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<br />HYDRAULIC ~NGlNEERlNG '<)4
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<br />for sediment an..lysis. All of the New York study sites arc coarse-grained channclli and.
<br />Ihercfore. were evaluated b) lhe melhod lor coarse-grained bed malcria~. Random gr~b
<br />samples of bed material were laken Oil equal incremems across Ihrcc 10 live cross sccuons
<br />within each study reach and alllhrcc dimensions of each panicle were mCilsu~d.
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<br />Major Flow-retarding Factors
<br />Most sites arc Iculhan ideal for hydraulic computations in cerlain aspects. In ihc New
<br />York. study, now-retarding factors such as variations in channel and cross-section shape.
<br />obslruclions. and meandering were avoided where possible. The one (acIDr thai could nOI
<br />be avoided was srrcambank. vegetalian. The generallypc and rclalive density of lhe vege-
<br />lillion, as well as seasonal changes. were recorded. The approximatc clevation al. which
<br />vegetation began on each bank. was DOted and u~cd to ~ompute the pcrce~tagc ot welled
<br />perimetcr that was vegctatcd for each discharge. and Uus percentage was I~ turn used as an
<br />indicalion of energy losses that could be allributcd to streamOOnk vegetallon. Researchers
<br />might intentionally select siles wilh other major now~retarding factors and aucmpt a com.
<br />parison llllalysis with n values from other sites 10 explain Ihe effect of the difference on the
<br />computed roughnc" cocfticiems.
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<br />Photographs
<br />The best way to document conditions lllld changes in a study reach is through 11110tog-
<br />raphy. Downstream and upstream views of each of the New Yolt study 'rcac~les were pho.
<br />togr.'phed to show channel alignment, streambank.vcgctalion type and dens).ty. channel
<br />sil.e in relation 10 the f1ow-resi~ing features of the channel, and, where poSSible. bed ma.
<br />tcrial. If before-and-afler or scaliOnal comparisons arc to be made, the oriemation and view
<br />of the photographs should be duplicated for each condition, and speci~1 anent ion give~ to
<br />any major Oow.retarding factor thai is being analyzed. Wherever poSSible, rcprcse~IaIIVe
<br />bed material should be photographed. and a referencc scalc, such as a person or sladla rod,
<br />should be included in each photograph.
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<br />Computation or HydrauUc Properties and Manning'!'t Roughness Coefficients
<br />A compulcr program developed by Jarrell and Pelseh (1985) was used 10 compute Ihe
<br />values of Mamliu~'li roughness coefficient. This program is based on the equation prescnt-
<br />cd in Barnes (1967, eq. 6) and is a modilkudon of the equation for computing discharge.lly
<br />the slope-area method as presemed in Dalrymple and Benson (1967). Roughness coeflto
<br />dents are computed for "an unsubdivided ch.mnel for a sin~le evenl of mca.'iured orlmown
<br />clear waler. Input data are discharge. ground elevalions and stationing to deJinc individual
<br />cross sections. walcr.surracc elevations at each cross section, and lhe distance downslrcam
<br />from a reference poim to each cross seclion" (Jarrell and Pelsch, 19H5). Outpul from Ihe
<br />program includes hydraulic data for each cross section and Ihe compule~ 11 valu~s for every
<br />pair of crOliS seelions as well as for the muhisecHon reach. The change an velOCity head tJc.
<br />tween adjaccRl sections is also labulaled for use in evaluating the assumption 01 channel
<br />unifonnity.
