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Lake and Reservoir Management

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Seasonal nearshore sediment resuspension andwater clarity at Lake Tahoe

Kristin E. Reardon, Patricio A. Moreno-Casas, Fabián A. Bombardelli & S.Geoffrey Schladow

To cite this article: Kristin E. Reardon, Patricio A. Moreno-Casas, Fabián A. Bombardelli & S.Geoffrey Schladow (2016) Seasonal nearshore sediment resuspension and water clarity at LakeTahoe, Lake and Reservoir Management, 32:2, 132-145, DOI: 10.1080/10402381.2015.1136013

To link to this article: http://dx.doi.org/10.1080/10402381.2015.1136013

Published online: 02 Mar 2016.

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LAKE AND RESERVOIR MANAGEMENT, VOL. , NO. , –http://dx.doi.org/./..

Seasonal nearshore sediment resuspension and water clarity at Lake Tahoe

Kristin E. Reardona,b,c, Patricio A. Moreno-Casasa,d, Fabián A. Bombardellia, and S. Geoffrey Schladowa,b

aDepartment of Civil and Environmental Engineering, University of California, Davis, Ghausi Hall, One Shields Avenue, Davis, CA; bTahoeEnvironmental Research Center, University of California, Davis, Country Club Drive, Incline Village, NV; cCHM, East Thirty-sixth Avenue,Anchorage, AK; dFacultad de Ingeniería y Ciencias Aplicadas, Universidad de los Andes, Mons. Álvaro del Portillo , Las Condes, Santiago,Chile

KEYWORDSLake Tahoe; nearshoresediment resuspension;STWAVE; water clarity;wind-waves

ABSTRACTReardonKE,Moreno-Casas PA, Bombardelli FA, SchladowSG. 2016 Seasonal nearshore sediment resus-pension and water clarity at Lake Tahoe. Lake Reserv Manage. 32:132–145.

Motivated by management challenges due to declining water clarity at Lake Tahoe, California–Nevada, we synthesized field observations and modeling of wind-driven nearshore sediment resus-pension to inform decision-makers. We present the first field observations of nearshore sedimentresuspension inboth summerandwinter and investigate seasonal differences in lakebehavior. In addi-tion, using a previously modified and validated wind-wave model, STWAVE, we developed manage-ment charts that illustrate relationships between fetch, wind intensity, and wave height and betweenwater depth, wave height, and the potential for mobilization of different-sized particles. For a repre-sentativegrain size of 150µm, thewind-driven surfacewaveswere found to influence the sediments toa maximum water depth of 9 m. Additionally, we evaluated the potential for wind-driven nearshoresediment resuspension with changing lake levels, considering a range of possible future scenarios.The areal extent of potential wind-driven sediment resuspension in Lake Tahoe’s southern nearshorezone is maximum (8.3 km2) at a lake water level equal to the natural rim (1897 m). The potential forresuspension notwithstanding, the results demonstrated no increase in particle loading of the sizeclass identified to most negatively impact water clarity (<16µm). Therefore, we corroborate previousfindings and conclude that wind-driven nearshore sediment resuspension does not contribute to thedeclining water clarity of Lake Tahoe and attribute this to a lack of available fine material within thelake sediments.

We typically assume that disturbances at the sediment–water interface negatively influence lake clarity. Par-ticulates and nutrients may be present in the watercolumn; they may enter a waterbody from diverseexternal sources; or they may be reintroduced fromwithin the system itself, such as when previouslydeposited sediments are resuspended from the sed-iment bed. The general concept of sediment resus-pension is predicated on the comparison of a desta-bilizing parameter (e.g., bottom shear stress) with acritical condition (e.g., the critical shear stress for arepresentative grain size; Parker 2004, García 2008).In keeping with standard practice, we too reason thatsediment resuspension occurs when the destabilizingparameter exceeds the critical condition by a certainvalue. Wind-driven sediment resuspension takes placewhen the water depth is shallow enough to favor thetransfer of sufficient momentum of the wind-waves

CONTACT S. Geoffrey Schladow gschladow@ucdavis.edu

from the water surface to the sediment–water inter-face (Håkanson and Jansson 1983, Kundu et al. 2012).Therefore, in an otherwise deep lake, only the shal-low nearshore zone is susceptible to wind-driven sedi-ment resuspension, in contrast to a shallow lake wherewind-driven sediment resuspension may occur overthe entire areal extent of the lake (Luettich et al. 1990,Chung et al. 2009a, 2009b).

Lake Tahoe is a deep, subalpine lakewith remarkablytransparent waters. Nevertheless, lake transparencyhas declined since long-term monitoring began inthe 1960s. In 1982, Lake Tahoe was designated animpaired waterbody by the US Environmental Protec-tion Agency. Under the Federal Clean Water Act, atotal maximum daily load (TMDL) was required toaddress its impairment. The agreed-upon water qual-ity objective for Lake Tahoe is deep water transparencyequal to the average annual Secchi depth measured

© Copyright by the North American Lake Management Society

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LAKE AND RESERVOIR MANAGEMENT 133

between 1967 and 1971 (29.7 m). The Lake TahoeTMDL identifies the pollutants responsible for the lossof transparency as fine sediment (<16 µm), nitrogen,and phosphorus (California Regional Water QualityControl Board, Lahontan Region and Nevada Divi-sion of Environmental Protection 2010). The LakeTahoe TMDL also identifies the load of each pollu-tant entering the lake, the contributing sources of theseloads, the reductions needed, the reduction opportuni-ties available for each source, and the implementationplan to achieve these reductions (California RegionalWater Quality Control Board, Lahontan Region andNevada Division of Environmental Protection 2010).Until now, a comprehensive investigation of the poten-tial for internal sources of fine sediment (i.e., resus-pended sediment) has been lacking, yet at other wind-exposed lakes, sediment resuspension can be a signifi-cant source of fine sediment (Evans 1994, Bloesch 1994,Chung et al. 2009a, 2009b).

