Rotating laser scan method to measure the transient free-surface topography of small-scale debris flows

We present an imaging method aimed at measuring the transient three-dimensional topography of rapidly deforming opaque surfaces. The imaging setup features a laser stripe scanned repeatedly across the surface using a spinning wheel with mirror facets. Acquisition rates of 20 scans per second are attained by spinning the wheel rapidly and imaging the resulting stripes using a fast camera. The algorithms adopted to capture the stripes, reconstruct the surface topography for each scan, and calibrate the camera and laser configuration are described. Error estimates for successive steps are evaluated and compared to results from repeat and independent measurements of the final deposit shape. The method is then used to acquire time-resolved measurements of small-scale debris flows. Specifically, we measure the free-surface topography of debris surges as they flow across an idealized canyon–fan transition. In addition to profiling the final deposit shape, the method successfully resolves the transient evolution of the free surface. Applied to textured surfaces, the method can also be combined with particle tracking velocimetry.