Recent Saltmarsh Changes and Sedimentation Rates in the Sado Estuary, Portugal
Maria Eugenia Soares de Albergaria Moreira
Centro de Est lidos (;eogrc.~lti('()s Faculdade de l.ei.ras Universidadc de Lisbon 1699 Lishoa Codex, Portllgal
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MOI{EIH,A, M.KS.A., ]!l~l~. Hen'1I1 salt marsh ehallgl'H lind sr dunent.at ion rateH in t hr- SlIdo EHluHrv, ,tflllllll:. Portugal. .luurn al of ('ul1l;fal U/'."I'{I,.("h. l'{C\), I);l] 1;40. Fori Lllllderdllk (Florida l, ISSN ()749-()~OH. . ~ The j idal mudflats (If t he r-st u.rrv of t hi' Sado l i ivr-r an' COtllpoSl'd (If silt and cia v. wit h intr-rspersed sandv units that mnv repn'spnt n'sl rictl'd "all'o I ida) chanll('L" [lr Wldl'Spft'Hd (Iuviul flood d"l\Osils. The min (·ralogy (If t hr- day is primarily mont motillouue. wlwft,!!.,-, tilt' silt and silnd grains .ire Jllq~f'ly quartz.
euus~ ~ ~ Estuarine and lagoollal salt marshes rovr-r 'i'll', 100' I (If the llppn m udflat , forming- narrow platforms -+- ;,-- disst'etf'ef hv neeks and pol,vgel\t'llt· sail pallS. The lowr-r mudtlnt is dis~wett'd liv Ilw
ADI>ITIONAL INDEX WORDS: Est uurin e ..,,,,d'llll'nllll/ull. ("U(/SIIl!u,I'f/IlTlds. J.:1'fJm(}r{J}w/ug\-', SI'{1··!('/'e(
IN'TR(llllJ(:'rI()N 1he marsh ('rosiollal pn)('('ssps (!\/l()HEIHA-LoI'Es, t ~r,~): M()HEIIL\, I !)Hi). The coastal wetlands of the Sado estuary", Oil the southwestern coast. of Portugal (Figure 1l, oc l{('('ognition of the h.\'drological rpgilne is irn cupy 7,170 km'. The wetlands are part. of a Na portend. tot he u ndr-rst and ing of t he sedimentarv ~H' tional Natural J{eserv(' whose surface has hr-en processes Oil t est uari IH' we-tlands (V EHC!':I{, 19()H; ~: decreasing because of progressive reclamat ion and HAN \V ELL, t Hi (' II A I)1\'1 A \J, 1974 ). r l' hp flu v iaI re ginH' of Sado i~ of a I\;']edit('rraIH'(\n-type (AM developtnent. on its rnargins (MOHEIHA and ()LI the VEIHA, 1988). rrhe estuary is sheltered seaward b.v HAH ('I ul., IDHO), hut at pres('nt the cool season ~In 9110<" rNl'il'('d {lnd (l('I'I'{)fl'd 29 NOI'I'f/lllI'r 1991 signiticant. 632 Moreira SURVEY METHODS ------1 1-- '''I, - \ --1 The linear horizontal retreat or progradation of '.[ J~~ ~j ::~: __ the salt.marsh scarp was observed by surveying i ~.:-' ~f",:"~--:---_J the distance from the scarp-line to fixed bench l~ __ marks. In some cases, profile lines were run to 2 -~lL JJ ~."'1 :', include aspects of the vegetation dynamics. Ver D 't.j . , r lJG tical accretion rates on the high and low marsh ~-.~.;~~:=:.:. surfaces were determined through a combination . of surveying, using marker horizons of brick dust ~~{~~:;,.,:} (STEERS, 19:38; RICHARD, 1978), and stakes to r'~ r, 0 .o identify specially-designated core sites in a range I 0 i.otdeu o , \:)\dPTrO'r;B of wetland morphologies (Figure 1). Two cores \:~" " were collected at each site and the results of the '11'Kld" A, --p analyses represent the average of the two samples. o «)"'01'0 -<;' Brick dust was applied to sites after the first spring tide in September in each of three different years (1978,1982, and 198~3). Recovery of the brick dust layer in the entire study area was incomplete because several of the sampling sites were dis turbed by fishermen, by cattle, and by other traffic through the marshes. Some of the other sites, al though not apparently disturbed, did not retain ~~~O'nporlr1 a recognizable brick dust layer in their cores. . .:...Jt'dOlt:Qtl'fT"\,t r. ...:...JSCllt-rnUr':::lh jdUflt! The inorganic component of the collected sam r 1 r 1 .