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<br />Analysis or Dala
<br />Analysis of the fleld-coUceled and ~ompulcd data entailed Ihe l'allowing sleps. (I) WoI'
<br />ler-surface ,pmliles were ploned and checked for discrepancies due to measuremenl errors
<br />or unlwo.".1 backwater dfecls. (2) TIle n-calculalion oull)\ll was checked for inconsisten-
<br />cies in lhe ,omputcd roughness coefficients. Ideally, n values for subrcac;hcs should agree
<br />closely. (3) The hydrauliC data generalcd by Ihe n-calculalion program. including Jlow
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<br />ROUGHNESS-C()~H1ClENT STUDY-DESIGN
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<br />6"
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<br />area. slream.top widlh, welled perimeter, hydraulic radius, and mcan velocity were labu-
<br />laled. (4) WOller-surface slope and energy gradicRI were computed. (5) These dala were
<br />plottcd against each olher and the computed n values to indicate I rends or variability in the
<br />dala_ (6) Analysis of streambed-panicle size and size distribution should include compu-
<br />tation and plotting of the cumulative percent finer for sueambcd panicles at each she and
<br />the calculation of characteristic diameler siles, such as the intermediate diameler that
<br />equals or cxceeds that of 50 and 84 percent of the panicles ("so and d84).
<br />Discrepancics in lhe data could be caused by (I) nonuniform hydrolulic conditions
<br />Ihrough the reach; (2) inaccurate estimalion of expansion losses in the reach; (3) variabililY
<br />in Ihe water-surface slope bclween rising and falling slages or delays in lhe timely mea-
<br />surement of wOller-surface elevalions; (4) seasonal variation in hydraulic conditions; and
<br />(5) movement of the water~surfacc-clevation recording gages, or aggradation or scour
<br />within the study reach.
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<br />SUMMARV
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<br />A well-designed roughness eoefUcieRl sludy can reduce data.collcction and n-valuc-
<br />compuUltional problems and yield valuable information on energy losses in channels. Site
<br />selection requires careful consideration. CompariliOR studies of two similar sires. orof the
<br />same site under differing conditions, provide the most infonnative approach to analyzing
<br />Ihe increment of "roughness" thai can be altribuled 10 a specific faclor, for example, vege.
<br />talion or obstructions. localion of sites near established streamOow~monitoring stations
<br />and inslrumentation of the cross sections wilh cresHuage gages or aUlOmatic recorders will
<br />facilitate the collection of discharge and water-surface-elevation data. respectively. A com-
<br />pUler program developed for the computation of roughness cocfiicients will facilitate data
<br />compulalion and analysis.
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<br />REFERENCES
<br />Barnes. H.H.. Jr., 1967. Roughness characleristics of natural channels: U.S. Geological
<br />Survey Waler.Supply Paper IM9, 213 p.
<br />Benson, M.A., aud Dalrymple, Tale. 1967. General field and office procedures for indirect
<br />discharge measurements: U.S. Oeological Survey Techniques of Water-Resources
<br />Invesligalions. Book. 3. Chapter A I, 30 p.
<br />Dalrymple, TalC. and Benson. M_A., 1967, Measurement of peak discharge by slope-area
<br />method: U.S. Oeological Survey Techniques of Waler.Resources Invesligations, Book
<br />3. Chapler A2, 12 p.
<br />Guy. H.P.. 1969. laboratory theory and methods for sediment analysis: U.S. Geological
<br />Survey Techniques of Water.Rcsourccs Investigations, Book. 5, Chapter CI, 5H p_
<br />Jarrett, R.D.. and Petsch, II. E., Jr., 19S5, Computer program NeAlC user's manual.
<br />Verification of Manning's roughness coefficient in channels: u.s. Geological Survey
<br />Waler-Resources Im-"::-iligations Rcpon 85-4317, 27 p.
<br />Kcllerhals, Rolf, and Bray. 0.1., 1970. Sampling procedures for Coarse nuvial sediment:
<br />American Sociely of Civil Engineers, Hydraulics Division Specialty Conference.
<br />Mlllneapolis, Minn., 31 p.
<br />Rantz. S.H.. and ulhcrs. 1982, Measuremem and compulalion of stream now-Volume I.
<br />Measurement of stage and discharge: U.S. GeOlogical Survey Water-Supply Paper
<br />2175, 2114 p.
<br />Wolman. M.G.. 1954, A melhod of sampling coarse river.,bcd material: American
<br />GL'Ophysical Union. Transactions. v. 35, no. 6, p. 951-956.
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