Reardon et al. (2014) investigated wind-drivennearshore sediment resuspension at Lake Tahoe inwinter and found instances of wind-driven sedi-ment resuspension as corroborated by simultaneousincreases in bottom shear stress and total measuredsuspended sediment concentration (SSC). In thatstudy, the authors developed a novel methodology todirectly estimate total bottom shear stress by its contri-butions from wind-waves, mean currents, and randommotions and found that the total shear stress exceedingthe critical shear stress was due to the overwhelmingcontribution of wind-waves (80%). In addition, theauthors successfully extended and validated modifi-cations to the US Army Corps of Engineers (USACE)wind-wave model STWAVE to simulate wind-drivensediment resuspension for viscous-dominated flowtypical in lakes. Previously, lake applications ofSTWAVE had been limited to special instances offully turbulent flow as a result of hurricane conditions(Bunya et al. 2010, Dietrich et al. 2010); in these cases,bottom-friction was accounted for using Manning’sequation, which was appropriate for fully turbulentflow. To accommodate a viscous-dominated flow,Reardon et al. (2014) introduced a key change to theSTWAVE subroutine (friction) whereby the bottomfriction coefficient was estimated with the expressionfrom Kamphuis (1975) for viscous-dominated condi-tions. Thismodificationwas necessary because the fielddata showed that lake flow near the bed was viscous-dominated with a wave Reynolds number<104. Lastly,Reardon et al. (2014) presented and analyzed in situ

measurements of SSC and particle size distribution forseveral instances of low-wind and high-wind periods;for those periods, the concentration of fine sedimentin suspension remained unchanged. Therefore, weconcluded that further consideration of winter andsummer conditions was warranted because we couldno longer presume that wind-driven disturbancesat the sediment–water interface contributed to anincrease in the fine-sediment load responsible for LakeTahoe’s declining water clarity.

Also of interest were the potential implications ofclimate change with changing water level and/or windintensity. Water level at Lake Tahoe over the next cen-tury is projected to vary within the range we have iden-tified for our model runs (Sahoo et al. 2012). The rangeextends from a water surface elevation 2 m below thenatural rim (1895 m) up to the maximum legal limit(1899m; all referenced toNAVD88). Also in the TahoeBasin, wind speed is projected to decrease 4–5% overthe next century relative to historical averages (Det-tinger 2012).

To investigate nearshore sediment resuspension andconsider seasonal differences in such phenomenon,we synthesized field observations of hydrodynamicand sediment variables, computations of bottom shearstress from field data, and simulations from a previ-ously modified and validated wind-wave model withthe aim of answering the following questions:

1. What are the differences in resuspension eventsbetween summer and winter in Lake Tahoe?How does bottom shear stress change from oneseason to another?

2. Because the surface layers of lakes experiencewarming, does the temperature difference affectthe water column dynamics and in turn sedi-ment resuspension?

3. To what water depth do wind-driven surfacewaves influence the sediments?

4. What lake area does this influence extend, andhow does it change with changing lake level?

5. What are the implications of wind-drivennearshore sediment resuspension for waterquality at Lake Tahoe during different times ofthe year?

Study site

Lake Tahoe, located in the western United States onthe California–Nevada border in the mountains of theSierra Nevada (Fig. 1), is an example of a deep, olig-

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134 K. E. REARDON ET AL.

Figure . Lake Tahoe (∼°N, °W) bathymetry and orientation.Meteorological data were collected at Timbercove station, andlake measurements were collected at the study site. Contours areshown at m intervals.

otrophic lake as characterized by low nutrient con-centrations, high dissolved oxygen concentrations, andexceptionally transparent water to great depths. LakeTahoe is large within a small watershed and has anextremely long hydraulic residence time (Table 1).

The nearshore study site was located at the south endof Lake Tahoe 1000 m offshore on a shallow, broad,gently sloping shelf (∼0.3%; Fig. 1). We collectedmeteorological data from the long-term meteorologi-cal station maintained by the University of California,Davis, Tahoe Environmental Research Center (TERC)on the pier at Timbercove near the City of South LakeTahoe. Timbercove station, the nearest long-termmeteorological station, was located 4.5 km from thestudy site (Fig. 1).

Table . Lake Tahoe summary information.

Maximum depth mAverage depth mLake surface area km

Watershed area km

Lake volume km

Shoreline kmMean hydraulic residence time yrElevation of natural rima mElevation of maximum legal limita, b mAnnual average Secchi depth (target)c . mAnnual average Secchi depth ()d . m

aReferenced to NAVD .bFixed according to the Truckee River Operating Agreement (https://troa.net/).cFrom California Regional Water Quality Control Board, Lahontan Region andNevada Division of Environmental Protection ().

dFrom UCD TERC ().All other information from Goldman ().

