Efficiency of the Nam Theun 2 hydraulic structures on water aeration and methane degassing

24 Pages·2012·3.54 MB·English

Hydroécol.Appl.
©EDF,2015
DOI:10.1051/hydro/2015002
Efficiency of the Nam Theun 2 hydraulic structures
on water aeration and methane degassing
Efficacité des structures hydrauliques de Nam Theun 2
sur l’aération de l’eau et le dégazage du méthane
S. Descloux(1)*, V. Chanudet(1), B. Taquet(1), W. Rode(2), P. Guédant(2),
D. Serça(3), C. Deshmukh(3), F. Guerin(4,5)
(1)EDF, Hydro Engineering Centre, Sustainable Development Department, Savoie-Technolac, 73373 Le
Bourget-du-Lac,France
[email protected]
(2)Nam Theun 2 Power Company Limited (NTPC), Environment & Social Division – Water Quality and
BiodiversityDept.–GnommalathOffice,POBox5862,Vientiane,LaoPDR
(3)Laboratoired’Aérologie,ObservatoireMidi-Pyrénées,14Av.ÉdouardBelin,31400Toulouse,France
(4)UniversitédeToulouse,UPS(OMP),LMTG,14Av.ÉdouardBelin,31400Toulouse,France
(5)IRD,LMTG,14Av.ÉdouardBelin,31400Toulouse,France
Abstract–Releaseofhypolimneticwaterfromman-madereservoirscanbeaproblemfor
downstreamrivers.Theseeffectscanbesignificantmainlyduringthefirstyearsafterthe
reservoirimpoundment,especiallyinthermallystratifiedreservoirsfavouringthereleaseof
anoxicmethane-richwater.Intropicalareas,highertemperaturesdecreasetheoxygensolu-
bilityandenhancechemicalprocessesresponsiblefortheriseofreducedcompounds.The
NamTheun2Reservoirwasfirstfilledin2008.Itexperiencedhypolimneticdeoxygenation
andsignificantmethaneconcentrationsduringthefirst2years.Dedicatedstructurestooxy-
genatewateranddegasmethaneweredesignedduringthestudyphase.Theoverallaerating
and degassing effects of these hydraulics structures varied from very good to moderate.
Resultsdependonthecontinuouswaterqualityimprovementwithtimeaswellasonthelimi-
tedrangeofupstreamoxygenandmethaneconcentrationstested.Thehollowjetvalveand
theconcretetoothshapedstructurewereveryefficienttogetherwithdownstreamnaturaltur-
bulenceinaerating/degassingcomparedtothestaggeredbaffleblocks.Contrarytoother
structures,theefficiencyofthebaffleblocksstructureisreducedwithhighdischarges.The
aerationweirshowedamoderateefficiencyinsupplyingoxygentothewaterduetothehigh
upstreamoxygensaturations(closeto100%).However,itwasveryefficientformethane
degassingevenatlowconcentrations.HydraulicsstructuresoftheNamTheun2projectare
an efficient, reliable and low maintenance way to improve oxygen content and to degas
methane.
Keywords–aeration,reservoir,sub-tropical,weir,degassing,oxygen,methane
ArticlepubliéparEDPSciences
2 S.Desclouxetal.
Résumé–Ladélivranced’eauprovenantdel’hypolimniondesréservoirsartificielspeutêtre
unproblèmepourlaqualitédel’eaudesrivièresenaval.Ceseffetspeuventêtreimportants
pendantlespremièresannéesaprèslamiseeneaud’unréservoiretplusparticulièrement
danslesréservoirsstratifiésfavorisantladélivranced’uneeauanoxiqueetricheenméthane.
Danslesrégionstropicales,lestempératuresplusélevéesdiminuentlasolubilitédel'oxy-
gèneetfavorisentlesprocessuschimiquesresponsablesdel’augmentationdesconcentra-
tions en composés réduits. Le réservoir de Nam Theun 2 a été mis en eau en 2008. Il a
présentéunedésoxygénationdelacouchehypolimniqueetdesconcentrationssignificatives
deméthaneaucoursdes2premièresannées.Desstructuresdédiéesàl’aérationetaudéga-
zageduméthaneontétéétudiéesetmisesenœuvresurlesite.L'efficacitéglobaledel’aéra-
tionetdudégazagedesdifférentesstructureshydrauliquesaétédetrèsbonneàmodérée.
