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<br />RcccntliclJ.mcasurcmcnt data have shed some light on the troublesome
<br />problem of inllirectly measured velocities resuIling in ProutIc numbers thai signify
<br />hydraulically supcrcrilical now. High vclocilics (up to 3.3 mls) were rcconkd during
<br />II current.meler measurement of :nreamnow in II steellly (ahout 2%) sloping ephcli'l'ral
<br />chunncl in suulh-central Nevada (Thoma.~ G. Kane, U.S, Geological Survey. written
<br />l.:UIUI11UI1., 1994). The resultant average depth and average velocity indic;alcd
<br />supcrcrilical now with a Froudc number of 2.5. This experience. combined with
<br />recent similar Hodings by Wahl (1993 and 1994) for some western stream channels,
<br />indicates that the apparent stale of supercritical now may not in itself be a
<br />disqualifying factor in the a.~'\essment of the hydraulic acceptability of indirect flow-
<br />measurement results.
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<br />Stratc2ies for Imorovement
<br />
<br />Although no simple, clear-cut panacea exists for the variety of measurement
<br />difficulties and uncertainties associated with indlrect measurements. of peak flows in
<br />stl'ep desert streams, the Manning equation is slill the best known mathematical
<br />expression of those nows. The challenge is to improve measurement techniques to
<br />strengthen conlidence in the application of this equation. Several Mrategies arc
<br />aVl.lilablc to improve detennination results or at least to increase confidence in some
<br />of the results.
<br />
<br />Knowledge of Ihe hydraulic state of gradually varied flow, or the degree of
<br />deviation from this assumed state, would be greatly improved lfffow-triggered video
<br />cameras could be inslalled at key sites. ResuUant video tapes would allow
<br />investigators to view. and review, varying strcamnows. This pictorial documentation
<br />would pcmlit reasonable estimates of surface velocities of flows for comparisons with
<br />average velocities calculated indirectly using the Manning equation. VisuaJ
<br />documentation urthe streamflow surface should aloo allow a rational judgement orthe
<br />validily of the high-water profiles. Streamflow surface configurations might also
<br />indicale the hydraulic character of the streambed (dunes. plane bed, ani i-dunes) at the
<br />lime of peak now and thereby improve confidence in the assessment of channel-
<br />boundary resistance to now. Overall and specific characteristics of the nuid motion
<br />might be interpreted to allow a reasonable assessment of inlernal fluid resistancc to
<br />flow caused by the entrained sediment load. Visual documentation of streamnow
<br />would also illustrate changes in now depths associated with changes in now regime!'
<br />accompanying transitions h,.tween subcritical and supercritical flow.
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<br />The selected measurement reach should be near, or should include, a recording
<br />streamfluw gage. The recurd of varying stream stage and the resultant record of
<br />stream discharge could then be referenced 10 Ihe video documentary. A basin nelwork
<br />of recording rain gages would allow precipitation-runoff modeling of individual
<br />slorms and thereby provide a semi-independenl check on logical ranges for
<br />detemlinations of peak-streamflow rates. Time synchronization of the min and stream
<br />gages and real-time. radio reporting would provide investigators with a notice of
<br />storm-runoff activity that might allow some direct observations and measuremenls of
<br />streamflow.
<br />
<br />. Additional current-meter measurements are needed to verify n values,
<br />particularly for lhe stale of supercritical now. Those mea'iuremcnl~ would also
<br />provide needed data on cross-sectional velocity distributions as Ihey relate to '1.
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<br />I'I~^K STREAMFI.()WS-I'R()lJI.EMS
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<br />. L>ct~r":,inatiolls of lIow depth can be improved through in~tallation of scour
<br />chams .wl~hm lhe ~cnsurc01en! reach to document the net depth of scour for specific
<br />runofk Scour ~'~arn'" are '.Iexlble lengths of stcel chain, buried vertically in the
<br />~trca~~hcd. ~ndtv~du~\1 chains can. be positioned, by optional numher and spacings,
<br />along a cross.sectlonaf ChiUlOCI ahgmnent The suhsequenl deplh I.'. I. I. I
<br />". '. oulsruplunotlc
<br />vcrtlclll ahgnmcnt IS a (Iuamitalivc mea.~ure of net scour depth at any given chain site.
<br />
<br />,The stream ~l.Ige, n~in gage, video recording, and scour-chain technologies
<br />reqUire ~re-rum)~ se'ec(um and aUlomated inslrumentalion of channel reachcs.
<br />Proper site .selectlon, therefore, requires some knowledge of runoff frequcncics to
<br />g~ar~n!~ timely rcsult~ and selection of an instrument sile that can be protected from
<br />vandahsm. perhaps as m parks or on publicly conlrolled lands.
<br />
<br />., ~.i.lta collccled for slope-arca determinations can also be amdY7.cd using rhe
<br />~~c~-b~e~watcr method (Rantz and others, 1982, p. 338). Stcp~backwaler analyses,
<br />c.'k~I~I~d for bOlh upst~am and downstream direclions, should initially uliIize the
<br />~rcIUl~mary srope-area-lhscharg~ resulls. Through subsequent discharge iteration,
<br />ste~-backw~ter analyses match Simulated high-water profiles to Ihe profiles defined hy
<br />mc.lsu~d high-water marks. The net resull'i of this trial-and.errnr process allow an
<br />evalual~on. of the reasonableness of slope-area resulls and e:1'1 help to reduce
<br />un~ertm.n~les regardin? t~e results. However, both the slope-area and stcp--backwarer
<br />methods share uncertalnlles related to now resistance and configuration oflhe channel
<br />bed and hanks.
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<br />Am>endix.-References
<br />
<br />Barncso' H. H.: Jr.. 1967, Roughness characteristics of natural channels: U.S.
<br />eulnglcal Survey Water-Supply Paper 1849,213 p.
<br />
<br />Benson, M.A., and Dalrymple, T-dte. 1967. General field and office procedures for
<br />indirect discharge measurements: U.S. Geological Survey Techniques ~f
<br />Walcl'-Resourccs Invcstigations, Book 3. Chap. AI, 30 p.
<br />
<br />Dalrymple. Talc, and Benson, M.A., 1967. Measuremenl 01 peak discharges hy the
<br />slope-~rc~ method: U.S. Geological Survey Techniq\lt.'~ ofWalcr-Resources
<br />Investigations, Book 3, Chap. A2. 12 p.
<br />
<br />Ranlz. S.E., a?d others, 1982, Measurement and eomputation of streamflow: U.S.
<br />Oenlogleal Survey Waler-Supply Paper 2195, 2 vols., 631 p.
<br />
<br />Wahl, ~.L., 1993, Variation nf Froude number with discharge for large-gradient
<br />streams: Amencan Society of Civil Enginec:rs Proceedings of National
<br />Conference nn Hydraulic Engineering, San Francisen Calif. July 26-30 1993
<br />p. 1517-1522. " "
<br />
<br />Wahl, K. L.. 1994, Ev.alu3tion of supercriticaVsubcritical nows in high gradient
<br />cha~nel: . American Society of Civil Engineers Journal of Hydraulic
<br />Englneerong: v. 120, no. 2. p. 270-272.
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