~ ,--./:hluf' l~]~·ke l~g(]tP 1 (1 r I n 9 <; I t ples was determined by treating the samples with Figure.I. Location of the core sites on the intertidal platforms H20~ and expressing the remaining sediment as of the Sarlo estuary. a percentage of the total dry weight. The major surficial geomorphological features and their distributions were identified on aerial The tidal regime is sernidiurnal and mesot.idal, photographs (1:25,000 and 1:15,000 scale) and on The tidal range at spring tide is ;1.7 m at the satellite imagery (Landsat MMS and TM), but entrance to the estuary (Troia) and 4 m at loca the micro-geomorphological features and the pro tions within the estuary (Comport.a, Palma, Zam cesses at the micro-scale were measured, moni bujal, and Mitrena). Wave heights in the estuary tored, and analyzed in the field. are usually less than 1.0 m. During the summer, maximum wave heights are associated with winds from the north, Nortada. In the winter, winds GENERAL GEOMORPHOLOGICAL AND from the northwest are funneled through the Ma SEDIMENTOL()GICAL CHARACTERISTICS rateca estuarine channel and generate waves OF THE STUDY AREA reaching 1.0 m. These waves produce considerable The intertidal platforms and shoals of the Sado erosion of the dikes that are exposed to these estuary develop between the elevations of - 1.8 greater fetches. Oceanic swell does not pass ill and +2.0 m along the margins of the lagoon through the inlet into the estuary. and the estuarine channels (Figure 1). Most of the The objective of this study is to describe and wetland system has been modified by human ac analyze the sedimentation rates of the last decade tion. The ecotone of the inland margin of the on the intertidal platform and to relate the rates intertidal platform is against a constructed dike to the variety of natural and anthropogenic pro along 80(';, of its length. The only remaining nat cesses active in the area. The terminology applied ural ecotone of the inner margin of the intertidal to describe the estuarine and intertidal forms, the platform is along the contact with the Troia spit micro-forms, and the processes follows the defi and along short sectors of the bluffs at Arrabidas nitions cited in CRONlN (1978), I)AVIl<~S (1978), Pinheiro and at Mitrena (Figure 1). MOHEIHA (1984), KELLEY et at. (1988), and WOOD Two levels of intertidal platforms can be dis et al. (1989). tinguished. The lower portion is usually an un- Journal of Coastal Research, Vol. 8, No. a, 1992 IGlJ 1989 Portugal Field Symposium 633 l-~-~--~- ,,1111'\11 I I L" ~ 11II ~~.~~~---- : ~ ~ i \ cr c ck h'IIIO 111'111< (t1nql(\lf,1 '1IIhlll.'l j ', Figure ;~. Retreating processes of the saltrnarsh scarp: (A) Troia Figure 2. Schematic diagram of Sado estuary tidal platform spit (Site If}), (H) l lha do Cavalo (Site ;j), and (C) Monte do morphology (Troia spit wetlands). Pinheiro (Site 7), vegetated, low-tide platform, slihhe, which is sub jected to the tidal flows during every tidal cycle, (Arthrocnemum ssp.). 