Comparison of 2D laser scanning with 3D MRI scanning; Analysis of graded gravel bed porosity

Surface/subsurface interaction in gravel beds plays a fundamental role in sediment mobility, surface/groundwater exchange and quality of the hyporheic zone. Porosity is a key parameter for all of these processes which requires an understanding of the 3D structure of the river bed; yet, this is difficult to determine using conventional invasive measurement techniques. This paper therefore compares porosity statistics derived using the non-invasive technique of surface-based laser displacement scanning (resolution ~0.1mm) with those calculated from 3D volumetric data acquired from Magnetic Resonance Imaging (MRI) (resolution =0.3mm). Four flume experiments exposed two grain size distributions [uniform (σg = (D84/D16)0.5 ~1.0); bimodal (σg = 2.08)] to two sub-threshold flow durations of 60 and 960 minutes. The samples were scanned with the laser, extracted from the flume and 3D images were acquired using MRI. Results consider: (i) the validation of the experimental procedure (Digital Elevation Models, DEMs); (ii) the limitations of the 2D imaging technique; (iii) 3D porosity estimations of the boundary surface/subsurface for the different beds and water-working periods. Keywords: MRI, Laser scanning, Sediment, Porosity.