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<br />~ <br /> <br />O'Brien (1984) studied the hydraulic and sediment transport <br />dynamics on a cobble bar within the Yampa River spawning site and <br />duplicated some of its characteristics in the laboratory. He <br />concluded that the finer sediments, primarily sand, were flushed <br />from the coarser cobbles down to a -depth of one.-half the cobble <br />diameter below the cobble surface during peak flows. He reported a <br />range in cobble size of 50-100 nun with an average ofr75mm~ <br /> <br />~'Brien calculated that discharges on the order of one-half the <br />iincipient motion of the cobble bed were necessary to accomplish the <br />)observed affect at the Yampa study site. Flume experiments by <br />L <br />Berry (1985) show that subsurface sand can not be flushed below the <br />median cobble size until the surface cobbles are moved. The <br />process of sand flushing in the cobbles occurred primarily as the <br />result of turbulent bursts within the interstices. The time <br />interval measured to flush sand from the cobble bed was 160 to 290 <br />minutes. Thus, the goal of the flushing flow is not to move the <br />bed material, but merely 'pluck' the fines from the surface down to <br />one-half the median bed surface material size. <br /> <br />4 <br />