RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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Reservoir Sedimentation Assessment GuidelinewhereQ b = sediment discharge, in t/dayq b = sediment discharge at a given point, in kg/(s.m)l = distance between the measured points, in mE r = equipment sampling efficiency.For that kind of measurement, the formula must take into consideration theequipment trap efficiency, the value of which is determined in laboratory.For indirect measurement, the sediment discharge computation is throughformulas. Stevens & Yang (1989) have studied the several formulas available andselected 13 as the most recommendable (Table 7.8). Besides that, they have preparedcomputer programs that are available in the above-mentioned publication.Table 7.8 – Summary of the main formulas for calculating sediment discharge and bedmaterial, as presented by Stevens & Yang (1989)Formula authorYearEntrainmentdischarge (B) orbed materialdischarge (BM)Kind offormula(1)Kind ofsediment(2)GranulometryAckers & White (*) 1973 BM D S S, GColby 1964 BM D S SEinstein (bed load) 1950 B P M S, GEinstein (bed material) 1950 BM P M SEngelund & Hansen (*) 1967 BM D S SKalinske 1947 B D M SLaursen 1958 BM D M SMeyer-Peter & Muller (*) 1948 B D S S, GRottner 1959 B D S SSchoklitsch (*) 1934 B D M S, GToffaleti 1968 BM D M SYang (sand) (*) 1973 BM D O SYang (gravel) (*) 1984 BM D O G(1) Deterministic (D) or Probabilistic (P)(2) Granulometric sand fraction (S), composition or mixture (M) or optional (O)(3) Sand (S) or gravel (G)(*) Regarded as the most reliable by Stevens & YangTotal sediment discharge computation – The approximate total sediment dischargemay be obtained by summing up the suspended discharge and the bed materialdischarge. Nevertheless, that procedure is questionable due to the inaccuracy reportedby non-sampled zones.The total sediment discharge may be obtained through computation processes ofEinstein’s modified method and by Colby’s simplified method. Otto Pfafstetterconverted the first method into the metric system, where the abacus depends on units,adjusted by Carvalho (1994). Stevens (1979) prepared a computer program for usingANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department30
Reservoir Sedimentation Assessment Guidelinethat method. Carvalho (1981) has also converted the second method into the metricsystem.If there are some measurements using Einstein’s modified method – which isvery hard to do – such values may be used for correcting Colby’s simplified method orfor obtaining correlations for correcting the total discharge (Yuqian, 1989).Establishing the sediment discharge value– Considering that the only data availableare for suspended sediments only, the calculator tries to establish the value of the nonmeasureddischarge, in order to have the total discharge required by sedimentationassessment. In Brazil, it is usual to determine such value as 10%, while there arecountries where utilities establish up to 30% of the suspended discharge. ICOLD (1989)presents a suggestion for selecting the method for obtaining sediment discharge, inrelation to bed material and sand percentages existing in the suspended sampling (Table7.9). The table shows the complexity of establishing just the %.Table 7.9 – Guide for correcting the sediment discharge and for orienting the methodfor obtaining such discharge (ICOLD, 1989)ConditionConcentration ofsediment load(mg/l)% of bed load inrelation to thesuspended loadBed material Granulometry ofbed material1 (1) < 1000 Sand 20 to 50% of sand 25 to 1502 (1) 1000 to 7500 Sand 20 to 50% of sand 10 to 353 > 7500 Sand 20 to 50% of sand 54 (2) Any concentration Compacted argyle,gravel, rolledstones or stonesAny amount up to25% of sand 5 to 155 Any concentration Argyle and silt No sand < 2(1) Special sampling for computations through Einstein’s modified method are required for thiscondition(2) A program for direct measurement using a Helley-Smith sampler, or other measurer, or even using theformulas for thick material7.5 Data processingData processing is intended to obtain average discharge and average runoff loadeither annual or for a period, as well as to obtain representative parameters for thephenomenon.The first step is an adequate revision of both field and laboratory documentationand, following, the tabulation of measurements performed. The table shall have thefollowing items: number of measurement, date, values for average level, section width,area, average depth, average speed, net discharge, concentration of dissolved solidmaterial, sediments concentration, suspended sediment discharge, entrainment bed loador bed material discharge, total sediment discharge and method for obtaining such data.Granulometric curves shall be always available for further use. One can also make aANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department31
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