RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

Reservoir Sedimentation Assessment GuidelineThe natural formation of river beaches provides leisure for riparian population.The effects of the reservoir are felt on those sand banks, both when the lake floods suchareas, making them disappear, and through effects at downstream. Once the reservoir isformed and the sediment is being settled therein, there is no sand feeding atdownstream, thus causing the disappearance of sand banks on that reach. The beacheswill appear just very downstream when the erosion on the downstream incrementalbasin – and the associate transportation of sediment along the stream – allows for theoccurrence of new sand banks, known as sand bars.The effects of the lack of sand provision at downstream are felt up to the outfallof those rivers, and the changes may appear in long-term. This phenomenon may be thereason for the changes occurring at the outfall of Paraíba do Sul and São FranciscoRivers.13.4 Erosion on reservoirs banksThe reservoir banks must be always protected with infusorians vegetation, or byusing conservative practices. Nevertheless, erosions may occur on its banks, be it due towaves impact or due to the high level of drenching during rainy periods, causing the fallof taluds. When that happens nearby the dam, it is necessary an immediate protection.Those sediments will become incorporated to sedimentation, while erosiondevelopment may bring several consequences.13.5 Deposit erosionThe settled sediment may undergo accommodation, slipping into the reservoirbed. When it is settled in the dead storage, it is regarded as benefic. There arecomputational models for reservoir operation using quicker depletion, thus facilitatingthe accommodation process, and enlarging part of the volume occupied by the sediment.However, when sedimentation is closer to the dam, the sliding of deposit maypose risk for its structure or suddenly reach the intake.13.6 Downstream effectsThe sediment trapping in the reservoir causes a runoff of clean water downstream.On the other hand, the regulation of downstream discharges causes major actions overbeds and banks of that channel.Jointly, those two effects – besides others – may cause the deepening of damdownstream channel’s bed and erosion of its banks. For small reservoirs, these areminor effects and may occur at the closest channel, while for large reservoirs thoseeffects are greater and may be felt even hundred kilometers downstream the barrier.ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department76