Thermal Mediation in a Natural Littoral Wetland:Measurements and Modeling

Thermal Mediation in a Natural Littoral Wetland: Measurements and Modeling

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Citation Andradóttir, Hrund Ó., and Heidi M. Nepf. “Thermal Mediation in a Natural Littoral Wetland: Measurements and Modeling.” Water Resources Research 36.10 (2000): 2937–2946. ©2000 American Geophysical Union.

As Published http://dx.doi.org/10.1029/2000WR900201

Publisher American Geophysical Union

Version Final published version

Citable link http://hdl.handle.net/1721.1/69012

Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. WATER RESOURCES RESEARCH, VOL. 36, NO. 10, PAGES 2937-2946, OCTOBER 2000

Thermal mediation in a natural littoral wetland: Measurements and modeling Hrund0. Andrad6ttirand Heidi M. Nepf Ralph M. ParsonsLaboratory, Department of Civil and EnvironmentalEngineering MassachusettsInstitute of Technology,Cambridge

Abstract. As a river flowsthrough shallow littoral regionssuch as wetlands,forebays, and side arms,the temperatureof the water is modifiedthrough atmospheric heat exchange. This process,which we call thermal mediation,can control the initial fate of river-borne nutrient and contaminantfluxes within a lake or reservoir.This paper presents temperatureobservations that demonstratethe occurrenceof thermal mediation and directlysupport the theoreticalresults derived by AndradOttirand Nepf [2000]. The measurementsshow that the wetlandwarms the river inflow by approximately1-3øC during summer and fall nonstormconditions. Less thermal mediation occursduring storms,both becausethe residencetime is significantlyreduced and becausethe wetland circulationshifts from laterallywell mixed (low flows)to short-circuiting(storms). The dead-zonemodel can simulateboth theseregimes and the transitionbetween the regimes and is therefore a good choicefor wetland modeling.

1. Introduction This paper presentsdetailed observationsof thermal mediation in a natural wetland that receivesunregulated river in- Wetlands can play an important role in improving down- flow. The major contributionsof the paper are to (1) demon- stream water quality. Numerous massbalance studieshave strate through field observation that wetland thermal shown that suspendedsediments, nutrients, metals, and an- mediationoccurs in a smallwatershed and how it is affectedby thropogenicchemicals are efficientlyremoved in natural and flow conditions,(2) comparewetland thermal structureand constructedwetlands through a variety of sink mechanisms, circulationduring low flowsand storms,(3) validatethe useof suchas bacterial conversion,sorption, sedimentation, natural the dead-zonemodel in wetlands,both by confirmingthe an- decay,volatilization, and chemicalreactions [Tchobanoglous, alytical dead-zone model results derived by Andrad6ttir and 1993].Yet other studieshave shown that wetlandsmay alsoact Nepf [2000] and by showingthat the model simulateswell as a temporalsource of nutrientsand pollutantsas they release wetland thermal behaviorduring variable flow conditions,and storedmaterials [Mitsch and Gosselink,1993, p. 157]. To date, (4) demonstratethe effect of wetlandthermal mediationon extensive research has been conducted to understand the lake intrusiondynamics. This paper is the observationalcoun- chemical,biological, and physicalprocesses underlying the terpart to the theorypresented byAndrad6ttir and Nepf [2000]. sink/sourcepotential of these complexecosystems. However, one processthat so far has receivedlittle attention is thermal mediation,i.e., the temperaturemodification of the water flow- 2. Theoretical Background ing through the wetland. For littoral wetlands the outflow Thermal mediation is a well-known process in cooling temperature determinesthe lake intrusion depth which, in ponds, in which atmosphericheat exchangeis exploited to turn, affects the initial fate of nutrients/contaminants in the attenuatethe waste heat from power plants.Andrad6ttir and lake, aswell as the residencetime and mixingdynamics within Nepf [2000] showedthat thermal mediationis also an impor- the lake. Using a dead-zone model, Andrad6ttir and Nepf tant processin littoral wetlands,when the river inflow follows [2000]showed that wetlandthermal mediation can profoundly a different seasonaltemperature cycle than the wetlandwater. affect intrusiondepth. For example,this processcan shift the This condition is met in small or forested watersheds, where timescaleof lake intrusion depth variability from predomi- the river followsa dampedseasonal cycle because of ground- nantly seasonalto synopticand diurnal. Moreover, wetlands water recharge[Gu e! al., 1996] and/or sun shading[Sinokro! can prolongsurface intrusions during summer, which can lead and Stefan, 1993]. Thermal mediation is less significantin to increasedhuman exposureto river-borne contaminants. largerwatersheds, in which the river has enoughtime to ½quil- Similarly,the increasednutrient supplyto the epilimniondur- ibrat½with the atmosphereand in which sunshading and wind ing the growing seasoncan acceleratelake eutrophication shelteringare lessprominent. [Carmacket al., 1986;Metropolitan Council, 1997]. On the other While the inflow condition sets the potential for wetland hand, more surfaceintrusions lead to quickerflushing, poten- thermal mediation, the degree of thermal mediation actually occurring is governed by the residence time distribution tially reducingthe long-termdeposits of nutrientsand contaminants in the lake. Wetland thermal mediation can therefore (RTD) withinthe wetlandand the thermalinertia of the water /heat'The thermal inertia representsthe heating timescaleof havecomplex short- and long-termeffects on lake water quality. the water columnand is definedas the ratio of the water depth Copyright2000 by the American GeophysicalUnion. H, and the surface heat transfer coefficient K. The ratio of the Paper number 2000WR900201. nominalresidence time J to the thermalinertia, also called 0043-1397/00/2000WR900201 $09.00 thermal capacity,

