class10 12thfeb.pptx

8/9/2019 Class10 12thFeb.pptx

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Water Resources

Engineering II

Water Storage


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What is a Reservoir? It is an area developed by water body due

to construction of dam.

Tarbela Dam ung!ua Dam

"Taiwan#


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Storage reservoir serve the followingpurpose $

Irrigation Domestic %se

Industrial

!ydroelectric power generation &lood control

'avigation

Recreation

Development of (sh ) wild life

Soil conservation


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*lassi(cation

Storage Reservoirs: Storage reservoirsare also called conservation reservoirsbecause they are used to conserve water.

Storage reservoirs are constructed to storethe water in the rainy season and torelease it later when the river +ow is low

Flood Control Reservoirs: , +oodcontrol reservoir is constructed for thepurpose of +ood control. It protects theareas lying on its downstream side from

the damages due to +ood.


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Retarding Reservoirs: , retarding reservoir is

provided with spillways and sluiceways which areungated. The retarding reservoir stores a portion ofthe +ood when the +ood is rising and releases itlater when the +ood is receding.

Detention Reservoirs : , detention reservoirstores e-cess water during +oods and releases itafter the +ood. It is similar to a storage reservoir butis provided with large gated spillways and

sluiceways to permit +e-ibility of operation.


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Distribution Reservoirs: , distribution reservoir isa small storage reservoir to tide over the pea

demand of water for municipal water supply orirrigation. The distribution reservoir is helpful inpermitting the pumps to wor at a uniform rate. Itstores water during the period of lean demand andsupplies the same during the period of high

demand.

Multipurpose Reservoirs: The are constructed formore than single purpose.

Balancing Reservoirs: , balancing reservoir is asmall reservoir constructed d/s of the main reservoirfor holding water released from the main reservoir.


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Investigations

Engineering surveys

0eological investigations

!ydrological investigations


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Engineering surveys

*onducted for dam1 reservoir and otherassociated wor.

Topographic survey of the area is carriedout and the contour plan is prepared

The hori2ontal control is usuallyprovided by triangulation survey1 andthe vertical control by precise levelling.


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&rom the prepared plan1 the following

physcial characteristics are prepared$"a#,rea3Elevation *urve

"b#Storage3Elevation *urve

"c#4ap of the area to indicate the landand property to be surveyed

"d#Suitable site selection for the dam


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0eological investigation

0eological investigations of the damand reservoir site are done for thefollowing purposes.

"i# Suitability of foundation for thedam.

"ii# Watertightness of the reservoirbasin

"iii# 5ocation of the 6uarry sites forthe construction materials.


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"iv# 0eological and structural features 1such as folds1 faults. &issures etc ofthe rocs of the basin

"v# 5ocation of permeable and soluablerocs

"vi# 0round water condition of the

region


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!ydrological investigations

The hydrological investigations areconducted for the followingpurposes $

"i# To study the runo7 pattern todetermine the re6uired storage

capacity of the reservoir.

"ii# To determine the ma-imumdischarge at the site to determine

the spillway capacity and design.


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Site selection

5arge storage capacity River valley should be narrow1

length of dam to constructed isless.

Watertightness of reservoir.

0ood hydrological conditions Deep reservoir


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Small submerged area

5ow silt in+ow

'o ob8ectionable minerals

5ow cost of real estate

Site easily accessible


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9ones of storage


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Normal Pool level (NPL): The normal poollevel "':5# is the highest water level to which

the water surface will rise during normaloperating conditions.

Maimum !ater level (M"L): The ma-imum

water level is the ma-imum level to which thewater surface will rise when the design +oodpasses over the spillway.

Minimum pool level: The minimum pool levelis the lowest level up to which the water iswithdrawn from the reservoir under ordinaryconditions.


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Dead storage: The volume of water held belowthe minimum pool level is called the dead

storage. It is provided to cater for the sedimentdeposition by the impounding sediment laid inwater. 'ormally it is e6uivalent to volume ofsediment e-pected to be deposited in the

reservoir during the design life reservoir.

Live#use$ul storage: The volume of waterstored between the normal pool level "':5# and

the minimum pool level is called the usefulstorage. It assures the supply of water forspeci(c period to meet the demand.


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Flood#Surc%arge storage: is storagecontained between ma-imum reservoirlevel and normal pool level. It varies withspillway capacity of dam for given design+ood.


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Ban& storage: is developed in the voids

of soil cover in the reservoir area andbecomes available as seepage of waterwhen water levels drops down. It increasesthe reservoir capacity over and above thatgiven by elevation storage curves.

'alle storage: The volume of water heldby the natural river channel in its valley upto the top of its bans before the

construction of a reservoir is called thevalley storage. The valley storage dependsupon the cross section of the river.


