The effect of imbrication on the porosity value of uniform gravel bed

Abstract

Existing empirical relations used to predict the porosity of gravel beds are mainly derived from laboratory-generated sediment beds with random grain packing. However, such relations could not adequately describe beds with non-random grain arrangements that occur widely in fluvial deposits. In this work, the effect of grain imbrication on gravel-bed porosity has been quantified using beds with variable strengths of imbrication generated by flume experiments. Mono-sized ellipsoids with specific shapes were used in experiments to remove particle size and sorting effects on porosity. Random bed packings were generated by settling of ellipsoids in still water whilst imbricated beds generated under flowing water. Beds were frozen using liquid nitrogen before extraction. A new relatively simple and time-saving workflow was developed to measure the orientation of particles and quantify the degree of grain imbrication in frozen beds from X-ray Computed Tomography images. Beds with the strongest grain fabric display a ca. 0.03 absolute reduction of porosity value on average (8–10% relative reduction) compared to that of random packing for undisturbed beds. Further, results were obtained for beds deposited under still-water conditions subject to disturbance by shaking, to mimic the potential effect of vibrations from currents, waves or other sources in the environment. A reduction in bed porosity of ca. 0.014–0.018 (ca. 5% relative reduction) is observed between beds with the strongest grain fabric and those with random packing that had undergone shaking after deposition. Hence, a significant proportion (> 50%) of the porosity loss observed for imbricated beds may be attributable to tighter packing due to turbulence-related vibrations from the flow. The small decrease in porosity value despite the formation of strong imbrication is considered to be due to the limited improvement in grain organization, as the results show that the flat shape of the ellipsoids and the uniformity of their size promote the formation of a stacking structure under gravity, leading to a similarly highly ordered grain organization in random packings compared to the imbricated packings. Graphical abstract