An Integrated Approach for Evaluating the Restoration of the Salinity Gradient in Transitional Waters: Monitoring and Numerical Modeling in the Life Lagoon Refresh Case Study

Abstract

Large lagoons usually show a salinity gradient due to fresh water tributaries with inner areas characterized by lower mean values and higher fluctuation of salinity than seawater-dominated areas. In the Venice Lagoon, this ecotonal environment, characterized in the past by oligo-mesohaline waters and large intertidal areas vegetated by reedbeds, was greatly reduced by historical human environmental modifications, including the diversion of main rivers outside the Venice Lagoon. The reduction of the fresh water inputs caused a marinization of the lagoon, with an increase in salinity and the loss of the related habitats, biodiversity, and ecosystem services. To counteract this issue, conservation actions, such as the construction of hydraulic infrastructures for the introduction and the regulation of a fresh water flow, can be implemented. The effectiveness of these actions can be preliminarily investigated and then verified through the combined implementation of environmental monitoring and numerical modeling. Through the results of the monitoring activity carried out in Venice Lagoon in the framework of the Life Lagoon Refresh (LIFE16NAT/IT/000663) project, the study of salinity is shown to be a successful and robust combination of different types of monitoring techniques. In particular, the characterization of salinity is obtained by the acquisition of continuous data, field campaigns, and numerical modeling.

Full text





Environments2022,9,31.https://doi.org/10.3390/environments9030031www.mdpi.com/journal/environments
Article
AnIntegratedApproachforEvaluatingtheRestorationofthe
SalinityGradientinTransitionalWaters:Monitoringand
NumericalModelingintheLifeLagoonRefreshCaseStudy
AlessandraFeola
1,
*,EmanuelePonis
1
,MicheleCornello
1
,RossellaBoscoloBrusà
1
,FedericaCacciatore
1
,
FedericaOselladore
1
,BrunoMatticchio
2
,DevisCanesso
2
,SimoneSponga
2
,PaoloPeretti
2
,MatteoLizier
3
,
LuigiManiero
4
,ValerioVolpe
4
,AdrianoSfriso
5
,MaurizioFerla
1
andAndreaBonometto
1

1
ISPRA,ItalianNationalInstituteforEnvironmentalProtectionandResearch,LocalitàBrondolo,
5‐30015Chioggia,Italy;emanuele.ponis@isprambiente.it(E.P.);michele.cornello@isprambiente.it(M.C.);
rossella.boscolo@isprambiente.it(R.B.B.);federica.cacciatore@isprambiente.it(F.C.);
federica.oselladore@isprambiente.it(F.O.);maurizio.ferla@isprambiente.it(M.F.);
andrea.bonometto@isprambiente.it(A.B.)
2
IPROSIngegneriaAmbientaleS.r.l.,CorsodelPopolo,35131Padua,Italy;matticchio@ipros.it(B.M.);
canesso@ipros.it(D.C.);sponga@ipros.it(S.S.);peretti@ipros.it(P.P.)
3
DirectionforSpecialProjectsforVenice‐VenetoRegion,CallePriuli—Cannaregio,99‐30121Venice,Italy;
matteo.lizier@regione.veneto.it
4
InterregionalSuperintendencyforPublicWorksinVeneto—TrentinoAltoAdige—FriuliVeneziaGiulia,
SanPolo,19‐30125Venice,Italy;luigi.maniero@mit.gov.it(L.M.);valerio.volpe@mit.gov.it(V.V.)
5
DepartmentofEnvironmentalSciences,InformaticsandStatistics,UniversityCa’FoscariVenice,ViaTorino
155,30172Mestre,Italy;sfrisoad@unive.it
*Correspondence:alessandra.feola@isprambiente.it;Tel.:+39‐347‐1814149
Abstract:Largelagoonsusuallyshowasalinitygradientduetofreshwatertributarieswithinner
areascharacterizedbylowermeanvaluesandhigherfluctuationofsalinitythanseawater‐
dominatedareas.IntheVeniceLagoon,thisecotonalenvironment,characterizedinthepastby
oligo‐mesohalinewatersandlargeintertidalareasvegetatedbyreedbeds,wasgreatlyreducedby
historicalhumanenvironmentalmodifications,includingthediversionofmainriversoutsidethe
VeniceLagoon.Thereductionofthefreshwaterinputscausedamarinizationofthelagoon,with
anincreaseinsalinityandthelossoftherelatedhabitats,biodiversity,andecosystemservices.To
counteractthisissue,conservationactions,suchastheconstructionofhydraulicinfrastructuresfor
theintroductionandtheregulationofafreshwaterflow,canbeimplemented.Theeffectivenessof
theseactionscanbepreliminarilyinvestigatedandthenverifiedthroughthecombined
implementationofenvironmentalmonitoringandnumericalmodeling.Throughtheresultsofthe
monitoringactivitycarriedoutinVeniceLagoonintheframeworkoftheLifeLagoonRefresh
(LIFE16NAT/IT/000663)project,thestudyofsalinityisshowntobeasuccessfulandrobust
combinationofdifferenttypesofmonitoringtechniques.Inparticular,thecharacterizationof
salinityisobtainedbytheacquisitionofcontinuousdata,fieldcampaigns,andnumericalmodeling.
Keywords:transitionalwaters;integratedapproach;numericalmodeling;restoration;monitoring;
salinitygradient;freshwater;lifeproject

1.Introduction
Coastallagoonsareecotonesbetweenmarineandterrestrialenvironments,receiving
variableamountsoffreshwater.AlongtheMediterraneancoastline,therearemorethan
400coastallagoons,coveringawiderangeofhydrologicalandbiologicalcharacteristics
[1,2].Mediterraneanlagoonsareusuallyshallowwaterbodies,and,duetotheirgeo‐
morphologicalandhydrologicalcharacteristics,environmentalconditionsinthem
Citation:Feola,A.;Ponis,E.;
Cornello,M.;BoscoloBrusà,R.;
Cacciatore,F.;Oselladore,F.;
Matticchio,B.;Canesso,D.;
Sponga,S.;Peretti,P.;etal.An
IntegratedApproachforEvaluating
theRestorationoftheSalinity
GradientinTransitionalWaters:
MonitoringandNumerical
ModelingintheLifeLagoonRefresh
CaseStudy.Environments2022,9,31.
https://doi.org/10.3390/environments
9030031
AcademicEditor:PaulC.Sutton
Received:27January2022
Accepted:25February2022
Published:1March2022
Publisher’sNote:MDPIstays
neutralwithregardtojurisdictional
claimsinpublishedmapsand
institutionalaffiliations.

Copyright:©2022bytheauthors.
LicenseeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsand
conditionsoftheCreativeCommons
Attribution(CCBY)license
(https://creativecommons.org/license
s/by/4.0/).

