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<br />I' <br />J' <br />~ <br /> <br />} <br />1. <br /> <br />~ <br /> <br />j <br /> <br />~, <br />, <br />{ <br />f <br />l'" <br />t <br />~ <br />~ <br />" <br />i, <br />f <br />~. <br />~~. <br />~ <br />, <br />~ <br />: ~ <br /> <br />L <br />i'i'I' <br />" , <br /> <br />ii: <br />,~ <br />'~ . <br />d1f <br />os: ... <br />~' f-, <br /> <br />~ <br />~ <br />'.(' <br />;~ .' <br /> <br /> <br /> <br />. <br />Scour and fill in steep, sand-bed ephemeral streams <br /> <br />Ic~D <br />JUl 22 1997 <br /> <br />Colorado Water <br />8 I _&.:_- 0__..... <br /> <br />MICHAEL G. FOLEY Department of Geology, University of Missouri-Columbia, Columbia, Missouri 65201 <br /> <br />ABSTRACT <br /> <br />The traditional idea that entire long reaches of alluvial stream <br />channels in semiarid regions are scoured at high flood discharges <br />and subsequendy filled in the waning flood phaseJmean-lxd scour <br />and fill) can be challenged. The alternative concepc chac mean-bed <br />elevacion varies buc little during a flood and chac both scour and fill <br />occur concurrently at different migrating loci within a reach (local <br />scouc and fill) is also consistent wich published field data. Field and <br />laboratory" investigations reported herein suggest mat mean-bed <br />scour and fill in a straight uniform channel is minor compared to <br />local scour and fill caused by bedform migration and, furthermore, <br />that maximum local scour and fill may occur during the waning <br />flood phase in some instances. <br />The field experiment. utilizing a rectilinear array of maximum- <br />scour indicators (scour-cords). produced data for concoured plots <br />of maximum scour and fill in an ephemeral stream bed during two <br />floods. In the first flood, 24 em of scour and fill was measured for a <br />bankfull flow depch of 23 em. In che s<cond, maximum scour and <br />fill was at lease 66 cm for a bankfull flow depch of 34 cm. Escimaces <br />of antidune amplitudes for che cwo floods, based on theocetical <br />models and laboratory and field obs<cvarions, ace 27 to 61 em and <br />44 to 92 em, respectively. This indicates that aU scour and fill mea~ <br />sured by the scour-cord array coUld have been caused by antidune <br />migration. <br />Laboratory experimems were conducted in an 18-m long non~ <br />recirculating flume with automated controls for rates of sediment <br />and water input.. A series of experiments in a 26.7-cm-wide sand- <br />bed channel wich rigid walls, at grade for a simulated flood pat- <br />terned after those typical of ephemeral streams, showed that <br />mean-bed scour and fill was less chan 3% of local <couc and filL For <br />thc:se experiments, mean sand size was 0.3 mm, channel slope was <br />0.009, maximum water depth was 40 mm, maximum local scour <br />and fill waiU mm; and maximum mean-bed scour and fill was 0.6 <br />mm. Maximum mean-bed elevation variation was thus only two <br />sand-grain diameters. Maximum local scour and fill took place <br />near che end of the simulated floods. when bed-form amplitudes <br />were me greatest'". <br />Ancidune. amplitudes calculated for flows in che Arroyo de los, <br />Frijoles) New Mexico, are larger than published values of scour and <br />fill for unit discharges greater chan 0.5 m'/m . s (5 dslftl_ Below chac <br />chreshold discharge, che antidunes thac formed at maximum flood <br />discharge are smaller than the dunes that probably form during the <br />waning flood, and calculated antidune amplitudes are less than re- <br />poered'scour and fill. ,::r:.l"",'n: .1,0 >l '; T. ~ _.'1 , , <br /> <br />.7 '~:_i I; I );<:f: l:!~: ,.l., ':, :.;~ ~ i~\: J ~l>\~", ;,':.'~~.~' ~;~\~~": :,'.:-,~,: ;':;~,~:,'.: :""~.:,,,:,,_: <br />~qDUCTIONi. L:,',.',Jd -1.W!" .~_. ."~I &. ...<~".J. u'-"" .' i '~_', <br />:;;.. '. >!~;:"'~::;". :. I)'}iJ-m;' ;ri "dl1ir!:':fP:;'.,~~ih .;,; ,.:;~J i1j~:!