An electromagnetic device for automatic detection of bedload motion and its field applications

Abstract

A new sensor for the continuous and unmanned detection and recording of bedload motion is described. The sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel. Artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor, replacing bed particles. The entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded. A field installation demonstrates the value of the sensor by revealing for the first time in coarse‐grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves. It also illustrates the importance of pebble clusters in delaying particle entrainment.

Modelling gravel‐bed topography from sediment transport

“…Features with bedform wavelengths of between 5 and 10 grains are superimposed on much longer bedforms with wavelengths of 30 to 40 grains. The microscale features are generated by the process of grain clumping and might be thought of as representing pebble clusters with a bedform length comparable to the field examples reported by Brayshaw (1984). The larger scale variations in the bed topography mirror the downstream variations in the flow and appear to be similar to the antidunes and step-pool features described by Whittaker and Jaeggi (1982).…”

The influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers

“…Second, there is the protection of the bed by microforms through the establishment of quasistable separation bubbles-the 'hiding' factor at pebble cluster scale. Brayshaw (1984) has reported clusters to be widespread on the beds of a number of channels ranging widely in sediment size and shape. He did not, however, quantify the distribution beyond estimating the proportion of the bed that they covered.…”

“…Brayshaw (1984Brayshaw ( , 1985 has drawn attention to the most prevalent type microform-the pebble cluster (Figure 1). However, while clusters have been shown to delay entrainment , there is no information about their continuing affect on bedload flux rates once the initial transport threshold has been exceeded.…”

An overview of underwater sound generated by interparticle collisions and its application to the measurements of coarse sediment bedload transport

“…The tagging of gravel particles radioactively was developed by Crickmore et al (1972) and applied with success to monitoring the movement of gravel. Reid et al (1984) utilised artificial pebbles constructed with a ferrite rod at its centre and deployed them in a brook. Particle mobility was detected using an electromagnetic sensing system installed in the bed and transport compared with the bed shear.…”