Environments2022,9,312of25


undergofrequentfluctuationsonaspatialandseasonalbasis[3].Theseareheterogeneous
andcomplexsystems,characterizedbydifferentphysicalandenvironmentalgradients,
involvingsalinity,marinewaterrenewal(e.g.,residencetime),nutrients,turbidityand
sedimentstructure,whosemagnitudeanddirectiondependmainlyonriverortidal
energy[4].Thedistributionofbiologicalcommunitiesintransitionalwatersisdrivenby
multipleenvironmentalfactors,includingsalinity[5],whichaffectsthecompositionand
abundanceoffishfauna[6],benthicfloraandfauna[7]andsaltmarshvegetation[8].
Salinityconditionsarerelatedtoacomplexinteractionbetweendifferent
hydrologicalprocessesthatinvolvedischargeoffreshwater,tidalregimeandthe
exchangeswiththesea,precipitation,interactionwiththeatmosphere(e.g.,heatflux,
evaporationrates)andwind‐drivenforces,whichvaryoverawiderangeoftime‐scales
andthatbecomerelevantforreallyshallowwaters,resultinginstrongdailyandseasonal
variability[9,10].Allthesedriverscontributetothehighdiversityintermsofsalinity
conditionsbetweenMediterraneanlagoons,rangingfromoligohalinetohyperhaline
waters[11].
Despitethishighdiversity,itispossibletoidentifysomecommonpatterns.In
particular,largelagoonsusuallyshowasalinitygradientduetofreshwatertributaries
thatexhibitestuarinefeaturesintheproximityoftheirlagoonalmouths[12],
characterizedbylowermeanvaluesandhigherfluctuationofsalinitythanseawater‐
dominatedareas[13–15].
IntheVeniceLagoon,thisecotonalenvironmenttypicaloftheinnerareas,
characterizedinthepastbyoligo‐mesohalinewatersandlargeintertidalareasvegetated
byreedbeds,hasbeengreatlyreducedbyhistoricalhumanenvironmentalmodifications,
amongwhichthediversionoutsidetheVeniceLagoonofmainriverssuchasBrenta,
Bacchiglione,SileandPiave(Figure1;[16]).
Thereductionofthefreshwaterinputsandtherelatedsedimentsupply,together
withotherconcurrentfactorssuchastheconstructionofinletjettiesandthedeepening
thelagooninlets(19th–20thcentury),theexcavationoflargenavigationcanals(20th
century),increasingofthelagoondepthduetothecombinedeffectofsubsidenceand
eustatism,causedamarinizationofthelagoon,withaprogressivelyincreaseinsalinity
fromaveragevaluesof10–14documentedin1926–1928tothecurrentvaluescloseto30
[16–18].
Theproblemofmanagingthehydraulicaspectsforlagoonshasbeenstudiedfora
longtimeworldwide.Thesestudiesincludethedescriptionoflagoonphysics,how
hydraulicsarelinkedtolagoonecologyandproductivity,howlagoonsmightbeexpected
torespondtohydraulicalterationsandprotocolforphysicalinvestigations[19].
Therestorationofbothabioticandbioticcomponentsoftransitionalwater
ecosystemsisrecognizedasastrategicapproachtobothenhancetheecologicalstatusof
degradedecosystemsandtacklethelossofbiodiversity.EcologicalEngineeringis
increasinglyusedtore‐createandrestoreecosystemsdegradedbyprevioushuman
activities.Inthiscontext,theso‐calledEcohydrologicalapproach[20]aimsatrestoring
naturalprocesses,especiallyregardingwaterandsedimentflows,byrecoveringthe
suitableconditionsforhabitatsandspecies.
TheLifeLagoonRefreshproject,startedinSeptember2017andlastingforfiveyears,
plannedthere‐introductionofafreshwaterflowintothelagoon,topartiallyrecoverthe
oligo‐mesohalineconditionssuitableforthereedbedcolonization[21].Thislow‐salinity
habitat,typicalofthebufferareasbetweenlagoonandmainland,providesvaluableand
diversifiedecosystemservices(see[22]andreferencestherein),includingwater
purification,nurseryfunctionsandbreeding,feedingorrefugehabitatforseveralfishand
birdspeciesofcommercialandconservationinterest.
Thecoreactionoftheprojectwasthediversionofafreshwaterflowofapproximately
1000ls−1fromtheSileRiverintotheVeniceLagoon.Othermeasuresincludedthe
implementationofstructuresmadebybiodegradablegeotextile,properlyarrangedto
slowdownthefreshwaterdispersion,andpermittingtheplantingofclumpsand

Environments2022,9,313of25


rhizomesofPhragmitesaustralis(Cav.)Trin.exSteud,toacceleratethedevelopmentofthe
reedbeds.
Theobjectiveofsalinegradientrecoverywasquantifiedinanexpectedchangefrom
asalinityvalueof30tolessthan5inatleast5ha,lessthan15inatleast25haandless
than25inatleast70ha.
Weappliedanadaptivemanagementby:
1. quantitativedefinitionoftheprojectobjectivesthroughnumericalmodeling,design
ofthehydraulicworksandidentificationoftheproperfreshwaterdischarge
necessarytoachievetheobjectives;
2. implementationoftheintegratedanalysisapproachthroughmonitoringand
validatednumericalmodeling;
3. assessmentofresultsandverificationofcomplianceofmanagementobjectives;
4. completionofconservationactionswithastep‐by‐stepapproachsupportedby
monitoringandmodelingresults.
Duringthedifferentphasesofthisadaptivemanagementapproach,anintensive
monitoringandmodelingactivitywascarriedouttosupporttheassessmentofthesuccess
oftheimplementedmeasuresandoftheidentifiedmitigationactions.Inmicrotidal
lagoons,thismonitoringactivityneedstointegratedifferentstrategiesabletocapture,
withthebestresolutionintimeandspace,variationsofwaterlevels,flowsandsalinity
thatareoftenencountered[11].
Inthiscontext,toolssuchasnumericalmodelscanusefullyintegrateobservations
andmeasurements,tomanagethevariabilityintimeandspaceofdescriptors[15],
providingamoreholisticdescriptionofthephysical,chemicalandbiologicaldynamics
[13].
ThroughtheresultsofthemonitoringactivitycarriedoutinVeniceLagooninthe
frameworkoftheLifeLagoonRefreshproject,thispaperillustrateshowthestudyof
salinityinestuarineenvironmentcanbecomesuccessfulandrobustcombiningdifferent
typesofmonitoringtechniques.
Inparticular,thecharacterizationintimeandspaceofsalinityvariations,performed
beforeandaftertheconservationactions,wasobtainedbytheintegrationofthree
differenttools:mooredsalinityprobesthatallowtheacquisition,infixedpositions,of
continuousmeasureddata;fieldcampaignswithCTDprofilersthatallowtheacquisition
ofinstantaneous/spatiallydistributedmeasureddata;theimplementationofnumerical
modelingthatallowsthesimulationofvariationsinspaceandtime.Argumentsforand
againsttheuseofthedifferenttoolsandmeasuresandtheimportanceofanintegrated
approach,thateffectivelycombinesmeasuredandmodeleddata,arediscussed.
2.MaterialsandMethods
2.1.StudyAreaandProjectDetails
TheVeniceLagoonisoneofthelargestcoastallagoonsintheMediterraneanSea
(approximately550km2).ItisconnectedtotheNorthernAdriaticSeabythreeinlets,and
experiencesmicrotidalconditionswithatidalrangeof±0.50mduringspringtides[23].
Thelagoonbasinismainlycomposedbyshallowwaterareas(averagedepthof1.2m)
intersectedbyanetworkofdeeperchannelsleadinginwardsfromtheinletsand
branchinginsideeachsub‐basin[6,24–26].
Thelagoonisalsocharacterizedbyseveralfreshwaterinputsfromthedrainage
basin,themostconsistentofwhicharelocatedinthenorthernsub‐basin[15].Thecurrent
supplyoffreshwaterfromthecatchmentareaisabout30m3/sfortheentirelagoonand
about17m3/sfortheNorthernLagoon[17].Mostofthefreshwatersthatoriginallyflowed
intheNorthernLagoon,throughtheSileRiverandotherminorrivers,weredivertedinto
theabandonedriverbedofPiaveRiverwiththeconstruction,onthenortheasternedgeof
thelagoon,ofthe“TagliodelSile”canal(1683).

