Engineering Hydrology : Introduction and measurements

It provides an overview of engineering hydrology in the field of engineering geology

1. Engineering Hydrology
NarayanGautam
Faculty : DOM, TC/TU
([email protected])
October, 2017
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2. EGE 554 Engineering Hydrology and Hydrogeology
(Credit: 2 / 30 hours)
1. Stream Flow
Waterstage,annualgages,creststagegages,miscellaneousstagegages,selection
ofstationsite.Discharge.
Reservoirevaporation,combinationmethodsofestimatingreservoirevaporation,
estimationofreservoirevaporationfrompanevaporationandrelated
meteorologicaldata,summaryandappraisaloftechniquesforestimatingreservoir
evaporation,increasedwatersuppliesthroughreducedevaporation.
2. Evapo-Transpiration
Factorsaffectingtranspiration,measurementoftranspiration.Waterbudget
determinationofmeanbasinevapo-transpiration,fieldplotdeterminationof
evapo-transpiration,lysimeterdeterminationofevapotranspiration,estimating
potentialevapo-transpirationfrommeteorologicaldata,estimatingactualevapo
transpirationfrompotentialirrigationwaterrequirements,controlling
evapotranspiration,equationsforevapo-transpirationcomputations.
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3. Introduction
ThewordhydrologyisderivedfromtheGreekwordshydor,means
waterandlogos,whichmeansscience(ortostudy).
Thusinbroadsense,hydrologyisthesciencewhichisconcerned
withallwatersonEarth,itsoccurrence,distributionandcirculation,
itsphysicalandchemicalproperties,itseffectontheenvironment
andonlifeofallforms.
Fromtheviewpointofpracticalapplications,WislerandBrater
havedefinedhydrologyasthesciencethatdealswithprocesses
governingthedepletionandreplenishmentofwaterresourcesof
thelandareasintheearth.
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4. Introduction…
Hydrologyisahighlyinterdisciplinaryscience.
Itisafactthatengineeringhydrologyisgenerallyfamiliarwith
applicationpointofviewwhereasscientifichydrologyisfamiliar
withacademicpointofview.
HydrologyinteractswithscienceslikeHydrometeorology
(emphasizingthemeteorologicalaspects),Hydrogeology
(emphasizingthegeologicaspects),Geohydrology(emphasizing
thehydrologicaspects),Hydrobiology(emphasizingthebiological
aspects),andsoon.
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5. Introduction…
Whendealingwithsurfacewater,thesupportingsciences
requiredarePotamology(scienceofsurfacestreams),Limonology
(scienceoflakes),Cryology(scienceofsnowandice)etc.
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Imjaglacier lake, Nepal

6. Stream flow &measurement of stage
Streamflow:Thewaterwhichconstitutestheflowinthesurface
streamiscalledstreamflow.
Stage:Theriverstagehasbeendefinedastheheightofthewater
surfaceintheriveratagivensectionaboveanyarbitrarydatum.In
manycases,thedatumistakenasmeansealevel.
Generallynon-recordingandrecordingsystemsareinusetofind
stageofariver.
Non-recordinggauge:verticalstaffgauge,inclinedstaffgauge,
Suspendedweightgauge,watersolublepaintsetc
Vertical staff gauge
Thestagecanbeveryeasilymeasuredbyinstallingaverticalstaffgauge
whichisnothingbutagraduatedscalesuchthataportionofitisalwaysin
thewaterallthetimes.
Whentheflowinthestreamissubjectedtolargevariationsseveralvertical
staffgauges(sectionalgaugesinsometexts)tobeestablished.
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7. Vertical staff gauge
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8. Gauging site selection (stream)
Thestationshouldbeaccessibleinallconditions,particularly
duringfloods.
Thestationshouldabletocoverthefullrangeofdischargewhich
mayoccur.
Itshouldnotbelocatedwithintheinfluenceofbackwater.
Thestationshouldbesosituatedthattheinstallationisalmost
permanentandisnotlikelytobedisturbed.
Oneoftheimportantfeaturesofstreamflowrecordisitsunbroken
length.
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9. Inclined staff gauge
•Sometimes the gauge may be placed in an inclined position upon the
stream bank.
•It must be properly anchored in the slope of the natural bank of the river
channel.
