Continuous Monitoring of Fluvial Dike Breaching by a Laser Profilometry Technique

A nonintrusive, high-resolution laser profilometry technique (LPT) has been developed for continuous monitoring of the three-dimensional (3-D) evolving breach in laboratory models of noncohesive fluvial dikes. This simple and low-cost setup consists of a commercial digital video camera and a sweeping red diode 30 mW laser projecting a sheet over the dike. The 2-D image coordinates of each deformed laser profile incident on the dike are transformed into 3-D object coordinates using the direct linear transformation (DLT) algorithm. All 3-D object coordinates computed over a laser sweeping cycle are merged to generate a cloud of points describing the instantaneous surface. The DLT-based image processing algorithm uses control points and reference axes, so that no prior knowledge is needed on the position, orientation, and intrinsic characteristics of the camera, nor on the laser position. Because the dike is partially submerged, ad hoc refraction correction has been developed. Algorithms and instructions for the implementation of the LPT are provided. Reconstructions of a dike geometry with the LPT and with a commercial laser scanner are compared in dry conditions. Using rigid dike geometries, the repeatability of the measurements, the refraction correction, and the dike reconstruction have been evaluated for submerged conditions. Two laboratory studies of evolving fluvial dike breaching due to flow overtopping have been conducted to demonstrate the LPT capabilities and accuracy. The LPT has advantages in terms of flexibility and spatiotemporal resolution, but high turbidity and water surface waves may lead to inaccurate geometry reconstructions.