Environments2022,9,314of25


AlongtheSileriverembankment,closetotheareaoftheproject,aspillwaywasbuilt
aftertheextremeeventof1966toprotectthetownofJesolo,locateddownstream,from
theriskofflooding.InadditiontothenewfreshwaterinputimplementedbytheProject,
duringsignificantoverflowevents,aconsiderablevolumeoffreshwaterentersthelagoon
throughthisspillway,withpeaksofflowratesoftensofm3/s.Between2002and2017,
aboutninesignificantoverfloweventswererecordedperyear,withanaveragevolume
ofwaterspilledintothelagoonofabout460,000m3perevent.
TheLifeLagoonRefreshprojectforeseesdifferentconservationactionsthatconsist
offreshwaterdiversionintheSileriverembankment,morphologicalreconstructionand
reedbedtransplantationinthelagoonalareaof70halocatedinfrontofthehydraulic
opera(Interventionarea)andaquaticangiospermtransplantationinabout1900ha(Pro‐
jectSite),asreportedinFigure1.
Theinterventionareaisashallow,innerportionofthenorthernsub‐basin,charac‐
terizedbyanaveragedepthof60cm,bythepresenceofresiduesofsaltmarshes,mainly
erodedbythewindactionandtherelatedwaveaction,andbythealmostcompleteab‐
senceofchannelsthatmakesthenavigationintheareaparticularlydifficult.
Itischaracterizedbyhigh‐waterresidencetimes(>20days[24])andeuhalinecondi‐
tions,withsalinitylevelsoftenhigherthan30[15,27].Fromatechnicalpointofview,the
areawaschosenbecauseitwasclosetothecourseoftheSileRiver,withflowratesand
waterqualityadequatetoallowthedeviationofapartoftheflowintothelagoon.
ThefreshwaterinflowfromtheSileRivertothelagoonispermittedbyahydraulic
infrastructure,realizedbetween08/2019and02/2020withintheproject,consistingina
canalforintakefromSileRiver,acrossingsectionoftheembankmentmadebytwopar‐
allelpipes(diameterof0.8m)equippedwithsluicegatestoregulatetheflowrateanda
returncanalinthelagoon.Thefreshwateroccursbygravity,accordingtothedifference
betweentheriverlevelandthetidallevelinthelagoon.
Thestructurewasequippedwithtwoflowmetersprovidingremotelyaccessibledata
inrealtime.Afterthecompletionoftheinfrastructure(postoperamphase),thefreshwa‐
terinflowwasgraduallyincreasedstartingfromapprox.300ls−1inMay2020upto1000
ls−1inFebruary2021.
Themorphologicalworkswerecarriedoutusingmodularbiodegradableselements,
positionedontheshallowlagoonbottomtoslowdownthedispersionoffreshwaterin‐
troducedintothelagoonandtoallowtheestablishmentofasuitablesubstrateforthe
developmentofreedbeds.

Figure1.(a)AmapoftheVeniceLagoonwithmainrivers(BacchiglioneandBrentainthesouth,
SileandPiaveinthenorth)divertedduringhistoricalhumanenvironmentalmodifications[16];(b)
projectsiteandthenewfreshwaterinputrealizedbytheLifeLagoonRefreshproject.

Environments2022,9,315of25


2.2.MooredProbes
Toinvestigatethevariabilityintimeofsalinityinfixedpositions,threemoored
probeswereinstalled.Inparticular,continuousmeasureswereacquiredusinginstru‐
mentedbuoys(IJINUS,ClaireGroup,Mellac,France)equippedwithconductivity(graph‐
iteandplatinumelectrodes)andtemperaturesensors.Salinitywascalculatedautomati‐
callybytheprobeusingtheUNESCOalgorithm[28].Theinstrumentedbuoyisabout50
cmlong(withouttheantenna)anditisadequateforthemonitoringofshallowwaters
subjectedtomicrotidalcycle.Theinstrumentedbuoyswereanchoredusingaballastand
themeasureswereacquiredataconstantdepthofapprox.25–30cmfromthewatersur‐
face.Datawerecontinuouslyacquiredwithafrequencyof10min.Aninternaldatalogger
storedthedataandaGSM/GPRSsystemwasusedfortheirdailytransmission.Thein‐
stalledprobeswereregularlymaintainedandcleanedinsitutoensurethequalityofthe
collecteddata,withafrequencyof15–30days,accordingtotheseasonalconditions.
Thestrategyofsalinitymonitoringwasmodulatedaccordingtotheprogressofthe
operativestateoftheproject.Inparticular,twophaseswereidentified:onebefore(ante
operam)andoneafter(postoperam)thestartofthefreshwaterinflowbythehydraulic
infrastructure.Thethreebuoysarelocatedalongatransectatincreasingdistancesfrom
theinflowlocation:P1closetothehydraulicinfrastructure,P3outsidethemorphological
structure,consideredtobeareferencelocation,andP2locatedinanintermediateposition.
P3bisanadditionalpositionusedinashortperiodtoinvestigatesuitableconditionfor
reedbedintermofmeansalinity.ThelocalizationofthemooredstationsisshowninFig‐
ure2.
Duringtheanteoperamphase,salinitywasmonitoredoveraperiodofoneyear
(May2019–April2020)usingP2andP3probes.Duringthepostoperamphase,starting
fromMay2020withfreshwaterof300ls−1andgraduallyincreasedupto1000ls−1,salinity
wasmonitoredalsoinP1.
AlldatatransmittedbythebuoyswerecollectedinanFTPserveranddailyincorpo‐
ratedinaspecificdatabase,whichcontainsmeteo‐marinedataimplementedbyISPRA
including,amongothers,anestimationofthetidallevelindifferentpartsofVeniceLa‐
goon.