Lesrésultatssontévaluésentenantcomptedel’améliorationcontinuedelaqualitédel’eau
etdelafaiblegammedevariationdeconcentrationsd’oxygènedissousetdeméthanedis-
sousamont.Lavanneàjetcreuxetlastructureenformededentenbétonontététrèseffi-
cacesaveclaturbulencenaturelleavalpourl’aérationetledégazagecomparativementaux
blocsdedissipation.Contrairementàd'autresstructures,l'efficacitédelastructureenblocs
de dissipation est réduite à fort débit. Le seuil aérateur a été modérément efficace pour
l'apportd’oxygènecomptetenudeshautessaturationsamont(prèsde100%)maisilaété
trèsefficacepourledégazageduméthane,mêmeàdefaiblesconcentrations.Lesstructures
hydrauliquesduprojetNamTheun2s’avèrentefficacespouraéreretdégazerleméthane.
Ellessontfiablesetnécessitentunefaiblemaintenance.
Motsclés–aération,réservoir,sub-tropical,seuil,dégazage,oxygène,méthane
1 INTRODUCTION concentration of reduced and dis-
solved compounds. Among them, it
Releaseofhypolimneticwaterfrom can be found ammonium (NH +), fer-
4
man-made reservoirs can alter the rous iron (Fe2+), hydrogen sulfide
physicalandchemicalqualityofdown- (H S),carbondioxide(CO )andmeth-
2 2
stream rivers, limit their uses and lead ane (CH ) (Galy-Lacaux et al., 1997,
4
to a loss of biodiversity. These effects 1999; Guérin et al., 2008). Concentra-
canbesignificantduringthefirstyears tions are generally higher when the
afterimpoundmentorunderstrongres- water body is stratified (Abril et al.,
ervoir stratification events and, are 2005).Undersuchconditions,hypolim-
generally enhanced in tropical areas netic waters are isolated from the sur-
where higher temperatures decrease face waters oxygenated from photo-
the oxygen solubility and enhance the synthesis and atmospheric gas
chemical processes of organic matter exchangebythepresenceofathermo-
degradation (Townsend, 1999; Abril cline. Waters in hydroelectric reser-
etal., 2005; Gregoire & Descloux, voirsaregenerallytakenfromthebot-
2009). tom layer, i.e. in the hypolimnion, to
Newly impounded reservoirs cause maximise energy production (optimi-
the degradation of the flooded organic sationofthelivevolume).Therelease
matterthatcanleadtoanoxiaandhigh of bottom waters can thus affect
EfficiencyoftheNamTheun2hydraulicstructuresonwateraeration 3
downstreamriversthroughouttheyear concentrations. Opposite results have
orduringspecificseasons,forexample been underlined with acute toxic gas
during the dry season when the river effects in marine fish starting at a con-
canbemostlyinfluencedonlybyreser- centration of about 1 mg.L-1 (Patin,
voir releases. As rivers are generally 1999). But methane has the indirect
used for various purposes (drinking detrimental effect of reducing DO con-
water, biodiversity, navigation and tentinoxicwaterbodies.
