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A . SCOUR AND FILL iN STEEP, SAND.BED EPHEMERAL STREAMS 569 At unit discharges greater than about O.5m1/m's (5 cfslfr), the as- ,ul11prion of fully rough now is valid, and the calculated flow v~. lacier is great enough ro reasonably assure char bed forms are an- (IJunes. At unit discharges below O.lm3/m's (1 dslEr) , the calcu- IJced flow velocities are so low that the assumption of anridunes becomes suspect. Thus, a transition of bed forms from anridunes to dunes probably occurs in the main projea reach at a unit dis(harge occween 0.1 and 0.5 m3/m's. Depch of interdune troughs below mean-bed elevation is nor an 3nalyrical function of flow parameters. One can expect, however, [hat dune size wit! nor be as sensitive (0 flow variations as antidune ~lze. Also, as can be seen for ripples in Figure 6b, ~d reworking by Junes will be greater than that by antidunes unless maximum dis- ..:hJrge is much greater than that for dune-regime flow. Thus, in a hypothetical sand-bed ephemeral stream where all bed reworking is by bed forms and where a discontinuous depth versus discharge re- lation exists. one would expect scour-chain dat3. to show a limited range of scour and fill below some threshold discharge. The rhreshold discharge would be that at which antidune amplitude is ~qual to the maximum depm of dune troughs, because dunes tormed on the waning flow would erase the effects of any smaller Jnridunes. Leopold and orhers' (1966) dara as presemed in Figure 10 can be interpreted as suggescing such a relation for rhe Arroyo de los Frijoles. Maximum scour data for unit discharges below 0.5 mJim's 5 cfslft) require an envelope that changes but little wich unic dis- ~'hi:lrge. Above 0.5 m3/m's, scour dara suggest a trend following the Jnridune zone, but of smaller amplitude. However, it was found jerh in Quatal Creek and the laborarory that flow estimations ?redicted the maximum antidune amplitude, nor the mean Jmplitude. Any flow tends to form high-velociry zones with widths ,hat are only a fraerion of the mean stream width. particularlY' in llluvia!-bank channels of grear widrh-deprh ratios (KennedY'. 1961; ~oley, 1975). The largest antidunes and greatesr scour and fill occur .,1 these zones, as suggested by Figure 4. Thus, average depth of -naximum scour for a cross section is not a reliable indicator of maximum flow conditions and lx:d reworking. Leopold and others' ! 966) average data do not conflier with the hypothesis mat all bed ~eworking in the Arroyo de los Frijoles was by bed forms. These calrulations for the Arroyo de los Frijoles are admittedly ~n.lde, since channel data used were not measured for the purpose )f antidune~amplirude estimation, and rhe possibiliry of a discon- 'Inuous deprh Versus discharge relation precludes quantitative bed- :nrm size estimation at lower dischatges. Thus, it is possible chat ,orne mean-bed scour and fill did occur. A definitive estimate of nean-bed versus local scour and fill would require a scour-chain \c'twork in the test reach, such as chat used in Quatal Creek but ':.xrending along a much longer reach, and measurements of flow j~prh in rhe test reach in addition to measurements of discharge. CONG.USIONS Conclusions concerning scour and fill in a steep. sand-bed 'phemeral "ream are based on combined field and laboratory ex. :-oerimems. Data were derived from floods of moderate discharge in I nacural channel in January and December 1974, a series of labo- ".lfOry floods in rigid-wall and alluvial-bank channels. and pub- I';oed field data. Scour and fill for simulared floods in a uniform sand-bed labora- 'ory channel at grade results predominandy from bed-form de- . dopmenc and migration. Scour and fill for moderate floods in a "ldJ ephemeral channel is c.:onsistcnr wirh theoretical estimates of "cworking of the bed by bed forms. Flood flow in the field stream was judged to be enrirely in (he a.ncidune regime because of steep gradient, lack of evidence for re. sidual bed forms rdared to a lower flow regime. and beha\'ior of scour-cords. Reasoning based on bed and bank sediment and chan- nel geometry suggests that the field stream is at grade despite net change in bed elevation over the shorr observarional period. S(our and fill measured by scour-cords was equal to bankfull water depth in the january, 1974 flood and rwice banklull warer deprh in the Decembu 1974 flood. In borh instances it was less than the upper limit of estimated antidune amplitude. Thus, scour and fill mea- sured by scour-cords for the [VIa major floods can be explained theoretically by bed-form migration, but this is a qualified conclu- sion since the floods were nor directly observed or gaged. For laboratory floods modeled after ephemeral stream floods, maximum change in mean-bed elevation resulring from mean-bed scour and fill was only 0.06 cm lor a rigid. wall channel a[ grade. For these floods, the depth to the bed-form troughs was 2.2 em, so mean-bed fill and scour was negligible (3%) compared ro maximum local scour and fill. Maximum ripple development oc- curred near the end of the flood, at minimum aischarge. In the laboratory, mean-bed fill and scour was strongly influenced by (I) the ad hoc sediment-input rate and (2) the dynamic effect of discontinuous deprh versus velocity relacions. Sediment-input rate decayed faster than sediment-transport rate., so the maintenance of channel grade tequired a surplus of sediment input during the waxing phase of a flood to compensate for the sediment~input deficiency on the waning phase. The rime required for deveIopmenc of finite bed roughness during transition from the upper to the lower flow regime was the cause of a greater. than- equilibrium velociry and greater sediment transporr-as the flood waned Both the ad hoc sediment-input rate and the bed~roughness time lag resulted in a small amoum of mean-bed fill and scour. Transirion from upper to lower flow regimes could produce (he same effeer in a natural scream with a discontinuous depth versus velocity telarion, provided that the rate of change of discharge is of the same order or greater than the rate of change of bed roughness. Since flow at the field sire was seemingly all in the upper flow re~ gime. this presumably did not occur. Application of [he antidune-amplitude estimation technique to the Arroyo de Ios Frijoles in New Mexico shows that Leopold and orhers' (1966) scour.and.fill dara lor rhe main project reach can be ocplained as well by bed reworking br antidunes as by mean-bed scour and fill for flows wirh maximum unit discharges greater than 0.5 mJ/m's. For flows with maximum unit discharges less rhan 0.5 mJ/m's, anridune bed reworking may be masked by dune re-.vorking during the waning flow. This last effeer is sperulaove since the stage versus discharge relation for the main projecr reach waS nor re- ported by rhose authors. ACKNOWLEDGMEi'\fTS This work ben~rted grearly from discussions and suggeStions by R. P. Sharp and V. A. Vanoni. [gratefully acknowledge rhe careful criticisms of the manuscript by V. -R. Baker, W. W. Emmett, and C. F. Nordin, Jr. Field work in the Los Padres National Forest waS perloemed wirh rhe permission 01 rhe U,$, Foresr Sel'Vice. Field work was supported by a research grant from the Geological Soci- ery of America, laboratory work was supported by grams from rhe Army Research Office and the National Science Foundation, REfERENCES CITED Baker. Victor R.. 1973. Paleohydrology Jnd scdimemology of !....;l!.:.~ .\1i:;- soula Rooding in eascern Washington: Geol. Soc. Amerio Spec. Paper 144,79 p. ;j " ;'! " 'I ili 'I 1.1. II: ,'. 1,1" :,Ii i'll II: , I' d 'I' ill II" , I , ' ',! 'Ii 'II ',II' I ':1 :j i ~ :ii! :Ii I 1', ,I :1;,1 :ilj, 'W:[ fir' I :II: I iil : I' . :,: I' ,., :iiiii' .,1111 'I' i,ll: II!! ! ;llll 'I:; ! " , '1_1 i "1 I Iii :-: I " j"ll 11, !