the wind tunnel and the field experiments it
V'
is found that shear velocity overshoots between 2-4m downwind of the
overshoot in mass flux. The distance between the overshoot in mass
flux and the overshoot in shear velocity is termed the 'separation
distance'.
The existence of a 'separation distance' between the overshoots of mass
flux and shear velocity questions the appropriateness of traditional
mass flux formulae. It is found that conventional mass flux
relationships with shear velocity, generated from data collected
simultaneously at the same site, have the lowest predictive capability.
The greatest confidence in the ability of shear velocity to predict the
rate of mass flux is shown to occur when shear velocity data are
collected downwind of mass flux data. The critical distance between
the data collection points is demonstrated to be defined by the
'separation distance'.
The downwind spatial development of the saltation system without a
point of initiation in the laboratory and the field is influenced by sand
entering from upwind. The existence of high energy bombardment by
saltation processes throughout the experimental area is shown to
produce an accelerated development of the saltation system. It is found
that the precise downwind development of mass flux and shear
velocity are dependent on the exact rate of sand entering from
upwind.
The temporal development of the saltation system is controlled
essentially by the availability of transportable grains from the sand
bed. In both the wind tunnel and the field experiments it is
demonstrated that the saltation system develops through time from a
transport-limited to a supply-limited system. The depletion of the sand
bed through time limits the existence of the state of equilibrium. The
equilibrium concept is thus shown to be inappropriate for the
universal prediction of mass flux