(:,,;,"~o::~ ~t..;'!;ii'1 <br />. -.Steep;'sand-bed screams 'are 'subject- to high~v~lodty.fiOOd t10~. <br />Large aInounts of suspended debris in these flows make-direct bed <br />obserVations' impossible, and high velocities make', soundings <br />difticulr and eXpensive. Thus, bed bchavioc during floods is poody <br />Wldecs;ood.:.Soundings from '.bridges or. gaging station cable- <br /> <br />crossings (for example, Pierce, 1916; Leopold and others, 1964) <br />indicate that stream beds at grade are lowered by scour during <br />flood-crest passage and raised by deposition to approximately their <br />former elevation on the waning flood - a behavior known as scour <br />and filL In discussions [Q follow, reference is made to "mean-bed <br />scour and fill" and "lohl scouc and filL" "Mean-bed scour" is <br />defined as scour occurring simultaneously OVer a stream-segment <br />length comparable [Q the flood-crest length; the subsequent <br />"mean-bed filling" is likewise defined. "Local scour and fill" is lim- <br />ited to a reach whose length is comparable to or less chan stream <br />width. Local scour and fill is related to flow disturbances occurring <br />in narrow gorges or near bridge abutments and other engineering <br />structUres, or to "bed-form scour and fill" related to bed-form de- <br />velopment and migration. Local scour and fill does not necessarily <br />occur simultaneously over the entire: transverse seerion of a stream <br />channe~ and it may Occur several rimes at the same location during <br />a single flood, "Total scouc and fill" is the combined effece of local <br />and mean-bed Scouc and fill. "Scouc and fill" will be used inter- <br />changeably wich "bed reworking" whece che specific nature of the <br />process is unknown. <br />The usual interpretation of gaging station measurements has <br />been that the entire flooded stream reach experiences mean-bed <br />scour and fill comparable to that in the measured cross sections. <br />However, Mitchelllin Leopold and Maddock, 1952) indicated chac <br />measurements at gaging stations may not be representative of the: <br />whole flooded reach, Lane and Borland (1954), obsecving chat gag- <br />ing stations are typically located in narrows, hypothesized that <br />deposition occurs where the'stream widens downstream. They also <br />proposed that restoration of the preflood c:levation of the bottom <br />involves deposition in scoured seroons of material removed from <br />such accumulation sections. From poine measurc:ments along cx- <br />tensive reaches of both ephemeral and perennial streams, Emmett <br />and Leopold (1963) found (1) thac SCOUt of che preflood bedoccucs <br />at some time during flood passage at all sections of these {eaches <br />and (2) chat in the two perennial scream; chey studied, chis scour <br />occurred simultaneously over a long teach. <br />These field observations are generally interpreted to indicate that <br />scour occurs continuously on all or part of the stream bed until that <br />process is replaced by deposition, when in fact scour and fiH may <br />alternate locally several times during a flood. with only a small per. <br />centage of the emire reach experiencing scour or fill at anyone time <br />(CulbectSon and Dawdy, 1964). This implies chat the scour-and.fill <br />process may be related to movement of bed forms such as dunes <br />during flood passage. Colby (1964) concluded chac the maximum <br />scour measured at a point is simply the elevation of the lowest in- <br />terdune "trough [0 pass during a flood; he: also 'concluded .that <br />mean-bed scour and fill occucs only in channel constrictions. He <br />reache&. this conclusion ~by . sedimenr~transport 1.analysis of a <br />hypotfi;;tical. flood in a unifonn sand-bed channel. However, his <br />. analysis assumed a continuous stage: versus discharge relanon and <br />slowly changing equilibrium flow condicions. <br />Under SOme conditions of slope and bed-material size, sand-bed <br />streams do not have continuous stage versus discharge relations. <br /> <br />.'.. <br /> <br />" <br /> <br />Geologia! Soci~ry of America Bulletin. v. sf. p. 559-570, 106gs., 1 ubl~ April 1978, Doc. no,.~0408. <br />~', i' ':', ,,<-,t, ". <br /> <br />559 <br /> <br />'"',, <br /> <br />'.'':' ',. <br /> <br />';1," <br />