Methods

Field observations and computations from field data

We took lake measurements in summer from 22 Julyto 3 September 2008 and in winter from 13 Novem-ber 2008 to 14 January 2009 with a total water depthvarying between 4.5 and 5 m. We measured verticalprofiles of water temperature with a thermistor chainof 8 Onset Stow Away TidbiT temperature loggers. Wemeasured nearbed velocity with 2 acoustic Dopplervelocimeters (ADV), a Nortek Vector, and a SontekADVOcean Probe, mounted on a sawhorse frame. Wemeasured nearbed SSC and particle size distributionwith a Laser In Situ Scattering and Transmissometry100X Type B instrument (LISST-100X) from SequoiaScientific, Inc. The LISST-100X was deployed on 2 alu-minum risers about 2 m from the sawhorse frame; theraw data were processed and converted according toAndrews et al. (2011). We characterized lakebed sed-iment from 2 grab samples processed using an LS 13320 MW particle-size analyzer from Beckman Coul-ter, Inc. All field data were collected at the study sitewith the exception of wind data. We measured windspeed and direction 5m above lake level at Timbercovestation.

Instrument sampling strategies in summer and win-ter were the same (and at the same location) with2 exceptions. Lakebed sediment characteristics weredetermined from grab samples collected solely in sum-mer (22 Jul 2008). We assumed that lakebed sedimentscharacteristics did not change from summer to winter,a reasonable assumption considering our visual esti-mation of the study site, the relatively small contribu-tion to the lake by the inflow streams, and the con-tinuous absence of bedforms. Measurements of SSCand particle size distribution were collected solely inwinter (11 Dec 2008 to 14 Jan 2009). Field measure-ments including the parameter(s) measured, instru-ment, sampling period, sampling interval, samplingduration, and burst interval, as applicable, were sum-marized (Table 2).

In addition to measurements collected during the 2study periods, we analyzed 10 years of wind data col-lected at Timbercove station from 1 June 2003 to 31July 2013 to characterize the seasonal wind exposureat the study site (Table 2). We define summer as Junethrough September and winter as December throughMarch.

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Table . Field measurements for Lake Tahoe in summer and winter.

Sampling period Sampling frequency

Parameter(s) measured Instrument Summer Winter Interval Duration Burst interval

Wind speed anddirectiona

Met One Windset-cup anemometer

ongoing ongoing min NA NA

Vertical profiles ofwater temperatureb

Onset Stow AwayTidbiT temperatureloggers

Jul– Sep Nov– Jan min NA NA

Nearbed velocityc,d Sontek ADVOceanProbe

Jul– Aug Nov– Dec Hz min h

Nearbed velocityc,e Nortek Vector Jul– Aug Nov– Dec Hz min hSSC and particle sizedistributionc,e

LISST-X NA Dec– Jan Hz min h

Lakebed sedimentcharacteristics

Grab samples () Jul NA NA NA NA

aWind speed and direction were measured at Timbercove station located at N°.′ , W°.′ .bEight temperature loggers were positioned at heights of ., ., ., ., ., ., ., and . m above lakebed and located at N°.′ , W°.′.cLocated at N°.′, W°.′.dSampling volume was . m above lakebed.eSampling volume was . m above lakebed.NA= not applicable.

We directly estimated bottom shear stress fromADV-measured velocity data collected from the Son-tek ADVOcean Probe. According to the method devel-oped in Reardon et al. (2014), we partitioned bottomshear stress into 3 components attributed to wind-waves, mean currents, and randommotions by filteringthe vertical component of the velocity for each high-frequency burst and considering its fluctuations as avariation of the turbulent kinetic energy method fromKimet al. (2000).Using a second-order Butterworth fil-ter and applying a band-pass filter that passed all fre-quencies between 0.03 and 2 Hz, a low-pass filter thatpassed all frequencies below 0.03 Hz, and a high-passfilter that passed all frequencies above 2 Hz, we sep-arated the vertical component of the velocity into 3contributing parts attributed to wind-waves, mean cur-rents, and randommotions, respectively. Reardon et al.(2014) demonstrated that with increasing total bottomshear stress, the relative contribution of each compo-nent became distinct due to the overwhelming contri-bution of wind-waves (80%).

To compute the value of critical shear stress, weused the formulation from Parker (2004) that incor-porates the nondimensional critical Shields parame-ter. The Shields diagram (Shields 1936) for thresh-old of sediment particle motion in unidirectional,steady, uniform flow adequately characterizes incipi-ent conditions under oscillatory flows (Raudkivi 1998,Chung et al. 2009a). This is a reasonable approachconsidering the surface wavefield of wind-exposedlakes.

Wind-wavemodeling

To estimate the wind-wave contribution to bottomshear stress, we implemented a previously modifiedand validated form of the full-plane version of theUSACEwind-wavemodel STWAVE (see Reardon et al.2014). STWAVE is a finite difference, steady-state,spectral wavemodel that solves thewave action balanceequation, including wave generation and transforma-tion by refraction and shoaling, wave breaking, genera-tion by wind-input, wave–wave interaction, and white-capping (Massey et al. 2011). Reardon et al. (2014)modified the bottom-friction formulation within thecode to reflect typical lake conditions with flow regimesthat were viscous-dominated. Earlier lake applicationsof STWAVE had been limited to special instances offully turbulent flow as a result of hurricane conditions(e.g., Bunya et al. 2010, Dietrich et al. 2010). The mod-ified STWAVE was validated and is an appropriate toolto calculate the effect of wind-waves on the develop-ment of bottom shear stress leading to sediment resus-pension in the shallow nearshore area of a deep lake(Reardon et al. 2014).

