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<br />J <br /> <br />'.. <br /> <br />u <br /> <br />564 <br /> <br />M. G, FOLEY <br /> <br />draulic radius because it gives maximum-flow conditions in the <br />channel - not just aVCr.lge flow conditions for the cross section. <br />Kennedy (1961) urged the use of local now conditions fot anodune <br />calculations in narural streams, where maximum-flow conditions <br />may vary greacly from the mean because of irregular channel cross <br />sections. The difference becwecn maximum depth and hydraulic <br />radius for the January 1974 runoff evenr is only 1.4%, so the dis. <br />tinction is not significanr. The transverse wavelength of stationary <br />waves is raken as the width of the largest trough formed by scour, <br />which was nearest the south bar (south channel margin in cross <br />seCtion AN, Fig. 4a), <br />Decembet 1974 Runoff Event. Calculated now behavior for <br />December is the same as for January (Table 1), if the initial channel <br />cross-section geometry is used. Maximum bed reworking from ini- <br />tial bed elevation in cross seaion AA' (Fig. 4b) was 82 cm. At the <br />maximum antidune amplitude of the January event, this amount of <br />reworking implies an additional 21 em of mean-bed scour. How- <br />ever, net scour of the center of the channel ocrurred during me De- <br />cember event and resulted. in a posdlood bed elevation there mat <br />was 17 cm lower man me preflood bed elevation (Fig. 4b, cross <br />section AA'). Peak-flow calculations are sensitive to flow depth, so <br />that it is imporranr to determine whether the net scour occurred be- <br />fore or after peak flow. <br />If the observed ner scour had occurred before or during peak <br />flow, che antidune amplitude for the December event should have <br />been in the range of 44 to 92 em (Table 1), enough to account for <br />all the bed tewotking, Net scour and net fill of the channel as a <br />whole support chis interpretation. Net fill, mostly fine sand, oc- <br />curred at the edges of the scoured channel (Fig. 4b, cross section <br />AA'). Scour-cords showed no reworking of the initial bed under <br />chis fill, which suggests mat filling occurred early in the runoff event <br />and was nOt disturbed by subsequem antidune development near <br />che banks at peak flow. This asymmetry of channel reworking with <br />net scour in one part and fill in another contrasrs wich channel be- <br />havior in the January 1974 runoff event. It appears that the Dc- <br />cember flow was modifying the channel toward a smaller width <br />depth rano. The smaller widlh depth ratio implies that the De- <br />cember flow carried a larger proportion of fine material than the <br />January now (Schumm, 1960), and the telaovely fine-grained na- <br />ture of the channel-edge deposits supports this implication. It is <br />more likely that this modification occurred at peak flow when the <br />sedimenr.transport rare was much higher, than during rhe waning <br />flow; therefore, the flow conditions shown in Table 1 for December <br />ptobably prevailed. <br />Comparison between calrulated and observed maximum scour <br />supporcs this conclusion. The January flow inferred from scour- <br />cord measurements is ar the lower end of the calculated range for <br />the friction factor, whereas the December flow is in the middle of <br />the calculated range (Table 1). This suggests chat the January flow, <br />if bankfull,. had a higher friction facror rario than the Decembe~ <br />now and hence a coatser bed load and sinaller proportion of silt <br />and day. These variations in load material can be caused by rainfall <br />over different tributaries, which then provide different sediment <br />sizes. However, if mOSt bed reworking is by anridunes as is <br />hypothesized here, gross sediment transport is roo small to allow <br />tributary input effects to be felt very far downstream during the <br />same flow event. Only the wash load and finest fraaions of salta. <br />ring bed load would be expeCted to move completely through the <br />drainage system. In this case, channel cross-seaion modification is <br />either a re'sult of transportation and deposition of only the finest <br />fraction of the bed load or, as