The Effect of Sand Grain Size in the Aeolian Transport Processes of Transgressives Dunefields of the Coast of the Santa Catarina State Brazil

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1 Journl of Costl Reserch SI ICS 2004 (Proceedings) Brzil ISSN The Effect of Snd Grin Size in the Aeolin Trnsport Processes of Trnsgressives Dunefields of the Cost of the Snt Ctrin Stte Brzil G. Vintem ; L. J. Tomzelli nda. H. d F. Klein Instituto de Geociêncis, Universidde Federl do Rio Grnde do Sul Av. Bento Gonçlves, 9500, CEP , Porto Alegre, Rio Grnde do Sul, Brsil, gvintem@cttmr.univli.br,ljt@vi-rs.net ABSTRACT Lbortório de Ocenogrfi Geológic, Centro de Ciêncis Tecnológics d Terr e do Mr, Universidde do Vle do Itjí, Ru Urugui, 458, , Itjí, Snt Ctrin, Brsil, klein@cttmr.univli.br VINTEM, G.; TOMAZELLI L. J. nd KLEIN, A. H. F, The effect of snd grin size in the eolin trnsport processes of trnsgressives dunefields of the cost of the Snt Ctrin Stte Brzil. SI 39 (Proccendigs of the 8th Interntionl Costl Symposium), Itjí, SC, Brzil, ISSN The eolin trnsport of sediment is result of the reltionship between wind energy nd locl sediment properties. The im t this pper is nlyze the effect of snd grins in the eolin trnsport processes of the dunefields of the Snt Ctrin cost plin. Along it's 538 kilometers the cost of Snt Ctrin presents trnsgressive dunefields. An inversion in the migrtion direction cn be verified in the centrl portion of the stte, in n distnce of pproximtely 50 kilometers between the cities of Florinópolis nd Plhoç. The existing dunefields south of Plhoç migrte towrds SW, whit the northern fields migrte towrds NW. In n ttempt to explin this divergence in the migrtion directions, 39 smples from 8 dunefields, between the ltitudes 26 58'37''S nd 29 18'18''S long in the costl. Grin sized nlysis ws conducted nd used to ccount the threshold velocity. The results show tht the men snd grin size increses from south to north s well s the fluid threshold sher stress ( ut * ), impct threshold wind velocity t 10 m u(10). The pttern shows tht the role of grin size is n importnt fctor in the nlysis of eolin trnsport processes. ADDITIONALINDEX WORDS: Trnsgressives dunes, grin size, drift potentil, Snt Ctrin. INTRODUCTION To know the wind regimen nd the texturl chrcteristics of the vilble sediments on the bech nd dunes is bsic to nlyze the trnsport nd depositionl processes of the costl Aeolin systems of Snt Ctrin. In the centrl portion of Snt Ctrin cost, in n intervl of pproximtely 50 km between the cities of Florinópolis nd Plhoç, evidence n inversion in migrtion direction of trnsgressive dunes cn be found. Existing dunefields south of Plhoç migrte southwest (SW) while tht the dune fields to the north migrte northwest (NW) (Figure 1). The im of this work is to nlyze the effect of snd grin size in the Aeolin trnsport processes of trnsgressive dunefields (ctive dunes) of the cost of Snt Ctrin Stte. STUDY AREA The study re encompsses eight (8) existing dunefields long of the Stte of Snt Ctrin between ltitudes 26 58'37"S nd 29 18'18"S. Being these: Pri Grnde (São Frncisco do Sul), Brr do Sul (Blneário Brr do Sul), Lgo d Conceição, Ingleses (Florinópolis), Siriú (Gropb), Ribnceir (Imbitub), Snt Mrt/Cmcho (Jgurun/Lgun), Morro dos conventos (Arrnguá) (Figure. 2). -3 Where: is ir density (1.22 kg.m ), s is snd grin density -3-2 (2650 kg.m ), gis grvity (9.8 m.s ), dis the grin dimeter in mm, A is n constnt equl to 0.1 (esteemed by BAGNOLD, 1940). To clculte the tension of sher stress relted to wind speed distribution of logrithmic speed ws used, expressed by eqution 2: * Z u( 10m) 5.75u tlog u' (2) Where: Z' is the roughness fctor of the snd grin surfce * determined by ( BELLY, 1964), Z = 10 d (mm) (plin surfce ws ssumed), Z is height stndrd of the wind dt (10 meters), u' is the sher speed 894 * d (mm), ( u*t) is the threshold sher -1 stress (m.s ), u(10) is the impct threshold wind velociy, mesured t 10 meters high. The wind regimen long the cost ws nlyzed from superficil winds registers from three meteorologicl sttions. Z METHODS Thirty nine smples were collected t the eight trnsgressive dunefields throughout the cost. Lbortoril grin sized nlysis ws crried out for determintion of sttisticl prmeters, using the grphicl moments method ( FOLK nd WARD, 1957). threshold of sher stress ( u* t) gotten by eqution 1: u * t A s gd (1) Figure 1. Divergence observed in migrtion direction of dune fields in the centrl portion of the cost of the Snt Ctrin Stte. A Ingleses, B Lgo d Conceição, C Pinheir e D Siriú.