2937 2938 ANDRAD(STTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITTORAL WETLAND

Table 1. Vegetation Drag Estimatesin the Upper Forebaya mixed reactorwhere the RTD has a large variancearound the meannominal residence time J (Figurela) [Kadlec,1994]. Vegetation Flow Re = UH/v Cfb A During highflows, however, the circulationbecomes inertia- or Yes low O (103) 0.2-2(1, 2) 28-280 jet-dominated(A, •, andFi -2 < 1), andsubstantial short Yes high O (105) 0.008-0.02(2, 3) 1.1-2.8 circuitingoccurs; that is, a large portion of the river flow exits No high O (105) 0.004(4) 0.55 the wetland in much less time than the nominal residence time abeddrag decreases significantly at highReynolds numbers, Re, due J (Figurelb). Boththese regimes, and especially the short- to pronationof vegetationstem. circuiting regime, produce less thermal mediation than the bEstimatesare adaptedfrom (1) Kadlecand Knight [1996, p. 201], ideal plug flow [Andrad6ttirand Nepf, 2000]. (2) Chen[1976], (3) Dunnet al. [1996,p. 54], and(4)Andrad6ttir [1997, pp. 69, 74]. 3. Methods 3.1. Site Description F = J/theat, (1) The Aberjonawatershed is a medium-sizedwatershed (65 km2 surfacearea) located in suburbanBoston, Massachusetts. is a major factor controlling how much thermal mediation Becauseof long term industrial activities the watershedis occurs. For "ideal" plug flow, r = 0 produces no thermal heavily contaminatedwith heavy metals such as arsenicand mediation,whereas r > 3 producesover 90% of the maximum lead which are routinelytransported downstream by the Aber- thermal mediationpossible in the system[e.g., Jirka and Wa- jona River until they are finally depositedin the Upper Mystic tanabe,1980]. Wetland flow regimes,however, are rarely ideal Lake [Solo-Gabriele,1995; •4urilio et al., 1994].Before entering [Kadlec,1994], and water circulation,which governs both the the lake, the river flowsthrough two littoral wetlands,first the skewnessand varianceof the RTD, will generallymodify how largerupper forebay and then the smallerlower forebay (Fig- efficientlythe wetland mediatesthe water temperatures. ure 2a). Both wetlandsare vegetatedwith water lilies and Wetland water circulationdepends on three meteorological submergedcoontail, and their mean water depth rangesbe- forces: wind, which scaleson the mean wind shear stressand tween 1.3 and 2.0 m dependingupon season.In comparison, wetland length as •wL; river buoyancy,which scaleson the the lake epilimnionis approximately5 m deep, and the ther- densityanomaly between the inflow and wetland water and moclineis 5 m wide during the summer. waterdepth as Ap#H2; and river momentum per unitwidth, In this paper, we focuspredominantly on the thermal me- pUSH, where Uo is the inflowvelocity and p is the water diation occurringin the upper forebay shownon Figure 2b. density.In addition,the water circulationis influencedby veg- This wetland providesmost of the thermal mediation in this etationdrag, which scales on the friction factor as • p . system,first, becausethe river dischargesinto it, second,be- The relative importanceof vegetationdrag, wind, and water causeits larger size (thus longerresidence time) allowsmore buoyancyto the river inertia canbe summarizedby the ratio of thermal alterations to occur, and, third, because it receives scales: limited return flow from the lake. In contrast, the lower fore- Vegetationdrag parameter[DePaoli, 1999] bay exchangessignificantly with the lake throughthe 60 m wide channelconnecting this wetland to the lake as illustratedon C•L A = 2 •q' (2) Figure 2a. The lower forebaycan thus be consideredto be an

Wind parameter

•w L n = (3)

(InternalFroude number) -2

Fi-2= IA P/P V•#H ' (4) Noticethat since both • andFi-2 cr U•-2, thenthe influence I I of wind and buoyancyon water circulationchanges with flow RTD(O RTD(O conditions.Similarly, A is indirectlyrelated to Uo throughthe frictionfactor Cf, whichdepends strongly on water depth and velocity, as well as vegetationtype and density [Petrykand _ ) t r : ) t BosmajianIII, 1975; Chen, 1976; Shih and Rahi, 1982]. In t t particular,A can decreasedrastically during storms, as a result a) b) of increasedwater depthsand stempronation. This is shownby the characteristicvalues of Cf givenin Table 1. Water circu- Figure 1. Bimodal circulationin free water surfacewetlands. lation is thereforeexpected to changebetween low flowsand (a) Under low flows,drag, wind, and/or buoyancy dominate the circulation(A, •, andFi-2 >> 1); thewetland is laterally well storms[DePaoli, 1999;•4ndrad6ttir, 1997]: During low flows, mixed producinga relativelysymmetric residence time distri- vegetationdrag, wind, and buoyancydominate the water cir- bution(RTD) aroundthe nominal residence time J. (b) Dur- culation(A, •, andFi-2 >> 1). Uniformvegetation drag will ing high flows the circulationis river-dominated(A, •, and help distributethe river flow overthe width of the wetland[Wu Fi -2 < 1); shortcircuiting occurs producing a skewed RTD and Tsanis,1994], and wind will enhancelateral and vertical with much of the flow exitingthe wetland in muchshorter time mixing.Consequently, the wetland behavesas a partiallywell than t. ANDRADOTTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITTORAL WETLAND 2939

extensionof the lake surfacelayer, and the outflowtemperature of the upper forebaycan be taken as representativeof the a) Upper temperatureof the lake inflow. Førebayj•J•....._2• Aberjona