Figure2.Mooredprobes:locationalongatransectwithintheareainfrontofHydraulicandMor‐
phologicalstructures.P1:closetothehydraulicinfrastructure;P2:locatedinanintermediateposi‐
tion;andP3:inathirdsiteoutsidethemorphologicalstructure,consideredtobeareferencelocation.
P3bposition,toinvestigatesuitableconditionforreedbed,isalsoreported.

Environments2022,9,316of25


Datawereprocessedtoeliminatespuriousvaluesduetomomentaryairexposition
ofthesensorsduringverylowtidesaswellastoinstrumentalissues.Atwo‐stepprocess
waschosen.Firstsalinityvaluesbelow0.1andover41wereeliminated.Thesecondstep
wastocalculatetheZ‐scorevalueintoamobilewindowof6h(halfofthetidecycle,37
values):
Z 
xμ
σ(1)
withμmeanvalueandσstandarddeviationwithinthemobilewindow.Valueswithas‐
sociatedZ‐scorebiggerthan1.96orsmallerthan−1.96,correspondingtothe97.5percen‐
tilepointofastandardnormaldistribution,wereeliminated.
Afterthecleaningprocess,datawereelaboratedtogetmaindailystatisticsasmean,
median,quartiles.Thepercentageoftimewithsalinityvaluesbelow5‐15‐25,chosenas
quantitativeobjectivesforprojectgoals,wascalculated.Inparticular,thevalueof15is
consideredathresholdforreedbedsuitability[29].
Timeserieswerealsoevaluatedinrelationtotidallevel,freshwaterdischargeand
riveroverflow.
2.3.CTDProfiles
DifferentfieldcampaignsperformedtoacquireCTD(Conductivity,Temperature,
Depth)profileswereplanned,toinvestigatetheinstantaneous/spatiallydistributedchar‐
acterizationofsalinitywithintheareaofinterest.
Acombinedstrategywithtwoboatsandrespectiveequipmentallowedtoacquire
datasimultaneouslyatalocalandlargescale.Inparticular,atthelocalscaleadetailed
gridof25verticalprofilesofmeasureswasusedtoevaluatelocaleffectsclosetotheinter‐
ventionarea(about1.3km2),whilefourtransectswereinvestigatedwithverticalprofile
collectionataregulardistanceof350mtoevaluatethesalinityatalargerscale(Figure3).
Themultiparameterprobeusedforthelarge‐scalesamplingisanOceanSeven
316PlusmodelbyIdronautSrl(Brugherio(MB),Italy),equippedwiththefollowingsen‐
sors:pressure,conductivityandtemperature.Pressuresensoraccuracyandresolutionare
0.02dbar,and0.008dbar,respectively;conductivitysensoraccuracyandresolutionare
0.003mS/cmand0.00025mS/cm,respectively;temperaturesensoraccuracyandresolu‐
tionare0.003°Cand0.0002°C,respectively.Timeacquisitionwassetat20Hzanddata
arestoredinaninternalnon‐volatilememory.DatawereformerlyprocessedwithIdro‐
nautsoftwareREDAS‐5.
ASonTekCastAway‐CTDmultiparameterprobeequippedwithpressure,conduc‐
tivityandtemperaturesensorswasusedforthelocalscalesampling.Italsohadaninter‐
nalGPSreceiver,usedtoassociatethefieldpositionwitheachcast.Theacquisitionsam‐
pleratewassetat5Hz,pressuresensoraccuracyis±0.25%andresolutionwas0.01dbar
andconductivitysensoraccuracyis0.25%±0.005mS/cmandresolutionwas0.001mS/cm.
Temperaturesensoraccuracyandresolutionwere±0.05°Cand0.001C,respectively.
Duringtheanteoperamphase,salinitywasmonitoredin2018overtwoseasonal
campaigns(spring,autumn)atdifferenttidalconditions(spring‐neaptide).
Duringthepostoperamphase,thefreshwaterwasgraduallyincreasedstartingfrom
300ls−1inMay2020upto1000ls−1inFebruary2021,andthedistributionoffreshwater
wasmonitoredondifferentoccasions.DetailsaresummarizedinTable1.
Inthispaper,onlyspringtidalconditionsareevaluatedandcomparedbeforeand
aftertherealizationoftheconservationactions.
Ineachcampaign,thedistributionofsalinitywaspossiblyinvestigatedintwodiffer‐
enttidalconditionstoevaluatetheminimumandmaximumdiffusionasafunctionof
tidalphase(flood/ebb).
Theverticalprofiles,acquiredwithacentimetricprecisionandpost‐processedwith
averticalresolutionof5cm,wereinterpolatedusingaKrigingmethodgivinganinstan‐
taneouscharacterizationofthespatial(planimetricandvertical)distributionofsalinityin

Environments2022,9,317of25


termsofmeanvaluewithinasurfacelayerofabout30cmandmeanvaluealongtheentire
watercolumn.
ThespatialinterpolationusingtheKrigingmethodwascarriedoutwithEsriArcMap
10.3.1andresultedinageotiffrastermapperdatasetwithacellresolutionof2m.
Frommapsofthespatialdistributionofsalinity,obtainedforeachcampaignand
eachinvestigatedtidalcondition,twotransectswereextractedtoverifythegradientof
salinitybetweenthelagoonsideandthelandside.

Figure3.CTDcampaigns:localandlargescalestrategies.Pointswithinformationrelativetothe
completeverticalprofilearealsoindicated.
Table1.CTDcampaignsbeforeandaftertheopeningofthefreshwaterflow.Meantidevaluesare
evaluatedaccordingtothenearestISPRA’smeteo‐mareographicstationlocatedinGrassabòarea.
Valuesoftidallevelarereferredtoma.s.l.andtheyareaveragedvalueswithineachtimewindow.
(*)Campaignsconsideredforthisstudy.
PhaseCTDCampaignInvestigatedTidalConditionDischarge
Ante
operam—no
flow
16April2018(*)
Springtide
(floodtidewithameanvalueof0.05ma.s.l.between8:25–12:00summertime,
ebbtidewithameanvalueof0.25between14:10–16:15summertime)
0ls−1
18November2018
(localscale)
31October2018(large
scale)
Neaptide
(Localscale:lowtideuniquephasewithameanvalueof0.15ma.s.l.between
10:00–16:30summertime.Largescale:hightideuniquephasewithamean
valueof0.44ma.s.l.between12:00–16:30standardtime)
0ls−1
Post
operam—
freshwater
flow
23June2020(*)
Springtide
(beginninghightidewithameanvalueof−0.15ma.s.l.between9:20–12:10
summertime,hightidewithavalueof0.30ma.s.l.between14:40–17:00
summertime)
300ls−1
28January2021
Springtide
(beginninglowtidewithameanvalueof0.65ma.s.l.between11:00–13:30
standardtime,endinglowtidewithavalueof0.15ma.s.l.between14:40–
17:00standardtime)
500ls−1
26February2021(*)
Springtide
Localscale:uniquephasewithflowtide,meanvalueof0.16ma.s.l.between
9:30–12:30standardtime
1000ls−1
10
J
une2021(*)Spingtide1000ls−1