2 Snd Drift Potentil by Wind Action 103 Tble 1. Limit sher stress nd impct threshold wind velociy determined in ccordnce to men grin size for ech dune field. Dunefield Men Grin Size (m.s -1 ) (m.s -1 ) (mm) P. Grnde Brr do Sul Ingleses Joquin Siriœ Ribnceir Frol M. Conventos Figure 2.Locliztion of dune fields nlyzed throughout the cost of Snt Ctrin n the meteorologicl sttions used in the present study. A situted one to the south, in the Cbo de Snt Mrt Lighthouse, nother one in geogrphic center of the cost, situted t the Biologicl Reserve of the Arvoredo Islnd nd plus nother one to the north, t the PXIV oil pltform in the Ltitude 26 46'22"S nd Longitude '02"W (~ 180 km of the cost) (see Figure 2). Dt ws nlyzed for speed (m/s) nd direction, directionl 16 proveniences of sectors of superficil winds. The meteorologicl dt ws given by BNDO Brzilin Nvy from the sttions of Arvoredo Islnd (period ) nd of the Snt Mrt Lighthouse (period ) with six hour intervl smpling The dt from Pltform PXIV, hd n one second during 10 minutes every hour (period ). The mount of snd crried by the wind ws clculted through the model considered by FRYBERGER nd DEAN, (1979) (Eqution 3): Q u ( u- ut)* t (3) 2 Where: Q is the proportionl mount of snd crried by the wind in time t. Its result is express in vector units (u.v.) it is equivlent to the snd drift potentil (DP), u is the verge speed of the wind t 10 meters (stndrd height of registers in meteorologicl sttions), ut is the mesured limiting impct threshold wind velociy t 10 meters (minimum speed to keep the snd in slttion) nd t is the time where the wind blew in the given direction (percentile vlue of the registers of frequencies of wind directions, registered in the meteorologicl sttions). RESULTS AND DISCUSSION The vlues obtined from equtions (1) nd (2) proposed by BAGNOLD (1941) nd the grin sizes found t the nlyzed dunefields cn be visulized in Tble 1. Figure 3 presents the threshold of sher stress ( u* t) obtined by eqution (1) proposed by BAGNOLD (1941). The results show tht the verge snd size increses from south to north nd consequently threshold of sher speed ( u* t) of the fluid necessry to begin grin movement lso increses northwrd. When used in the eqution (2) these vlues of threshold sher speed t 10 meters limiting impct velocity (minimum necessry to keep the grin in slttion) shows vrition t n order of 3 m/s between Pri Grnde e Morro dos Conventos. The wind regimen found t the Pltform PXIV during the period of 1995 to 1999, highlights the predominnce of northnorthest (NNE) nd northest (NE) winds with frequency of 24%, nd verge speed of up to 8.22 m/s (Figure 4). The est winds (E-NE) follow in frequency, with n verge speed of 7.40 m/s. The SSE-S-SSW winds, together dd 17% of occurrence, with speeds of up to 10m/s. For the meteorologicl sttion of the Biologicl Reserve of the Arvoredo Islnd in the center-north of the cost, the predominnt regimen of winds occurred from the south qudrnt (s) with 18.47% nd verge speed of 6.52 m/s, the winds of the north qudrnt (N) nd northest (NE) correspond 12.35% nd 15.88% respectively, of the occurrences, with verge speeds of 5 m/s. For the south region of the cost wind nlyzes obtined from the meteorologicl sttion of the Snt Mrt Lighthouse, presented winds proceeding from northest sector (NE) with percentge of 21.69% of occurrence, nd verge speed of 7.63 m/s is the predominnt one. The second highest occurrence of wind is from the south qudrnt (S) with 10.62% of verge occurrence speed of 6.11 m/s (Figure 4). The wind regimen long the cost of Snt Ctrin presented difference between the sttions. This difference cn be ttributed to the dt registers of cquired superficil winds, for still presenting differences in of sttions nemogrphic