3.2. Field Observations (• ,• River Loweri• 0.053 •\ Water temperatureswere monitoredevery 5 min at different locationsin the Upper Mystic Lake systemfrom July to No- vember in 1997 and 1998. Temperature measurementswere UpperMysti• • made 10-20 cm below the surface and 10-20 cm above the bed at each site in the upper forebay shownon Figure 2b using Onset temperatureloggers with 0.2øCresolution. In 1997 the temperatureloggers at the outlet of the wetlandwere repeat- edly stolen, so the following year, the measurementswere '97,'98 repeatedwith additionalloggers deployed at the outletand in the shallowvegetated zone. Lake water temperatureswere measuredin 1997 at 1-1.5 m depth intervalswithin the surface mixedlayer and thermocline(1.8-10.9 m depth) at the loca- b) tions shownon Figure 2a usingthermistor chains with 0.1øC resolution[Fricker and Nepf, 2000]. Lake intrusiondepth with and without the wetlandswas estimatedby matchingthe aver- ageof the near-surfaceand near-bedwetland channel temper- J•Coontail ature and the river bed temperature,respectively, to the lake thermistordata. Surfaceintrusions within the top 2 m in the .8m lake were not resolved because no thermistor was located there. Changesin suspendedsediment levels did not significantly affect the water densityand were neglectedin the cal- culations. Wind speed and direction were measured 10 m above groundat the southernend of the lake at 10 min intervalsin DeadZone 2 ¾•__••_-•'• 1997 and 1998 (Figure 2a). Hourly Aberjona River flow was 0.8 metres measuredby the U.S. GeologicalSurvey approximately 800 m upstreamof the inlet to the upper forebay. In addition, air Qr 0 100 200 300 temperature,relative humidity, and solarradiation were mon- itored every5 min during 1997 (Figure 2a). Cloud coverwas Figure 2. Monitoring program in the Upper Mystic Lake measuredat 3 hour intervals at the Boston Logan Airport system,Winchester, Massachusetts. (a) Site overviewand sur- located 15 km east of the studysite. face areas.(b) Detailed map of the upper forebaywith mean water depthsduring the 1998 monitoringperiod. Thermistor 3.3. Dead-Zone Model Application locationsare denotedby T, the anemometeris denotedby W, 3.3.1. Model formulation. The dead-zone model was de- and the weather stationis denotedby S. scribedin detail byAndrad6ttir and Nepf [2000].In short,the wetlandarea is dividedinto twozones, a flowzone (or channel) with cross-sectionalarea A c and a stationarydead zone with t) = r0(t), (7) cross-sectionalareaA a. As shownon Figure2b, while the river O•c traversesthe wetlandchannel (shaded area), it exchangescon- Ox =0. (8) tinuouslywith two stationarydead zoneson either sideof the x=L channel. Note that the southern dead zone is smaller, shal- Here u = Q•(t)/Ac is the averagespeed in the channel,L is lower, and more vegetatedthan the northern dead zone. The the length of the wetland, T O is the wetland inflow tempera- circulationregime in the wetland is describedby the areal ture,rk is thenet surfaceheat flux per surfacearea, and Cp is ratio, q = Ac/(A c + Ad) , and the nondimensionallateral the specificheat of water. exchangecoefficient a* - AQ/Q r, where AQ is the total The parametersq and a* are site-dependentand are typi- lateralexchange rate betweenthe channeland deadzones and cally evaluatedby conductingdye experiments[e.g., Bencala Qr is the inflow flow rate. Neglectinglongitudinal dispersion, and Walters,1983]. For preliminary design and assessment, the simplifiedgoverning equations for the depth-averagedwa- however, it is useful to derive expressionsbased upon the ter temperaturein the channel,Tc(X, t), anddead zone, Td(X, inflow and site geometry alone. First, consider the river- t), are dominatedcirculation regime, when the river flowsthrough the a7'c a7'c a*u rk wetlandlike a jet (Figure lb). The laboratoryexperiments of --+ u --=- rc) +- (5) at Ox L pCpHc' Chu and Baines[1989] suggestthat the jet spreadslinearly from its initial width Wo, with spreadingangle/• • 0.1, such O•'d a * U q q• that the channelwidth is W• = Wo + 2/•x. This yields the at= L 1- q(•d-- •c) + pCpHd. (6) mean wetland areal ratio of The boundaryconditions at the inlet (x - 0) and outlet(x - L) are q = (Wo+I•L)W He• •/• LW' (9) 2938 ANDRADOTTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITTORAL WETLAND

