Journal of Coastal Research 1119-1129 Fort Lauderdale, Florida Fall 1997 Field Measurements of Longshore Sand Transport and Control Processes on a Steep Meso-Tidal Beach in Portugal Paolo Ciavola]", Rui Taborda']; Oscar Ferreira] and joao Alveirinho Dias] t Unidade de Ciencias e t Departamento de Geologia Tecnologias dos Recursos Universidade de Lisboa Aquaticos Cidade Universitaria Uni versidade do Algarve Faculdade de Ciencias Campus de Gambelas Bloco C2, 5° Piso 8000 Faro, Portugal Campo Grande 1700 Lisboa, Portugal ABSTRACT _ CIAVOLA, P.; TABORDA, R.; FERREIRA, 0., and DIAS, J.A., 1997. Field Measurements of Longshore Sand Trans­ .tllllllll:. port and Control Processes on a Steep Meso-Tidal Beach in Portugal. Journal ofCoastal Research, 13(4),1119-1129. Fort Lauderdale (Florida), ISSN 0749-0208. ~.~ A field experiment was carried at Culatra Beach in Algarve (Southern Portugal) to determine longshore transport ~ ~ 7.J rates and sand mixing depth on a steep (slope 0.11) meso-tidal beach. The experiment was undertaken over one and -. b--- a half tidal cycles using sand tracers in conjunction with wave and current monitoring. Variation of mean significant wave height during the experiment was limited (0.34-0.37 m) with mean zero-up crossing periods of 5.1-5.8 sec. Mean longshore current velocities in the breaker zone reached a peak in the second tide (0.28 m sec 1), while they were one order of magnitude smaller during the first (0.02 m sec 1) and third tide (0.04 m sec 1). The increase in current speed was due to a moderate wind that was blowing along shore during the second tide. Average advection velocity of the tracer cloud and longshore currents showed a good correlation, leading to calculation of much larger transport rates for the second tide (1.38X 10 2 m'' sec I) than for the other two (0.23X 10 2 m" sec I). Average depth of sand mixing of 10.6 cm in the beach face was 29% of breaking wave height and showed a marked uni-modal distribution, with maximum of 15 em in the breaker zone. Previously published empirical formulae do not predict satisfactorily this behavior in depth of sand mixing that seems to be peculiar of steep beaches under plunging waves. Empirical formulae were used to compute theoretical longshore transport and compare it with field observations. They all underestimated measured transport rates of about one order of magnitude, thus confirming that the morphodynamics of steep beaches are characterized by relatively high sediment transport even in relatively low energy regimes. ADDITIONAL INDEX WORDS: Meso-tidal beach, plunging waves, fluorescent sand tracers, mixing depth, longshore transport, longshore wind, empirical models. INTRODUCTION for predicting longshore sand transport mainly based on mod­ ifications of the original work of KOMAR and INMAN (1970). The prediction of littoral sand transport has important ap­ The main problem in using these empirical notations is that plications both in earth sciences and coastal oceanography they contain an empirical coefficient which shows large vari­ and in the past 40 years efforts by oceanographers, sedimen­ ability due to environmental factors (KOMAR, 1988) that in­ tologists and coastal engineers have greatly improved the un­ clude beach characteristics such as grain size (DEAN et al., derstanding of the physical forces that control the process. Net rates of longshore transport under carefully monitored 1982; KAMPHUIS et al., 1986) and slope (KAMPHUIS et al., physical conditions have been previously studied using large­ 1986). Values of this empirical coefficient as proposed in the scale sediment traps (DEAN et al., 1982; KAMPHUIS et al., international literature varied by a factor of four since intro­ 1986), portable sediment traps (KRAUS, 1987), dispersion of duction (BODGE, 1989). sand tracers (INGLE, 1966; KOMAR and INMAN, 1970; INMAN Recent reviews of field data on longshore sediment trans­ et al., 1980; KRAUS et al., 1982; TABORDA et al., 1994) and port have concluded that there are large uncertainties re­ beach profiles (BEREK and DEAN, 1982). Many of these beach garding the distribution of sediment movement across the experiments have led to the production of empirical formulae surf zone (BODGE, 1989), and the behavior of beaches for transport rates exceeding 0.2 X 106 mvyear, significant wave 96023 received 25 March 1996; accepted in revision 8 August 1996. height larger that 1.8 m, grain size coarser than 0.6 mm and Contribution n. A76 to the Projecto DISEPLA (Sediment Dynamics beach slope steeper than 0.06 (SCHOONEES and THERON, of the Portuguese Shelf), 1993). The lack of information on steep beaches seems to be I Present address: Dipartimento di Scienze, Geologiche e Paleonto­ logiche, Universita di Ferrara, Corso Ercole I d'Este, 32, 44100 Fer­ of particular importance, since their hydrodynamic behavior rara, Italy. is radically different from that of low-gradient beaches 1120 Ciavola el al. Height (em) 500','- - - - - - - - - - - - - - - - - - --, Berm Top 400 Tra cer Injection 30 _ 1 Rig ~----.... 