RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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Reservoir Sedimentation Assessment Guidelinethe basin)Control of erosion atwater courses andreservoir banksControl of sedimentaffluence in the flumeControl of sedimentsdeposition12.2 Corrective practicesSediment control on roads, cities,several civil works, control of bothurban and rural erosion;Erosion on the channelFilling in gulliesDams at upstream (may besubmerged, if desired)VegetationBy-pass derivation channelsDeviation of floods for inundationareasDischarger with gate (plannedoperation)Reservoir depletionReservoir planned operationMechanical practices:- Rational distribution of ways- Counter crops- Leveling- Groove and raised bed- Drainage channels- Refrain or protection of taluds- Drainage works- Control of erosion in ravines andgullies- Protection with infusorians vegetation- Structural protection (break-waterconstruction, etc.)Periodically remove the sedimenttrapped- Channel- DuctDecantation basins- Density streams- Bottom sedimentdead storageThere are computer software aimed atsediment accommodationSedimentation corrective practices are performed during the operation stage.Usually, deposits surprise the operator, since they are submerged and increase slowly. Ifthere is no monitoring the surprise happens. It is tried to recover the lost volume withmitigating measures, which are expensive and repetitive. Table 12.2 presents a summaryon corrective practices measures.Table 12.2 – Corrective measures for controlling sediment andreservoir sedimentationRemoval of sediment from thereservoirDam raisingCorrective measuresDredging (the deposition site isimportant)By-pass derivation worksSiphon FilteringBottom dischargerPerform an adequatedimensioningEventualAlmost permanentChannelDuctAt the dam or sometimesremoving farer sediments throughthe bottom dischargerSometimes is must be built afterthe dam is readyWhenever possible, because itwill increase the level and waterbodyANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department72
Reservoir Sedimentation Assessment Guideline12.2.1 Discharge of dredged sedimentsThe removal of a reservoir’s sediments by dredging it is expensive. Sometimes,it is cheaper to raise the dam or another solution. Therefore, costs must be reviewed inrelation to the convenience of dredging. Usually, this solution is applied to smallreservoirs to relieve problems caused by deposits in given sites, as for example, at theintake basin.One of the major problems involving dredging is the material settlement.Usually, the dredged material is not economically used because of several costs andother factors, such as sediment pollution or issues concerning the material transportationfor reservoir sources. One could suppose that the coarse settled in the delta area couldbe used for construction, and the fine material closer to the dam, containing nutrientscould be put into agricultural areas. However, in small reservoirs, that natural selectionis not so good and the deposits may have too many impurities, such as waste and others.There is experience enough and suitable solutions for each kind of problemreferring to dredging – width, depth, consolidated sediment, presence of materials suchas stones, gravel, tree trunks and rigorous limitation of disposal – for all cases, (ICOLD,1989).There are several kinds of equipment for removing deposits, which are basicallythe airlift system, the mechanical system (drag-line or clam-shell) and suction anddeposition dredge, using centrifuges pumps to perform the material hydraulicstransportation (Engevix, 1980). Therefore, one must look for the suitable equipment foreach case, allowing greater savings.The disposal of dredged material is a topic involving economic andenvironmental issues. The simple disposal at the reservoir bank, at the area closer to thedredging site, or is thrown at the dam downstream channel, may be an inadequatesolution. For the first case, most of the sediments may come back in short time, duringthe first rains, for the reservoir. For the second case, there will be several problems atdownstream, including channel sedimentation.Many countries have laws regulating water quality, prohibiting the disposal ofdredged material in the stream. Countries as China and Formosa, where sites for dambuilding are scarce, have improved agricultural fields by putting selected materialderiving from dredging, simultaneously recovering the reservoirs’ water storagecapacity. The material may also be used for building marginal dikes to the rivers wherethere is the need for protection against floods (ICOLD, 1989)13. SECONDARY EFFECTS DUE TO SEDIMENTBesides the physical effects deriving from reservoir sedimentation over itsobjective functions, there are several additional secondary impacts that must be taken73ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department
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RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

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