Optical method for measuring bed topography and flow depth in an experimental flume

Abstract

Abstract. We describe an optical method known as moiré for acquiring quasi-simultaneous measurements of bed topography and flow depth in laboratory experiments. The moiré method is based on projecting a fringe pattern (grating) on the bed and analyzing the deformation of the pattern caused by the topography with respect to a reference plane. The height of the object is encoded in the phase of the pattern and can be retrieved either through Fourier transform or phase shifting algorithms. The methodology enables image-based non-contact measurements over a continuous surface at very high spatial and temporal resolutions. We use a commercial software package of a moiré method called Light3D to map bed topography and flow depth in an experimental braided channel and demonstrate how the method can be used to characterize a full range of statistics not previously possible.

Diffusive evolution of experimental braided rivers

“…To avoid edge effects near the inlet and outlet of the experimental table, we used a subset region of 0.82 m downstream by 0.61 m cross-stream area in the center of the table for our analysis. We first removed negative (nonphysical) values for water depth following the procedure of Limare et al [30]. We then found empirically that isolating the third statistical quartile of depth values reliably identified flow locations.…”

“…All sediment arriving at the downstream end of the box was allowed to exit, and thus the initial slope of the granular bed was free to evolve to an equilibrium value. In the first run (R1), we took scans of topography and water depth at 5 min intervals for 6.7 h; methods are described in detail in Limare et al [30]. This run had a constant incoming sediment discharge of Q s = 7.56 × 10 • .…”

The Use of Imagery in Laboratory Experiments

“…While the mathematics behind the method is rather complex, the good news is the methodology is relatively easy to implement and user-friendly commercial software that perform the calculations automatically are available. For details about the theory and mathematical operations readers should refer to Sansoni et al (1999), Pouliquen and Forterre (2002), and Limare et al (2011).…”

“…A more elaborate but more precise implementation of the moiré method uses a procedure known as phase shifting to calculate the phase and robust phase unwrapping combined with grey coding to assess the geometric parameters (details in Limare et al, 2011). A more sophisticated phase-to-height conversion is based on a calibration using a plane tilted at a known incline rather than a stationary object (Figure 13.18).…”

“…Incidence angles can be calculated for any point using simple trigonometry:i p , i c = f (i, j). In the case of parallel projection and observation beams, the relation between true and distorted bathymetry has a simplified form:H = n water h. A moiré projection method was used to study the evolution of microscale braided channels (Figure 13.20; Limare et al, 2011). Experiments similar to the one here are described in detail in Metivier and Meunier, (2003).…”