In-situ measurement of river-bed sediment porosity using Structure-from-Motion image analysis

Abstract

Due to the difficulty and inaccuracy of in-situ porosity measurement methods in fluvial deposits, knowledge about spatial variation of river bed porosity is limited. The objective of this study was to develop a new accurate method to improve and facilitate the in-situ porosity measurement of fluvial deposits. For this purpose, a Structure-from-Motion (SfM) photogrammetry-based method is applied to determine the in-situ total volume (Vt) of the sampling pit. The total volume is then combined with the solid fraction volume (Vs) measured by the water displacement method to calculate the porosity. Several sets of experiments were carried out to evaluate the systematic and stochastic uncertainties of this new method. Combination of Vt and Vs uncertainties using standard error propagation showed the absolute uncertainty of the porosity to be less than 0.01. Compared to the existing methods, this new SfM method increases the measurement accuracy significantly and strongly speeds up the porosity measurements, as it is independent from field calibration and initial sediment sample stabilization.