20 100 Mean Sea Level o -100 I, ii ', I o 10 20 30 40 50 60 70 80 Distance Irom benchmark (m) Figur e I . Ind ex map of th e Alga rve barrier island sys tem and continen­ ta l she lf (depth contours are in met er s below Mean Sea Level), Figure 2. Beach profile at Culatra Beac h at th e beginning of th e exper­ imen t and location of tracer injection a nd ocea nogra phic rig. (HUNTLEYand BOWEN, 1975). On steep beaches the surf zone tend s to be narrow and swash processes and wav e character­ in files corres ponding to a duration of 10.24 minutes, thus istics (e.g, plunging) become particularly important for sa nd giving 3072 data points for each time series. remob ilization (JACKSON and NORDSTROM , 1993 ). The pressure tran sducer was calibrated in th e laboratory Th e field experiment described in this pap er took place to esta blish a lin ear relationship between pressure and water over one and a half tid al cycles on 7 and 8 October 1993 on level and to determine the offset typi cal of the sen sor as sug­ th e beach of Cul atra Island in Southern Portugal (Figu re 1). gested by DAVIDSON (1992).A Fast Fourier Tr an sformation Th e experiment was part of a field study (LUAR-Culatra '93 ) was applied to each time series following the methods of undertaken between 7 and 12 October 1992 by several Por­ EARLE and BISHOP (1984) before calculating significant wave tuguese and European universities (Algarve, Lisbon, South­ height (Hs) and mean zero-up crossing period (T) with spec­ ampton, East Angli a, Liverpool, Bordeaux) with the support tral methodologies using routines availab le in th e popular of local and national authorities (lnstituto Hidrografico, MATLAB@J environment (digital signal processing toolbox ). Parque Natural da Ria Formosa, Capitania do Porto de Faro­ Angles of wave approach at breaking point were calculated Olh ao ), by refr action of deep water waves hindcasted for the time of Culatra Islan d is part of the 60 km long barrier isla nd sys­ th e experime nt using a wind stress model (PIRES and Ro­ tem of th e Algarve, that exists under meso-tid al conditions DR IGUES, 1988 ) and validation by field observations. Tim es (about 4 m maximum spring ran ge), and moderate to high when the rig was ins ide th e surf zone were also recorded in wave ene rgy regime (PILKEYet al., 1989). Th e studied beach th e field. Voltage read ings of longshore and cross-shore cur­ had at th e time of the experime nt a reflecti ve profil e (Figure rents collected in th e field were converted into speed units 2) with a stee p foreshore (slope was 0.11), falling th erefore (rn sec ') using a laboratory calibration carried out at th e within a category wh ere publi shed field st udies a re particu­ Univers ity of Southampton following th e methodology of larly scarce ( S C H OO N ~; E S and THERON, 1993 ). G UZA et al. (1988). Hourly average wind speed and direction for th e time of METHO DOLOGY th e experi ment wa s supplied by the weather station of the A multi-instrument rig was deployed on th e beach on 7 Instituto de Meteorologia located at the nearby Faro airport October 1993 at about 17 em above local Mean Sea Level­ (15 km distant, see Figure 1), being collecte d by an anemom­ MSL (Figu re 2). Th e rig included a Sensym LX piezoresistive ete r located at 17 m above MSL. pressure tran sducer with onboa rd a mplificat ion a nd temper­ A composite sediment sample of about 120 kg was collected ature compensation to record waves as in HARDI STY (1988 ). at th e field site on the lower foreshore and a sub-sa mple of Currents were recorded by employing two discus type bi-axi al 120 g was an a lyzed for particle size determination by dry electromagnetic Valeport 800 current meter s mounted verti­ sieving using a set of meshes ranging betw een 4 mm (- 2 Phi ) cally in th e structure at 17 ern (EMCM-1) and 45 ern and 0.063 mm (4 Ph il, followin g the methodology of INGRAtVI (EMCM-2) above sea bed . Data from three successi ve high (1971). Mean gr ain size, sorting and skewness of th e sa nd tid es were logged on th e morning of 7 October 1993 (05:20­ population was calculated using th e FOLK'S (1974) gr aphical 08:23), hereafter indicated as 7/10am , evening of 7 October parameters.