Table 1. Vegetation Drag Estimatesin the Upper Forebaya mixed reactorwhere the RTD has a large variancearound the meannominal residence time J (Figurela) [Kadlec,1994]. Vegetation Flow Re = UH/v Cf• A During high flows,however, the circulationbecomes inertia- or Yes low O (103) 0.2-2(1, 2) 28-280 jet-dominated(A, •, andFi -2 < 1), and substantialshort Yes high O (10s) 0.008-0.02(2, 3) 1.1-2.8 circuitingoccurs; that is, a large portion of the river flow exits No high O (10s) 0.004(4) 0.55 the wetland in much less time than the nominal residence time abed drag decreasessignificantly at high Reynoldsnumbers, Re, due J (Figurelb). Boththese regimes, and especially the short- to pronationof vegetationstem. circuiting regime, produce less thermal mediation than the bEstimatesare adaptedfrom (1) Kadlecand Knight [1996, p. 201], ideal plug flow [Andrad6ttirand Nepf, 2000]. (2) Chen[1976], (3) Dunn et al. [1996,p. 54], and (4)Andrad6ttir[1997, pp. 69, 74]. 3. Methods 3.1. Site Description r -' J/theat, (1) The Aberjona watershedis a medium-sizedwatershed (65 km2 surfacearea) located in suburbanBoston, Massachusetts. is a major factor controlling how much thermal mediation Because of long term industrial activities the watershed is occurs. For "ideal" plug flow, r = 0 produces no thermal heavily contaminatedwith heavy metals such as arsenic and mediation,whereas r > 3 producesover 90% of the maximum lead which are routinelytransported downstream by the Aber- thermal mediationpossible in the system[e.g., Jirka and Wa- jona River until they are finally depositedin the Upper Mystic tanabe,1980]. Wetland flow regimes,however, are rarely ideal Lake [Solo-Gabriele,1995;/1urilio et al., 1994].Before entering [Kadlec,1994], and water circulation,which governsboth the the lake, the river flowsthrough two littoral wetlands,first the skewnessand varianceof the RTD, will generallymodify how largerupper forebayand then the smallerlower forebay(Fig- efficientlythe wetland mediatesthe water temperatures. ure 2a). Both wetlandsare vegetatedwith water lilies and Wetland water circulationdepends on three meteorological submergedcoontail, and their mean water depth rangesbe- forces: wind, which scaleson the mean wind shear stress and tween 1.3 and 2.0 m dependingupon season.In comparison, wetland length as •wL; river buoyancy,which scaleson the the lake epilimnion is approximately5 m deep, and the ther- densityanomaly between the inflow and wetland water and mocline is 5 m wide during the summer. waterdepth as Ap#H2;and river momentum per unitwidth, In this paper, we focuspredominantly on the thermal me- pUSH, where Uo is the inflowvelocity and p is the water diation occurringin the upper forebay shownon Figure 2b. density.In addition,the water circulationis influencedby veg- This wetland providesmost of the thermal mediation in this etationdrag, which scales on the friction factor as •l pCœU•L. system,first, becausethe river dischargesinto it, second,be- The relative importance of vegetationdrag, wind, and water causeits larger size (thus longerresidence time) allowsmore buoyancyto the river inertia canbe summarizedby the ratio of thermal alterations to occur, and, third, because it receives scales: limited return flow from the lake. In contrast, the lower fore- Vegetationdrag parameter[DeP•oli, 1999] bay exchangessignificantly with the lake throughthe 60 m wide channel connectingthis wetland to the lake as illustrated on C•L A = 2 H' (2) Figure 2a. The lower forebaycan thusbe consideredto be an

Wind parameter

•w L 1•= pU•H' (3) (InternalFroude number) -2 Fi-2=IAP/PI# H U• ' (4) Noticethat since both • andFi-2 crU• 2, thenthe influence I I of wind and buoyancyon water circulationchanges with flow RTD(O RTD(t) conditions.Similarly, A is indirectlyrelated to Uo throughthe frictionfactor Cf, whichdepends strongly on water depth and velocity, as well as vegetationtype and density [Petlyk and _ > t r ,_ > t BosmajianIII, 1975; Chen, 1976; Shih and Rahi, 1982]. In t t particular,A can decreasedrastically during storms,as a result a) b) of increasedwater depthsand stempronation. This is shownby the characteristicvalues of Cf givenin Table1. Watercircu- Figure 1. Bimodal circulation in free water surfacewetlands. lation is therefore expectedto changebetween low flows and (a) Under low flows,drag, wind, and/or buoyancy dominate the circulation(A, •, andFi-2 >> 1); thewetland is laterally well storms[DePaoli, 1999;Andrad6ttir, 1997]: During low flows, mixed producinga relatively symmetricresidence time distri- vegetationdrag, wind, and buoyancydominate the water cir- bution(RTD) aroundthe nominalresidence time t. (b) Dur- culation(A, •, andFi-2 >> 1). Uniformvegetation drag will ing high flows the circulationis river-dominated(A, •, and help distributethe river flow overthe width of the wetland[Wu Fi -2 < 1); shortcircuiting occurs producing a skewedRTD and Tsanis,1994], and wind will enhancelateral and vertical with much of the flow exitingthe wetland in much shortertime mixing. Consequently,the wetland behavesas a partially well than t. ANDRADOTTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITI'ORAL WETLAND 2939

extensionof the lake surfacelayer, and the outflow temperature of the upper forebaycan be taken as representativeof the a) Upper Forebay,,.J t,•Aberjon a temperature of the lake inflow. .ive 3.2. Field Observations Lower(•0.053i••• '•-• Water temperatureswere monitoredevery 5 min at different locationsin the Upper Mystic Lake systemfrom July to No- vember in 1997 and 1998. Temperature measurementswere UpperFørebayMysti• •1•••'• • / made 10-20 cm below the surface and 10-20 cm above the bed Lake / • at each site in the upper forebay shownon Figure 2b using Onset temperature loggerswith 0.2øCresolution. In 1997 the temperatureloggers at the outlet of the wetlandwere repeat- edly stolen, so the following year, the measurementswere repeatedwith additionalloggers deployed at the outlet and in the shallow vegetatedzone. Lake water temperatureswere measuredin 1997 at 1-1.5 m depth intervalswithin the surface mixed layer and thermocline(1.8-10.9 m depth) at the loca- b) tions shownon Figure 2a using thermistor chainswith 0.1øC resolution[Fricker and Nepf, 2000]. Lake intrusiondepth with and without the wetlandswas estimatedby matchingthe aver- age of the near-surfaceand near-bedwetland channeltemper- • Coonta• ature and the river bed temperature,respectively, to the lake thermistordata. Surfaceintrusions within the top 2 m in the lake were not resolved because no thermistor was located there. Changesin suspendedsediment levels did not significantly affect the water densityand were neglectedin the cal- culations. Wind speed and direction were measured 10 m above ground at the southernend of the lake at 10 min intervalsin IDead Zone•* ' ' 1997 and 1998 (Figure 2a). Hourly Aberjona River flow was ' 0.8 metres measuredby the U.S. GeologicalSurvey approximately 800 m upstreamof the inlet to the upper forebay. In addition, air 0 100 200 300 temperature,relative humidity, and solar radiation were mon- itored every 5 min during 1997 (Figure 2a). Cloud coverwas Figure 2. Monitoring program in the Upper Mystic Lake measured at 3 hour intervals at the Boston Logan Airport system,Winchester, Massachusetts. (a) Site overviewand sur- located 15 km east of the studysite. face areas.(b) Detailed map of the upper forebaywith mean water depths during the 1998 monitoring period. Thermistor 3.3. Dead-Zone Model Application locationsare denotedby T, the anemometeris denotedby W, 3.3.1. Model formulation. The dead-zone model was de- and the weather station is denoted by S. scribedin detail by Andrad6ttirand Nepf [2000]. In short,the

