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Sediment Resuspension in the Wakes of Hurricanes Edouard and Hortense

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Sediment Resuspension in the Wakes of Hurricanes Edouard and Hortense GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO.18, PAGES 3533-3536, SEPTEMBER 15, 1998 Sediment resuspensionin the wakes of Hurricanes Edouard and Hortense T. D. Dickey,• G. C. Chang,•Y. C. Agrawal,2A. J. Williams,III,3 andP.S. Hill4 Abstract. A unique set of physicaland optical observationsof The presentsite is locatedin a region known as the "Mud sedimentresuspension was obtainedduring the passageof two Patch"on the Mid-Atlantic Bight shelf off the eastcoast of the hurricanes,Edouard and Hortense. The eyes of thesehurricanes U.S. (Fig. 1) where extensiveobservations have been made in the passedwithin approximately110 and 350 km, respectively,of past(e.g., Biscaye et al. [1988]). The Mud Patchis a depositof 2 our study site on the continentalshelf (100 km south of Cape to 14 m thicknesscovering an areaof roughly100 km by 200 km Cod, Massachusetts)within a two-week period in the fall of composedof relativelyuniform fine-grained material overlying 1996. Sedimentswere resuspendedto more than 30 m abovethe coarsersand-sized sediment [Twichelt et at., 1987]. The oceanbottom during both hurricanes. The sedimentresuspension sedimentconsists primarily of fine-texturedmaterial. processesassociated with the two hurricanesare shownto differ primarilybecause of the separationdistances between the eyesof Methods the hurricanesand the observationalsite (e.g., local versusremote forcing). Observedparticle-size distributions were shiftedtoward The presentexperiment consisted of four deployments smaller scalesduring HurricaneEdouard because of flocculate (spanningsummer 1996 through summer 1997) of a mooringand disaggregationcaused by high levels of localized shear and two bottomtripods located at approximately40.5øN, 70.5øW in turbulence. watersof about70 m depth(Fig. 1). The time seriesdescribed herewere obtained during the first deployment period. Introduction Several physical and optical measurementsystems were deployedon the mooring(Fig. 1). Theseincluded bio-optical The present report describes physical and optical systems(BIOPS) (as described in Changet al. [1997];Dickey et measurementsrelevant to sedimentresuspension on a continental al. [1998]),which were placed at 14, 37, and 52 rn depthsto shelf during the passageof two hurricaneswithin a two-week quantifyoptical and physicalvariability. A subsetof BIOPS period. Our resultsare especiallyimportant as few observations sensorsused for thisstudy included 1) ac-9s,2) fluorometers,and of resuspensionhave been made under extreme atmospheric 3) temperaturesensors. The samplingrate for the ac-9 wasonce forcing (e.g., Madsen et al. [1993]). Such observationsare of per hour for 30 secondsand the samplingrate for all other greatinterest as they enablethe studyof episodicprocesses that sensorswas eight times per hour. Vector winds were measured modify the near-bottomenvironment and control the fate of from a nearbysurface buoy with an anemometerat 3.3 m above bottommaterials including sediments, organisms, and pollutants the surface.In addition,wave data were measured by National that are transported,deposited, and transformedon continental Data Buoy Center (NDBC) buoy 44008, locatedat 40.5øN, shelves(e.g., Churchill [1989]; Cacchioneand Drake [1990]; 69.43øW(90 km east of theCMO site). Nittrouer and Wright [1994]). Physical processesthat can A nearby optical bottom tripod consistedof a suite of contributeto sedimentresuspension and transportinclude wind- instrumentsdesigned to measureproperties of particles(Fig. 1). generatedsurface waves, internal gravity and solitary waves, The BIOPS, as describedabove, was mounted at 1.5 meters tides, mean interior and boundarylayer currents,eddies, and abovethe bottom (mab) (-68 m depth).A photographiccamera buoyancy-drivenplumes (e.g., Cacchioneand Drake [1990]; was placedon the tripod at 1 mab and was usedto measuresize Nittrouer and Wright [1994]). Further, there is a complex distributionsof particleslarger than 500 gm in the bottom interplaybetween physical, geological, and biologicalprocesses boundary layer. The final set of instrumentsmeasured true involving particlesand organismsof varying size, shape,and volumetric concentrationof fine particlesderived from size buoyancy(e.g., Nowell [1983]). These processesresult in a distributionusing the LISST-100 [Agrawal and Pottsmith, 1994]. changein particle-sizedistributions. The presentmeasurements The LISST-100, measuringthe size distributionat 1.6 mab, was provide insights into the dynamical processesthat result in sampledat 15-minuteintervals and on the hour. sedimentresuspension during intenseforcing and the relaxation The otherbottom tripod (also located at a depth'of70 m and to more