A model of two‐phase bedload transport in an oregon coast range stream

Abstract A descriptive model for bed material routing in small sand and gravel bedded channels with sequences of pools and armoured riffles was developed. Aspects of the model are demonstrated at Flynn Creek, a 202‐ha, third‐order drainage in the Oregon Coast Range. Assuming that channel geometry, sediment transport competence, and the availability of sediments for transport are all non‐uniform in the downstream direction, the model described bedload transport as occurring in two phases. Phase I involves the transport of fine, predominantly sand‐sized bed materials over stable gravel‐armoured riffles. Phase II occurs at higher flows that can entrain riffle armour and transport riffle sediments, in addition to Phase I sediments. Phase I bedload transport was sampled during three moderate flow events. A power function related Phase I bedload transport to water discharge. The relationship was consistent between storms and between two successive years. Phase II bedload transport, sampled during a 1.8‐yr return period streamflow event in February, 1979, proved non‐uniform in a downstream direction and unsteady over time at a given stream discharge. Transport rates closely correlated with the rapid scour and redeposition of the riffle at the bedload sampling cross‐section; transport peaks corresponded to scour and transport decreased greatly during deposition. The transport of large (> 12.5 mm diam.) bed material increased up to 12‐fold during riffle scour.