Developments in photogrammetric remote sensing for grain-scale fluvial morphology studies

Abstract

THE AIM OF THIS STUDY is two-fold. Firstly, it uses the latest advances in photogrammetric engineering and computer vision to develop a remote-sensing technique customised for grain-scale fluvial morphology research in both the laboratory and the field. Secondly, the recorded gravelbed elevation data are processed by means of geostatistical methods, providing new observations on fluvial microtopography, and its interaction with flows. Initially, a series of flume tests is presented, which allowed to test, develop and optimise a solution for measuring water-worked gravel beds using digital photogrammetry, a growing technique used in the Earth Sciences. The thesis presents in detail the use and evaluation of digital cameras and off-the-shelf calibration and stereo matching methods for non-proprietary close-range remote sensing applications. Substantial developments are made (i) to customise the camera arrangement to the measurement tasks, (ii) to improve the measurement of submerged topographies in flumes, (iii) to identify and quantify photogrammetric errors using purposely-designed 3D-printed ground truths, (iv) to design optimisation strategies to minimise calibration and stereo matching errors and their propagation to digital elevation models (DEMs), and finally (v) to allow merging of overlapping DEMs to optimise surface coverage and measurement resolution. This results in a photogrammetric technique that is capable of efficient and effective DEM collection in both the laboratory and the field, with sub-millimetre resolution and accuracy, needed for precise measurements of gravel beds and roughness properties. In the second part, the presented photogrammetric developments are then used to study riverbed armouring manifestations of water-worked gravel beds. A range of geostatical analysis methods are introduced to determine (i) bed-surface sediment size and preferential orientation, (ii) bedelevation distribution moments and spatial correlations, (iii) distribution of surface slope and aspect angles, and (iv) grain inclination and imbrication. Analytical results show that stable armour layers are replicable for similar formative flow and sediment and that armour layer formation and break-up involves a complete and consistent re-arrangement of the bed material, beyond the generally reported surface coarsening. For an initial field application, using the developed photogrammetric and geostatical analytical techniques, it is shown that sedimentological and geomorphic contrasts exist between gravel bars of the same river reach, and advances are made to relate these differences to the flow history.