Environments2022,9,318of25


(uniquephasewithebbtide,meanvalueof0.20ma.s.l.between14:00–16:00
summertime)
15November2021
Neaptide
(Localscale:lowtideuniquephasewithameanvalueof0.23ma.s.l.between
15:00–16:10summertime)
1000ls−1
2.4.NumericalModeling
Anumericalhydrodynamicmodelwasimplementedtocharacterizesalinityvaria‐
tionsintimeandspace.Thenumericalsimulationswerecarriedoutusingthefiniteele‐
mentnumericalmodel2DEF[30–32],whichwasdevelopedattheDepartmentICEAof
UniversityofPadovaandwidelyappliedandvalidatedintheVeniceLagoon[33–37].The
modelsolvesthetwo‐dimensionalshallowwaterequationsinasuitableformtodealwith
floodinganddryingprocessesinveryirregulardomains[31].Theintegrationinspace
usesasemi‐implicitstaggeredfiniteelementmethod[38]whereastheintegrationintime
ismadethroughasemi‐implicitscheme.Thehorizontaldiscretizationofthedomainis
obtainedusinganunstructuredtriangularmesh.Waterlevelsarecomputedatthenodes
ofthegrid,whiledepthintegratedvelocitiesarecomputedatthecenterofeachtriangular
element[39].
Themodelcanbealsoappliedin3Dbaroclinicmode(3DEFmodel),i.e.,usingthe
samehorizontalgridofthe2Dmodel,withthewatercolumndiscretizedinlayersofvar‐
iablethickness[40,41].Watertemperatureandsalinityaretransportedsolvingthe3Dad‐
vection‐diffusionequation[42].Theformulationofthe3DEFmodelisparticularlysuita‐
bleforsimulatinghydrodynamics,salinitytransportandmixinginveryshallowtidalen‐
vironments.Allowingtheuseofverythinlayersnearthesurface,suitableresolutioncan
beachievedaroundthepycnocline,evenincasesofstrongverticaldensitygradients.
The2DEFand3DEFnumericalmodelswereusedinsequencetosimulatetidalhy‐
drodynamicsandsalinitydisplacementsintheprojectsite.For2DEFsimulations,amesh
withabout83,600gridpoints(nodes)and160,600triangularelementswasused,including
thewholeVenicelagoonandaportionoftheNorthernAdriaticSea(Figure4).Thereso‐
lutionofthegridwasgreatlyvariable,anditwasveryrefinedintheprojectsitewhere
sidesoftrianglesare<10m.Lagoonbathymetrydatawerethesameusedinprevious
studies[35,43],exceptfortheareaoftheprojectsitewheremoredetaileddataacquired
withintheLagoonRefreshprojectwereavailable,whilethe2DEFsimulationswereforced
byimposingthetidalsignalattheseaboundaryandthetime‐varyingwindblowingon
thewholelagoon.Thewindshearstresscoefficientwasthesameasinpreviousvalidation
studies[33,34].
The3DEFmodelwasappliedtoasub‐domainofthe2DEF(Figure4),anditwas
forcedbytidallevelsobtainedby2DEFsimulationalongthesouthernboundaryofthe
domain(thebluelineinFigure4)andbywind.Boundaryconditionsincludeddischarges
ofthesmallriversenteringthelagoonandinflowsfromtheSileRiverthroughthewater
intakestructurewhichwasbuiltwithintheLagoonRefreshproject.

Environments2022,9,319of25



Figure4.Unstructuredtriangularmeshesusedformodelsimulations;(a)domainofthewholeVen‐
icelagoonusedwiththe2DEFmodel(83,586nodesand160,650triangularcells);(b)sub‐domainof
thenorthernlagoonusedwiththe3DEFmodel(34,826nodesand67,272triangularcells,14layers).
Tidalgaugelocations(LeTrezze‐LT,Grassabò‐GB,CanalAncora‐CAeLeSaline‐LSstations)are
depicted.
Correctdefinitionofsalinityinitialconditionsinthemodeldomainiscriticalforthe
3DEFdensitydrivensimulations.Eachsimulationstartedfromasalinityinitialdistribu‐
tionderivedfromtheavailablefieldmeasurementsinthelagoon,andthemodelwasrun
forasuitablestart‐uptimeinterval(about5days)beforetheanalysisperiod.Thecalibra‐
tionofthemodelconsistsofthetuningoftheparametersandcoefficientsinvolvedinthe
simulatedprocessesuntilthebestpossibleagreementbetweenmeasuredandmodeled
dataisobtained.
Differentscenarioswereperformed(Table2):foraperiodincludingtheCTDcam‐
paignof16April2018inanteoperamconditionsofabsenceofflow;foraperiodincluding
theCTDcampaignof23June2020withaflowrateof300ls−1.Duringbothcampaigns,
waterlevelandcurrentdatawerealsoacquired,bymeansofexistingtidalgaugesbelong‐
ingtotheISPRARealTimeTidalGaugeNetworkoftheLagoonofVenice(Grassabò‐GB,
CanalAncora‐CAeLeSaline‐LSstations)andbytemporarytidalgaugeplacedinthe
studysite(LeTrezze‐LT)(Figure4);moreover,ADCPmeasurementswerecollectedwith
aboatrepeatedlynavigatingthroughfourtransectsduringthewholetidalcycle(ebband
flood).Withanaccuraterepresentationofthemorphologyoftheseabed,theonlycalibra‐
tionparameterwasthecoefficientofroughness(theStricklercoefficient)thatwasat‐
tributedtotheelementsofthecomputationalmeshwithaconstantvalueof40m1/3s−1.
Withthisassumption,themodelaccuratelyreproducesboththewaterlevelsandtheflow
dischargesmeasuredduringthecampaigns.
Asimulationscenariowasthencarriedoutwiththecalibratedmodeltovalidate
modeledresultswiththemeasurementsobtainedintheCTDcampaignof26February
2021(flowrateof1000ls−1).
Table2.Scenariosofnumericalmodelingsimulation.
TypeofScenarioPeriodDischarge
ConditionCTDCampaign
Calibration3April–18April20180ls−116April2018
Calibration20
J
une–5
J
uly2020300ls−123
J
une2020