'I'he lower and estuarine whereas the higher portion is covered by a salt ward marsh is dominated by horse grass (8par marsh, forming a high-tide platform, schorre, tina maritima) (MOHEIHA, 1987). The high marsh which is usually inundated only during spring tides is cut by meandering tidal channel networks which (Figure 2). The two portions of the platform are drain the upper surface and is pocked by salt pans separated by a scarped rise of 0.5 m-2.0 m that (CHAPMAN, 1974; BEEFTINK, 1977). constitutes a micro-cliff in the saltmarsh profile. RESlJLTS AND DISt:USSI()N It is the processes related to this micro-cliff or saltmarsh scarp, the rates of change, and the sed Horizontal Retreat of the Saltmarsh Scarp imentation in its vicinity that form the focus of In general, the salt.marsh scarp is present in all this paper. of the profiles measured in the Sado estuary. In The low tidal flat component is wide and is most cases, the scarp is retreating inland, but in comprised of sandy sediments adjacent to the creek a few instances, the salt.marsh is extending es channels (tidal sandflat) but of silty sediments tuarineward over the low tidal fiat. The salt.marsh away from the channels (tidal mudflat). Quartz is scarp retreat is due to several processes that de the dominant mineral making up the sand and pend on the vegetation cover and on the frequency silt particles, whereas most of the clay particles of excavating fauna, as well as the sedimentolog are montmorillonite (MOREIRA and LAPA, 1985). ical characteristics of the marsh. Within the tidal fiats, sandy levees and spits are generated by tidal currents. These features are U ndcrcutting and Collapsing produced by the flood tides in the Comporta chan The erosion and scarping of the high marsh is nel and by the ebb flows, augmented by Iluvial associated with the creation of a notch in the outer discharge, in the Marateca and Sado channels. In edge of the marsh surface. Tidal currents, es several locations (Comporta, Carrasqueira, and pecially those occurring during flood stages, sap Palma) small oyster reefs colonize the outer edges and undercut the micro-cliff and eventually lead of the tidal flats and estuarine shoals, creating to the collapse or sliding (or both) of the top :30 small, irregularly-shaped bioherms that trap or ;')0 em section of the root-bound marsh surface. ganic detritus and fine sediment. On the basis of observations throughout the Sado, The high-tide platform is composed of compact the mechanisms for failure of the marsh margin silt and montmorillonitic clay accumulations and are related to the dimensions of the notch. When has two sectors. The inland sector is completely the notch extends very far (60 to 1:30 cm) into the submerged only during the spring high tides, leading edge of a deep and densely-rooted high whereas the estuarineward sector is su bmerged marsh and the notch is very thin (10 to 20 em), during every ordinary high tide. The former sector the overlying material shifts forward by a rela is the high marsh portion of the platform and is tively slow rotational movement and does not col colonized with semi-prostrate Chenopodiaceae, lapse completely. The result is a scarp with a con among which the dominant species are sea-purs vex profile (Figure ;)A). In this situation, there is lane tHtuimione portulacoidesj and glassworts no abrupt cliff face and the scarpline is protected Journal of Coastal Research, Vol. 8, No. ;j, 1992 634 Moreira -- • ". -; i - , , ~: ...... ~.' .. ~.. : :. - ~ ...... -:-- • w ' ~ '1 ~ j,'r.. Fi gur e 4. Salt marsh scarp retr eat by rotation al sliding and vegetat ion destru ct ion in th e so ut hern side of Ilha do Cavalo. and masked by vegetation (T able 1,Sites 1 an d migration in those locations where there is little 2, Figure 4). However, the scarp is being under cut interference by humans and where the scarp is and is undergoing a progressive retreat. The T roia laterally exp osed to tid al curre nts (F igure 5). sp it marsh is sh ifting in thi s manner (Ta ble 1, In similar situa tions as described