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<br />IIYUt{AlIU<' EN(ilNI:EK1N(j '1)-1
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<br />Measurement Prnhlcms . . I' d
<br />.' '. I I the slope~area tcchfilllUi; (Dalrymp e an
<br />Most indirect deternllnauons, p~l~U ar 1 h Manning equation (Kantz and others,
<br />Benson, 1967), Ulili~fsomcak' va.~la':ft~~ r~les at a given stream site. The Manning
<br />1982 p. 274) 10 quanti y pe' s re
<br />, 2/3 ,/2
<br />, " ) _ AR S where
<br />equation, expressed in terms of stream discharge, IS Q - " .' .
<br />. ' . '-seelional area, R is hydraulic radius, S IS, f~lctton
<br />Q is stream dlscharg~. A IS croghss ffi . eot This equation is an empmcal
<br />slope. and" is ManDl.ng's rOO ~ss.coe uC~tion' Q=VA, where V is aYera~e slream
<br />expression of the haste now..con'.lDul~Y.~ orlbe Manning equation rct.luu'cS steaoy,
<br />velocity. A proper and c~nfident ap~hca:~onian fluid (water-dominated rather Ih.m
<br />uniform to ~radually vane~ now~. aN ro idl varying, dcbds-chargcd n~w ?Ccur~
<br />dcbris-doffilOatcd). Rccau:tc unsteady, P bf is arc often assodated With held
<br />orten in steep desert .:hanncls, recurrent pro en.
<br />2/3 1/2 .
<br />I . ( R S ) lerro' of the Manmng
<br />measurements of both the area (A) and ve oclty ~- '. .
<br />. . Id situations the Manning equation is sullthe best, ami
<br />cquation. Howevcr,.lD most fie . . bl 'ndirect assessment of peak streamflow.
<br />most practical, solu1Ion for a reasona e In' I lI'
<br />f h ,. 'ctional area (A) of stream ow me u t;
<br />Field mcaJtUr~mcnH. to ~e me t e ~~~:h~~he distance between high.watcr marks
<br />those of channel wldth and depth, ~op . II ' 'y to measure and relatively free of
<br />on opposite banks of the channel, l~ usu~ :.s e.:: distinct and steady-now, or nearly
<br />serlous measurement eaors ~hen t e mar widlh is uncertain dudng highly
<br />steady-flow conditions prevail; howc'wcr, even. If rapidly varying flow condilion:-.
<br />ft . FI depth is usually less CCnalO. d b
<br />unsteady ow. . ow .' rod cd by the wave-like surges of fluid cause Y
<br />Prevail a false high-water hne IS p UC . d commonly recurring error of
<br />, ft lses However a more senous un . f h
<br />the unstable ow pu. '. t arding the vertical position 0 t e
<br />depth measurement results f-:om u~ce~: Y rc~ccuratc depth mea~,hi~mcnt rCllu.irc'
<br />channel-floor surface at the time 0 pe .ourfw. d' peak flow. If evidence 01 ncl
<br />evidence or the location of the streambed s ace uflng
<br />scour or fill is obscure, depth-measurement errors can result.
<br />
<br />R2I'SII2 . s alld
<br />. . ( . ) can introduce sC'nouS error.
<br />Field mea"iuremcnts to dehne velOCity -n-
<br />. f' k d' . 'har e The measurements that ddinc ,
<br />uncertainty in the calculau~n ~ p~a lSC I' ~ ~ofthe fall of high-water lines. whll"h
<br />velocity include (I) .aquantltat~ve ocuor::~~u~~hc channel reach l'oClecled f()r
<br />approximates the fnctlon slo~ (~) thJet g 'natilJn of the (1)ughness cocflil:lcnl. '1,
<br />measurement, and (2) a qUcu;'ll~auT~ h e~~~tic rddius (R.) is derived from the, wll]lh
<br />through the measurement reac. c: y. 's-sectional area (A) ,Uld is, thcr~torc.
<br />depth measurements made 10 ~~I~nuntetf,;r~:o~ parameters, as PH'" I~)u~ly lh:-'(uS~ll.
<br />subject to measurement errors III ren 0 . .