We implemented the modified STWAVE lake-wideusing 50 m square grid cells to which we specifiedbathymetry. The bathymetry data were prepared bymerging US Geological Survey multibeam sonar andUSACE SCHOALS bathymetry datasets, both with agrid spacing of 10m (T.E. Steissberg, University of Cal-ifornia, Davis, 2009, unpubl. data). We used the mod-ified STWAVE to consider a range of wind conditionsand the resulting bottom sediment dynamics.We based

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136 K. E. REARDON ET AL.

Figure . Wind statistics separated bywind direction at Timbercove station for (a) summer –, (b) summer during the study period,(c) winter –, and (d)winter during the study period. Bars represent percent ofwind data separated bywind direction.Wind speeds(m/s) are given as mean (closed circle) and maximum (open circle).

our analysis on the work of Norrman (1964), who pre-sented relationships between wave height, wind con-ditions, effective fetch, and sediment response at LakeVättern (Sweden; Håkanson and Jansson 1983). Usingthe modified STWAVE, we developed similar relation-ships based on Lake Tahoe and suggest that other lakescould be likewise considered.

In particular, we reproduced wind events rangingfrom 3 to 21 m/s coming from different directions.Seven wind velocities (3, 6, 9, 12, 15, 18, and 21 m/s)and 5 wind directions (0, 30, 45, 60, and 90, degreesmeasured clockwise from north) represented a rangeof realistic wind events at Lake Tahoe. Considering thestudy site in the southern part of the lake, the longestfetch (∼30 km) results from a northerly wind (i.e.,a wind direction equal to 0 degrees clockwise fromnorth). Conversely, the shortest fetch (∼1 km) resultsfrom a southerly wind (i.e., a wind direction equal to180 degrees clockwise from north). The angles cov-ered a full range of northeasterly winds,which pro-duced a similar effect as corroborated by simulations(not shown). We simulated northerly winds becausethese have the longest fetch and therefore generate the

greatest resuspension potential for these wind intensi-ties. We implemented 35 simulations in the modifiedSTWAVE and calculated significant wave heights, waveperiods, bottom orbital velocities, and bottom shearstresses for each cell, based on wave motions alone,(i.e., neglecting the presence of mean currents). Wethen used the results of modeled bottom shear stress tocalculate the corresponding maximum particle diam-eter that could be resuspended from the lakebed con-sidering the formulation of critical shear stress. Oncewe obtained the potential resuspension of the maxi-mum particle diameter for each incremental bottomshear stress, we computed equation coefficients usingthe multivariate regression tool in the commerciallyavailable software package MATLAB. In this way, wedeveloped predictive equations relating wind, waterdepth, and the potential for mobilization of different-sized particles.

To quantify the areal extent of wind-drivennearshore sediment resuspension at Lake Tahoewith changing lake water level, we ran the modifiedSTWAVEmodel with water surface elevations between2 m below the natural rim (1895 m) and the maximum

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LAKE AND RESERVOIR MANAGEMENT 137

Figure . Water temperature observed from (a) July to August (summer) and (b) November to December (winter) atthe study site for top and bottom thermistors. The temperature at the top thermistor remains nearly equal to or greater than that of thebottom thermistor, implying thermodynamic neutrality or stability.

legal limit (1899 m) in increments of 0.5 m (the naturalrim of the lake is 1897 m, all referenced to NAVD88), and for wind speed and direction that generatedthe maximum shear stress. In this way, we definedan area of maximum potential wind-driven sedimentresuspension. Lake level is influenced by stream inflow,groundwater inflow, direct precipitation, evaporation,outflow, and overflow, as applicable, and is thereforeaffected by climate change. Considering these pro-cesses, Sahoo et al. (2012) projected water levels atLake Tahoe over the next century. Future climate con-ditions drier than normal will likely result in a watersurface elevation below the natural rim; conversely,future climate conditions wetter than normal will likelyresult in water surface elevations above the natural rim,up to the maximum legal limit.

We did not consider future scenarios for which thewind intensity changes. In general, researchers pre-dict a decrease in wind intensity as average globaltemperatures increase (Ren 2010). Focusing on theTahoe Basin in particular, Dettinger (2012) spatiallydownscaled meteorology output from the coupledocean-atmosphere general circulation model (CM2.1)of the National Oceanic and Atmospheric Adminis-tration’s Geophysical Fluid Dynamics Laboratory inresponse to 2 greenhouse-gas emissions scenarios (A2and B1). Considering both scenarios for the TahoeBasin, wind speeds are projected to decrease 4–5%over the next century relative to historical averages(Dettinger 2012); therefore, our consideration of windintensity from recent meteorological data (2003–2013)

represents our understanding of the greatest potentialfor wind energy input into the lake. Future scenariosconsidering decreased wind speeds would result in lesspotential for nearshore sediment resuspension.

Results and discussion

Wind exposure

From the Timbercove wind record from 1 June 2003to 31 July 2013, we observed predominantly southerlywinds (SW, S, SE) representing 60% of the winds overthe 10 yr period (not shown) and 60–70% of the windsmeasured in summer and winter (Fig. 2). Northerlywinds (NW, N, NE) represented 22–31% of the windrecord measured in summer and winter (Fig. 2).

Averaging over the summer period, the mean windspeed by direction ranged from 2.1 to 5.9 m/s and wasmost frequently from the southeast direction (Fig. 2a).For the study period, themeanwind speed ranged from2.0 to 5.7 m/s and wasmost frequently southeasterly, asobserved in dailymid-afternoon peaks (Fig. 2b and 4a).Averaging over the winter period, themeanwind speedby direction ranged from 2.1 to 6.1 m/s, and the great-est mean wind speed was associated with northeast-erly winds (Fig. 2c). The greatest value of wind speedover the entire winter record was from the northeast(18.1 m/s; Fig. 2c); the greatest value of wind speedover the winter study period was from the southwest(13.9 m/s; Fig. 2d and 5a). The maximum wind speedobserved during the study periods of 2008 and 2009

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138 K. E. REARDON ET AL.