Environments2022,9,3110of25


Validation23February–10March
20211000ls−126February2021
2.5.DataEvaluation
2.5.1.MooredProbesData
Toobtainacharacterizationovertimeofmeanvaluesofsalinityintheareaofinter‐
ventionatincreasingdistancefromthefreshwater,datacollectedbythemooredprobes
duringperiodsof14daysatthethreedifferentfreshwaterregimes(Table3)wereana‐
lyzed.Theseperiodswereconsideredlongenoughtoincludevariationsoftidalcondi‐
tionsduringspringandneaptide.TemporalseriesofdailymeansalinityandBoxand
Whiskerplotstocompareresultsarealsopresented.
Table3.Periodsoftwoweekswithdifferentregimesoffreshwaterdischarge,duringwhichmoored
probesdatawerecomparativelyevaluated.
PhaseDischargePeriod
Postoperam—freshwaterflow
300ls−112–25
J
une2020
500ls−129January–11February
2021
1000ls−112–25February2021
2.5.2.CTDProfileData
CTDprofiles,obtainedduringthedifferentcampaignswithdifferentdischargere‐
gimes,wereanalyzedtoevaluate,forspecificinstantaneousconditionscorrespondingto
theacquisitionduringfieldcampaigns,thedistributioninspaceandthethicknessofthe
freshwaterlenstransportedbytheinteractionofthefreshwaterflowandthetideinside
thelagoonalwaterbody.
Verticalprofilesacquiredfordifferentpositions(Table4)chosenintheCTDlocal
scalegrid(Figure3)atincreasingdistancefromtheinputarecompared.
Table4.SpecificpositionschosenintheCTDlocalscalegrid(Figure3)toevaluateverticalprofiles
atincreasingdistancefromthefreshwaterinput.
PositionDistancefromInput
GRD3120monwestdirection
GRD29105monsouthdirection
GRD23500monsouthdirection
GRD101400monsouthdirection
ParticularattentionwasfocusedtoevaluatethesurfacelayertocompareCTD
measureswithcontinuousmeasuresacquiredbythemooredprobesanddataproduced
bynumericalmodelingsimulations.
2.5.3.NumericalModelData
Inparticular,frommodelresults,timeseriesofsalinitywereanalyzedwithauto‐
matedDrEAMtools[44],originallyimplementedforthequantitativecharacterizationof
dredgingplumedynamicsandhereadaptedtoevaluatesalinitytemporalandspatialvar‐
iations.Foreachnodeandateachverticallayerorcombinationoftheminthedomain,
timeseriesofsalinitycanbeextracted.
Duetotheshallownessofthearea,indifferentpositionsofthedomainasafunction
ofthelocalbathymetry,thedifferentverticallayersalongthewatercolumnmayexperi‐
encedryconditionsdependingonthetidallevel(H,blackdashedlineinFigure5),with
deeperlayersdryingearlier.ArealisticmeanvalueofsalinityforaSurfaceLayer(bold

Environments2022,9,3111of25


bluecontinuouslineinFigure5)canbeobtainedbyevaluatingthemeantimeseriesfor
Layers11–14(verticalpartofthewatercolumnbetweenthesurfaceand−0.42m).
Statisticalparameters(i.e.,mean,standarddeviation,standarderror,etc.)usefulfor
thequantitativecharacterizationofthesalinityvariationinspacecanbederivedintegrat‐
inginformationovertime.Inparticular,mapsofmeansalinityduringtheentireduration
ofsimulationforthesurfacelayerwerecalculatedbyevaluatingforeachpointinthedo‐
mainthemeanvalueovertime(bluedashedlineinFigure5)ofthespecifictimeseries,
andcomparingittothethresholds(i.e.,5‐15‐25),establishedasreferencevaluestobe
achievedbytheproject.Theextractedtimeseriescanbeeasilycomparedwiththresholds,
byidentifyingevents(duration,ti,Figure5)whensalinityconditionislowerthanspecific
values(e.g.,15,thresholdofinteresttoevaluatereedbedsuitabilityconditions).
Mapsofthepercentageoftimewhensalinityislowerthanathresholdcanshow
areaswithmoreorlessstableconditionsintermofmeansalinity.

Figure5.Numericalmodeledtimeseriesofsalinityfordifferentverticallayers(11–14,wetordry
asafunctionoflocalbathymetryandtidallevel)andtheirmeansalinityvalues(SurfaceLayer).
Identificationofevents(duration,ti)withsalinitylowerthanadefinedthreshold(15).Evaluationof
meansalinityvaluesovertime.
3.Results
3.1.MooredProbes
Thevalueofsalinitychangesduringthedaybecauseofthetidallevelanditsinter‐
actionwithfreshwaterflows(Figure6).Inanteoperamconditions,withnofreshwater
flow(Figure6a),thesalinityfluctuatesduringthedayinarangeof5,while,inpost
operamconditionwithafreshwaterinflowof300ls−1establishedafterMay2020(Figure
6b),salinitycanexperiencesharperdailyvariability(uptoadailyrangeof30).Overall,
thedailyminimumsalinityoccursduringthelowtideattheendoftheebbphase.
Moreover,duringfloodconditions,correspondingtohigherleveloftheSileRiver
(Figure6b,boldgreyline,7–10June2020)andstrongerdifferencebetweenSileRiverlevel

Environments2022,9,3112of25


andlagoonmeanlevel,salinityundergoeslonglastingdropsduetotheoverflowofthe
freshwaterfromtheriver(spillwaypresentclosetotheProjectopera)intothelagoon.
Despitetheminordailyfluctuations,intheanteoperam,monthlyaveragedsalinity
showedaclearseasonalpattern,withhighervaluesinsummer(closeto30)andlower
salinityinlateautumn/earlywinter(Table5).Thelowsalinityvalues,intheanteoperam,
wererelatedtotheseveraloverfloweventsoccurredinthatperiod.
AfterMay2020,afreshwaterflowwasestablishedandprogressivelyincreasedfrom
300ls−1upto1000ls−1reachedinFebruary2021.
Lookingattheresultsrelativetoeachprobe,themeanvaluesofsalinityisdecreasing
withtheincreasingofdischarge.Intheanalyzedperiods(Table3),datafromtheprobe
closertothefreshwaterinput(P1)decreasedfromameanvaluehigherthan20(discharge
300ls−1)tomeansalinitylowerthan5(1000ls−1).Similarly,attheprobeP2,salinityde‐
creasedapproximatelyfrom25to15.MinorvariationsoccurredintheprobeP3,located
atamajordistancefromthefreshwaterinput.

(a)

(b)
Figure6.Dataofsalinity(10minoffrequency)registeredin10/2019((a),anteoperamcondition)
andJune2020((b),postoperamcondition)inP2mooredprobe.Comparisonwithlagoontidallevel
andSileRiverleveldataisreportedtoshowthecomplextrendofsalinityasafunctionofthetidal
levelanditsinteractionwithfreshwaterflows.



Environments2022,9,3113of25


Table5.Statisticsofmonthlymeanvaluesofsalinitybeforetheestablishmentofaregularfresh
waterflow(May2020)acquiredbyP2mooredprobeinstalledinthemiddleoftheareaofinterest.
PeriodMeanSalinity StandardDeviation
May201921.963.95
June201931.295.78
J
uly201933.693.79
August201929.431.58
September201932.344.03
October201931.462.01
November201921.575.60
December201916.723.35
January202017.162.15
February202021.452.97
March202024.493.08
April202032.854.40
Thedifferencesamongtheprobesconstantlyincreasedwiththeincreasingofthe
freshwaterinputrate(Figure7).With300ls−1regime,allsiteshavecomparablevariation
aroundthemedianvalueandtheinfluenceoffreshwaterislimitedattheP1andP2sites.
Increasingthedischarge,theinfluenceofthefreshwaterincreasesasexpected:measures
atP1andP2sitesshowanincreaseinlowersalinityvalues.Finally,with1000ls−1regime,
thedecreaseinsalinityandamarkedgradientbecomesaconsolidatedresult:P1isalmost
alwaysbelow5,P2showsaverylargevariationinsalinity,whileP3hasalowvariation
aroundthemedianvalue(25.6).Theabove‐mentionedpatternsareconfirmedbythetime
series,acquiredfromthethreemooredprobes,ofdailymeansalinityfortheperiodbe‐
tween12–25/02/2021withdischargeoffreshwaterof1000ls−1(Figure8).

