RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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Reservoir Sedimentation Assessment Guideline11.10 Bed scanning and geophysicsThis work is performed by using special equipment. An echobathymeterworking with high-frequency ultrasound allows for emissions that cross the settledlayers, returning to the equipment and recording the changes on thickness (Figure11.15).Figure 11.15 – Seismic recorder, displaying the reservoir bed bottom line and lowerlayers.– UHE Funil/FURNAS, on 02.02.1993 (Conage)Another very useful equipment is the lateral scanning using audiography, whichshows the bed conformation. The equipment is submerged and a sonorous signal is sent,at regular time intervals, through two transductors located on coated water vehicles,named towfish, which carries the side scan sonar. The emission beams are addressed tothe bottom surface sides. Each transductor acts independently, being responsible alsofor receiving the reflected signal. The signals from the bottom surface are recorded asthey are received, on electro-sensitive sheet, therefore making up an image of thebottom of the area being surveyed, named sonogram (Geomap, 1991).12. CONTROL OF A <strong>RESERVOIR</strong> <strong>SEDIMENTATION</strong>Forecast studies and the entire process of sedimentometric measurements aim atverifying the likely reservoir sedimentation and the need for sediment controlling, inorder to mitigate its effects. The sediment control presents several implications forseveral Engineering fields, as a way of protecting works and patrimony involved.Several measurements are complex, since sediment is derived from erosion along thewhole drainage area at the dam site, being hardly accessed by the entity responsible forthe reservoir. Most of the time, only a governmental planning may establish and executea program on erosion control along the whole hydrograph basin.ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department70
Reservoir Sedimentation Assessment GuidelineMany programs on sediment control by the reservoir owners are restricted to itsaction area, where they try to protect river and reservoir banks, in order to diminish theentry of sediments into the system. Programs on reservoir sedimentation prevention arethe most important, and corrective measures are adopted just when there is no otheralternative.12.1 Preventive ControlAccording to CIGB (ICOLD, 1989) the most obvious preventive measure forcontrolling sediments is, most times, disregarded by designers. It regards to the regionsat the rivers headwaters - the upper basin – that have great runoff contribution but smallparcel of bed sediment. It is extremely important to have the forests preserved at thoseregions, so that they do not become responsible for great production of sediments.Summarily, preventive measures may be listed as shown in Table 12.1,following the proper selection of work and reservoir sites, basin erosion control,sediments trap before entering into the fluvial system, and the automatic removal ofsediments. They are used for all stages – inventory, feasibility, project and operation.Table 12.1 – Preventive measures for sediments control andreservoir sedimentationPreventive measuresSelection of reservoir site If there is more than one siteavailable for the dam and reservoirformation, select the onepresenting the lowest allocation ofsedimentsSuitable dead storage forecastForecast on volume set apart forFor the reservoir projectControl of erosion in thebasin(brings several benefits,where the most efficient ishard to be applied by thedam worker; it is necessaryto ask for support fromother entities to managesedimentForecast of sediment dischargerwith gates (for density streams andbed sediment)Soil conservation and managementin agriculture (Bertoni andLombardi Neto, 1990)The site selection depends on financialcosts including protection for the mostdeprived areaIncrease the dam heightIncrease the dam heightPlants far from the dam needs a desanderafter the intakeVegetative practices:- Forestation and reforestation- Grazing grass- Coverage plants- Strip crops- Vegetation beltsPedology practices:- Fire control- Green fertilization- Chemical fertilization- Organic fertilizationANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department71
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RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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