wetlandarea is dividedinto two zones,a flow zone (or channel) _ _ with cross-sectionalarea A c and a stationarydead zone with Tc(O,t) - To(t), (7) cross-sectionalareaA a- As shownon Figure 2b, while the river traversesthe wetlandchannel (shaded area), it exchangescon- Ox ' tinuouslywith two stationarydead zoneson either side of the ø?cIx=L =0 (8) channel. Note that the southern dead zone is smaller, shal- Here u = Q•(t)/Ac is the averagespeed in the channel,L is lower, and more vegetatedthan the northern dead zone. The the length of the wetland, To is the wetland inflow tempera- circulationregime in the wetland is describedby the areal ture,45 is the net surfaceheat flux per surfacearea, and Cp is ratio, q = Ac/(A c + Aa), and the nondimensionallateral the specificheat of water. exchangecoefficient a* - AQ/Q r, where AQ is the total The parametersq and a* are site-dependentand are typi- lateral exchangerate betweenthe channeland dead zonesand cally evaluatedby conductingdye experiments[e.g., Bencala Qr is the inflow flow rate. Neglectinglongitudinal dispersion, and Walters,1983]. For preliminary design and assessment, the simplifiedgoverning equations for the depth-averagedwa- however, it is useful to derive expressionsbased upon the ter temperaturein thechannel, •c(X, t), anddead zone, •a(x, inflow and site geometry alone. First, consider the river- t), are dominatedcirculation regime, when the river flowsthrough the O?c a*u 0 wetland like a jet (Figure lb). The laboratoryexperiments of o-7-+u = L - ?c)+ pCdqc' (5) Chu and Baines[1989] suggestthat the jet spreadslinearly from its initial width Wo, with spreadingangle/3 •-- 0.1, such OTa a*u q 0 that the channelwidth is Wc = Wo + 213x.This yields the rO + (6) Ot L 1 - q pCpHa' mean wetland areal ratio of The boundaryconditions at the inlet (x = 0) and outlet (x = L) are q = Wo+W13L ) Hc• •/3 W'L (9) 2942 ANDRADOTTIR AND NEPF:THERMAL MEDIATION IN NATURAL LITTORAL WETLAND

ticular,during the initialstage of the storm(J.D. 305.7-306) a) the water temperaturein the channelincreases abruptly, whereasthe water temperaturein the dead zone risesmore gradually.This is consistent with short circuiting (Figure lb) in whicha largeportion of the river advectsrapidly through the 24 wetlandchannel while the remaindermixes slowly with the T deadzones. The timelag between the temperatureincrease in (øC) the river andwetland channel (1 hour)matches the advective 20 timescaleestimated for the riverjet [Chuand Baines, 1989]. Also, note that the water column in the flow zone is well mixed duringthe risinglimb of the hydrograph,as expectedwhen 16 river inertiadominates buoyancy and wind (Fi -2 and 12 < 0.5). However,this one-layerflow structurebreaks down on 220 230 240 250 260 270 thereceding limb shortly after the onsetof a southerlywind at Julian Days J.D. 306.0.The colderwater that appearsat the channelbed is October November likelyadvected from the northerndead zone by the subsurface b) returnflow associatedwith the wind setup. 1998 - Next considerthe Auguststorm illustrated on Figure5b. In

16 _ T a) 14 .... I , , , .... , .... 20 November19'97' ...... ß (øC)

12 _ 13 T...... '...... ""' "" ?------( 15 12 ..R ,,T• ,, ,, _ T : T • 11 10QR (øC) (m3/s) I 5 270 280 290 300 310 320 10. .,._.>.•_/, O _ Julian Days 8 , , , , I -•', , , I I 0 Figure 4. Water temperaturesin thewetland channel Tc and 305 305.5 306 306.5 31•7 the northernand southern dead zones, Ta• and Ta2, respec- Julian Days tively,during (a) early and (b) late fall 1998.The bold line b) depictsthe near-surfacetemperatures, and the light linesde- 20 pictnear-bed temperatures. Cross-hatched areas denote large 22:.:'_;'•.' .... '''' Adg-•t'l,•9'1 stormoccurrences when Qr > 1 m3/s.Wetland stratification 21- ' _., T• -', 15 and diurnalsurface temperature fluctuations decrease pro- gressinginto the fall becauseof convectivecooling and increas- T 20 ing winds. Observationresolution is _+0.2øC. 10 Q• (øC)19- "'".,,:"""T• (m3/s)

