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<br />GRANT: CRITICAL FLOW CONSTRAINS FLOW HYDRAULICS <br /> <br /> <br />a <br /> <br />~-~.:--~, <br />,. <br /> <br />_~_ ~,:c~ :,--=l. <br /> <br />351 <br /> <br /> <br />b <br /> <br />Figure 1. Examples of standing wave trains in fine-grained channels, (a) Sludy site at Big Creek, Oregon, <br />showing building standing waves. Seagulls in background for scale. (b) Braided reach of the Pasig~Potrero <br />River, Philippines, at low flow after i<.lhar passage the previous day. Notc standing wave trains in each of the <br />anabranches. <br /> <br />These observations are not limited to coastal sand-bed chan- <br />nels but can readily be observed in other fine-grained channels <br />with slopes in excess of about 0.01. Dynamics virtually identical <br />to lhusIJ of the Oregon b~8Ch channcl.!l were described by <br />Sdwmm t:l al. [1982J for Mcclane Creek, a sand-bed channel in <br />Colurado with similar slope. Clear water and fine-grained hy- <br />pcn.:oncentrated flows in channels subject to lahars in volcani- <br />cally affected landscapes also display similar behavior [Pierson <br />und SCOfl, 1985; Simon, 1992; Simon and Hardison, 1994]. For <br />example, I measured a lahar-runout reach of the Pasig-Potrero <br />River draining Mount Pinatubo in the central Philippines (Fig~ <br />ure 1 b). Because of the 1991 eruption of Mount Pinatubo and <br />subsequent channel changes, the Pasig~Potrcro now carries an <br />extremely high sediment load, consisting: of coarse sand and <br />gravel derived from the 1991 pyroclastic field, along with pU4 <br /> <br />miceous cobbles and boulders up to 20 ern in diameter. During <br />the wet season, from June through September, thunderstorms <br />with high-precipitation intensities produce frequent lahars <br />[Pil:!fSOtl t!/ al., 1992]. Following each lahar. the low-flow chan- <br />nel reworks lahar-derived deposits into a 50- to IOU-m wide <br />shallow, braided channel with a gradient of 0.012 (Figure Ib). <br />Individual braids of this channel have hydraulics identical to <br />those of the Oregon coastal channels, including wave trains of <br />building and breaking standing waves, periodic bores, and flow <br />osciHating between subcritical and supercritical. I measured <br />velocities by using floats and depths by using a folding stadia <br />rod. In a straight, uniform 30-m reach, surface velocities <br />ranged from 1.7 to 2.6 mis, averaging 2.0 mis, and depths <br />ranged from 0.16 to 0.22 m, averaging 0.20 m. Because of the <br />methods used, I could not instantaneously measure FrouJe <br />