•Proper care should be taken while installing the staff gauge to protect
them from damage by boats, debris etc.
Suspended weight gauge
•Theriverstagecanalsobemeasurednormallybysuspendedweightgauge
inwhichaweightattachedtoaropeisloweredfromafixedreference
pointonabridgeorotheroverheadstructuretillittouchesthewater
surface.Bysubtractingthelengthoftheropefromthefixedreference
point,thestageisobtained.
•Beyondthem,watersolublepaintsarealsoappliedtoknowthestage
height.
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10. Manual and automatic stage recorder
•Meritsanddemeritsofmanualstagerecorder:Thoughthemanualgauges
describedabovearesimpleandinexpensive,itislikelythatthepeakstagemay
bemissedwhenitoccursinbetweentheobservations.Recordinggaugesmaybe
installedtoovercomethesedifficulties.
Recording gauge: Float type automatic stage recorder, bubble gauge
Floattypeautomaticstagerecorder
Itnormallyconsistsofafloattiedtooneendofacablerunningoverapulley.To
theotherendofacable,acounterweightisattached.
Thefloatwouldberestingonthewatersurfaceandthecounterweightalways
keepsthecableintension.Anychangeinwatersurfacemakesthefloateitherto
riseorlowerandthisinturnmakesthepulleyrotate.
Themovementofthepulleywouldactivateapenarmwhichrestsonaclock-
drivendrum,wrappedwithachart.Thecircumferenceofthedrumrepresents
thetimeaxiswhiletheheightofthedrumrepresentsstage.Theclockandthe
drummaybesodesignedthatthechartrunsforaspecifiedtimeperiodlikea
dayoraweekoramonthetc.
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11. Float type automatic stage recorder
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12. Bubble Gauge (Rel
n
bet
n
pr
r
and gas)
•Inthisgauge,compressedairorgasismadetobleedoutatavery
smallratethoughanoutletplacedatthebottomoftheriver.
•Apressureguagemeasuresthegaspressurewhichinturnisequal
tothewatercolumnabovetheoutlet.
•Asmallchangeinthewatersurfaceelevationisfeltasachangein
pressure.
•Thepressuregaugereadsthenewwaterdepthwhichistransmitted
toarecorder.
Stagehydrograph
•Aplotofstageagainstchronologicaltimeisknownasstage
hydrograph.Stagedataitselfisofimportanceinfloodwarningand
floodprotectionworks.
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13. Measurement of river velocity to get discharge of
a river
•Someofthetechniqueslikefloatmethod,currentmetersareusedtomeasurethe
velocityatvariousverticalsacrossthewidthofthestream.
•Discharge(Q)ofarivercanbeobtainedbymultiplyingwaterflowvelocity(V)*
Cross-sectionareaoftheriver(A).
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14. Evaporation
Itisaprocessbywhichwaterfromliquidorsolidstatepassesinto
vaporstatethatdiffusingintotheatmosphere.
Factorsaffectingevaporation
Solarradiation
Intheregionsnearequatorwheretheannualsolarradiationislarge,
theannualevaporationisalsolarge.
Temperature(atmospheric)
Theprocessofevaporationalsodependsupontemperature.Ifthe
temperatureismore,theevaporationincreases.Thus,insummer
seasonorhotcountries,theevaporationwillbemoreascomparedto
thatinthewinterseasonorcoldcountries.
Windvelocity
•Ifthevelocityoftheairincontactwithwatersurfaceismore,the
watervapourwillmoveeasily,causesmoreevaporation.
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15. Evaporation…
Qualityofwater
•Thequalityofwaterinthewaterbodyalsoaffectstherateof
evaporation.Thepresenceofanydissolvedsaltsinwaterreduces
therateofevaporation.Forthisreason,usuallytheevaporation
decreasedby1%forevery1%increaseinthesalinityofawater
body.
AtmosphericPressure
•Iftheatmosphericpressureismore,therewillbelessevaporation.
Whereas,thedecreaseinatmosphericpressurewouldincreasethe
rateofevaporation.
Size of the water surface
•Theamountofevaporationisdirectlyproportionaltothesizeof
watersurfacearea.Iftheareaislarge,theevaporationwillbemore
andvice-versa.
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16. Measurement of Evaporation
Evaporationisusuallymeasuredeitherbyatmometersorbythe
evaporationpansalsoknownastheevaporimeters.