normalnear-bottom conditions, as well as variability in about400 m from the mooring)was used to measurenear-bottom near-bottomparticle-size distributions. These observations allow currents.BASS currentmeters [Williams et at., 1987] recorded investigatorsto testmodel hypotheses and performance. vector velocities at seven depthsfrom 0.38 to 7 mab. These measurements were made on the hour and half-hour with •OceanPhysics Laboratory, University of Californiaat SantaBarbara, samplingat 2 Hz for 28 min and 49 sec. Mean currentbottom Santa Barbara, California. shearstress (xc), wave-orbital velocity (u,,), current-wavebottom 2SequoiaScientific, Inc., Mercer Island, Washington. shearstress (To+,,), and dissipation rate (•½+,,) were computed using 3WoodsHole Oceanographic Institution, Woods Hole, Massachusetts. time seriesvelocity data collected by the BASS currentmeters to 4Departmentof Oceanography,Dalhousie University, Halifax, Nova characterizethe near-bottomflow field responsiblefor sediment Scotia, Canada. resuspension.Wave-orbital velocity was estimated by takingthe Copyright1998 by the AmericanGeophysical Union. squareroot of the integralof energyunder the wavepeak in the velocityspectra (periods from 8 to 25 s). Mean currentbottom Papernumber 98GL02635. shearstress was computed from BASS currentmeter data and the 0094-8534/98/98GL-02635505.00 "law of thewall" formulation[Tennekes and Lumley, 1972]. 3533 3534 DICKEY ET AL.: SEDIMENT RESUSPENSION IN THE WAKE OF TWO HURRICANES ß m s-] (Fig. 2a). The water columnwas highly stratifiedbefore the 10m I passageof Edouard as indicatedby the temperaturetime series shownin Fig. 2b. The mixed-layerdepth (MLD; computedusing 14m! BIOPb a 1ø temperaturedifference criterion) deepened from 12 m to a depthof 67 m shortlyafter the passageof Edouard(Fig. 2b). The ! I generalmixing effect of the hurricaneled to entrainmentof cool, I deeperwaters into the upperlaye? and to mixing of upperlayer warm waterswith cooler,deeper waters. The warming of deeper 37m[ BIOP$ waters (see 68 m record) continued for about 10 days when ! coolingbegan again. i The physicaleffects of HurricaneHortense at the mooring site I are less obvious than those associated with Edouard because of the much greaterdistance between the eye of Hortense and the site, and possiblybecause of the greater translationalspeed of Hortense. Winds associated with Hurricane Hortense were more I I sporadicat the mooring site, occasionallyexceeding 10-15 m s4 from September 14 through September 19, 1996. Hortense 68mBIOPS on• resulted in further mixing of the water column with the MLD increasingfrom 18 m to near 50 m. The passageof Hortenseis more evidentin the sedimentresuspension event (seenin optical from time series)than in the physicalrecord. The time series of beam attenuation coefficient at 676 nm Figure 1. Geographicmap indicatingsite of mooringand tripods used for the presentstudy. Map showingtracks of Hurricanes (hereafterbeam c) collectedwith the ac-9 on BIOPS packagesat Edouard and Hortense. Mooring and optical tripod schematic 37, 52, and 68 m depthsare illustratedin Figs. 2c, d, and e (note diagrams. differences in ordinate scales for the three panels). This wavelengthwas selectedfor beam c becauseof its covariance with suspendedparticulate matter and its minimal absorptiondue A number of models have been formulated to characterize to dissolved humic acids. Sediment resuspensioncaused by near-bottomcombined wave and current flow (e.g., Grant and Madsen [1979], and Christoffersenand Jonsson[1985]). More advancedversions of bottom boundarylayer modelsthat include effectsof armoring,moveable beds, and/or stratifiedflows have beendescribed by Trowbridgeand Madsen [1984] and Glenn and Grant [1987]. These advancedversions were not used in the presentstudy due to the absenceof bottomsediment condition data and the presenceof a well-mixed bottom boundarylayer. The Christoffersenand Jonsson [1985] model was chosen to computebottom shearstress from combinedcurrent and wave motionat 0.38 mab. The modelis quitesimilar to the commonly used Grant and Madsen [1979] model, but utilizes an iterative 68m approachfor computingfriction factors. Dissipationrate was estimatedusing the relationshipdescribed in Grosset al. [ 1994]. •..2[ c. 37m E [ H Observations o.5 The present observations were obtained during the deploymentperiod from July 8 throughSeptember 26, 1996. On o August 29, 1996, Hurricane Edouard'swinds reachedtheir maximumspeed of 140 mph (category4 on the Saffir-Simpson HurricaneScale), and the translationalspeed peaked at 1000 km d4 approximately700 km south of the mooring site (Fig. 1). 30 • , Hurricane Edouard maintained category 4 status for 2O approximatelyseven days. HurricaneHortense reached category 4 statuson September13, 1996, roughly 800 km south of the mooringsite, but decreasedin intensityto category3 on the same 235
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