above, but Site 1).T his appears to be th e pro cess of scarp whe re the clay has been subjected to drying, with surface cra cks and holes excavated by animals, the retreat of the sca rp takes place along a line Table 1. Horizontal sedime ntation rate.' orth e saltmarsh scarp, of weakness. The edge is characterized by a small step or series of ste ps as thi ck as th e root band Mea n Annu al Core Ho rizonta l Change (Fi gure 38) and the process results in a progres Locati on sive recession of th e vegetation zones (F igure 5, Sites Mo nitoring Retreat Accret ion T able 1, Sites 3 and 4). The en tire pr ocess can be (Figure I) P eriod em/year em/year accele rated by anthropogeni c destruction of the 1 1978-1989 4.7 vegetation cover. Near the Carrasqueira fish ing I 1978- 198:1 1.2 1 1 9 8 ~- 1989 7.7 harbor, several islands of saltma rsh wer e almost 2 1983-1989 0.8 comp letely destroyed because of th e retreat pro 3 1978-1989 4.7 cesses combined with the use of the marsh sur face :l 1978-1 98:l 2.7 for boat repairs (Table 1, Sites 5, 6, and 7; Figure 3 1 9 8 ~ - 1 9 8 9 6.3 6). 4 1978-1989 5.0 5 1978 - 1989 10.0 Rota tional Sliding and Peat Block Collapsing 6 1978 -1 989 :1 1.8 7 1978-1 989 4fi.5 Where there is no veget ati on cover or where it 8 1978-1989 72.7 is very scarce « 10%), where the desiccation :l6.4 9 1978-1 989 cracks are deep and there is a high density of salt 9 1978-198:l 64.0 9 1 9 8 ~ - 1 9 89 I:l.:l pans and holes , where the notch is not too deep 10 198:J-1989 ~ 7. fi (20 to 70 ern), and th e flood tid al current is st rong, 11 198:1- 1989 8:1.:l there are two possible outcomes of edge failure 12 1 9 8 ~ - 1 9 8 9 100.0 depending on the lateral infiltration of water into J our nal of Coastal Researc h, Vol. 8, No. 3, 1992 IGU 1989 Por tugal Field Symposium 635 Figure 5. Sa ltmarsh sca rp retr eatin g by collapsin g in the 'Troia mar sh . Figure 6.Horizontal an d vertical retrea t of th e saltmarsh in Ca rrasqueira -Moitinhas, du e to activity of fishermen . Jo urnal of Coastal Research, Vol. 8, N o. :1, 1992 636 Moreira Figur e 7. Ret reat of the saltmarsh scarp by gulleying followed by th e collapse of blocks of peat, in th e marsh of Mont e do Pin heiro. th e marsh clay . One case occu rs in th e wet winter this type of retreat, the pan s are rap idly dissected peri od du ring spring tides. At this time, the fai lure by the tidal exchanges (Figure 3C). This is the is by the rot ati onal sliding of large blocks of the situation for the most rapid retreat of the salt mar sh edg e. If sa lt pan s ar e encountered during marsh scarp (Figu re 7; T able 1, Site 7). This type Figur e 8.Advance of th e Spar/ina marsh of Monte do P inheiro over t he low mudfl at. Jo urnal of Coas tal Research , Vol. 8, No.3, 1992 IGU 1989 Portugal Field Symposium 637 Figure 9. Organic and mineral detritus accumulated around the semi-prostrate stems of Halimion e portulacoides . is found most frequently in areas where the salt 1, Sites 10, 11, and 12) due to a sandy accumu marsh is very narrow and is pocked by numerous lation on the mud flat (Figure 8). This phenom salt pans, and where the marsh scarp is exposed enon of migration of the sand tidal flat landward to very strong tidal currents. covering the low mud flat is found throughout the Another instance occurs during the dry summer estuary. This transgressive characteristic may be period when the clay sediments dry rapidly and a consequence of the recently-described sea-level are marked by deep desiccation cracks. At this rise along the