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Suspended Load : Sampling and Measurement

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Suspended Load : Sampling and Measurement GOVERNMENT Of THE REPUBLIC Of INDONESIA MINIST';:", Of PUBLIC· WORKS DIRECTORAlE GENERAL Of WATER RESOURCES DEVELOPMENT PROGRAMME Of ASSISTANCE fOR THEIMPROVEMENT Of HYDROLOGIC DATA COLLECTION, PROCESSING AND EVALUAT ION IN INDONESIA SUSPENDED LOAD Sampling and Measurement by M. Travaglio' .. ~ SOCrETI'~ C~NTRA.LE ~I~ POUR r:<,QUl?EMENT Il ~ DU TERRITOIRE IN1[RNAlIONAL INTERNATIONAL SUSPENDED LOAn Samp1ing and Measurement by M. Travag1io Bandung, May 1981 Table of Contents Page List of Symbols used in the text 1. Introduction • 1 II. Vertical Distribution of Suspended Sediment 5 General considerations 5 Depth integrating method • 8 Point integrating method 9 III. Intcgrating Samplers •••••• •• 10 Depth integrating samplers •• 11 Transit rate •••• .11 U~S. Depth integrating samplers (DH 48, DH 59, D 49) 13 point integrating samplers (US P6l, US p63, Turbidisonde,belft bottle) •• •• .17 IV. Practical Observations about the Use of the U.S. silt Samplers 23 Transit rate . .. · 23 Nozzles . ... ·· 27 Gaskets . .. ·· 28 BottIes .. .... .. 29 V. Lateral Distribution of Suspended Sediment 31 Site selection •• 32 Number of sampling verticals 36 The Equal Discharge Increment IlIethod 38 The Equal Transit Rate method •• 41 VI. Other Measurement Deviees .• .. •• 43 Instantaneous sampIers ••• 43 Single stage sampler (US 59, Gluschkoff bellows, Pumping samplers). • ••••• •.• ••••• 45 ORSTOM procedure •. •• 47 Photoelectric turbudity method • 48 \ VII Concluding Remarks ~9 Annex l 51 Annex II 56 Annex III 59 References ., 63 List of Symbols Q.,..., suspended sediment discharge for the whole cross section in weight per unit time =ls suspended sediment discharge at a given vertical in weight per unit time and unit width Q water discharge for the whole cross section in volume per unit time q water discharge at a given vertical in volume per unit time and unit \~idth u velocity at a given point u mean velocity at a given vertical c concentration at a given point in the cross section c mean concentration at a given vertical C mean concentration for the whole cross section y distance above channel bed x distance from an origin along the cross section for a given vertical a sampler nozzle area T..... channel width D total depth at a given vertical R transit rate in the depth-integrating method t w weight of sediment in a sample v volume of water-sediment mixture of a sample U--or V mean velocity for the whole cross section D- mean depth for the whole cross section 1 \ 1. INTRODUCTION The suspended load is made up of particles which move in suspension with the flow at the same average velocity. It includes the suspended part of the bed material load and the washload, the latter being made up of grain sizes finer ~~an the bulk of the bed material. Einstein suggested that the limiting sizes of washload and bed material load may be chosen quite arbitra­ rily as the grain diameter of which 10 percent of the bed material is finer. The washload is supplied from the watershed through a shcet erosion process and is usually not a function of the stream discharge, therefore