Morphodynamics of Fetch-Limited Beaches in Contrasting Environments

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN Morphodynamics of Fetch-Limited Beaches in Contrasting Environments P. Freire, Ó. Ferreira, R. Taborda, F. S. B. F. Oliveira, A. R. Carrasco, A. Silva, C. Vargas, R. Capitão, C. J. Fortes, A. B. Coli and J. A. Santos National Laboratory of Civil Engineering, Lisbon , Portugal {pfreire, foliveira, cvargas, rcapitao, jfortes, FCMA/CIMA Algarve University, Faro , Portugal {oferreir, LATTEX, Lisbon University, Lisbon , Portugal {rtaborda, CEPEMAR Meio ambiente Vitória, Espírito Santo , Brasil ABSTRACT FREIRE, P.; FERREIRA, Ó.; TABORDA, R.; OLIVEIRA, F. S. B. F.; CARRASCO, A. R.; SILVA, A.; VARGAS, C.; CAPITÃO, R.; FORTES, C. J.; COLI, A. B. and SANTOS, J. A., Morphodynamics of Fetch-Limited Beaches in Contrasting Environments. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN Sandy beaches can be found in fetch-limited environments that are protected from ocean generated waves, as estuaries, lagoons, and backbarriers, and where fetch characteristics allow local wind-generated waves to develop and maintain a beach. The morphodynamics of these low-energy beaches present a peculiar behaviour and general open-ocean models are inappropriate for their study. The main objective of this work is the development of a fetch-limited beach classification scheme based on the relative importance of wave, tidal and river flow forcing. This objective was pursue through the study of the evolutionary pattern of two fetch-limited beaches located in two different Portuguese systems, Alfeite (in Tagus Estuary) and Ancão backbarrier (in Ria Formosa barrier system). Results show that both beaches display a typical fetch-limited profile, with a narrow reflective beach face and a wide dissipative tidal flat, and that profile shape exhibits small changes without a seasonal pattern typical of ocean beaches. At Alfeite, profile changes are restricted to the beach face and result from low-frequency extreme events (strong wind generated waves, Hs>0.5m). At Ancão, although major morphologic changes are also related to the beach face, which is typical behaviour of wave dominated beaches, sediment transport, mainly driven by tidal currents, extends through the entire profile. A tentative fetch-limited beach classification, based on the relative importance of wave and tide effects, is presented. ADDITIONAL INDEX WORDS: locally-generated waves, morphological evolution, beach classification INTRODUCTION Sandy beaches, resulting exclusively from local wind waves, can be found in fluvial and in marine fetch-limited systems as large dam reservoirs, estuaries, lagoons and backbarriers. Without influence of ocean waves, these beaches occur where sediment is available and fetch characteristics allow local wind-generated waves to create and shape a beach (NORDSTROM, 1992). In the last two decades, several studies were dedicated to beaches in fetch-limited environments, mainly in estuaries and enclosed bays. This scientific interest resulted from the increase of the human pressure in these systems and the awareness of their environmental value. The research undertaken pointed out the differences between fetch-limited beaches and beaches affected by ocean waves (NORDSTROM, 1977; NORDSTROM, 1980; NORDSTROM, 1992; NORDSTROM and JACKSON, 1992; JACKSON and NORDSTROM, 1992; JACKSON, 1995; JACKSON et al., 2002; TRAVERS, 2007): (1) limited fetch generates small waves with short periods that promote high ratios between tidal range and wave height in macro to mesotidal systems; (2) physical processes as wave generation and propagation and shoreline evolution are site specific dependent, by factors as water depth, tidal currents, wind and human interferences; (3) beach morphologic changes are mainly associated to high-energy events with low-frequency and no seasonal/cyclical evolutionary pattern is evident; and (4) beach morphodynamics differs from the established open-ocean models. Considering wave characteristics and their effect in beach morphologic features and profile dynamics, fetch-limited beaches can be considered as low-energy shorelines characterized by (JACKSON and NORDSTROM, 1992; NORDSTROM, 1980; HEGGE et al., 1996; NORDSTROM et al., 1996; JACKSON et al., 2002): non-storm significant wave heights, Hs, smaller than 0.25 m, with Hs during storm winds not exceeding 0.50 m; beach faces are narrow (e.g. less than 20 m in microtidal environment), planar, usually without backshore, limited seaward by an extensive sub-horizontal tidal flat; morphologic features include inherited forms from previous energy events. Fetch-limited differs from other low energy environments that are sheltered from offshore wave energy, e.g. situated on the landward of ocean islands and on windward of estuarine basins, by the dominance of locally generated waves, comparatively to ocean waves, which heights depend primary on wind conditions and basin dimensions (JACKSON et al., 2002). Besides the locally-generated waves, other forcing factors, as tidal currents or river flow, can play an important role in determining beach morphology and sedimentary characteristics. In this context, the present paper aims to discuss a possible classification for fetch-limited beaches based on the relative importance of waves, 183