Portuguese coast (T ABORDA and time, spring tide elevations are accompanied by DIAS, 1989), as well as a consequence of the sub collapses of large blocks of peat. This process is sidence of the Sado estuarine region. especially prone to occur on the northward facing marsh scarps which are exposed to the greatest Vertical Accretion Rates on the Saltmarsh Surface fetches across the estuary associated with either and on the Sandy Levees the N artada of the summer, or to the winter storm The vertical accretion of sediment on the salt waves coming from the northwest. marsh surface is not so spectacular as is the lateral The mean annual values of retreat on the ex erosion. The values of vertical change (Table 2) posed saltmarshes were greatest on the northern demonstrate that accretion is not regular nor ho side of Ilha do Cavalo and in the fishing harbor mogeneous in either space or time. Local factors of Carrasque ira (Table 1, Sites 8 and 9), especially such as vegetation cover and floristic changes, the due to the anthropogenic activity. position of the core sites relative to the source of sedimentsupply (distance of tidal or fluvial trans Horizontal Progradation of the Low Marsh port) create differences in the quantities of ac In several sheltered sectors of the estuary, a cumulation and the composition of the sediments. positive sediment budget on the low tidal flat The suspended sediment load in the Sado es caused the Spartina marsh surface to prograde. tuary is derived from several sources. One very Rates of buildout reached as high as 1.0 m/yr in important source is the erosion of the tidal flats Mitrena (Figure 9) and Monte do Pinheiro (Table within the entire estuary. That is obviously a re- Journal of Coastal Research, Vol. 8, No.3. 1992 6;~8 Moreira ------~-_._--~.. Table 2. Vertical sedimentation ruff'S on the sult tnurslu:s in cupied by a dense vegetative cover cornprised of the Seide, E«! (10'-.\ ~);) ". Spurt ina maritinia and [)('( Suaeda mari tima. Ve-rti- The low rate of accumulation at Site 18, cal At' al ('OfP Locatioll crvt iou Coruposit iou though in a low energy environment. seems to be ------of Sl'di- related to the low plant density at the site, less (;l'O- l\ nnunl nu-nt s Ill()rph(,l\'gil' \-'lolli- l\'ll'illl t.han .so 1'( , as well as a possible lack of sediment Sitp I<~ Il \' i ro 11- tOfing (mrn/ (f', (f', supply. (Fig. I) rne nt Ppri(ldunes. almost entirely tine-grained organic det.ritus. The The sand accumulations on the levees of the rates of accretion on the high marsh surface vary Cur r asq uei r a. Marateca, and Ilha do Cavalo from 0.8 to ;~.1 rnrn/yr Crable :2). The highest val marshes are derived from sandstones located in ues are found along 1he sheltered tidal creek berms t.he drainage basin of the Sado estuary and are veget.at.ed with Halimionv portulacoidv« (Site l:n part of the fluvial load COining into the estuary. and in t.he high marsh concave depressions cov (:()NCLUSI()N ered with Huli mionc portulacoides and Arthro cnemum glaucum (Site 17), These dominant semi A It.hough there is a great variat ion within the prostrate species an' very important in trapping obse-rvations and measurements of the erosion and the tine serl iment.s (Fig"ure 9). accretion rates in the Sado estuary, there are some In general, 1h« accrvt ion rutr-s are slight Iy high general conclusions that rnay be drawn from the er on the lower, vpget,IU'd port.ions of 1he marsh data set s, surface. Thi» mav rplat.t' to plant d('llsity as we ll The mean horizontal retreat of the salt.marsh as to the .rvailahilitv of' sedillleJ11 and t ho daily scarp for the J 97H-1 ~)89 and 198;)- J 989 periods inundnt ion. The ratf'S of <)('(Tf't ion in the S/)([r( ina oj" 17 cm/vr gives evidence of' a general loss of the marsh. whet.her on t hp higher or lowe-r v('getat,ed snll.