RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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Reservoir Sedimentation Assessment Guidelinenew reservoir contour, the pivot point, as well as several additional pieces ofinformation (please refer to the item on measurement of reservoir sedimentation). Itwould be advisable to have small reservoirs surveyed at every two years; the mediumsizeones at every five years, and the large ones at every 10 years. It the new surveypresents small variation concerning sedimentation, so the survey interval may be longer,and the changes taking place in the basin due to land occupation and consequentincrease of erosion should be monitored.Comparative satellite-based studies for different periods of time allow forobtaining several pieces of information on changes occurring in the concerned reservoirarea.Data obtained from both operation of sedimentometric network and survey datamay allow for the assessment of the reservoir remaining useful life. For thoseassessments, the surveys used for forecast shall be repeated.5. FACTORS AFFECTING SEDIMENTS YIELDSediments reaching the reservoir come from the inflow drainage area and aretaken mainly through the major fluvial channels network.The production of sediment deriving from drainage area – or corresponding to awhole hydrograph basin – depends on erosion, rainwater runoff with the transportationof sediments, and characteristics of sediment transportation along streams as well.1989):The main factors affecting the sediments yield at the drainage area are (ICOLD,• Precipitation – quantity, intensity and frequency;• Kind of soil and geological formation;• Soil coverage (vegetation, apparent rocks and others);• Soil management (cultivation practices, grazing grass, forest exploitation,building activities and conservation measures);• Topography (geomorphology);• Nature of the drainage network– density, slope, shape, size and channelsconfiguration;• Surface runoff;• Sediments features (granulometric, mineralogical etc.);• Channels hydraulics.Additional factors may be added, as well as likely combinations among the nineabove-mentioned factors. For the assessment of sediments yield in a drainage areainflowing the dam position, it is necessary to have an expert assessment on the mostinfluencing factors. It shall, necessarily, lead to the measurement conclusions requiredANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department12
Reservoir Sedimentation Assessment Guidelineto accurately define the sediments amount, available techniques for foreseen suchsediment production or even to assess the quantity of sediments at basins where duemeasurements have not yet taken place.6. RESERVOIR SEDIMENTATION ASSESSMENTThe assessment on the sedimentation of the reservoir total volume and useful lifeis essential for surveys about the lake formation, as well the evaluation of the reservoiroperation. The end of its useful life - in sedimentological terms - is considered as whendeposits come to interfere on the regular operation of either the plant or of the reservoirpurpose. Additional evaluations shall be performed, according to the time taken by thesediment to reach the intake sill (useful life), sediments distribution along the reservoir -corresponding to a given period -, the pivot point development and delta building (upand frontal slope).For the preliminary sedimentation computation, the following mathematicalexpressions are used:SDxE365xQxEst rst r= =(6.1)γapγapVresT = (6.2)Swhere:S = volume of sediment trapped in the reservoir (m 3 /year);D st = annual average for total bed load inflowing the reservoir (t/year);E r = trap efficiency for the sediment inflowing the reservoir (decimal);γ ap = deposits specific weight (t/m 3 );Q st = total average sediment discharge inflowing the reservoir (t/day);T = sedimentation time for a given volume (years);V res = reservoir volume, total or dead storage (m³).For items 7, 8 and 9, equations 6.1 and 6.2 indicate how to determine theparameters required for evaluating the sedimentation.6.1 Reservoir dataThe main project data required for such forecasts are:• Maximum normal water level, in m;• Minimum normal water level, in m;• Intake sill height, in m;• Volume of maximum normal water level, in m 3 ;• Volume of minimum normal water level (dead storage), in m 3 ;• Volume of intake sill, in m 3 ;• Natural discharges series;ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department13
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RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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