towers heights nd for the difference of dt collection methods, in reltion to smpling intervl. However, in generl wy, it ws possible to verify tht the predominnt wind is northestwrd, represented by the pltform PVIX nd Snt Mrt sttions. Where similrity in wind regimen ptterns between these two sttions ws evidenced, due to the bsence of obstcles surrounding these, s the of pltform PVIX sttion is locted 100 miles offshore nd the Snt Mrt Lighthouse sttion in costl plin with low relief. The results of the DDR (Direction of Resultnt Drift) obtined by the snd roses gree with the generl migrtion direction of the dunefields (Figure 5). The Pri Grnde dunefield presented concordnt DDR with the rel direction of migrtion of dunes. The drift potentil (DP) found ws of uv, tht ccording with the wind energy clssifiction proposed by FRYBERGER (1979), is low wind energy Aeolin environment. A high directionl vribility, with RDP/DP rte of 0.076, represents complex regimen of wind.at Brr do Sul dunefield, vlue of DPof 324 uv shows intermedite energy wind regime with high directionl vribility, nd RDP/DP rte of 0.12, presenting the complexity in the wind regimen tht cts in this field. However the DP mgnitude ws considerbly higher, since the verge snd grin size of this field is smller nd the wind mkes the snd grin trnsport more effective. For superficil wind dt corresponding to pltform PVIX, DP vlues were obtined t the Ingleses nd Lgo d Conceição dunefields were 330 nd 304 uv respectively. The DDR from snd rose does not ccurtely represent the direction of migrtion evidenced in the field. At Lgo d Conceição (Joquin) dunefield, nlyzed with registers of superficil winds of the meteorologicl sttion of the Biologicl Reserve of thearvoredo Islnd, DDR corresponds to migrtion route of the northwest (NW) fields, presenting DP of 85 uv. sesonlly this field hs the most effective trnsport during winter nd spring, however, the directionl vribility is intermedite throughout

3 104 Vintem et l. Figure 3. Men grin size nd threshold of sher stress ( u *t ) found for ech field of trnsgressives dunes long the cost of Snt Ctrin. ll the sttions, wht represents contrry winds to migrtion direction of this field, thus chrcterizing, wht BIGARELLA (1972) nd BIGARELLA (2000) cll reverse dunes. The sme uthor does not ssocite the migrtion of these dunefields to the NE winds, s these would be hindered to ct in the trnsport due to high topogrphicl grnites tht borders the system. The Siriú dunefield nlyzed from the dt of PXIV pltform shows DDR disgreeing with the rel migrtion direction of the dunefield. This probbly occurs becuse the south winds of this sttion re intense s north-est ones, with verge speed of circ 8m/s. Since the necessry speed to plce the grin in movement is of 5.96 m/s wht occurs is blnce of the resultnt forces of these wind directions generting resultnt vector (DDR) tht it does not ccurtely correspond to the relity observed in field. In the sme wy, the results found here gree-with those obtined by BIGARELLA et l. (1972) tht, on bsis of the nlysis of the sedimentry structures, showed tht the direction of migrtion of the dunefields is southwest (SW) t the south of the Stte. The Ribnceir dunefield presented DP of 103 uv, being considered regimen of low energy ccording to clssifiction of FRYBERGER (1979). Winter nd spring re the most effective in trnsport due to the northest (NE) winds tht re the predominnt ones for this region; the sme ws evidenced by MARTINHO (2003), for this dunefield. The Snt Mrt Lighthouse/Cmcho dunefield presented DP of 285 uv, considered n intermedite energy wind, but very next to the limit of high energy winds ccording to clssifiction proposed by FRYBERGER (1979). The directionl vribility, with 0.37 