2 Morphodynamics of Fetch-Limited Beaches Figure 1. Location of Alfeite beach in Tagus estuary and Ancão backbarrier in Ria Formosa. 1.3 m during neap tides. The field site extends over ~150 m, is located at the backbarrier, and therefore protected from all oceanic direct influence and subject to a different wave and currents regime. The sandy beach is typically characterized by a low, narrow and reflective morphology, exhibiting small cuspate forelands. The steep beach face (D 50 =0.8 phi) and the low wave energy result in a narrow surf and swash zones. A gently sloping tidal flat of medium sand (D 50 =1.0 phi) mixed with fine sediments (silt and clay = 10%) is present, ending at a parallel sand spit (D 50 =0.9 phi) cut-off by a small transverse secondary tidal channel (Figure 1). The overall beach profile exhibits a convex-curvilinear shape. Main forcing mechanisms acting at the backbarrier include tidal currents and waves generated by local wind over a short fetch distance. Waves are considerably small (H mean ~0.05 m, T mean <1 s), being on the order of few centimetres, with the exception of waves generated by exceptional strong winds. No river influence is observed at this study site. tidal currents and river flow in beach characteristics and evolution. Two different fetch-limited beaches in Portugal (Alfeite and Ancão) were investigated through the medium-term monitoring of cross-shore profiles complemented with meteo-oceanographic data, and the observed evolution and morphodynamic characteristics are discussed. STUDY SITES Alfeite beach Alfeite beach is located in the inner bay of Tagus estuary, one of the largest estuaries in Europe (Figure 1). The estuary is characterized by a complex morphology with a broad and shallow inner domain separated from the ocean by a narrow fault-controlled inlet channel. The inner estuary, protected from ocean waves incursion, extends northwards with wide mudflats and salt marshes resulting from fluvial fine sediment input. Sandy beaches, developed along the southern margin, are affected by a low energy wave climate resulting from locally generated wind waves that rework sediment from local sources (FREIRE and ANDRADE, 1999; FREIRE et al., 2007). The estuary presents a semi-diurnal mesotidal regime with a mean tidal range in Lisbon of 3.2 m and 1.5 m, respectively, in spring and neap tides (PORTELA and NEVES, 1994). Typical tidal current speed is about 1.0 m.s -1, with maximum values at the inlet of 2.5 m.s -1 (MARETEC, 2001). The Tagus river is the main source of fresh water into the estuary with c. 300 m 3.s -1 of mean flow ( Due to the shoreline s main alignment, WNW-ESE, the directional wind sectors that affect Alfeite beach, by wave generation, are NW and N, predominant during Spring and Summer, each with 20% of frequency of occurrence; and NE and E, predominant during Autumn and Winter, with 15% and 3% of frequency of occurrence, respectively (OLIVEIRA et al., 2009). The beach profile at Alfeite includes two different geomorphologic units separated by a residual gravel deposit: a narrow and steep beach face with sediment median diameter, D 50, of 0.2 phi, and a broad and sub-horizontal tidal flat limited offshore by a tidal channel. Locally dominated by fine grained sediments (between 2% and 60%), dissipative conditions prevail on the tidal flat. Ancão beach Ancão beach belongs to the Ria Formosa, a multi-inlet barrier island system located in southern Portugal (Figure 1). Tides in the area are semi-diurnal, average ranges are 2.8 m for spring tides and METHODS Beach cross-shore profiles were measured in both sites using a total station, from April 2005 to July 2007 at Alfeite, and from March 2006 to March 2008 at Ancão. The morphological mobility of both sites was evaluated based on the maximum vertical variability in each cross-shore profile over successive surveys. Mean beach face slope was also determined in order to attest the sediment displacement through the upper beach. Surface sediment samples were collected at the main morphological units, and the classical grain-size parameters were obtained using the Moment Method according to FRIEDMAN (1961, 1967). The typical beach profile variability is confronted with the main acting forcing mechanisms. Mean wave heights (locally-generated), mean current flow and river influence were characterized in attempt to evaluate their relative dominance at both sites. At Alfeite beach, the water column height was measured with resistive wave gauges (at a frequency of 25 Hz) and pressure transducers (at frequencies of 25 and 2 Hz), and the wave parameters were computed through classical spectral analysis based on the surface elevation measurements. The energy attenuation induced by depth was corrected based on Airy linear theory. Also the influence of the ferry-boats crossing the area of interest was removed from the original data measurements. Local wind data was obtained with an anemometer in each campaign. The influence of the tide and the Tagus river flow in the current velocity and water level at the beach was estimated by numerical modeling (VARGAS et al., 2008) and field observations. At Ancão beach, estimative of wave parameters were performed at the beach face, for a wide range of wind conditions, based on visual observations, supported by a marked ruler, and video records. To attest the medium term influence of wave generation, wind data was obtained from the closest wind station (Faro airport), between March 2006 and March Tidal current data was recorded at Ancão beach, during spring tidal cycles, in the two most important morphological units: at the beach face (with a bidirectional Electromagnetic Current Meter), and at the sand spit (with an Aquadopp Profiler). RESULTS Alfeite beach Alfeite beach is relatively stable in a medium-term analysis (months to years): maximum vertical variability of the beach profile is 0.30 m and beach face slope varies between 0.08 and 0.12 (Table 1). Episodically significant morphologic changes are 184

