The Use of Imagery in Laboratory Experiments - Fluvial Remote Sensing for Science and Management - page 307

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Figure 13.17 Stepsinvolvedinamoire method analysis: (a) spectrum of the reference image obtained using the Fourier transform,
(b) an image of the bed topography reconstructed from the phase shift between the reference image and an image with a straight
channel carved into the bed and an object of known dimensions (consisting of three stairs), (c) a profile from top to bottom of one of
the transects in b, (d) phase-to-height calibration based on the known heights of the three stairs.

fully braided morphology spontaneously developed and
reached steady state (sediment input equaled sediment
output) after a few hours. Sediment and water were sup-
plied continuously at the upstream end of the flume at a
constant rate. Flow discharge was around 2.5

phase field was calculated with 8 phase-shifted images and
robust unwrapping using a series of 8 grey code images.
The computer generated patterns were projected from a
Sanyo PLV-Z5 video-projector. A black and white μ eye

10 − 5 m 3 /s
and sediment discharge was approximately 8 × 10 − 8 m 3 /s.
Glare caused by flow was reduced by using two cross
axis polarisers placed in front of the projector and the
camera respectively. A moire method was implemented
to measure bed topography and flow depth using a com-
mercial software package called Light3D - developed at
the Laboratory of Solid State Mechanics, University of
Poitiers, France (Breque et al., 2004). The program uses
phase shifting to calculate the phase and robust phase
unwrapping combined with grey coding to assess the
geometric parameters. The user can choose the number
of phase-shifted patterns to be projected (3, 8, 16 or 32)
and phase unwrapping can be performed with or with-
out 8 grey code images. For the results shown here the

× Stemmer Imaging CCD camera (1280 1024 pixels) was

used for image acquisition. Prior to each experiment a
calibration procedure was carried out using a flat, white,
highly diffusive board that could be tilted following the
procedures outlined by the software. In addition to the
phase-to-height conversion, the software uses the calibra-
tion to automatically calculate all the required geometrical
parameters which can be difficult to measure accurately
manually (Figure 13.18).
Over the course of an experiment bed topography and
water depth were measured every 10 minutes in the fol-
lowing manner: 'wet' bed topography was measured with
the flow on, the flow was turned off, 'dry' bed topography
was measured after the flow was off for 1 minute (the
time required for the bed to be uniformly drained). Water

×