Figure7.BoxandWhiskerplotsrelativetothreeperiodsoftwoweekswithdifferentregimesof
freshwaterdischargearecompared.Datarelativetothethreepositions(P1‐P2‐P3)arereported.

Environments2022,9,3114of25



Figure8.Dailymeansalinityacquiredfrommooredprobesfortheperiod12–25February2021with
dischargeoffreshwaterof1000ls−1.
Differencesofmeansalinitybetweenthedifferentlocationsalongthetransectwith
increasingdistancefromthefreshwaterinputareclearlyreported.Valueslowerthan5
areregisteredclosetotheinput(P1probe),valuesaround15areregisteredinthemiddle
ofthearea(P2probe)andvaluesaround25areregisteredoutsidethestructures(P3
probe).
3.2.CTDProfiles
Thespatialdistributionofsalinity,capturedduringtheCTDcampaignof16/04/2018
beforethefreshwaterinput,showsthealmostcompleteabsenceofagradientofsalinity,
withvaluesbetween26.7and31.5(Figure9a).

(a)(b)
Figure9.MapofKriginginterpolationperformedonresultsofCTDcampaigns:(a)campaignof16
April2018withnofreshwaterinput;(b)campaignof10June2021withadischargeof1000ls−1.In
bothmaps,thesurfacelayer(0–30cm)isreported.

Environments2022,9,3115of25


Theestablishedgradientwithintheareaofinterest,withdischargeof1000ls−1canbe
verifiedanalyzingtheresultsoftheCTDcampaignof10June2021(Figure9b).
Comparingtheconditionbeforefreshwaterflow(CTDcampaignof16April2018)
andaftertheconsolidateddischargeof1000ls−1(10June2021),theresultsshowastrong
reductionofsurfacesalinitywithinadistanceof500mfromthefreshwaterinputalong
transect2,withsalinitylowerthan5within200mandsalinitylowerthan15within400
m(Figure10).
Toinvestigatethechangesinverticaldistributionofsalinitywithincreasingdis‐
chargesasafunctionofthedistancefromtheinputoffreshwater,verticalprofileswere
collectedduringtheCTDcampaignsrespectivelyinanteoperamconditions(16April
2018,AC)andpostoperamconditions(10June2021,PC)atincreasingdistancefromthe
input(GRD3,GRD29,GRD23,GRD10)(Figure3andTable4).
Beforefreshwaterinput,theverticalprofileishomogeneous,withconstantvalues
around28–30foreachposition.Duringthepostoperamphasewithdischargeof1000ls−1,
inpositionsclosertothefreshwaterinput(around100–120m,GRD3andGRD29)astrong
verticalstratificationcanbefoundwithasharpreductionofsalinityfordepthbetween20
and30cmthatcorrespondstoafreshwaterlensfloatingonheaviersaltwater(Figure11).
Homogeneousverticalprofilecanbefoundonlyatdistancesgreaterthan500m
(GRD23‐GRD10),butmeansalinityvalueisdefinitelylower(withvaluesof22andaround
25).

Figure10.Variationofmeansurfacesalinityalongspatialtransect1and2comparingthetwoCTD
campaignswithoutflow(16April2018,blacklines)andwithadischargeof1000ls−1(10June2021,
bluelines).

Environments2022,9,3116of25



Figure11.VerticalprofilesofsalinitiesatfourdifferentstationscollectedduringtwoCTDcam‐
paignsinanteoperamcondition(opensquaresymbols,16April2018)andpostoperamcondition
(closedcirclesymbols,10June2021),respectively.Thecolor‐codedstationsGRD3,GRD29,GRD23
andGRD10arelocatedatincreasingdistancefromthefreshwaterinputs,seeTable4andFigure3.
3.3.NumericalModeling
Thenumericalstudywascarriedoutforevaluatingmodelperformances,bycompar‐
ingcomputedsalinityandobservationsbythemooredprobesanddatacollectedduring
theCTDsurveys,andforinvestigatingindetailthehorizontalandverticaldisplacement
ofthefreshwaterinthestudyarea,duringthedifferentphasesofthetidalcycle(flood
tideandebbtide).Forthispurpose,a15‐daysimulationwasruntoreproducetheperiod
23February2021–10March2021,whentheaveragefreshwaterdischargeintroducedin
thelagoonfromtheSileriverwasmaintainedat1000ls−1.
3.3.1.ModelPerformance
Thecomparisonofmodeledandmeasuredsalinityissatisfactorybothforthemoored
probesdataandtheCTDsamples.Despitethecomplexityoftheflowfield,whichis
drivenbytide,windanddensitydifferencesandisstronglyinfluencedbythecomplicated
morphologyofthearea,modelresultscomparefavorablywiththemeasures.Dailyaver‐
agedsalinitytrendatthemooredprobesisveryclosetomeasuresandwellrepresentsthe
persistenceofthestronghorizontalsalinitygradientthatisestablishedbetweenthepoint
wherethefreshwaterisintroducedandtheopenpartofthelagoon(Figure12a).Com‐
parisonwiththeCTDsamplesalsodemonstratesthatmodelresultscanreproducethe
actualflowpathsandcangivearealisticrepresentationofthefreshwaterspreadinginall
thestudyarea(Figure12b).

Environments2022,9,3117of25



(a)

(b)
Figure12.23February–9March2021simulation.(a)Comparisonofdailyaveragedsalinitycom‐
putedandmeasuredatmooredprobes;(b)mapofdepth‐averagedsalinityobtainedbymodelre‐
sultsandbyCTDmeasurementsof26February2021.
Thevariabilityofthesalinityinspace,thatisatdifferentdistancesfromthefresh
watersource,andintime,thatisatdifferenttimesduringthetidalcycle,iswellillustrated

Environments2022,9,3118of25


bytheplotsinFigure13,representingcalculatedsalinityattheP1‐P2‐P3mooredprobes.
Plotsrefertothelastfourlayersoftheverticalmodeldiscretization(layers11–14),which
approximatelyrepresentsthe40cmsurfacelayerofthewatercolumn(seeFigure4).It
canbeobservedthatinthepointclosertothefreshwatersource(P1)theaveragesalinity
remainsaroundzero,whereasatthefarthestpoint(P3)itvariesaround20÷25.Inthein‐
termediatepoint(P2),thesalinityassumesintermediatevaluesasexpected.However,it
isevidentthatinP2thefluctuationsinsalinityduetotidalvariationsaremuchmore
pronouncedthanintheothertwopoints,sincesalinityapproximatelyvariesbetween0
and20ateachtidaloscillation.Basically,aswillbebetterexplainedlater,theareaslocated
halfwaybetweenthestationsP1andP3showthegreatestvariationsofsalinityintime.