tourism, etc) and fulfill the function of TomaintainDOandgasconcentra-
self-purification,itisofhighimportance tions compatible with aquatic life and
to preserve and guaranty their water other uses, many solutions have been
quality. tested to supply DO (namely aeration;
Theminimumlevelsof(i)dissolved Baylar et al., 2009). Solutions range
oxygen (DO) to be maintained and (ii) from hypolimnetic oxygen injection,
reducedgastobeeliminatedvaryfrom reservoir clearing and selective with-
sitetositeandwiththephysico-chemi- drawaltotheconstructionofdedicated
cal characteristics of the water, the hydraulic structures like weirs or baffle
morphologyoftheriverandthedilution blocks. Creation of turbulence by
effect from downstream tributaries. It hydraulic structures and subsequent
alsodependsontheecologicalrequire- aeration is a principle utilized by aera-
mentsoftheaquaticcommunityandon tion weirs and seems to be the most
the latitude (e.g. oxygen solubility is efficientsolutionforaeration(Chanson,
a function of water temperature). The 1995; EPRI, 2002; Emiroglu & Baylar,
downstream DO consumption by ben- 2003). DO transfer occurs when air is
thiccommunityrespirationorbiological entrained into the flow in the form of a
andchemicalDOdemandmustalsobe large number of bubbles. These bub-
considered. For instance, salmonids bles greatly increase the surface area
swimming is altered below 2–5mg.L-1 available for mass transfer (Baylar
DOwhereastilapiaandcarpscanswim etal., 2009). Very few man-made res-
atDOlevelsof1–2mg.L-1(Kutty,1972; ervoirs are equipped for aeration with
Kutty & Saunders, 1973). Tropical respect to the total number of reser-
fishes generally have a higher toler- voirs throughout the world. Structures
anceoflowDO.Nevertheless,DOthat built with the objective of degassing
remains below 1 mg.L-1 (and corre- reducedgasesareevenmorerareand
sponding DO saturations equivalent to thecurrentliteraturedataarebasedon
~ 10% at 20°C) for a few hours can theonlycaseofthePetitSautReservoir
affect biodiversity and generally result in French Guiana (Gosse & Gregoire,
in large fish kills (Baylar et al., 2009). 1997; Gosse et al., 1997). Moreover,
Studiesofthedirecteffectsofmethane efficiencyofotherstructureslikehollow
on water organisms are very limited. jet valves that are not specifically
SackettandBrooks(1975)haveshown designedtoincreaseDOconcentration
that under experimental conditions, ortodegashaveneverbeenassessed
hydrocarbons (methane and others) onsite.
didnotcauseharmfuleffectsonmarine The creation of the Nam Theun 2
phytoplankton even at high dissolved Reservoir(NT2)intheLaoPDRhasled
4 S.Desclouxetal.
to the submersion of 489 km² of forest Intakeintheupperpartofthereservoir,
and agricultural soils. Descloux etal. channelledthroughthePressureTunnel,
(2011) have estimated the quantity of thePowerHouse,thenreleasedintothe
floodedcarbonincludingaboveground RegulatingPondequippedwithaReg-
andbelowgroundbiomassto5.1MtC. ulating Dam (used to buffer discharge
Itwasforeseenthatthemajorpartofthe variationsofthePowerHouse)andinto
DOconsumptionwouldcomefromthe the 27 km long Downstream Channel
decomposition of this organic matter (throughtheAerationWeir)whichjoins
(mainlyfromthesoils).Inaddition,other theXeBangfaiRiverabout30kmsouth
processes can increase DO consump- of the Power House (Descloux et al.,
tion, such as respiration from aquatic sameissue).TheRegulatingDamwhich
plantsandchemicaloxidation(ironand also receives water from the upstream
other metals such as Mn in soils). The NamKathangRiveralsodeliversamin-
residencetimeoftheNT2Reservoiris imum flow towards the downstream
about6months,whichisrelativelylong, Nam Kathang River, a tributary of the
and the water body is stratified about Xe Bangfai River. The Nakai Dam is
half a year during the warm season locatedinthelowerpartofthereservoir
(Chanudet et al., same issue). These far(around40km)fromthewaterintake
characteristics may affect DO concen- and delivers a riparian release to the
trations and generate reduced gases NamTheunRiverfor environmentalcon-
and it was decided during the study siderations. The mean annual inflow in
phase to build dedicated structures to theNamTheunReservoiris238m3.s-1
increaseDOconcentrationsandtodegas (Desclouxetal.,sameissue).TheNT2
supersaturated gases. Moreover, for Reservoir is relatively shallow with an
theNT2Project,theguidelineformini- averagedepthof8mandischaracter-
malDOisfixedat5mg.L-1,atargetfor izedbyalargedrawdownarea.Theres-
allthere-aerationstructures. ervoirsurfacefluctuatesfrom86km²at
The objectives of this study were i) theminimumoperatinglevel(525.5masl)
todeterminetheaerationandmethane to 489 km² at the full supply level (538
degassing efficiency of dedicated and masl;DTG,2012;Desclouxetal.,same
non-dedicated structures of the NT2 issue).Threeseasonsaredistinctinthis
projectover5yearsfollowingthefirstfill- sub-tropical environment; the warm-wet
ingofthereservoirandii)tocomparethe (WW: July to October), cool-dry (CD:
efficiencyofthesehydraulicstructures. November to February) and warm-dry
(WD:MarchtoJune)seasons(Descloux
etal.,sameissue).