Figure. Comparisonofobservedvariables in summer from July throughAugust : (a)wind speedand (b)winddirection (degreesclockwise fromnorth)measured at Timbercove station, burst-averaged nearbed current speed in the horizontal plane at (c) .m (Vector)and (d) .m (ADVOcean Probe) from the bottom, and (e) total bottom shear stress at a water depth of m computed from data collectedby the ADVOcean Probe. The horizontal dashed line at . Pa indicates the critical shear stress for incipient motion for a representativeparticle size of µm.

was less than for the 10 yr record; however, the meanwind speed was comparable.

Observed nearshore patterns in summer andwinter

Patterns of physical phenomena were different in sum-mer and winter, as expected. In summer, we observeda reliable diurnal pattern of nearshore water warmingand cooling with a daily difference of 2 C (Fig. 3a).In winter, the diurnal pattern was more subtle, and, ingeneral, the temperature of bottom and top thermistors

tracked closely (Fig. 3b). We saw a greater temperaturedifference between bottom and top thermistors in sum-mer (3 C on average; Fig. 3a) than in winter (1 C onaverage; Fig. 3b). These temperature differences, how-ever, were usually sustained for 2–4 h and nomore than6 h.

During the summer study period we observed apattern of daily, elevated, mid-afternoon wind speeds>6 m/s in a south–southeasterly direction. Windspeeds �9 m/s were observed several times in anortherly direction (see Fig. 4a and 4b; days 208–209

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LAKE AND RESERVOIR MANAGEMENT 139

Figure . Comparison of observed variables in winter from November to December : (a) wind speed and (b) wind direction(degrees clockwise from north) measured at Timbercove station, burst-averaged nearbed current speed in the horizontal plane at (c). m (Vector) and (d) . m (ADVOcean Probe) from the bottom, and (e) total bottom shear stress at a water depth of m computedfrom data collected by the ADVOcean Probe. The horizontal dashed line at . Pa indicates the critical shear stress for incipient motionfor a representative particle size of µm.

and 216) and south–southwesterly direction (Fig. 4aand 4b; days 221, 231–232, and 238). For the winterstudy period, we observed lowwind speeds punctuatedby high-energy wind events; no regular daily wind pat-terns were observed in winter (Fig. 5a and 5b).

During the summer study period, we observedand average nearbed current speed of 0.03 m/s mea-sured with the Vector and ADVOcean Probe. Peaksin nearbed current speed were <0.10 m/s (Fig. 4cand 4d), in contrast to winter conditions with nearbed

current speed peaks of 0.15–0.20 m/s (Fig. 5c and5d).

Spectra from different high-frequency, nearbedvelocity bursts collected using the ADVOcean Probeduring high-wind events throughout the summer andwinter have a significant difference associated with thepeaks in energy, yet yield a generally similar form(Fig. 6). The peak of the power spectral density rangedfrom 0.28 to 0.37 Hz, corresponding to wave peri-ods between 2.7 and 3.2 s. For the 2 summer bursts,

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140 K. E. REARDON ET AL.

Figure . Power spectral density for -min bursts in for sum-mer on (a) August beginning at h and (b) August begin-ning at h, and winter on (c) December beginning at h,(d) December beginning at h, and (e) December begin-ning at h. The vertical dashed lines demarcate the so-calledwind-wave band from . to Hz (corresponding to wave peri-ods between . and s). Note that the spectra are offset for illus-trative purposes.

for instances when bottom shear stress exceeded thecritical shear stress, we observed the most energeticpeak in the power spectral density plots at 0.28 Hz(Fig. 6a) and 0.37 Hz (Fig. 6b), corresponding to thewind-wave contribution fromwave periods equal to 3.6and 2.7 s, respectively. For high-wind events in winter,more pronounced peaks in power spectral density of0.30–0.35 Hz were observed (Fig. 6c–e), correspond-

ing to the wind-wave contribution from wave periodsof 2.9–3.3 s.

Sediment characteristics

Generally, we found that the sediment at the studysite was well-sorted, noncohesive, and with no appar-ent bedforms. We verified that the presence of organicmaterial was negligible. In addition, we assumed thatflocculation processes were negligible because specificconductivity was extremely low (typically<0.1µS/cm;D. Roberts, University of California, Davis, 2015,unpubl. data). Considering the grab samples collectedat the study site, the lakebed sediment was about 92%sand, 7% silt, and <1% clay (Wentworth 1922), con-sistent with a substrate map developed to assess fishhabitat suitability byHerold et al. (2007), which showeda predominance of sand in the entire south shore ofLake Tahoe. We assumed a representative grain sizeof 150 µm, corresponding to ∼d10 from the particlesize distributions of the 2 grab samples (Reardon et al.2014). Furthermore, 150µm corresponded to the peakin SSC by mass concentration during sediment resus-pension events (Reardon et al. 2014). Following theformulation found in Parker (2004), the critical shearstress for incipient motion for a particle size of 150 µmis 0.081 Pa.

