RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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Reservoir Sedimentation Assessment Guidelinemethod, thought based on Stokes law, has its restriction for reaching the desiredaccuracy when surveying the percentage of sediments for a given granulometry in liquidmeans. Table 7.7 presents the major restrictions to be obeyed.Table 7.7 – Amplitude of several granulometric analysis methods forfine material using water-dropping velocity(SUBCOMMITTEE ON <strong>SEDIMENTATION</strong>, 1943)MethodApproximate limit of theparticle diameterApproximate limit inconcentration(mm)(ppm)Settling tube 0,001 to 1,0 300 to 10.000Decantation 0,001 to 0,0625 1.250 to 19.000Pippeting 0,001 to 0,0625 3.000 to 10.000Hydrometer (densimeter) 0,001 to 0,0625 60.000 to 116.000Siltmeter (TAV, visual accumulation tube) 0,0625 to 2,0 125 to 25.000Bed load analysis is performed mainly through siftering, using the Tyler seriesof sifters. For small quantities of sandy material, the TAV method can be used. If theremainder for the last sifter – the finer material – is equivalent to 5% of the material, orhigher, it is necessary to complement the analysis by defining the curve lower segment.One of the methods presented in Table 7.7 should be used for that. The analysisprocedures may be consulted in Normas e Recomendações Hidrológicas – Anexo III,Sedimentometria (DNAEE, 1970).7.4.3 Sediment discharge computationOnce all field and laboratory data are available, sediment dischargecomputations may be performed. The required data are obtained from net dischargemeasurement and sediments sampling, sediments concentration, granulometricdistribution and others. For calculating the bed discharge by using formulas, one mustobtain some additional values, such as water temperature, energy line slope, shearingtension, kinematic viscosity, particle-dropping velocity; usually, those last ones areincluded in the computation programs available.The maximum error expected for sediment discharge determinations is 10%,even including the bed discharge collection, which is very inaccurate. Suspendeddischarge is usually the prevailing part of the total discharge, representing more than90% for most measurements. However, the bed discharge may report values from 10 to150% in relation to the suspended discharge, according to ICOLD (1989). On the otherhand, the sedimentometric data consistency analysis is very hard, due to the severalprocesses required for determining it, mainly, the phenomenon complexity. Therefore, itis essential to try to eliminate errors during the measurement and for the laboratorywork. Consequently, the sediment discharge measurement shall be performed asaccurately as possible in the field, by a successful hydrometologist, using the suitableANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department28
Reservoir Sedimentation Assessment Guidelineequipment, and the analysis should be performed by an experienced chemistryexpert/technician. That shall allow repeating the computations, if required. If field andlaboratory services report errors, the adjustment of the sediment discharge valuebecomes impossible.Suspended sediment discharge computation – In both direct and indirectmeasurement for suspended discharge, the concentration value is obtained. Thecomputation is performed by multiplying the net discharge by the concentration.Usually, the Q ss value is presented in t/day, and it requires a unity transformation factor.For the average concentration obtained through ILL and IID sampling methods:where,Q ss = 0,0864.Q.c s (7.4)Q ss = suspended sediment discharge, in t/dayQ = net discharge, in m 3 /sc s = concentration, in mg/lIf c s is a high value, presented in kg/m 3 , the equation is:Q ss = 86,4.Q.c s (7.5)If the samplings are for several verticals separately analyzed, the followingequation is used, with the due constant on unity transformation:Q ss = Σ q ss = Σq.Δl.c sv (7.6)whereq ss = suspended discharge for width unity corresponding to the segment beingconsideredq = partial net discharge for width unity corresponding to the segment beingconsideredΔl = distance referenced to q ss and qc sv = sediment concentration at vertical.The average concentration on the vertical is equivalent to:∑ qssQsscs = =(7.7)∑ q QComputation of sediment discharge and bed load – The direct measurementdetermines the dry sediment and the bed discharge is calculated as:(7.8)ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department29
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RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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