Figure13.23February–9March2021simulation.SalinityinthepositionofP1‐P3mooredprobes
calculatedbythemodelforthesurfacelayers(11–14).

Environments2022,9,3119of25


3.3.2.DrEAMTool
ApplyingtheDrEAMtool,amapofmeansalinityduringtheentiredurationofsim‐
ulation(23February–10March2021)wascalculatedfortheSurfaceLayerevaluatingfor
eachpointinthedomainthemeanvalueovertimeofthespecifictimeseries(Figure14).
Thismapwasproducedbyidentifyingdiscreteclassesofvaluesofsalinity(0–5,5–15,15–
25,>25)andclearlyshowstheextensionofareaswiththenewestablishedconditionasa
consequenceoftheinteractionbetweenthefreshwaterflowandthetidaloscillation.In
particular,anareaofabout8hahasameansalinityvaluelowerthan5,anareaofabout
34hahasameansalinityvaluelowerthan15andanareaofabout96hahasamean
salinityvaluelowerthan25.

Figure14.Mapsofsalinitydistribution,evaluatedasthemeanvaluecomputedbythemodelwithin
theverticallayers11–14ineachnodeofthedomain,fortheperiod23February–10March2021,with
adischargeof1000ls−1.
Comparingthemeanvalues(respectivelyaround5forP1,15forP2and25forP3,
Figure8)fordailytimeseriesandthevaluesofmeansalinityintheirpositionsinthemap
ofsynthesisofnumericalmodeleddatareportedinFigure14,itcanbestatedthatthese
probeswellrepresenttheirsurroundings.
Amapofthestandarderrorofmeansalinityduringtheentiredurationofsimulation
(23February–10March2021)wascalculatedforthesurfacelayer,byevaluatingforeach
pointinthedomaintheratiobetweenthestandarddeviationandtherootofthenumber
ofelementsinthespecifictimeseries(Figure15).Witha1000ls−1discharge,theareacloser
tothefreshwaterinlet(nearP1sitewithmeansalinityfrom0to5)andthefarthestone
(nearP3sitewithmeansalinityfrom15to25)arelessvariableintermsofsalinity,while,
inthetransitionalarea,wherefreshwaterandmarinewatermixthemselvesaccordingto
tidalcyclesandtheSileriverlevel,themaximummeanvariabilityisdetected.Themodel
simulatesadetailedspatial‐distributedpictureofwhatwasalreadyreported,bymeans
ofBoxandWhiskerplotsforthethreepositions(P1‐P2‐P3)inFigure7,intermofvaria‐
tionsofsalinityinspace.Meanvariabilityattherightsideofthehydraulicstructureis
higherthanattheleftsidealsobecausebathymetryisveryshallow(presenceoftidalflats
aroundsaltmarshes).Standarderror,insteadofstandarddeviation,wasadoptedasa

Environments2022,9,3120of25


measureofvariabilitytoconsiderinthecalculationofthedifferentamountsofavailable
datarelatedtowaterdepthandthenumberofverticallayers.
Amapofthepercentageoftimewithconditionofmeansalinitylowerthan15,com‐
putedusingtheDrEAMtoolwithintheverticallayers11–14ineachnodeofthedomain,
wasproduced(Figure16).

Figure15.Mapsofstandarderrorofsalinityvaluescomputedbythemodelwithinthevertical
layers11–14ineachnodeofthedomain,fortheperiod23February–10March2021,withadischarge
of1000ls−1.

Figure16.Mapsofpercentageoftimewithconditionofsalinity,evaluatedasthemeanvaluecom‐
putedbythemodelwithintheverticallayers11–14ineachnodeofthedomain,<15(conditionfor
reedbedsuitability).

Environments2022,9,3121of25


Intheareainfrontofthefreshwaterinflowanddelimitedbythemorphological
structure,about8ha,theconditionofmeansalinitylowerthan15canbeconsideredtobe
stable,beinginplacefor75%–100%oftime.Correspondingly,anareaofabout15haun‐
dergoesameansalinitylowerthan15foratleast50%oftime.
4.Discussion
Dependingonthenature(hydraulicworks,naturalconnection)andthecharacteris‐
ticsoftheirconnectionswiththesea,theirwatershedsorsurroundingrivers,shallow
coastallagoonscanexhibitverysignificantspatialandtemporalheterogeneityinboth
salinityandwaterlevel.Theimportanceofimplementingdifferentmonitoringstrategies
tounderstandthehydrosalinefunctioningoftheselagoonsisreportedintherecentliter‐
ature[11].
Intheplanningphaseofrestorationprojects,theobjectivesshouldbeidentified,
alongwithspecificindicatorstomeasureprojectprogress[21].Consideringtheobjectives
oftheLifeLagoonRefreshproject,torestorethetypicalsalinitygradientonatransitional
environmentwithintheNorthernVeniceLagoon,andthespatialandtemporalcomplex
variabilityofthesalinitytobeinvestigated,amonitoringstrategyofsalinitywithdifferent
toolswasdeveloped.
Theeffectivenessoftheconservationactionswaspreliminarilyinvestigatedandthen
verifiedthroughthecombinedimplementationofenvironmentalmonitoringandnumer‐
icalmodeling.Inparticular,inthiscasestudy,despitethesmallsizeofthearea,avery
higheffortofinvestigationwasadopted,consideringthat:
 therestorationofthesalinegradient,mainobjectiveoftheproject,requiredaprecise
quantitativeanalysis;
 thehighspatialvariability,inducedbytherealizationoftheinterventionwiththe
introductionofafreshwaterflow,andtemporalvariabilityonashortandmedium
scale,duetotheinteractionofthetideandtheseasonalvariabilityoftheboundary
conditions,requiredadetaileddescriptionwithadequateresolutionintimeand
space;
 eachtoolhasdifferentfeaturesandonlyanintegratedapproachcanidentifypros
andconsanddefineacombined,effectiveandefficientstrategy;
 theconsolidatedsmall‐scaleapproachshouldberobustandapplicabletoallscales.
Lookingatmooredprobes,therecordingofcontinuousdataisparticularlyimportant
inthepresenceofhighvariabilityintime.Continuousprobesallowtodetectfluctuations
relatedtothetidallevelandphase.Ingeneral,minimumsalinityvaluesareobservedin
conditionsofebbtide,inwhichthefreshwaterflowencountersadecreasingobstacleto
propagationandadecreasingamountofwater,whileinflowtidecondition,salinityis
higherduetotheincreasingtidallevelthatholdsthefreshwaterfrontneartheinput.
Continuousdataareimportantnotonlytoanalyzeshort‐termfluctuations,butalso
tohaveareliableaveragevalueonalongtimescale(monthly/annual),freefromtheran‐
domnessofthesamplingmomentorfromthesystematicselectionofcertainbiasedsam‐