<br />f II f the water surfac~ and cornputallun
<br />The measurenu:nt of the d?wnstream ,a 0 . lhe hi'gh-water protUe!> alung
<br />of the friction slope IS accomphshed by ~~~:~~~; the result of impropedy
<br />both channel hanks. Measure~nt e"or:. ar high water marks as previously
<br />recognizing, mislocating, or nnsmterpre 109 - ,
<br />
<br />described. 'hemeral channels ute vcry
<br />posl-fm:to determinations of ~ghDess l~ slee~'t~ result of channeH>l.lUndar)
<br />difficult The dominant energy loss IS assume to
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<br />PI::AK STREAMH.()WS~~I)IU)BI.I::MS
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<br />6.\7
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<br />friclion losses caused mainly by protrusions of vegetation and rocks and channel-
<br />geumetry irregularities; collectively, these obstructions to flow are generdlly referred
<br />10 as channel-boundary roughness. A proper value for channel-boundary roughness
<br />can be assigned subjectively using a handbook procedure (Bames, 1967). This
<br />procedure relales values of n 10 typical examples of differing degrees of roughness for
<br />a wide variely of combinations of obstructive conditions and variable channel
<br />geometries. Careful and experienced application of Ihe handbook procedure
<br />minimizes errors and thereby results in generally consistent assessmenls of channel-
<br />boundary roughness.
<br />
<br />Channel-boundary roughness probably is not the only cause of energy loss in the
<br />sleep, l'phemcml channels typical of the arid southwestern United States. During
<br />Iiigh.energy runoff, which is characteristic oflhis hydrologic regime, strcamUows are
<br />variably charged wilh entrained sediment. If the cntraincd.scdimenlload dominates
<br />the nuid,the now behaves as a debris flow and the Manning equal ion is not applicable.
<br />If waler dominale!llhe fluid, varying combinations of streamftow hydraulics amI
<br />entrained-sediment transport can alter flow resislance by modifying channel bed
<br />forms and by varying boundary roughness. That is. sediment transport can either
<br />increase or decrease flow resistance (energy losses) by mcxlifying the streambed.
<br />Flow resistance is also increased Ihrough energy head losses cau,cJ by intense
<br />imerat:llUns of large sediment particles during transport. Thus, for sleep desert
<br />channels, lolal now re!liSlatlCC is a combination of channel-boundary resistance
<br />(roughness) and internal resistance. This combined resislance to nuid now, which
<br />lIlay vary greatly over the course of a specific runoff, does not leave evidence that can
<br />he rationally factored inln the post-faclc) assessment of Manning's n. The result may
<br />he either an undereslimatc or an overestimate of n by including only the channcl-
<br />huundary componenllhat is discernible after the peak flow has receded. As yel, no
<br />method has been devised that can reliably account for variations in the dynaqtic
<br />component of resistance.
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<br />Carefully executed Held measurements of all available flow evidence for steep,
<br />ephemeral slremns often result in estimates of peak discharge wherein the calculated
<br />velucities yield Froude numbers (RUDlZ and others, 1982, p. 549) greater than one.
<br />1llesc large Froudc l1U1l1l)Cr!lthcorelkally indicate Ihat hydrdulically supcrcritical now
<br />t'Undilions prevailed at and near the times of peak nows. The concepl of sustained
<br />Ul;L'Urrcnce or prevalence of !lupercritical flow in natural alluvial channels is nol
<br />n:.ldily accepted or believed by some hydraulic practitioners. This sk~plicism either
<br />I.'an lead lu greally reduced confidence in the computed results or can cause the results
<br />In he rcjccli.:J as hydruulically implausible. Computed discharges arc sometimes
<br />Illilpproprialely reduced to, or less than. the hydraulically critical-flow rate without
<br />rc:-;()Iuliun of field evidence to the contnuy. As an example of Ihis dilemma, Ihe peak
<br />llow ora severe nODd in a steep (3.8%), ephemeral wash during 1992 was delemlined
<br />hy lhe sJopc.area technique utilizing the Manning equation. The resultant discharge
<br />Ill' about 570 m3/s had an apparent mean velocity of S.2 mls and a Froude number of
<br />1.5. When six U.S. Geological Survey hydrologists, with a cumulative ISO years
<br />experience, examined Ihe flood site, they found no major errors with the field survey
<br />ur calculations, hut were unable to agree regarding the results. The major point of
<br />I:tlllcern to some was the high apparent velocity and resultant supercrilical Froude
<br />number.
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