W. W. Emmett (1976, written com- <br />mun.) has suggested, a result of passage of more slowly moving <br />slugs of tributary sediment derived from previous runoff eVents. <br /> <br />Resolution of this question will require many more flood observa. <br />tions than the two reported here. <br />It is possible [Q account for the asymmetrical bed reworking in <br />the December 1974 runoff event entirely by antidune development, <br />if it is assumed that the transported load was more fine-grained <br />than usual; nevertheless, a remote possibility is that the December <br />flew asymmetry developed because part of the Stream bed was fro- <br />zen. The southern half of the stream bed is shaded by a diff, and in <br />episodes of cold winter weather it escapes diurnal thaw. Under such <br />conditions a midday runoff eVent would scour the northern part of <br />the channel more easily than the southern parr. However, the flow <br />would probably thaw the thin (less than 5 em thick) frozen layer on <br />the southern part of the channel before the end of peak flow, and an <br />initially partly frozen bed does not explain the fine-grained fill on <br />both the northern and southern channel edges. The assumption of <br />line-grained load is thus the preferred explanation. . ~. <br /> <br />lABORATORY EXPERIMENT <br /> <br />i:. <br />J ~~i" <br /> <br />:l~ <br />A series of laboratory experiments on scour and fill was con-.'; <br />ducted in the W. M. Keck Laboratory of Hydraulics and Waret ae.:.~, <br />sources, California Institute of Technology. These were sand-bed,~ <br />experiments in a nonrecircularing. rigid-wall flume with rim~:: <br />varying rates of water and sediment input patterned on rhose of1 <br />floods in ephemeral streams. The purpose was to observe scour and ~_ <br />fill caused by noods in a steep, sand,bed stream in a controlled 1a1>-1 <br />orarory environment. These experiments were intended as"~ <br />analogues of the field environmem, not scale models of any parriai~! <br />larnarural stream. .'":J), <br />i~~ <br />Equipment and Procedure _:.-~ <br />-.,j, <br />ExperimentS were conducted. in a rigid-wall channel18.J m long, ~ <br />26.7 em wide, and 25.4 em deep, installed in a tilting steel flume.;~ <br />Flwne slope was adjustable from 0% to 1.5%. Time-varying walte~.. ". <br />and sediment input was conrrolled by prc::programmed moror: ' <br />driven cams that controUed a pneumatic .......ater inlet valve and a <br />variable.speed wet sand feeder. Water supply for the flume was <br />constant-head tank supplied. from a large reservoir. . ~ <br />Flume oudet was a frec-overfaU into a settling tank, where sari '. <br />was separated for recycling before the water was returned to the i <br />reservoir. Total sediment discharge was determined at the outl1l <br />using a travetsing vertical-slot sampler, Water-surface and bed e1e:Jiii <br />vacions were determined using a point gage mounted on a carriag~ <br />that could be moved the length of the nume on tails mounted at~PI', <br />the flume walls. The point gage permitted location of any point In. t <br />the flume to within 0.5 mm horizontaUy and 0.05 mm vertically: . <br />Determinations of water-surface and sand-bed elevations reproduc.lt <br />ible within 0.05 mm could only be made for srill water, or for sand ' <br />saturated with water but with no standing water. Sand.surface de::. <br />vation was rhus determined from the elevation of the water he.ld <br />between the grains by surface tension. Watet-surface clevatio <br />were recorded continuously at seven locations along the flume ~., <br />pressure transducers attached to piezometric taps in the flume ho~- ;it <br />tom. Sttip-chatt rerordings of transducer output were used to d <br />termine mean water.surface elevation and slope during unstead <br />flows with violent surfao:-wave activity. Pressure.transducec d~ <br />terminations of water~surface elevations were accurate co wich~ <br />0.2mm. <br />Bed and input load material used in these experiments w,as-. <br />natural sand wich geometric mean size d, = 0.28 mm and geom~~ <br />ric standard deviation cr = 1.42. This sand consisted almost e~~,~. <br />tirely of subcounded to well-rounded quartZ grains, wich traces -~- <br />magnetite. ~ <br /> <br />