10 to 1000. Under these conditions the wetland can exhibit a 17 complex,transient, three-dimensionalflow behavior. Since A 18i • 05 rangesfrom about30 to 300 (Table 1), vegetationdrag is also 233 233.5 234 234.5 235 expectedto play an importantrole in distributingthe river JulianDays laterally[DePaoli, 1999]. Dye experimentsshow that the inte- C)17 ' ' ' , ' ' ' I ' ' ' , ' , , , , , , 120 gratedeffect of all theseprocesses over the residence time (>3 October1998 /:::;:::.. 1 weeks)is that the wetlandbehaves as a partiallywell mixed 16 R '' ":" '%'•"" reactor[Andrad6ttir, 1997]. This supportsthe useof a simple one-dimensional(l-D) modelsuch as the dead-zonemodel for T 15 low-flowconditions, even though the instantaneouscirculation 10 Q• may be far from 1-D. (øC)14 (m3/s) 4.2.2. Storms. The cross-hatchedareas on Figure4 show that the diurnal thermal fluctuations are reduced or erased 13 duringstorms (e.g., J.D. 229,265, and 282) as the wetland heat 12 budgetis morestrongly influenced by the river heat input than 281.4 atmosphericfluxes. Other importantstorm features are dem- 281.6 281.8 282 282.2 282.4 onstratedin the threelarge storms on Figure5. First,consider JulianDays thelargest storm in 1997,portrayed on Figure5a. As is typical Figure 5. River and wetland water temperaturesat x = for late fall storms,the river is a sourceof warm water to the 0.4L duringlarge storms in (a) November1997, (b) August wetland.Unlike low-flow conditions the water temperatures in 1997,and (c) October1998. Bold lines depictnear-surface the channeland dead zone deviate from one another. In par- temperatures,and light linesdepict near-bed temperatures. ANDRAD(STTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITTORAL WETLAND 2943

the beginningof the stormthe river is a sourceof coldwater to a) 26 the wetland,the reverseof the previousstorm. Short circuiting ß z = -0.2m.'" x = 0.4L is again observedas the channel temperature drops more ' I , ' I ' I . ' I ' abruptly than the dead zone temperaturearound J.D. 234.0. 24 -- Furthermore,the water columnis well mixed on the risinglimb of the stormbut becomesstratified on the recedinglimb. Un- T like the previousstorm, this transition cannot be easily ex- 22 plained based upon the available data, becauseit does not (øC) coincidewith wind setupnor a significantincrease in buoyancy '•,, • ' i , forcing. After J.D. 234.0 the colder river flows through the 20 simulated'• andT a wetland as an underflow until around J.D. 234.4, when the river becomes warmer than the wetland water. The river then .... observedTI 18 , I I I , I I I1'/ , switchesto an overflow that appears to be short-circuiting, indicatedby the suddendrop in the near-surfacetemperature 240 242 244 246 248 250 in the channel that is not mirrored in the dead zone around Julian Days J.D. 234.7. b) 26 ' I ' I ' I ' I ' Finally, considerthe largeststorm in 1998, depictedon Fig- T atx=L c ure 5c. The wetlandthermal response is verycomplicated, both because this event consists of a series of storms and because it 24 occurson windydays (peak wind speed7 m/s).In the following we focussolely on the secondstorm (J.D. 282.2), when the T 22 river is warmerthan the wetlandwater and the wind is blowing (øC) from the north, producingless interference with the jet structure than in the first storm (J.D. 281.6). The temperature recordson the rising limb of this storm showthe warm river pulsemoving through the wetlandas an overflow,reaching the channel station more rapidly than both the northern and 18 , I I southerndead zones.This is in agreementwith the conceptual 240 242 244 246 248 250 pictureof shortcircuiting portrayed on Figure 2b in whichthe Julian Days channel cuts through the middle of the wetland. The river continuesto short-circuiton the recedinglimb (J.D. >282.2), Figure 6. Low-flowmodel simulations.(a) Comparisonbe- as seenby the fact that the water in the channelis significantly tween dead-zone model simulations and field measurements at warmer than that in the northern dead zone. Furthermore, in x = 0.4L. (b) Wetland outflowtemperature predicted by the contrastto the stormson Figures5a and 5b, the flow is strat- dead-zone and stirred reactor models. ified on the rising limb of the storm but is well mixed on the recedinglimb. To summarize,although the temperature records during convectivecooling can be explainedby the fact that the model storm eventsare quite complex,they consistentlydemonstrate does not account for wind sheltering.During this low-flow that the river short-circuitsthrough the wetland (Figure lb). period the residencetime in the wetland is so long, r w •- 8, Our observationsthus support the hypothesisthat wetland that the upperforebay approaches the limit of functioninglike circulationchanges during storms from partially well mixed to a stationarywater body [Andrad6ttirand Nepf, 2000]. At this short-circuiting.In addition,the upper forebayexhibits vertical limit the thermal budget is predominantlygoverned by the structureboth duringlow flowsand storms.This suggeststhat surfaceheat flux and not the river heat input, and the detailed the use of a depth-averageddead-zone model is at times an circulationregime in the wetland is not so important in pre- oversimplification,especially during storms. This will be inves- dicting thermal mediation. This is demonstratedby the fact tigated in greater detail in section4.3. that both the stirredreactor and the dead-zonemodels predict very similarwetland outflow temperatures (Figure 6b). Note, 4.3. Dead-Zone Model Simulations however,the wetlandcirculation is still importantfor predict- To further validate the applicationof the dead-zonemodel, ing chemical or solid removal which is unaffected by atmo- we now consider how well the model simulates the thermal sphericexchange [Thackston et al., 1987]. responsewithin the upper forebayduring both low flowsand 4.3.2. Storms. Figure 7 summarizesmodel simulations storms. during the three large stormsin 1997 and 1998. Recall from 4.3.1. Low flows. Figure 6 illustratesmodel simulations section4.2 that these stormsall exhibit short circuitingeither and observationsduring a 10 day period in 1997, when the as a one layer-flowor as an overflow/underflow.An important AberjonaRiver flow rate was0.1 m3/s.Since the dead-zone function of the model with respect to lake transport is to model is l-D, we comparethe simulateddepth-averaged tem- predictthe temperatureof this short-circuitingflow. Sincethe peratures(solid lines) to the observednear-surface and near- model is l-D, this may involve simulatingonly the surface bed water temperatures(dotted lines), which define the upper wetland temperatureduring overflows(or bed temperatures and lower limit for an observeddepth-averaged temperature. duringunderflows). First, considerthe 1997 Novemberstorm, Figure 6a showsthat the model simulateswell low-flowcondi- which has the strongestsignature of shortcircuiting. Figure 7a tions, producingvalues that lie within the observedtempera- showsthat the dead-zonemodel predictswell the propagation ture range,except on windydays (J.D. 246-248) when simu- of the warm short-circuitingriver flow to the middle of the lations are colder than observations.This overpredictionof wetland(x - 0.4L), evenafter the flowis no longer1-D (J.D. 2944 ANDRADOTTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITTORAL WETLAND