Sediment resuspension

For the summer study period, we observed a patternof daily, elevated, midafternoon bottom shear stress(computed with the modified turbulent kinetic energymethod; Reardon et al. 2014). At a water depth of5 m, we observed 2 instances when the bottom shearstress exceeded the critical shear stress: 8 August 2008(day 221) and 18August 2008 (day 231; Fig. 4e). For thewinter study period, we observed bottom shear stressthat demonstrated no regular daily patterns; similarto the wind record, low measures were interrupted byabrupt increases (Fig. 5e). At a water depth of 5 m,we observed 3 instances where the bottom shear stressexceeded the critical shear stress: 8 December 2008(days 343–344), 13 December 2008 (days 347–348),and 19 August 2008 (day 354; Fig. 5e).

For summer, the mean bottom shear stress over thestudy period ranged from 0.001 to 0.007 Pa. The max-imum bottom shear stress was 0.093 Pa and associatedwith wind of southwesterly direction, the only wind

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LAKE AND RESERVOIR MANAGEMENT 141

Figure . Mean andmaximum bottom shear stress at a water depth of m separated by wind direction for (a) summer and (b) winter. Thehorizontal dashed line at . Pa indicates the critical shear stress for incipient motion for a representative particle size of µm.

direction for which the bottom shear stress developedin excess of the critical shear stress (Fig. 7a). Gen-erally, values of bottom shear stress were greater inwinter than summer, attributed to greater wind inten-sity observed in winter. For winter, the mean bottomshear stress ranged from 0.003 to 0.036 Pa. The maxi-mum bottom shear stress was 0.182 Pa and associatedwith wind of southwesterly direction; bottom shearstress also exceeded the critical shear for southerly(0.119 Pa) and westerly (0.106 Pa) wind directions(Fig. 7b). The wind directions associated with maxi-mum bottom shear stress were the same wind direc-tions as for the maximum observed wind speeds.

During winter we collected in situmeasurements ofSSC with an LISST-100X. We observed that SSC didnot change with increasing total bottom shear stressfor particles of median diameter 1.25–16 µm (Fig. 8a).The Lake Tahoe TMDL attributes the loss of deep-water transparency, in part, to fine sediment (<16µm).Furthermore, Swift et al. (2006) showed that particles

with the greatest potential to negatively impact lakeclaritywere inorganic and<10µm. Fine inorganic par-ticles strongly scatter light, which results in decreasedlake transparency (Davies-Colley and Smith 2001, Kirk2011); however, we observed an increase in SSC forthose instances when the bottom shear stress exceededthe critical shear stress for particles ofmedian diameter100–250 µm (Fig. 8b).

Predictive tools developed fromwind-wavemodeling

The process of wind-driven nearshore sediment resus-pension results in the entrainment of bed sediment intothe water column, with wave height and water depthinfluencing the potential for resuspension (Fig. 9).Increasing wave heights resulted in increased bottomshear stress, leading to greater potential for resus-pension; conversely, increasing water depth resultedin decreased bottom shear stress, leading to thediminished potential for resuspension. Considering

Figure . SSC vs. total bottom shear stress at a water depth of m for particles of median diameter (a) .– µm and (b) – µm.The vertical dashed line at (a) . Pa and (b) . Pa indicates the critical shear stress for incipient motion for a particle size of . µmand µm, respectively (following Parker ). The data point indicated by the open gray circle was disregarded.

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Figure . Particle resuspension with varying water depths andwave heights. Solid lines show the limit at which different particlediameters are resuspended from the lakebed. Similarly, the brokenline shows the case for particles with diameter�µm.

our representative grain size of 150 µm and a maxi-mum wave height of 0.7 m, the maximum water depthfor which wind-driven surface wavesmay influence thelakebed sediment is 9 m (Fig. 9).

The potential for particle resuspension for 3 dif-ferent water depths comes from the combination ofwind speed and direction/fetch (Fig. 10). For the waterdepths shown, the greatest resuspension potential wasachieved with the longest fetch, as expected. At thestudy site, the longest fetch (∼30 km) resulted from anortherly wind. The no-resuspension area (gray area)increased because the water depth was no longer shal-low enough to favor the transfer of sufficient momen-tum of the wind-waves from the water surface to thesediment–water interface (Fig. 10).

Due to the complex bathymetry of the nearshore ofLake Tahoe, the relationship between lake level and thepotential areal extent of wind-driven nearshore sed-iment resuspension is not linear. The areal extent ofwind-driven nearshore sediment resuspension at LakeTahoe’s south nearshore zone changes with changinglake level (Fig. 11). For a lake level equal to thenatural rim (1897 m), the potential area affected is8.3 km2 (Fig. 11b); for a lake level equal to the maxi-mum legal limit (1899 m), the potential area affectedis 7.2 km2 (Fig. 11c); and finally, for a lake level equalto 2 m below the natural rim (1895 m), the poten-tial area affected is 4.7 km2 (Fig. 11a). As the lakelevel drops below the natural rim, the potential arealextent of wind-driven nearshore sediment resuspen-sion decreases (Fig. 11d) because with dropping lake

Figure . Resuspension curves considering different wind speedand direction for water depths of (a) . m, (b) . m, and (c) . m.The corresponding fetch is indicated on the secondary axis. Solidlines show the limit at which different particle diameters are resus-pended from the lakebed. Similarly, the broken lines show the lim-iting case for particles with diameter�µm.

level, the extent of the wetted area at Lake Tahoedecreases. As the lake level increases from the natu-ral rim up to the maximum legal limit, the potentialareal extent of wind-driven nearshore sediment resus-pension also decreases (Fig. 11d) because as lake levelincreases, less of the nearshore is sufficiently shallowfor the transfer of momentum of the wind-waves fromthe surface to the sediment–water interface. As a result,

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Figure . The areal extent ofwind-driven sediment resuspension (denoted by black) in Lake Tahoe’s southern nearshore zone, consideringlake water level equal to (a) m below the natural rim ( m), (b) the natural rim ( m), and (c) the maximum legal limit ( m).The area potentially affected by wind-driven sediment resuspension is (a) . km, (b) . km, and (c) . km. Contours are shown at mintervals. (d) Potential resuspension area with changing water surface elevation.

the potential for wind-driven sediment resuspension isreduced.

