Measurement of the temporal progression of scour in a pool-riffle sequencein a gravel bed stream using an electronic scour monitor

A relatively inexpensive prototype monitor was designed and developed to re cord temporal variation in scour depth and was field-tested in a gravel bed stream. The device consists of plastic practice golf balls that are fitted internally with ring magnets and strung on a two-conductor cable enclosing a small reed switch. The balls are installed and oriented near-vertically in the streambed. As each ball is disturbed and released, it slides along t he cable past the reed switch, and the time of circuit closure caused by pa ssage of the magnet is recorded by a data logger. The device can be applied in arrays that span large areas of the streambed, including in wide channe ls that are inaccessible during a flood. Data obtained from 19 devices inst alled in an aggrading site described scouring processes in a pool-riffle in terface during a bed load transport event. Substantial bed excavation occur red in the region of the pool edge during the rising stage, indicating exis tence of a local, temporally varying imbalance in bed load transport rate. Bed disturbance in the rest of the site prior to aggradation was limited to the surface and immediate subpavement layer.