14 T at x = 0.4 L ½ T at x = 0.4 L 13 •....ß T=atx=0.4 L 0.2 C 12 T z = -0.2m ,,, ., 11 (øC) z = -0.2m lO t s,mu,ai,11 ...,•• --simulation• r-•.z = -1.3mj .... il•'zI 5m '" øbservatiøn//'" observation',..•., " "- -" observation 8 ',' , , , I =•-,',, I , , , , I , , , ,/ / J J , , I , • , , I , , I I • I I I I

13 23 T at x = 0.4 L T atx= L Td•at x = 0.4L ß'. dl c ,. 0.3øC 12 22

11 T 21 Z = -0. (øC) 10 z•-0.2m ..:.";' ß 19 _ -- simulation z = -1.3m observation •, ,, 8 18

13 ' ' ' ' I ' ' I T atx= L c 12 T•atx=L • 22 T=atx=L t _ _ 11 21 - T (øC) lO

• • dead-zone model • dead-zonemodel stirred reactor 19- __ dead_zonemodel-- I stirred reactor - - stirredreactor I 18 .... I , , , , I , , , , I , , , , , , , , I , , , , I , , , , I , , , • 305 305.5 306 306.5 307 233 233.5 234 234.5 235 281 281.5 282 282.5 283 Julian Days Julian Days Julian Days a) b) c)

Figure 7. Stormsimulations for the (a) 1997November, (b) 1997August, and (c) 1998October storms. Dead-zonemodel simulations (solid lines) are compared with observations (dotted lines) and stirred reactor simulations(dashed lines) at differentlocations within the wetland. The horizontalbars indicate when the flow is jet-dominated,i.e., Fi -2 < 2 and 12 < 0.5.

>306). The model,however, slightly underpredicts the water reactormodel simulations,given at the bottomon Figure7, temperaturein the northerndead zone, probablybecause it highlightsthe strengthof the dead-zonemodel relativeto the does not accountfor lateral exchangegenerated by wind- stirred reactor model. The dead-zonemodel capturesthe drivencirculation. For the 1997 Auguststorm portrayed on sharprises/drops in water temperatureassociated with short Figure 7b, the model again simulateswell the channeland circuiting,whereas the stirredreactor model predictsa dif- dead-zonetemperatures in the middleof the wetland(x = fused front because it assumes that the river mixes instanta- 0.4L). Similarly,Figure 7c showsthat the dead-zonemodel neouslythroughout the wetland.This diffusionof frontalsig- doesa goodjob predictingthe movementof the thermalfront naturescan affectthe predictionof lake intrusiondepth. This alongthe wetland channel both at x - 0.4L andx = L during can be seenby comparingthe model simulationsto the mean the 1998 October storm. Recall that the resolution of the temperaturein the lake epilimnionTt., ason Figure7a. Notice thermistor probes is 0.2øC, and the differencebetween the thatbecause of the largerreceiving volume of the lake,the lake observedand simulatedwater temperatures(0.2-0.3øC) is water temperaturedoes not follow the samesharp tempera- thereforenot significant.These three simulationsthus demon- ture increaseobserved in the wetland.During this 1997 No- stratethat the l-D, quasi-steady,dead-zone model gives reli- vemberstorm the stirredreactor model predicts a 4 hourdelay ablepredictions of shortcircuiting and the transitionbetween in surfaceintrusions (Tx=z. > Tz.) duringthe peakflow rates. shortcircuiting and laterallywell mixedflow duringstorms, Since the highestconcentrations typically occur during the evenin systemssuch as the Upper MysticLake forebayswhich risinglimb of a storm,the stirredreactor model will underpre- exhibitvertical structure (i.e., non-l-D behavior). dict the contaminant/nutrienttransport to the lake surface, Finally, the comparisonbetween the dead-zoneand stirred whichcan haveimportant implications for humanhealth and ANDRADOTTIR AND NEPF: THERMAL MEDIATION IN NATURAL LITTORAL WETLAND 2945

reservoirmanagement as will be discussedin greater detail in section 4.4.

