RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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Reservoir Sedimentation Assessment GuidelineThe rate of sediment transportation increase for the period is computed as:E c=r2− r1r1= 0,82It means that there was an increase of 82%on the sediment transportationbetween 1979 and 1982 - a very high amount for the short period being surveyed, whatmay come to jeopardize the reservoir due to very quick sedimentation.The annual rate computation, considering the small sampling for 4 years, isperformed by using the following equation:(1+ R i)4 =1,82thus resulting in the value of 16.15% for the annual increase on sediment transportation.For the percentage sedimented along 10 years or in a given period, t, thecomputation is as follows:10(1 + 0,1615) −1= 3,47 = 347% and ( 1+ R )t −1P11. MEASUREMENT OF A <strong>RESERVOIR</strong> <strong>SEDIMENTATION</strong>i=All reservoirs will become sedimented sooner or later. The main issue is toensure that there will be no problem that comes to hinder the reservoir operation withinits economical useful life. On the other hand, one should try to minimize the secondaryeffects resulting from sediment.Therefore, the sedimentation forecast is performed during the planning stage,and deposit formations, as well as sedimentation effects, are monitored during theoperation stage, no matter the reservoir size.Surely, this kind of study always brings about experience and new knowledge inthe field of Sedimentology. Therefore, the sedimentometric monitoring of gagingstations along the stream, the verification of erosion problems at the lake bed anddownstream channel, as well as the topo-bathymetric survey for reservoirs bringsubsidies for both undertakers and Science.11.1 Purpose of the surveyThe survey includes both terrestrial and underwater parts, as may be relevant tothe studies. The comparison between two surveys performed at different times is made,even if using aerophotogrametric interpretation map for the planning stage. TopographicANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department52
Reservoir Sedimentation Assessment Guidelinereferences must be the same. The surveys must report the same accuracy level in orderto grant comparable results.The determination of the new capacity and sedimentation level is the mainpurposes of the topo-bathymetric survey. Summarily, the following survey outputs maybe mentioned:• Determination of either the reservoir volume or capacity under current conditions(by the time of the survey), as the remaining capacity;• Determination of the new water body area;• Drawing new level x area and level x volume curves;• Drawing of the new bed geometry for the reservoir;• Drawing the new pivot point curve;• Verification of physical characteristics of accumulated sediments;• Quantification of sedimentation volume for the period, by comparing with previoussurveys or with the map at the reservoir formation time;• Determination of the reservoir trap capacity;• Determination of average tributary sediment discharge;• Verification of percentage of sediment settled in reservoirs, dead storage and thevolume lost in useful volume area.11.2 Frequency of surveysThe frequency of surveys in reservoirs depends on several factors; the main onesare: the reservoir’s total capacity and the likely amount of sediment deposit due to theriver bed sediment. The small reservoirs, as well as those whose tributary bed sedimentis high, shall be more frequently surveyed. On the other hand, the reservoirs withreduced tributary bed sediment shall have their survey frequency reduced. That is thecase, for example, when the drainage area is reduced due to the construction of a dam atupstream (Vanoni, 1977), or even when the tributary basin had its sediment dischargevalue reduced due to protective measures.The financial cost is a factor that greatly influences the reservoirs surveyfrequency. Usually, resources for such works are limited, mainly because the sedimentis submerged, where managers cannot reach.. Considering that the survey cost isjustified by an update on the reservoir capacity verification and its sedimentationvolume, the criterion indicated in Table 11.1 can be considered. It is evident that thesurveys shall be performed more frequently in reservoirs where high indexes ofsediments deposit are reported.ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department53
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RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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