2 METHODS
2.1 Studysite 2.2 Aeratinganddegassing
structuresoftheNT2Project
The NT2 Project, located in Lao
PDR,wasimpoundedinApril2008and
2.2.1 The Nakai Dam
commissionedinApril2010.Thereser-
voirislocatedontheNakaiPlateau.The The concession agreement stipu-
turbinated water is captured at the lates that the Nam Theun 2 Power
EfficiencyoftheNamTheun2hydraulicstructuresonwateraeration 5
Fig.1.PictureoftheriparianreleasestructureoftheNakaidamsiteontheNamTheunRiver(hollow
jetvalve).
Fig.1.PhotographiedudébitréservérelâchéausitedubarragedeNakaisurlarivièreNamTheun
(vanneàjetcreux).
Company (NTPC) has to release a theDownstreamChannelwaterrelease
minimumenvironmentalflowof2m3.s-1 (passingthroughpointDCH1inFig.5B).
intothedownstreamNamTheunRiver. Based on the water level difference
This flow is taken from the reservoir betweenthereservoirandthechannel,
surface, 2 meters below the surface, around40MWofenergy(whichcorre-
through a multi-level off-take structure sponds to approximately 315 m3.s-1)
(diameter 1.54 m). The flow then goes have to be dissipated downstream of
through a hollow jet valve (section of the gates. A model was built to design
5x3.7m)thatcreatesturbulenceoxy- adequatefeatureswiththeobjectiveof
genating the flow (Figs. 1 and 5A) dissipatingthisflowenergy.Adecision
whichreachesadesignedstillingbasin wasmadetodissipatethisenergywith
before its release into the Nam Theun staggered baffle blocks installed in a
River. Air is also entrained into the convergentsection(7blocks3mhigh,
basin and it is the combined efficiency 12 m width and 4 m long), made with
of the two devices (jet and basin) and high strength concrete (60 MPa), as
the natural aeration along the 0.7 km showninFigure2.Thehighturbulence
downstream river stretch that was created by the energy dissipation
assessed in this study. During high devices is deemed to produce a good
floods, flow can be spilled out through aeration of the water, for all the range
flapandradialgates. ofdischarges.
The second outlet is through the
‘restitution gate’ which releases flows
2.2.2 The Regulating Dam
higher than 1 m3.s-1 dedicated to the
TheRegulatingDamhasthreeout- Nam Kathang River (Descloux et al.,
lets. The first structure is dedicated to sameissueandpointNKT3inFig.5B).
6 S.Desclouxetal.
Fig.2.PictureofthestaggeredbaffleblocksinstalleddownstreamoftheRegulatingDaminaconver-
gentsectionintheDownstreamChannel.
Fig.2.Photographiedesblocsdedissipationinstallésenavaldubarragededémodulationdansle
chenalaval.
It is capable to release the natural Damtoaerate/degastheflowandthus
upstream inflow plus 10 or 15 m3.s-1, improve water quality in the Down-
dependingonthepowerstationopera- streamChannelbeforeitflowsintothe
tionandnaturalinflows(fromtheupper Xe Bangfai (Fig. 5C). It is a dedicated
Nam Kathang) into the Regulating structuredesignedtoprovidemaximum
Pond. It has a capacity of 30 m3.s-1at aeration at the maximum flow of
theminimumoperatinglevel.Inorderto 330m3.s-1.ThestructureincludesaU-
provide a good aeration, a dedicated shaped weir made of reinforced con-
concretetoothshapeddevicehasbeen crete walls. The total overflow crest
designedtosplittheflowjetin2parts, lengthisapproximately340m.Sub-hor-
andspreadit,inordertocreatealarge izontal perforated wooden devices are
areaofwaterincontactwithair(Fig.3). attached to the total overflow crest
The third outlet is a single canal that length (allow to spread the flow over
delivers the minimum environmental 1.2m). They divide the flow over the
flow(below1m3.s-1)totheNamKathang weirintomultipledischargestoincrease
during the dry season. The dedicated thewatersurfaceareaasmuchaspos-
concrete tooth shaped device is not sible to provide maximum aeration/
usedwiththisconfiguration. degassing potential. The weir height
from crest to the downstream water
levelisgreaterthan1.5matnormalflow
2.2.3 The Aeration Weir
(330 m3.s-1), with a water mattress
A weir was built approximately depthalmostequalto3m(Fig.4).The
8.3kmdownstreamfromtheRegulating structureperformancewasfirstlytested
EfficiencyoftheNamTheun2hydraulicstructuresonwateraeration 7
Fig.3.PictureoftheconcretetoothshapedstructuredownstreamoftheRegulatingDamintheNam
KathangRiver.