4.4. Lake Intrusion Dynamics and Water Quality -4 Wetland thermalmediation is an importantphysical process in part becauseit can alter lake intrusiondynamics which, in turn, impactdownstream water quality.Figure 8 describesthe z -6 expectedintrusion depth in the Upper Mystic Lake with and without the upper forebay,based on the water temperature (m)-8 measurements in 1997, as discussedin section 3.2. The sea- sonaland synoptictrends presented in Figure 8a showthat in the absence of the wetland, the colder river water would -10F -- w,wetland ".'\,•,.! ,..• :. plungewhen enteringthe lake throughoutthe fall exceptfor a -12 few daysof short-termheating (e.g., J.D. 284 and 305-310). However,with the littoral wetlandpresent, episodes of surface 250 260 270•' •80 290 300 - 310 320 intrusionare prolongedand occur during 28 daysor 40% of . Julian Days the monitoringperiod (e.g., J.D. 256-265, 279-291, and 306- b)-2 ...... ' .... ' .... 312). Therefore substantiallymore river-bornefluxes are de- 1997 livered into the surface water of the lake with the wetland -4 present,which can have a high impact on the nutrient and ,,, chemicalbudgets of the lake. A close-upon diurnal timescales i i (Figure 8b) showsthat the intrusiondepth can alsovary sig- nificantlyover the courseof a day, especiallyduring synoptic heating periodswhen the inflow insertsinto the lake epilim- (m)-• nion. During these times a small temperaturevariation can .... w/oweftand "'" lead to a large changein intrusiondepth becauseof the low thermalgradient in the lake surfacelayer. In the Upper Mystic -•0 • • w/weftand ',:',,. ,, Lake thesevariations can be as large as 4-6 m. Overall, the temperaturemeasurements in the Upper Mystic Lake system -12 suggestthat the presenceof a littoral wetland shiftsthe time- 280 285 290 295 300 scale of lake intrusion depth variability from predominantly Julian Days seasonal(no wetland)to synopticand diurnaltimescales (with Figure 8. Intrusion depth with and without a wetland in wetland).A similarresult was predicted by the linearizeddead- 1997. Wetland thermal mediationincreases (a) synopticand zone model [Andrad6ttirand Nepf, 2000].The shiftin intrusion (b) diurnalintrusion depth variabili•. Bold arrowsdepict large depth variabilitycan impact lake water quality.For example, (Q• > 1 m3/s)storm occurrences, and light arrows depict small nutrientsand contaminantsmay be more homogeneouslydis- (Q• < 1 m3/s)storm occurrences. Unccrtain• is •0.8 m for tributed over depth as the river inflow coversa larger vertical surface intrusions and •0.3 m for intrusions into the thcrmo- domainin the lake. In addition,lateral and vertical transport cline. processesin the lake canbe enhanced.Both of thesewill affect the managementof reservoiruse and withdrawals. The intrusion depth during stormsis of particular impor- determine the downstream fate of land-borne nutrients and tance for lake water quality. Becauseof surfacerunoff and contaminants.Measurements in the Upper MysticLake system remobilization of channel sediments, river nutrient and con- in Massachusettsduring summer and fall 1997-1998 demon- taminant concentrationsusually increase during storms. For strate that thermal mediation occurs in a natural littoral wet- example,storms account for 30-60% of the total annualfluxes land located in a small-to-medium-sized watershed, where sun of heavymetals such as arsenic,chromium, iron, and copperto shadingproduces a different water temperaturefor the river the Upper Mystic Lake [Solo-Gabriele,1995]. The arrowson than the wetland and lake. In addition, thermal mediation is Figure 8a depict the storm occurrencesin the Upper Mystic affected by flow conditions.During low flows the wetland lake systemduring fall 1997. As expectedfrom the previous raises the temperature of the lake inflow by approximately discussion,the presenceof a wetland does not significantly 1-3øC,which is sufficientto changethe lake intrusiondepth by modify the lake intrusiondepth duringstorms because of the 1-6 m and shift the timescaleof intrusion depth variability low effectivethermal capacityassociated with the high flow from seasonal(no wetland)to synopticand diurnal(with wet- rates. Figure 8a also showsthat sevenout of nine of the late land). In contrast,during storms almost no thermalmediation summerand fall stormsoccur during periods of synopticcool- occurs.These observationsare both in agreementwith the ing and produceintrusions in the thermocline.These measure- theorypresented by Andrad6ttirand Nepf [2000]. The change ments thus suggestthat elevatedsurface concentrations hap- in thermal responseassociated with flow conditionscan par- pen infrequentlyduring summer and fall in the Upper Mystic tially be explainedby a shift in wetlandcirculation: During low Lake. flows the river inflow fills the entire wetland volume, and the wetland behaveslike a partially well mixed reactor. During storms,however, the temperature records demonstratethat 5. Conclusions the river short-circuitsacross the wetland, and thus only a Thermal mediationin littoral wetlandsis an importantpro- fractionof the wetlandvolume is availableto producethermal cessthat can alter the intrusiondepth in lakes and thus can mediation.A simple l-D, quasi-steady,dead-zone model can 2946 ANDRAD•TYIR AND NEPF:THERMAL MEDIATION IN NATURAL LITTORAL WETLAND

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Acknowledgments.This work was funded by the NationalInstitute Solo-Gabriele,H., Metal transportin the AberjonaRiver system: of EnvironmentalHealth Sciences,Superfund Basic Research Pro- Monitoring,modelling, and mechanisms.Ph.D. thesis,Mass. Inst. of gram,grant P42-ES04675.Special thanks go to the Winchesterboat Technol.,Cambridge, 1995. clubfor theirinterest in ourresearch and for allowingus to launchour Tchobanoglous,G., Constructedwetlands and aquatic plant systems: boatsfrom their docksand to the Medfordboat club for providinga Research,design, operational and monitoring issues, in Constructed pole for our anemometer.The authorswould also like to thank Eric Wetlandsfor WaterQuality Improvement, edited by G. A. Moshiri, Adams for his insightand feedbackon this work and Laura DePaoli A. F. Lewis, New York, 1993. for her commentson the manuscript. Thackston,E. L., F. D. ShieldsJr., and P. R. Schroeder,Residence timedistributions of shallow basins, J. 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