Measurement of cobble abrasion in natural streams.

Abstract

The particle size of sediment in natural streams is modified by sorting, mixing, solution, and abrasion. Abrasion is the physical process(es) occurring during stream-flow events that causes wear (weight loss) of sedimentary particles. The deterministic cobble abrasion model assumes wear is proportional to total frictional work done on the particle (Etot ). Etot equals the work done by bed friction (Eb) plus the work done by dynamic bed-load friction caused by the motion of nearby particles. Eb is estimated by Eb = (3 x 10⁻⁴)Ww x •⁴⁵y•⁵⁵ where Eb = work done by bed friction (ergs) Ww = particle weight in water (grams) y = fall (cm) x = distance moved (cm) Dynamic bed-load friction is not calculable but is assumed to vary directly with size and concentration of moving bed load. Wear is related to total work by abradibility, a factor inversely proportional to sphericity, roundness, and inherent rock strength. Traceable cobbles were abraded in ephemeral streams in southern Arizona. The apparent abrasion efficiency (weight loss per erg of work done by bed friction) varied approximately with discharge squared. Field wear measurements compare reasonably well with prior laboratory measurements. Roundness changes, transport over bedrock, and en route weathering affect the apparent abrasion efficiency observed in natural streams.