Chapter Nine.ppt

Study

1. CHAPTER NINE
RESERVOIRCAPACITYDETERMINATION
By Ayansa T.( B.Sc. In Met. And Hydr., M.Sc. In WRE)

2. Introduction
•Thereservoircapacityisatermusedto
representthereservoirstoragecapacity.
•Itsdeterminationisperformedusinghistorical
inflowrecordsinthestreamattheproposeddam
site.
•Thereareseveralmethodstodeterminea
reservoirstoragecapacity.

3. Methodstodetermineareservoirstoragecapacity
Mass curve (ripple's) method:
•Amasscurve(ormassinflowcurve)isaplotof
accumulatedflowinastreamagainsttime.
•Amasscurvecontinuouslyrisesasitshows
accumulatedflows.
•Theslopeofthecurveatanypointindicatestherate
offlowatthatparticulartime.

4. •A demand curve on the other hand is a plot between
accumulated demand and time.
•If the demand is at a constant rate then the demand
curve is a straight line having its slope equal to the
demand rate.
•However, if the demand is not constant then the
demand will be curved indicating a variable rate of
demand.

5. •Thereservoircapacityrequiredforaspecifiedyieldordemand
maybedeterminedbyusingmasscurveanddemandcurve
usingthefollowingsteps.
1.Amasscurveispreparedfromtheflowhydrographfora
numberofconsecutiveyearsselectedfromtheavailablestream
flowrecordsuchthatitincludesthemostcriticalorthedriest
period.
2.Correspondingtothegivenrateofdemand,ademandcurveis
prepared.

6. 3.Linesaredrawnparalleltothedemandcurveandtangentialtothe
highpointsG,Fetc,ofthemasscurve
4.ThemaximumverticalinterceptsX
1Y
1,X
2Y
2,etc.betweenthe
tangentiallinesdrawninstep3andthemasscurvesare
measured.
5.ThelargestofthemaximumverticalinterceptsX
1Y
1,X
2Y
2etc.,
determinedinstep4representsthereservoircapacityrequiredto
satisfythegivendemand.
•However,therequirementofstoragesoobtainedwouldbethenet
storagethatmustbeavailableforutilizationanditmustbe
increasedbytheamountofwaterlostbyevaporationand
seepage.

7. Figure: Mass curve and demand curve

8. •Example:Thefollowingtablegivesthemeanmonthlyflows
inariverduringcertainyear.Calculatetheminimumstorage
requiredformaintainingademandrateof40m
3
/s:(a)using
graphicalsolution(b)usingtabularsolution.

10. Mass Curve

12. Example:Determinethereservoircapacityrequiredifa
hydropowerplantisdesignedtooperateatanaverageflow.
Solution:Theaverageflowis340.93m3/s.
i)Firstoption:Storageissameasthehatchedareaunderflow
durationcurve.

16. Reservoirs and sediments
•Inthedesignofdam,itisimportanttoassessthemagnitudeof
sedimentdepositioninthereservoir.
•Theproblemcanbedividedintwoparts:
1.Howmuchsedimentsenterthereservoir
2.Whatisthetrapefficiencyofthereservoir
•Inadetailedstudy,thesedimentsizedistributionsalsohaveto
bedeterminedforquestion1.
•Question2mayalsoinvolvedeterminingthelocationofthe
depositsandtheconcentrationandgrainsizedistributionofthe
sedimentsenteringthewaterintakes.

17. •In general, there are two approaches to the sedimentation
problem:
1.The reservoir is constructed so large that it will take a very long
time to fill. The economical value of the project will thereby be
maintained.
2.The reservoir is designed relatively small and the dam gates are
constructed relatively large, so that it is possible to remove the
sediments regularly by flushing. The gates are opened, lowering
the water level in the reservoir, which increases the water
velocity. The sediment transport capacity is increased, causing
erosion of the deposits.

18. •A medium sized reservoir will be the least beneficial.
•Then it will take relatively short time to fill the reservoir, and
the size is so large that only a small part of the sediments are
removed by flushing.
•The flushing has to be done while the water discharge in to the
reservoir is relatively high.
•The water will erode the deposits to a cross-stream magnitude
similar to the normal width of the river.
•A long and narrow reservoir will therefore be more effectively
flushed than a short and wide geometry.

19. •Forthelater,thesedimentdepositsmayremainonthesides.
•Theflushingofareservoirmaybeinvestigatedbyphysical
modelstudies.
•Anotherquestionisthelocationofsedimentdeposits.
•Figurebelowshowsalongitudinalprofileofthereservoir.
•Thereisadeadstoragebelowthelowestlevelthewatercanbe
withdrawn.
•Thisstoragemaybefilledwithsedimentswithoutaffectingthe
operationofthereservoir.

20. Figure:Longitudinalprofileofareservoir.HRWisthehighestregulated
waterlevel.ThereservoirvolumebelowLRWiscalledthedeadstorage,as
thiscanbeused.

21. Sediment Load Prediction
•Roughestimatesofsedimentloadmaybetakenfromregional
data.
•Oftenthesedimentyieldintheareaisknownfromneighboring
catchments.
•Itisthenpossibletoassesstheseriousnessoftheerosioninthe
presentcatchmentandestimateroughfiguresofsedimentyield.
•Thelanduse,slopeandsizeofthecatchmentareimportant
factors.

23. Q
sisthesedimentload,Q
wisthewaterdischargeandaandb
areconstants,obtainedbycurvefitting
•Thetotalsedimenttransportmaybeestimatedbyadding10-22%
tothesuspendedsedimenttransporttoallowforthebedload
contribution.

24. Reduction in reservoir capacity
•Theusefullifeofareservoirgetsreducedduetosediment
depositioncausingadecreaseinitsstoragecapacity.
•Thepercentoftheinflowingsedimentwhichisretainedina
reservoiriscalledthetrapefficiencyanditisafunctionofthe
ratioofreservoircapacitytototalannualsedimentinflow
•Therateatwhichthecapacityofareservoirisreducedby
sedimentdepositiondependson
1.The rate of sediment inflow i.e. sediment load
2.The percentage of the sediment inflow trapped in the reservoir,
i.e. trap efficiency