-marsh area over this period. Much of this loss units, are related to t he plant density and t.o t.he can be at Lrihuted to the human activity in the relnt.ive ex posur« of t.lu-sit ('s. Si {.(-. J ~), for e xumpie. arc-a, causing erosion directly or contributing to has t he high('st uurrshlund (lcculnulatloll rate. 11 all increased rnu- oferosion. In addition, it is Ii kely is located t() t ht' 1('(' of' a s.mdv lvve« and is 0('- that t.he pust at ir: rise or sea level is an important .lournul of ('oastal H,PSPiuch, Vol. H, No. ;~, I ~J9:2 IGU 1989 Po rt ugal Fie ld Symposium 639 Figure 10. Sa ndy accretion on marg inal levees of th e Comporta channel; severa l geomorphological char acteristics of the Troia spi t wetlands can be obse rved . contributor.T he upw ard shifti ng of th e sedime n CRONIN, L.E. (ed .), 1978. Estuarine Research , ll-Ge tary units, primarily t he sa ndy acc retion on the ology and Engineering. Ne w York: Academic, 389p. DAVIS, R.A. (ed .), 1978. Coastal S edimentary En viron tidal mudflat, demonstrates a transgressi ve situ ments. Ber lin: Sp ringer-Verl ag, 420p . ation attributa ble to a risin g wa ter level. MOREJI( A-Lol'l';S, M.E.S.A., 1 9 7 ~) . 0 Estutirio do Sado , Significantly, the accumulation rate of 1.9 mm / Paisagem e Diruimica. Lisbon: Centro de Estudos yr on the mar sh surface is sufficien t to keep pace Ceograficos, 89p. with the sea-level rise estima te d for t he Port u MORF: IRA, M.E.S.A., 1984. Glosstirio de T ermos Usados em Geom orfologia Litoral. Lis bon: Centro de Estu dos guese coast. Geogra ficos. 167p . MOHEIRA, M.E.S.A., 1987. Estudo fitogeogr afico do ecos ACKNOWLEDGEMENTS sistema de sa pal do Estu ario do Sado. Fi nisterra, Re vist a Po rtuguesa de Geografia, 22(44) , 247- 30.1. Th e author wish es to tha nk all the perso ns a nd Morucmx, M.E.S .A. a nd Or.rvrc mx. E. M., 1988. Estuario inst itu tions tha t contribute d to th e contents of do Sado: lden tificacao de unidades Iisiograficas a par ti r de imagens LAN DS AT . Infornuictio Tccnica de this paper. Special appreciati on is exte nded to Detecctio Remota, IT DR :1, 47p. Professor Suzanne Daveau -Rib eiro for revising the MOREIRA, M.E.S.A . and LAJ' A,A., 1985. Nota pre limi nar French summary of thi s pa per. A specia l th a nk sobre 0 estudo dos sed imentos a rgilosos das platafor you is offered to Professor Norbe rt P. Ps uty for mas de mare do Es tuario do Sado. Un iversity of Coirn providin g backgroun d literature on thi s to pic and bra, un published report. RANWI·;I.I., D.S., 1972. Ecology o] Salt Marshes and Sand for reviewin g the English manuscript. Dun es. Lon don: Chap ma n an d Hall, 258 p. RICHARD, G.A., 1978. Se asonal a nd env ironmental va ri LITERATURE CITED a tions in sed iment accret ion in a Long Is land sa lt mars h. Estuaries, I , 29- 35. AMB AR, I.; FIUZA, A.; SOUSA , M.F., and LOUREN ,:n, I., STEEIlS, .J.A., 1938. T he ra te of sed ime ntation on salt 1980. General circulation in the lower Sado River es mar s hes on Scolt Head Island , Norfolk. Geological tuary un der drough t conditions. NAT O Seminar on Magazine, 7fi, 26-: 19. Problemas Actuais da Oceanografia Portuguese, sc i T AIlOIWA, R. and DIAS, J.M.A., 1989. Ti de-gauge da ta entific paper. Lisbon: 13p. in deducing sea level rise and cru sta l movements rate; CHAP MAN, V.J .. 