rte, cn be clssified s intermedite to low. The sesonl nd nnul snd roses hs DDR tht fully reflects the route of migrtion of this field. Sesonlly the observed pttern is similr to the ones of the Ribnceir dunefield. The obtined results were similr to the found ones by GIANNINI (1993) tht pointed similr drift pttern in this region. The Snt Mrt Lighthouse sttion ws consistent with the snd rose nd the DP found for the Siriú dunefield, with DP of 279 uv, considered n environment with intermedite energy winds nd RDP/DP rte of 0.37 indicting n intermedite vribility. The obtined DDR in the snd rose greed with the field's migrtion direction. Dunefields nlyzed t the south portion of the cost correspondent to the Snt Mrt Lighthouse sttion, presented concordnce with the results obtined in the clcultions of drift potentil. The Morro dos Conventos dunefield presented DP of 297 uv, being clssified s of intermedite energy the high one. The vribility, with 0.37 rte ws clssified s intermedite to low. This field lso possesses nnul s sesonl snd roses, concordnt with the route of observble migrtion over the terrin. Sesonlly the results obtined in drift potentil clcultions confirmed the pttern found by GIANNINI (1993), where the winter nd spring winds re most effective in the trnsport. CONCLUSION Some fctors must be tken in considertion when nlyzing the processes of Aeolin trnsport, s: qulity of superficil winds dt; the choice of the meteorologicl sttions for the verifictions in reltion to the dunefields to be nlyzed nd minly the verge snd grin size, when significnt differences between the dunefields. The pttern found revels tht the effect of verge snd grin size to be considered in the nlyses of Aeolin trnsport Figur 4.Wind roses of nlyzed meteorologicl sttion.

4 Snd Drift Potentil by Wind Action 105 Figure 5. Wind roses obtined for the studied re t the nlyzed dune fields. A corresponds to meteorologicl sttion t pltform PXIV, B corresponds toarvoredo Islnd nd C corresponds to meteorologicl sttion t Snt Mrt Lighthouse. processes nd not only the winds directionl vribility, but lso the intensity of the superficil winds must be lwys nlyzed together in reltion to the verge grin size when considered vribility between dunefields. Thus, it ws verified tht the migrtion direction of the dunefields grees with the predominnt wind (highest occurrence) when the verge speeds surpss the limiting velocity of impct of the snd grin. However when this speed is lower the verge snd grin size of strts to determine the migrtion direction of the dunefield in greement with the previling winds (highest intensity). LITERATURE CITED BAGNOLD, R. A. The physics of blown snd nd desert dunes. Methuen & Co., London

5 106 Vintem et l. BIGARELLA, J.J. Eolin environment - their chrcteristics, recognition nd importnce. In: RIGBY, J.K. nd HAMBLiN, W. K. eds. Recognition of Ancient Sedimentry Enviroments. Spec. Publ. 16: 12-62, BIGARELLA, J.J. Prbolic Dune Behvior under Effective Storm Wind Conditions in: Revist Brsileir de Geomorfologi. Vol. 1, n p FOLK, R.L.; WARD, W.C. Brzos River Br: A Study in the* Significnce of Grin Size Prmeters. In: Journ. Sediment. Petrol., 27(1), pp. 3-26,1957. FRYBERGER, S.G. nd DEAN G. Dune forms nd wind regime. In: McKEE, E.D. ed. A Study of Globl Snd Ses. U.S. Geol. Surv. Prof. Pper, 1052: , GIANNINI, P. C. F. Sistems Deposicionis no Quternário Costeiro entre Jgurún e Imbitub, SC.,2v.(Tese- Doutormento) - Instituto de Geociêncis, Universidde de São Pulo. São Pulo, MARTINHO C. T. GIANNINI P. C. F. nd SAWAKUCHIA. O. Fácies Morfológics e Fácies Deposicionis de Cmpos de Duns Trnsgressivos Ativos d Região de Jgurun- Imbitub, SC. In: IX Congresso d Associção Brsileir de Estudos do Quternário (ABEQUA), Recife, Brsil, 2003.