RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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Reservoir Sedimentation Assessment Guidelineinto consideration, and that may expand beyond the reservoir limits and the actuation ofthe responsible company. Such secondary impacts shall be previewed, assessed andconciliated, both during planning, project and construction stage and during thereservoir operation stage (ICOLD,1989).13.1 Effects on the reservoir backwaterThe bed sedimentation in the reservoir entry with delta formation causesdeformations on the river channel which, along time, becomes strangled. The depositsadvance downstream and a little upstream, the channel gradient becomes reduced, whilethe underground water table remains in high level, thus hindering the dredging. Uponthe channel narrowing, as the delta increases, the effects on the reservoir backwater alsoincrease, thus increasing the frequency of floods upstream (ICOLD, 1989).The effects may be analyzed by surveying the formation and expansion of thedelta; however, the study is complex due to reservoir operation, quantity of sedimentaffluence and other factors. The use of HEC-6 model for computation backwater, takinginto consideration the sediment affluence, may display water line profiles for floodsreporting different recurrence times.Delta formation is represented in 13.1, where the top layer, the sliding point, thefront layer and the overbanks are displayed.Figure 13.1 – Typical delta formation: (1) top layer slope,(2) coarse, (3) thalweg original slope, (4) sliding/pivot point,(5) front layer slope,(6) overbanks slope, (7) normal/averageoperation water level, (8) maximum water level, (9) intake,(10) fine sediments (ICOLD, 1989)ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department74
Reservoir Sedimentation Assessment GuidelineFor preliminary evaluations, the delta formation starting point is considered asbeing on the intersection of bed line with the maximum level of the reservoir; and thesliding point (pivot) is on the intersection with average operation level. In this case, thevalue 1.5 of that bed is used for the top layer slope, and for the front layer slope, a valueequivalent to 6,5 times that top layer slope is used. Once having that volume, one maycalculate the formation time for that condition. When the sliding point reaches the dam,the top slope disappears (ICOLD, 1989, and Strand/Pemberton, 1982).13.2 Changes on water qualityThe impacts of sediments on the reservoir and on the quality of downstreamwater have not yet been fully explained or surveyed. Eutrofization is the term applied todescribe the effects and changes on waters confined by the increase on nutrient level,reduction of dissolved oxygen and increase on biologic productivity.The torrents deriving from precipitation carry many types of sediment forstreams and, together with such sediments, nutrients, agro-toxic and whatever thosewaters may carry. Once in the reservoir, those substances undergo several changes andmay, inclusively, affect the downstream water quality. Proliferation of algae and othereffects are consequence of such transformations.13.3 Ecological effectsBoth fauna and flora suffer ecological effects. The deposits in reservoirs modifythe bed quality, thus affecting the life of fishes by changing their natural habitat.Species disappears and only the strong ones survive.The sediment load in water also reduces the penetration of sunlight, thushindering the required transformations for life existing there. On the other hand, the fullremoval of sediments with nutrient, through disposal on the bed, also causes changes. Inanyway, Nature suffers, loosing some species that cannot overcome the changes.Concerning flora, the formation of macrophytes at the reservoir banks, due to thedisposal of fine sediments with nutrients, may be highlighted. Vegetation quicklyspreads and is wrenched by the water level increase and, following, is carried towardsthe dam and intakes.Some vegetal species, upon the fluviometric level raising, may be quicklydisplaced to the lake bottom, thus raising the quantity of flooded terrestrial biomass.Later on, that biomass decomposes through aerobic and anaerobic processes, startingthe process of gas emission to atmosphere, mainly the CH 4 (Methane) that maycontribute to worsen the thermal heating of low terrestrial atmosphere – GreenhouseEffect (UNEP, 1997).75ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department
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RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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