Fig. 3. Photographie des dents en béton installées en aval du barrage de démodulation dans la
rivièreNamKathang.
Fig.4.PictureoftheaeratingweirdownstreamoftheregulatingdamintheDownstreamChannel.
Fig.4.Photographieduseuilaérateurinstalléenavaldubarragededémodulationdanslechenal
aval.
byaphysicalmodelstudythatfocused speedintheDownstreamChannelfrom
onbubbletrajectories,andallowedthe theRegulatedDamtotheAeratingWeir
designerstoimplementdeflectionwalls wascalculatedtoapproximately1.7m.s-1
(vertical concrete wall attached to the at330m3.s-1.TheDownstreamChannel
main structure; Fig. 4). The water flow is 56 m width (20 m horizontal and on
8 S.Desclouxetal.
eachside:2x3mwitha3/1slopeand station (Descloux et al., same issue).
2x6mwitha2/1slope). Theanalyticalmethods,withtheasso-
Other small weirs are situated all ciatedlimitsofdetectionanduncertain-
alongtheDownstreamChannelandare ties (2% for DO saturation and 5% for
abletoincreaseDOconcentrationsand CH ) are summarized in Deshmukh
4
degas remaining reduced gases with etal.(2014)andChanudetetal.(same
lower efficiencies. Nevertheless, there issue). In the reservoir, between three
isnosmallweirbetweentheRegulating andsixsamplesweretaken(peristaltic
Dam (DCH1) and the Aeration Weir pumporsamplingbottle)ateachsam-
(DCH2)andtheAerationWeir(DCH2) pling point according to water column
and the sampling station downstream andoxyclinedepths.Intherivers,only
theAerationWeir(DCH3).TheTunnel surfacewaterwascollected.
outletstructureattheendoftheDown- Alltheupstream/downstreammeas-
stream Channel is also dedicated to urements were done between 11 am
dissipate the energy of the flow while and 2 pm within 2 hours (excepted for
creatinghighturbulences.Thesestruc- REG1/NKT3wherethemeasurements
tures are not included in the present were done within 24 hours due to field
study as no water quality monitoring constraints; this does not affect the
wasconductedthere. quality of the analysis as the transfer
timeisgreaterthanthelagtimebetween
thetwomeasurements).Theupstream
2.3 Physico-chemical
measurement has always been done
measurements
before the downstream measurement.
Theoxygenvaluesfor100%saturation
The physico-chemical water quality
at 20°C, 25°C and 30°C are respec-
characteristicshavebeenmonitoredin tively9.07,8.24and7.54mg.L-1.
theNT2system(hydroelectricreservoir
and downstream rivers) since the first
fillingofthereservoirinApril2008.The 2.4 Dataanalysis
results emphasize that the impound-
ment of the reservoir induced a sub-
2.4.1 The Nakai Dam
stantialmodificationofthewaterquality
in the whole aquatic system which is Foraerationanddegassinganalysis
partiallycontrolledbythehydrodynam- at the dam site, upstream concentra-
icsinthereservoir.DuringtheWDand tions were calculated from measure-
WW seasons, the reservoir water col- mentsatRES1(situatedinthereservoir
umnisthermallystratifiedwithawarm around700mupstreamofthedam)and
oxic epilimnion and a cooler anoxic downstreamconcentrationsweremeas-
hypolimnion (Chanudet et al., same uredabout700mfromthereleaseatthe
issue). NTH3station(Fig.5A;Desclouxetal.,
DO and methane (CH4) concentra- same issue). At RES1, the thickness
tions in water were measured weekly, andthedepthofthewaterlayerusedfor
fortnightly or monthly between April calculationoftheupstreamconcentration
2008andApril2013ateachmonitoring dependonthedischargeandrestitution
EfficiencyoftheNamTheun2hydraulicstructuresonwateraeration 9
A
B C
Fig. 5. Localisation of the sampling stations and length between stations and structures at A) the
NakaiDam,B)theRegulatingPondandC)theAeratingWeir(©NTPC).
