High-resolution Measurements of Cohesive Sediment Erosion Using Photogrammetric Methods

The erosion of cohesive sediments and non-cohesive/cohesive sediment mixtures represents a crucial issue for many engineering and ecological applications. Consequently, the erosion behavior has been intensively studied over the last decades in laboratories, as well as in the field. Despite the considerable variety of devices, information about the spatial and temporal variability of erosion rates and the capability of the devices to resolve the spatial and temporal variability is rare in literature.

This work presents a novel high-resolution photogrammetric measuring technique (PHOTOSED) to study in detail the erosion behavior of cohesive sediments, or cohesive/non-cohesive sediment mixtures. The experimental setup uses a semiconductor laser with a diffraction optic to project a pseudo-random pattern of light points on a sediment surface, a CMOS-camera for image acquisition, and a dense optical flow (DOF) algorithm with the OpenCV library that evaluates the displacements of the light points of two consecutive images during the erosion process to assess the erosion volume.

Intensive calibration and verification experiments were conducted to test the accuracy and applicability of the method. The calibration and verification procedure showed that the PHOTOSED method allows the detection of erosion volumes for several orders of magnitude with a minimum detection limit of approx. 15 mm³ and enabling high-resolution measurements of erosion rates, as well as in-depth investigations of the erosion behavior of cohesive sediments and cohesive/non-cohesive sediment mixtures.

The PHOTOSED method was subsequently applied to a sediment surface consisting of a cohesive/non-cohesive sediment mixture at two different flow rates. The results identify a high variability of the erosion rates within time intervals of 30s and variability factors up to 10 between the median erosion rate and the maximum erosion rate. The high variability of erosion rates distributed over the entire sediment surface emphasizes the need to study the erosion phenomena of cohesive sediments, or cohesive/non-cohesive sediment mixtures, in detail using high-resolution measurements.