1974. Sa lt Marsh es and Salt Deserts of T he Por tuguese case . IG U-CCE An nual Regional the World. London: Leonald Hill Boo ks, 392p. Sy mposium Guide Booh. Lisbo n: 130p. Journal of Coasta l Research. Vol. 8, No. 3, 1992 640 Moreira VERGgR, F., 1968. Marais et Wadden du Littoral Fran WOOD, M.E.; KJ.:LLI<:V, ~J.T., and BI<;LKNAP, D.F., 1989. cais, Etude de Geomorphologic. Bordeaux: Biscaye Patterns of sedimentaccumulation in the tidal marsh Freres Imprimeurs, 541p. es of Maine. Estuaries, 12(4),2;37-246. [J RESUMO [J Este artigo ref'ere a dinamica actual da plataforma de mare alta, no estuario do Sado, durante 0 periodo de 1978-1989. 0 balance da sedirnentacao nesta plataforma caracterizou-se por urn recuo generalizado da sua esearpa, de 0.7 a 10.5 cm/ano, devido a processes de deslizamento e de desmoronamento da parte superior da escarpa posteriores ao subescavamento de uma sapa basal, abaixo do nivel das raizes do sapal. Quando a taxa de cobertura da vegetacao do sapal e baixa, a densidade de tocas de animais escavadores e elevada, e as pocas de mare sao abertas pelo recuo da escarpa ou dos canais de mare, 0 ravinamemto da escarpa e 0 processo dominante, desprendendo se blocos de vasa compacta aglutinada pelas raizes das plantas do sapal. 0 valor de 45.5 em/ano foi 0 maximo atingido, por este processo de recuo. Porern, os valores maxirnos do recuo da escarpa do sapal, de 76.7 cm/ano, foram ocasionados pela intervencao antropica, Em sectores abrigados do estuario verifica-se 0 avanco do sapal sobre a plataforma de mare baixa, tendo-se registado 0 valor maximo de 1 m/ano. Durante 0 mesmo periodo a acumulacao de materials sobre a superficie da plataforma do sapal, apesar de nao ser uniforme, atingiu valores medios anuais de 0.8 e 5.:1 mm/ano, enquanto nos hancos arenosos das margens dos principais esteiros, este valor atinqiu 106 mm/ano. Sobre os lodacais e tarnbem sohre alguns sect.orcs do sapal, observou-se a acumulacao de uma estreita camada de areia. Este fenorneno e 0 recuo generalisado da escarpa cia sapal atestam uma recente elevacao local do nivel do mar. o Rf~SlJMf~ 0 Cet article presente la dynarnique sedimentaire actuelle de Ia plate-forme du schorre de l'estuaire du Sado, pendant la periods 1978 1989. Le bilan sedimentaire de cette plate-forme est caracterise par Ie recul de la micro-falaise du schorre, de 0,7 a 10,5 em/an, a cause du glissement de la corniche de la microfalaise, apres sapement basal. Quand la vegetation n'est pas tr es dense et que la densite des animaux fouisseurs est grande, quand les mares du schorre sont ouvertes par I'erosion, Ie processus Ie plus etficace est le ravinement suivi d'un eboulernent de biouies de vase (recul moyen de 45,4 ern/an). La valeur maximale du recul de cette micro-falaise (76,7 em/an), resulte l'arnenagement des marges des etiers. Dans les secteurs Ies plus abrites de l'estuaire, on verifie des phenomenes daccretion, a cause de la progression de la vegetation du schorre sur la slikke sablo-vaseuse. L'accretion verticale qui a ete observee n'est pas reguliere; des valeurs moyennes de 0,8 a5 mm/an sont observees sur la surface du sehorre et de :11,5a 106 em/an sur les levees sableuses. Presque partout, l'ensablement des slikkes vaseuses et de quelques secteurs du schorre est un phenomene aetuel. Ce fait assode au recul generalise du schorre resulte de l'activite hurnaine et de l'elevat.ion locale du niveau de la mer . •Journal of Coastal Research, Vol. 8, No.3, 1992