it is necessary to measure suspended load rates for numerous flood hydrographs both during rising and falling stages and during different seasons. To illustrate the foregoing two figures are presented.hereafter. Fig. l shows a clear rel,ationship between the bed material discharge and the mean velocity, however from fig. l' no definite relationship between washload and water discharge can be established, it c~~ only be said that the general trend is that the washload is small when the stream flow is low and conversely. , 1 1 i , i , , 1 1 1 : i ; ! i 1 1 1 ! 1 i 1 j ! ; 1 ! , 1 1 1 1 i 1 1 1.51 ! : ! i 1 , ; 1 , , , 1 , , 1 i 1 ! 1 1 1 ! i ~. 2 ~ '. l,! ] 1 i i 1 i .. ! 1 • 1 i , l • 1 .1 i i , ~ 1 I 1 1 i ..- \ 1 i i '. .J_ Il 3 , ! : J ~. , » i .. 1 1 j ! 1 1 i 1 : 1 l5 4 i ! 1 1 1 c j - i •,~. 1 1 1 2 1 1 i 1 1 : 1 , , 1 1 1 i : ~ i 6 , j ~: i 1 1 i ; i 1 , , , 1 1 1 : i 6 8 10 15 20 30 40 60 SO 100 150 200· Bell ,,",oteriel discl'lorqe, tons 1 (dayl (ft Of w,é!l'l) FiC' 1 Bed material Joad ~·s. mean velocity-Niobrara River. (Afler CoLBY (196]).) 2 100,000.. ., i ,. j j E c. 1 c. ::=J ~ '. .t ':'! E ~ .•• '. ~I E ' .. ,. ; ! .' , ,11 1 • 1 lOC! .: 1 ''':'O~-'---J----'1O::-:0:---'-~-'--'-''''1,''''0''''OO~-~''';'''-'''10"'"',000 S"!Omtlow, C'S. 01 1''"'eS ot somei1n9 for PC'!':!! s,ze c~o:ysls Fig. l! Washload \'S, ';ln:amflO'!"-Powdc:r Ri\cr. [A{ur COLD\' (/963).J Let us consider a cross section in a given stream (fig. 2). on the assumption that suspend particles are carried along by the stream with the same average velocity then for a small area where both concentration and ve­ locity may be considered as constant the sediment discharge may be expressed as the product of the water discharge and the concentration. Since the water discharge is the product of the area times the velocity it follows that the sediment discharge in weight through the small area surrounding a point M can be expressed in the forro of : ,,__~'__:l>~X~)!-.- --l~aJJ A Figure 2 3 = cu 6. x ~y (1) and by integrating through the who1e cross-section A we obtain = SSA dudxdy (2) Let us make a partition of the cross section A with the vertica1s· so that to express Qs in the forro of a twofo1d iterated integra1, namely . SD(X) = cudy (3) a where D (x) cudy is the sediment discharge in weight per unit width for Sa the vertical x which is denoted q. s 50 if for a sufficient number of points in the cross section both ve10­ cities and concentration are know the sediment discharge can be calculated by means of graphica1 integration. Actually measuring concentrations and velo­ cities at the saine time is both awkward and time consuming. Moreover in most cases a stage-discharge rating curve is available and the point is to get the mean concentration for several verticals without having to measure the velocities. Remarks The suspended sediment cannot he measured right to the bed since the sampler nozzlecannot reach the lower 10 to 15 cm of a verti­ D cal so the low'er limit in the integral S cudy is a and not a zero. 50 there is an unmeasured part (see fig. 3) fram the bottom to a height, a , which may or may not be a sizeab1e part of the total 10ad. 4 ••••••••.. :.. e ••••••t. .. t •••••••.... ..:. .. .. .:- ::: ~:>~:::::I::·:·:.:· •• _ tt .. :: .. .. .. " .. .. .. ... .. .. tt : ........ t ' ....." .. t. , ... tt: .. t, .: :: ,', ~.. : .. " .....................:.:::.. ;.. ;: .. ,~ :~)." : ',' " :••...:...•..... :. --:". : : : .. .. ;, , : : ~ .. .... : ~:: ::: .': ; ',':t.":.':.:: .c: Flow .. '" . .. .." .. .." " .. S.mplod or '~ .. .. .. .. :" •• 01,.: :-..:'.. t .. mer:sured d~pth : D- Cl ::' ; •• ::: .. :.'.0 ~:.: : Sampled z...",· t ••••• I ••• ~ •• 0 :. .. .. .. .. " .. " ••r _ .. '" • 1. .. .. :.. .. .. 1 .... t ..."• \. e.: e. .... ... .. .. t. ... .. ...... I.~ : •••••••·:.·.·· •• • •• t '• ToUl' depth ,1) .... .:.".. .. 1-' .." :.. 'tt· .- ~.. ...t ' , tt :.. .. .. e,: .. ;:..:. .It',' : .... '.'t' :•••••; ~ .. ~ tt tt ',' : .:.:." .. : ~ .. :.. ...........:.~ :: ~•. ::.:= , -. .... - CL ---- -- "1-~. -~:.·?j{;.!·;:~;~f~.!~;~t:~~~~.~:~*~>;'\\\\\~.. -- Unumpl~d ~ ~- -l :01\" 0.3 leetID;!"., ....' -..•• :.'.. •• .;;(./.. 'J' .,. L .....,.':",. J, "'ttr\ "~·~:·t;.: .. ,~~···.,; &~ U:.:.,·:: •. ,·; .\. ~ " ~ ~'" ~ ~ i:f;'~ ~lÎ'illl~I;;l;ii;;;;iiJJ:;;;J:I!!!,I.ili,l!i:.·;~lilil'111!:IIIII·i\'111!1'!'~'i;lrii!1.•,... ... 4 .. 1.. .•.•. •.•-' • ••. po J lb-_ ... .Fig.3 ·-Measured and unmorasu.ed sa'1loling zones in 0 stream sampling vertical with respect to velocity of Aow and seaiment concentration. J. K. Cuibertso" (Written commun., May 1968). • rr. llERTlt:1>.L DISTP.IBUTION OF SUSPENDEn SEDIMENT General Considerations In most rivers the fine fractions, namely, ~~e silt and clay are usually uniformly distributed over the dept." of a stream or nearly so where­ as the coarser fractions of the suspénded sediment, which are usually sand, exhibit the greatest variation in concentration fram the stream bed to the 'Water surface. The foregoing can be explained by considering the Rouse equation, namely, c (D-v) a z· (4) C = [ y ( D-- a ) 1 a V V settling velocity of s s z = a given particle (5) [4ku* f andk coefficients u. ~ : friction velocity and which gives the concentration, c , at any level, y , in terms of C ë\ which in the concentration at an arbitrary level, a. Figure 4 shows a graph of the Rouse equation for several values of . the exponant z. It is seen that fcr low values of z the concentration tends toward becoming uniform over the depth and that for large z "ralues 'the concentration is small near the surface and relatively high neu the bcttom. L~O~,\--r-........~,,~-==-,,"""""---""":'~~\"""""'--' 0.9 \ \ '(}."'- 0.81--\'\--+--11\'-\--+--+--\t--+-~~ ", 1\ .\ \ .\ 1 0.7 H--+-+-+---+---'l:t--t-~--+--ti----t:t---t \ r\ ~\ 0.6 ~-I-_-\+_..J...._+\-_.J--+--+-+---A--I---.,j ~~ 0.5' ' -\~~~ \\ ~ 0.4 \\ ~\ \\ \ ~~ \ ::t-\ ~r'.Ir--;,o; l~~'\-+--+-~r\.--+-T-+-\+\t-+--t .~ '" ,,~ "'~ 1\ ~\ t=~tt -,---,~ '~~ .........,- 1 ...-i--!_.c' o 0.1 il.2 0.3 0.4 0.5' 0.6 0.7 0.8 0.9 1.0 Relal'" ~ncentralion.f FIG. 4 "-Graph of Rouse Suspended Load Distribution ECjuatlon for ald - 0.05 In figure 5 it is Deen that the data weIl fit the fOl.ïn of the theo­ retical curves for a large range of z values in different rivers of the U.S.A. ....'l'1 1 ~. 0.7 3 Je 0.6 ~ i .!!.. , '1 ~ , .1 ..;; o.~ fi E 0 ~ 0.4 e' ..~ ~ '6 '3 ~ ~ .: 0.1 0.00\ 0.005 0.0\ 0.05 C Rei'!lVe ctlnce"tratl0r'? E' FIG. S.-Vertical Cistribation of Relative Concf:ntratlon C IC.
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