RESERVOIR SEDIMENTATION ASSESSMENT GUIDELINE - Aneel

Reservoir Sedimentation Assessment Guideline9.2 MeasuredThere are two processes for measuring the specific weight, namely direct andindirect. For the indirect process, or in situ, it is used the nuclear measurer, densityradioactive-type. For the indirect process, it is used to collect a non-deformed sampling,by using equipment like gravity or piston-core; to measure the sampling volume, take itto the stove and determine the dry weight. Such measurements shall be performed alongdifferent positions in the reservoir, in order to verify the variation of specific weight andobtain the average value.9.3 EstimateAccording to the equations, one may assess the variation of initial specificweight as follows:- If sediment is exclusively argyle, then γ i shall range from 0,42 to 0,96;- If sediment is exclusively silt, then γ i shall range from 1,12 to 1,17;- If sediment is exclusively sand, then γ i shall be equivalent to 1,55;- If there is a composition reporting similar portions of argyle, silt and sand, there is avariation from 1,02 to 1,22.In small reservoirs, sand is the main material settled; therefore, the initialspecific weight is established as ranging from 1,4 to 1,5 t/m 3 ; medium-size reservoirsmay report a composition with specific weight ranging from 1,2 to 1,4 t/m 3 , while forlarge reservoirs, where just a few quantity of fine sediments passes through the ductsand spillways, that amount may range from 1,1 to 1,3 t/m 3 . Obviously, knowing thebasin and quality of existing sediments, as well, may allow for the technician to performbetter assessments.For a more accurate assessment on apparent weight, one may use those valuespresented by Zhide (1998), Tables 9.4 and 9.5.Table 9.4 – Average initial specific weight of deposits in reservoirs, in t/m 3 (Zhide,1998)Kind of reservoir operationArgyle( < 0,004mm )Silt(0,004-0,062mm)Sand(0,062-2, 0mm)Sediment always, or almost always submerged 0,416 1,120 1,550Little to medium depletion of the reservoir 0,561 1,140 1,550Reservoir reporting significant level variations 0,641 1,150 1,550Reservoir usually empty 0,961 1,170 1,550ANEEL – Brazilian Electricity Regulatory Agency / SIH – Hydrologic Studies and Information Department45