Geoscience Reference
In-Depth Information
a
20
10
0
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x(mm)
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0
b
800
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Fig. 8 Bed surfaces measured using a handheld 3D laser scanner: (a) rippled bed and (b) dune-
covered bed. Bed elevations are in mm, with flow from
left
to
right
correct these surfaces for any magnetic field distortions. The corrected sweeps were
subsequently stitched together to give a final measured bed surface for each test,
where the relative inter-sweep offsets required were determined based on cross-
correlations of adjacent sweeps.
Figure
8
shows two surfaces obtained for a flow depth of 0.15 m. The first
surface is a rippled bed obtained after 19 h for Froude number
F
0.37, transport
¼
0.79 W/m
2
, and
stage parameter (van Rijn
1984
)
T
2.46, stream power
O
¼
¼
u
*
/
u
*c
¼
3.28. The dunes of the second surface were obtained after 30 min for
2.04 W/m
2
, and
u
*
/
u
*c
¼
F
4.33. As reflected by the results
of Fig.
8
, bed-surface details over a large domain can be accurately (and quickly)
captured by the 3D scanning system, where as discussed in the following section,
the geometry of the 2D autocorrelation function (or the closely related second-
order structure function) can provide an effective means of assessing the three-
dimensionality of the waves of the measured bed surfaces.
0.55,
T
6.65,
O
¼
¼
¼
3.2 Bedform Four-Dimensionality and Flying Probes
In an attempt to address the noted absence of investigation of flow and bed
development for sediment-transporting flows and naturally mobile and vari-
able bed waves (e.g. Best
2005
), the central focus of the SWAT.nz programme
(Coleman et al.
2008a
) was measurement and analysis of the 3D development of
sand waves and associated flow from plane-bed conditions to equilibrium bed-form
magnitudes. The 96 erodible-bed experiments carried out were undertaken in two
glass-sided tilting recirculating (water and sediment) laboratory flumes, measuring