Atmometers
Theatmometersareprovidedwithsomespecialsurfaceswhichare
keptwetandfromthesesurfacestheevaporationtakesplace.
Therewillbesomesourcetocontinuouslysupplywaterandkeep
theevaporatingsurfacewet.Thewaterlossobservedatthesource
isanindicationoftheevaporation.
Theatmometersarenotascommonastheevaporationpans
becausetheydonothavethelargeexposure.Therateof
evaporationrecordedbyatmometersisgenerallyhigherthanthe
evaporationrecordedbyevaporationpans.
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17. Measurement of Evaporation…
Evaporation Pans
Evaporationpansareevaporationmeasuringdevices.Inevaporation
pansthedepthofevaporationduringanytimeintervalismeasuredas
thedropinwatersurfacelevelinthepan.Anycylindricalvesselof
1.2to1.8mdiameter,0.3mhighwithopentopandmadeof
galvanizedironsheetcanbeusedasanevaporationpan.Thewater
levelwithinthepanismaintainedbetween5to7.5cmbelowtherim
ofthepan.
An example of US class A pan
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18. Measurement of Evaporation…
Advantages
Cheap,easytoreadandmaintenancearetheadvantagesofaevaporationpan.
Disadvantages
•Maindisadvantageofapanistheevaporationfromthepanwillnotbethesameas
lakeorreservoirduetothedifferentexposurecondition.
•Theheatstoringcapacityofthepanwillbedifferentfromthelake.
Theshadowcausedbyrimonthewateralsoaffectsincomingradiation.
Pancoefficient
Pancoeff.=Actualevaporationfromreservoir/Measuredevaporationfrompan
Generally,thepancoefficientdoesn’tchangemuchfromanannualbasisforthe
giventypeofpan.
Thisaspectmadetheuseofpanspopularandmanymeteorologicalobservatories
includetheevaporationpan.
Therearethreetypesofpansthatareincommonuse,namelySunkenPan,Floating
Pan&SurfacePan.
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19. Measurement of Evaporation…
Sunken Pan
ThistypeofpanisusedbyU.S.Bureauofplantindustry.It’s
diameteris183cm,depthis61cmandrimabovethegroundis10
cm.TheColoradosunkenpanissquareinshapeandit’span
coefficientis0.95.
Advantages:Certainboundaryeffectslikedirectsolarheatingon
sidesandheatexchangefromsidewallsarenegligible.
Disadvantages:
Itisdifficulttomaintain.Rustingmayoccur.
Itisdifficulttoinstall
Difficulttodetectleak
Difficulttoestimateheatlosstosoilfromsidesandbottom
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20. Measurement of Evaporation…
FloatingPan
Thispanissupportedbydrumfloatsandiskeptfloatinginalake.
Thewaterlevelsinlakeandpanaremaintainedassamewhilethe
rimofthepanis7.5cmabovethewaterlevel.
Squarepanisofside90cmand45cmdeep
FloatingpanisgenerallyusedbyU.S.Geologicalsurvey
Generallyit’spancoefficientisrecommendedas0.8.
Drawbacks
Itisdifficulttousebecauseofaccessibility
Splashingofwaterintoandoutofthepanisaproblem
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21. Measurement of Evaporation…
Surface (land) pan
Itismorepopular,realistic,easyandcommonpan.
It’sdiameteris122cmand25.4cmdeepandismadeofgalvanized
iron.
Itshouldinstall15cmabovefromgroundleveloverawooden
framewithfreeaircirculationunderthepan.
It’sannualpancoefficientof0.7butseasonalpancoeff.varyfrom
0.6to0.8.
Watersurfacelevelismeasureddaily(inthepan)
Advantages
It has stable pan coefficient
Easy access for observation
It is more stable than floating pan
It is relatively free from dirt and trash
It is easy to install
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22. Empirical evaporation equations
Therearemanyempiricalequationsthataresuggestedtoestimate
evaporationfromfreewatersurface.Someofthemareasfollows:
Fitzgerald’sequation
Horton’sequation
Meyer’sequation
Rohwer’sequation
LakeMeadequation
(Alsocheckwaterbalance,energybalance,masstransferand
combinedmethodstoestimateevaporation)
(For details, W.B.)
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