Fig.5.Localisationdesstationsd’échantillonnageetdistancesentrelesstructuresetlesstationsau
A)barragedeNakai,B)bassindedémodulationetC)seuilaérateur(©NTPC).
10 S.Desclouxetal.
structure(riparianrelease,flapgatesor (29%, 17.0 m3.s-1 in average). The
radial gates). For low discharges (e.g. remaining measurements(45%), done
the riparian release; Q ≈ 2 m3.s-1), fordischarge<1m3.s-1,werenotcon-
velocitymeasurementsandsimulations sidered as water was not released
(Fabre et al., 2010) show that only the through the restitution gate: the dedi-
uppermost 3 meters are withdrawn cated concrete tooth shaped device is
through the multi-level off-take struc- notusedwiththisconfiguration.Forthe
tureusedfortheriparianrelease.Most twodischargeclasses,theflowiswith-
ofthemeasurements(70%forDOand drawn through a sector gate opening
93%forCH )werecarriedoutwithsuch from 9 to 11 m below the surface. For
4
a low discharge. The depth and thick- the two classes, upstream concentra-
ness of the water layer to consider for tionswerecalculatedbyintegratingthe
higher discharges entering the multi- whole water column at REG1: meas-
level off-take structure are more deli- urementsshowthatthephysico-chem-
catetoestimate.Inthispaper,onlythe icalparameters(DOortemperaturefor
flowpassingthroughtheintakesurface instance) are almost constant over the
structureandthehollowjetvalveused water column (Chanudet et al., same
fortheriparianreleaseareconsidered. issue).Forinstance,sincethecommis-
DO measurements at RES1 (vertical sioning,thedifferencebetweensurface
resolution: surface level (0.2 m), then anddepth-averagedDOsaturationdid
every 0.5 m in the uppermost 5 m and notexceed3%inaverage.Thishomo-
every 1 m downwards) were directly geneityprecludedanyattempttotrace
integratedovertheuppermost3meters upstream water with physico-chemical
togetanaverageDOvaluewhileCH parameters such as conductivity for
4
measurements were linearly interpo- instance to assess the exact water
latedtocalculatetheCH concentration thickness to consider. Therefore, the
4
in the uppermost 3 meters (based on choiceofthewaterlayerdoesnotaffect
CH measurements made at the sur- significantly the results and it justifies
4
faceand1mabovetheoxyclineusually the use of a unique upstream concen-
found between 5 and 10 m; Chanudet trationcalculationmethodwhateverthe
etal.,sameissue). discharge.Downstreamconcentrations
were measured at the surface at the
NKT3station(Fig.5B;Desclouxetal.,
2.4.2 The Regulating Dam and the
Aeration Weir sameissue).
ReleasesintheDownstreamChan-
The aeration and degassing effi- nel were also divided into three dis-
cienciesoftheRegulatingDam(intothe charge classes: 0-100 m3.s-1 (24%,
Downstream Channel and in the Nam 38.9m3.s-1inaverageforDOanalysis),
KathangRiver;Fig.5B)wereanalysed >100-200m3.s-1(16%,147.1m3.s-1in
considering several discharges classes. average) and > 200 m3.s-1 (59%,
ReleasesintheNamKathangusingthe 280.3m3.s-1 in average). For these
‘restitution gate’ were divided into two threeclasses,upstreamconcentrations
discharges classes: 1-5 m3.s-1 (26%, werecalculatedbyintegratingthewhole
2.3 m3.s-1 in average) and > 5 m3.s-1 watercolumnatREG1,consideringthe