The emerging use of magnetic resonance imaging to study river bed dynamics

Abstract

The characterization of surface and sub-surface sedimentology has long been of interest to gravel-bed river researchers. The determination of surface structure is important as it exerts control over bed roughness, near-bed hydraulics and particle entrainment for transport1. Similarly, interpretation of the sub-surface structure and flow is critical in the analysis of bed permeability, the fate of pollutants and maintaining healthy hyporheic ecology 2.For example, many invertebrates (e.g. mayfly, caddis) and fish (e.g. salmon) lay their eggs below the river bed surface, and rely on sub-surface flows to supply the necessary oxygen and nutrients. Whilst turbulent surface flows drive these small sub-surface flows, they can also convey sand and silts that clogs the surface and sub-surface pore spaces. Reduction in sub-surface flows can starve eggs of oxygen such that larvae or juveniles do not emerge. This is particularly critical in Scottish gravel-bed rivers as the rising supply and deposition of fine sediment (silts and sands) is contributing to the dramatic decline in wild salmon. In order to gain a better understanding of such flow-sediment-ecology interactions in river systems, laboratory experiments are conducted using long rectangular flow tanks called “flumes”, see figure 1a,1b. Here, traditional techniques for analysing sediment structure are typically constrained to 1D or 2D approaches, such as coring, photography etc. Even where more advanced techniques are available (e.g. laser displacement scanning), these tend to be restricted to imaging the surface of the sediment bed. Using Magnetic Resonance Imaging (MRI) overcomes these limitations, providing researchers with a non-invasive technique with which to provide novel 3D spatio-temporal data on the internal pore structure. In addition the important sub-surface flows can be investigated by adding MRI contrast agents to the flowing surface water