Summary and conclusions

To elucidate the relationship between nearshore sed-iment resuspension and water clarity at Lake Tahoeover summer and winter, we synthesized field obser-vations of hydrodynamic and sediment variables, com-putations of bottom shear stress, and simulations frompreviously modified and validated wind-wave modelSTWAVE. This paper reported the first comprehen-sive, process-based study of sediment resuspension atLakeTahoe throughout the year, offering insight to oth-ers facingmanagement challenges at deep, oligotrophiclakes.

In summer, we observed that the time series ischaracterized by a strong diurnal pattern with windintensity peaking each day at mid-afternoon. By con-trast, in winter we observed low-wind periods punctu-ated by high-energy storm events. Both low-wind andhigh-wind periods were sustained for days at a timewith no regular pattern. Generally, the onset of windevents ended the thermal stratification of the water col-umn within several hours. Based on these results, thethermal structure did not greatly impact the transfer ofmomentum in the water column.

Considering the nature of the wind-waves, weobserved that the peak in the power spectral densityplots corresponded to a wave period of about 3 s inboth summer and winter. In summer, the form of thepower spectral density plot is less pronounced thanthose for winter. In winter, we see strong peaks of sim-ilar form for each burst and attribute this to the greaterwind intensity in winter and therefore themore evidentdifferentiation of the high-frequency, nearbed velocitysignal.

We considered wind conditions that may derive themaximum bottom shear stress and found that wind-driven surface waves resulted in potential sedimentresuspension up to a water depth of 9 m. This influ-ence extended to a lake area of 8.3 km2 for a lake waterlevel equal to the natural rim and decreased both as thelake water level increased to the maximum legal limit(7.2 km2) and decreased to 2 m below the natural rim(4.7 km2).

We determined that wind-waves do not greatly dis-turb the sediment–water interface at the study site (i.e.,the south end of Lake Tahoe 1000 m offshore at a waterdepth of ∼5 m). The processes we observed were sub-tle and nuanced compared to similar phenomena atshallow, wind-exposed lakes like the Salton Sea (Chunget al. 2009a, 2009b). With increasing bottom shearstress attributed to wind-waves, the SSC of particles

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of median diameter 1.25–16 µm did not change. Forparticles of median diameter 100–250 µm, we saw anincrease (same order of magnitude) in SSC for thoseinstances when the bottom shear stress exceeded thecritical shear stress. In general, we attribute the relativelack of fine material available for resuspension to thecoarse and granular lakebed sediment at the nearshoreof Lake Tahoe.

An important consideration is how these researchfindings can be translated to management strategies.The results suggested that wind-driven nearshore sed-iment resuspension does not produce an increase inparticle loading of the size class identified to most neg-atively impact water clarity; however, the resuspensionof coarser material has the potential to increase theinternal loading of nutrients and contaminants fromthe lakebed. Alexander and Wigart (2013a, 2013b)demonstrated an association between the average dailyincrease in turbidity and high-intensity boating inthe southern nearshore of Lake Tahoe. The authorsattributed the decline in apparent water quality toresuspended sediment and released nutrients fromboating-induced wave action and turbulence (Alexan-der and Wigart 2013a, 2013b). We recommend assess-ing the potential for internal loading of nutrients byboth resuspension and hypolimnetic anoxia, which hasnot yet been investigated. In addition, it would beworthwhile to extend the geographic scope to includefieldmeasurements ofwind-driven sediment resuspen-sion from other nearshore areas of the lake, particularlyto assess the availability of fine sediment.

Acknowledgments

The Vector was provided as a Nortek 2008 Student EquipmentGrantAward. TheADVOcean Probewas provided on loan fromthe US Geological Survey. The Western Regional Climate Cen-ter, Desert Research Institute, provided the meteorological datathat was collected at the Timbercove station by TERC. We alsowish to acknowledge key students and staff from the Environ-mental Dynamics Laboratory and TERC, both of the Universityof California, Davis. In particular, we thank Erik Maroney, anundergraduate at the University of California, Davis, for clean-ing 10 yr of meteorological data.

Funding

This research was supported by the University of California,Davis, and a grant from the US Department of Agriculture For-est Service Pacific Southwest Research Station using funds pro-vided by the Bureau of Land Management through the sale of

public lands as authorized by the Southern Nevada Public LandManagement Act.

References

Alexander MT, Wigart RC. 2013a. Effect of motorized water-craft on summer nearshore turbidity at Lake Tahoe,California-Nevada. Lake Reserv Manage. 29:247–256.

Alexander MT, Wigart RC. 2013b. Effect of motorized water-craft on summer nearshore turbidity at Lake Tahoe,California-Nevada. Lake Reserv Manage. 29:256a–256b.

Andrews S, Nover D, Schladow SG, Reuter JE. 2011. Limi-tations of laser diffraction for measuring fine particles inoligotrophic systems: pitfalls and potential solutions. WaterResour Res. 47:W05523.

Bloesch J. 1994. A review of methods used to measure sedimentresuspension. Hydrobiologia. 284:13–18.