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Safe yield

;ield is the volume of water whichcan be withdrawn from a reservoir ina speci(ed period of time.

Safe yield is the ma-imum 6uantityof water which can be supplied from

a reservoir in a speci(ed period oftime during a critical dry year.


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Secondar ield: is the 6uantity of water whichis available during the period of high +ow in the

rivers when the yield is more than the safe yield.

verage ield: The average yield is thearithmetic average of the (rm yield and thesecondary yield over a long period of time.

Design ield: The design yield is the yieldadopted in the design of a reservoir. The designyield is usually (-ed after considering the urgency

of the water needs and the amount of risinvolved.


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4ass In+ow *urve

Where to is the time at the beginning ofthe curve.

!ence1 4ass curve is an integral or

summation curve of a given hydrograph. The slope of the curve at any point is e6ual

to the rate of stream+ow at that time.


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The slope of the mass curve is always

either positive or 2ero


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Demand *urve

•

:lot between accumulated demandand time


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Reservoir capacity

depends upon the in+ow available anddemand

in+ow in the river is always greater thanthe demand1 there is no storage re6uired

if the in+ow in the river is small but the

demand is high1 a large reservoircapacity is re6uired


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The re6uired capacity for a reservoircan be determined by the followingmethods$


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0raphical method


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>. Draw the lines AB, FG, etc. such that

"i# They are parallel to the mass demandcurve1 and

"ii# They are tangential to the crests ,1 &1

etc. of the mass curve.

. Determine the vertical intercepts *D. !1etc. between the tangential lines and themass in+ow curve. These interceptsindicate the volumes by which the in+owvolumes fall short of demand.


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,ssuming that the reservoir is full at point,1 the in+ow volume during the period ,E

is e6ual to ordinate DE and the demand ise6ual to ordinate *E. Thus the storagere6uired is e6ual to the volume indicatedby the intercept *D.

@. Determine the largest of the verticalintercepts found in Step "#. The largest

vertical intercept represents the storagecapacity re6uired.


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The following points should be noted.

"i# The capacity obtained in the netstorage capacity which must be available

to meet the demand. The gross capacity ofthe reservoir will be more than the netstorage capacity. It is obtained by addingthe evaporation and seepage losses to the

net storage capacity.


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"ii# The tangential lines ,A1 &0B etc. when

e-tended forward must intersect thecurve. This is necessary for the reservoirto become full again1 If these lines do notintersect the mass curve1 the reservoir will

not be (lled again. !owever1 very largereservoirs sometimes do not get re(lledevery year. In that case1 they may becomefull after =3> years.

"iii# The vertical distance such as &5between the successive tangents

represents the volume of water spilled


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,nalytical method

•

capacity of the reservoir isdetermined from the net in+ow anddemand.

•

storage is re6uired when the demande-ceeds the net in+ow.

• the total storage re6uired is e6ual tothe sum of the storage re6uiredduring the various periods.


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>. Determine the direct precipitation

volume falling on the reservoir during themonth.

. Estimate the evaporation losses whichwould occur from the reservoir CThepanevaporation data are normally used forthe estimation of evaporation losses

during the month.

@. ,scertain the demand during various

months.


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. Determine the ad8usted in+ow duringdi7erent months as follows$

,d8usted in+ow Stream in+ow F:recipitation 3 Evaporation G DownstreamDischarge

H. *ompute the storage capacity for eachmonths.

Storage re6uired ,d8usted in+ow G

Demand

. Determine the total storage capacity of

the reservoir by adding the storages


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>. 4easure the slopes of all thetangents drawn in Step =.

. Determine the slope of the +attesttangent.

@. Draw the mass demand curvefrom the slope of the +attest tangent

"see insect#. The yield is e6ual to theslope of this line


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Reservoir Sedimentation

is a diJcult problem for which aneconomical solution has not yet beendiscovered1 e-cept by providing a Kdead

storageL to accommodate the depositsduring the life of the dam.

Disintegration1 erosion1 transportation1 andsedimentation1 are the di7erent stagesleading to silting of reservoir.


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*auses of sedimentation

'ature of soil in catchment area

Topography of the catchment area

*ultivation in catchment area

Megetation cover in catchment area

Intensity of rainfall in catchment area


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Sediment 4anagement

•

4a-imum e7orts should water shouldbe released so that less sedimentsshould retain in reservoir. &ollowingoptions are$

–*atchment Megetation –*onstruction of co7er dams/low

height barriers

–&lushing and desilting of sediments –5ow level outlets / sediment

sluicing


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*atchment vegetation


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*atchment vegetation


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Wooden barriers


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Wooden barriers


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Stepped watershed for sedimentcontrol


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&lushing of sediments fromreservoir


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4echanical desilting fromreservoir


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Sediment sluicing


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Sediment sluicing


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class10 12thfeb.pptx

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