Bunya S, Dietrich JC, Westerink JJ, Ebersole BA, Smith JM,Atkinson JH, Jensen R, Resio DT, Luettich RA, Dawson C,et al. 2010. A high-resolution coupled riverine flow, tide,wind, wind wave, and storm surge model for southernLouisiana and Mississippi. Part I: Model development andvalidation. MonWeather Rev. 138:345–377.

California Regional Water Quality Control Board, LahontanRegion and Nevada Division of Environmental Protec-tion. 2010. Lake Tahoe total maximum daily load technicalreport.

Chung EG, Bombardelli FA, Schladow SG. 2009a. Sedi-ment resuspension in a shallow lake. Water Resour Res.45:W05422.

Chung EG, Bombardelli FA, Schladow SG. 2009b. Modelinglinkages between sediment resuspension and water qual-ity in a shallow, eutrophic, wind-exposed lake. Ecol Modell.220:1251–1265.

Davies-Colley RJ, Smith DG. 2001. Turbidity, suspended sedi-ment, and water clarity: a review. J AmWater Resour Assoc.37:1085–1101.

Dettinger MD. 2012. Projections and downscaling of 21st cen-tury temperatures, precipitation, radiative fluxes and windsfor the Southwestern US, with focus on Lake Tahoe. Cli-matic Change. 116:17–33.

Dietrich JC, Bunya S, Westerink JJ, Ebersole BA, Smith JM,Atkinson JH, Jensen R, Resio DT, Luettich RA, Dawson C,et al. 2010. A high-resolution coupled riverine flow, tide,wind, wind wave, and storm surge model for southernLouisiana andMississippi. Part II: Synoptic description andanalysis of hurricanes Katrina and Rita. Mon Weather Rev.138:378–401.

Evans RD. 1994. Empirical evidence of the importance of sedi-ment resuspension in lakes. Hydrobiologia. 284:5–12.

García MH. 2008. Sediment transport and morphodynamics.In: García MH, editor. Sedimentation engineering: mea-surements, modeling and practice. Reston (VA): ASCEManual of Practice 110.

Goldman CR. 1988. Primary productivity, nutrients, andtransparency during the early onset of eutrophication in

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a D

avis

] at

22:

42 0

2 M

arch

201

6

LAKE AND RESERVOIR MANAGEMENT 145

ultra-oligotrophic Lake Tahoe, California-Nevada. LimnolOceanogr. 33:1321–1333.

Håkanson L, JanssonM. 1983. Principles of lake sedimentology.Berlin (Germany): Springer-Verlag.

Herold M, Metz J, Romsos JS. 2007. Inferring littoralsubstrates, fish habitats, and fish dynamics of LakeTahoe using IKONOS data. Can J Remote Sens. 33:445–456.

Kamphuis JW. 1975. Friction factor under oscillatory waves. JWaterw Harbors Coastal Eng Div. 101:135–144.

Kim SC, Friedrichs CT, Maa JPY, Wright LD. 2000. Estimatingbottom stress in tidal boundary layer from acoustic Dopplervelocimeter data. J Hydraul Eng. 126:399–406.

Kirk JTO. 2011. Light and photosynthesis in aquatic ecosystems.3rd ed. Cambridge (UK): University Press.

Kundu PK, Cohen IM, Downling DR. 2012. Fluid mechanics.5th ed. San Diego (CA): Academic Press.

Luettich RA, Harleman DRF, Somlyody L. 1990. Dynamicbehavior of suspended sediment concentrations in a shallowlake perturbed by episodic wind events. Limnol Oceanogr.35:1050–1067.

Massey TC, Anderson ME, Smith JM, Gomez J, Jones R. 2011.STWAVE: Steady-state spectral wave model user’s manualfor STWAVE. Version 6.0. Vicksburg (MS): US Army Engi-neer Research and Development Center.

Norrman JO. 1964. Lake Vättern: investigations on shore andbottom morphology. Geogr Ann 46:1–238.

Parker G. 2004. 1D sediment transport morphodynamics withapplications to rivers and turbidity currents. Minneapolis(MN): National Center for Earth SurfaceDynamics. E-bookavailable from http://cee.uiuc.edu/people/parkerg/

Raudkivi AJ. 1998. Loose boundary hydraulics. 3rd ed. Oxford(UK): Pergamon Press.

Reardon KE, Bombardelli FA, Moreno-Casas PA, Rueda FJ,Schladow SG. 2014. Wind-driven nearshore sedimentresuspension in a deep lake during winter. Water ResourRes. 50:8826–8844.

Ren D. 2010. Effects of global warming on wind energy avail-ability. Renew Sustainable Energy Rev. 2:052301.

Sahoo GB, Schladow SG, Reuter JE, Coats R, Dettinger M,Riverson J, Wolfe B, Costa-Cabral M. 2012. The response ofLake Tahoe to climate change. Climatic Change. 116:71–95.

Shields IA. 1936. Anwendung der ahnlichkeitmechanik undder turbulenzforschung auf die gescheibebewegung. Berlin(Germany): Mitt. Preuss Ver.-Anst.

Swift TJ, Perez-Losada J, Schladow SG, Reuter JE, Jassby AD,Goldman CR. 2006. Water clarity modeling in Lake Tahoe:linking suspended matter characteristics to Secchi depth.Aquat Sci. 68:1–15.

[UCD TERC] University of California, Davis Tahoe Environ-mental Research Center. 2015. Tahoe: State of the LakeReport 2015.

Wentworth CK. 1922. A scale of grade and class terms for clasticsediments. J Geology. 30:377–392.

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