3 Freire et al. associated to strong wind generated waves (Hs>0.5 m, Table 1). During these low-frequency extreme events sediment is removed from the upper beach face, with an erosive scarp formation (Figure 2), and deposited on the lower beach face without significant changes in profile slope. Erosive morphologic features can persist as relict landforms during non-storm conditions. The different magnitude of the energy dissipated, per unit surface, in both morphologic units restricts sediment transferences between them. The active part of the profile is the steep beach face, especially during the high tide. Long-term (years to decades) beach evolution is mainly associated to human interferences in the system, as land occupation and dredging activities (FREIRE et al., 2007). Although sediment content presents high textural and compositional spatial variability, it was not possible to relate grain size statistics to different energetic events. In normal conditions, tidal currents are not significant at the beach; although, under extreme storm events, the average velocity of the water column due to the tidal current during ebb, obtained by numerical modelling, can reach 0.3 m.s -1 in the channel in front of the beach (VARGAS et al., 2008). Numerical results also pointed out that the river has low impact in either the currents or the water level at Alfeite beach. Nevertheless, field observations show that when a flooding event, normally associated to low pressure conditions, occurs simultaneously with high tidal levels overtopping of the beach high-berm can occur, promoting morphologic changes in the upper foreshore and backshore areas (FREIRE, 2003). Strong SW winds (also associated to low pressure events) can also be also no visible response to strong events (e.g. storms, wind) during the studied period. Nevertheless, small short-term volumetric changes (between months) within profiles can be observed, being related with changes in wind direction, by morphological realignment to the prevailing wave induced characteristics. Most of the wind occurrences for the study period are related with S-SW wind conditions (mean 4 m.s -1 ). Local waves generated by S-SE wind are frequently associated to small erosion at the beach face, while transition to the SW domain is associated with accretion. There is no evidence of net alongshore sediment transferences within the study area, indicating that morphological changes are mainly dependent on cross-shore exchanges at short-term scale. This cross-shore exchange dot not imply exchange of sediment between different morphologies (e.g. between beach face and tidal flat), but a simple cross-shore readjustment within each morphology limits (e.g. Figure 3). The evident absence of sedimentary exchange between morphologies is mostly related with Figure 3. Cross-shore transport evidence at the beach face, Ancão; displacement of small sand waves. Figure 2. Erosive morphologic features in Alfeite beach resulting from storm wind generated waves. the predominant low energetic wave climate (H max =0.10 m, Table 1). Tidal currents assume particular importance when explaining other magnitude variations, at a longer scale of analysis (long-term scale). The beach proximity to the tidal channel (Figure 1-B), and the inherent vulnerability to the tidal current dynamics (speed/duration of ebb and flood tide) lead to other patterns of sediment transport (long-term scale). Sediment exchange follows the preferential current flow and respective fluctuations. For instance, the occurrence of a differentiate transport (ebb dominated, Table 1) caused by flow asymmetry, can lead to a dominant SE longshore transport. responsible for important sediment mobility in the higher units of the beach profile (FREIRE and ANDRADE, 2000). Ancão beach The overall volumetric changes at medium-scale are generally very small (vertical variability and slope), with maximum vertical displacement observed of 0.5 m (Table 1). The global evolution points towards an overall stability (from months to years) with several accretion/erosion changes for the studied period. There is no seasonality or cyclical behaviour in the obtained results. There is DISCUSSION Both Alfeite and Ancão beaches are affected by locally generated waves and protected from the influence of ocean waves. Their morphologic features, a steep beach face and a gently sloping tidal flat, are typical characteristics of fetch-limited beaches. In a medium-term analysis, both beaches present morphologic-sedimentary stability with no seasonal evolutionary pattern typical of ocean beaches. 185

4 Morphodynamics of Fetch-Limited Beaches Table 1: Characteristics and morphodynamics constrains of the study beaches (n. negligible). Alfeite Ancão Morphology Beach location estuary backbarrier Beach orientation E-W W-E Max. fetch (m) (NE) (NW) Tanβ (aver., max., min.) 0.10, 0.12, , 0.07,0.06 Beach max. vertical variability (m.y -1 ) Beach face sediments Median diameter ( ) Standard deviation ( ) Silt-clay fraction (<0.063mm) (%) < Forcing factors Predominant wind direction NE SW Waves (m; H mean, H max ) 0.11, , 0.10 Tidal currents at the beach (m.s -1 ; max) n River influence in water level n. none At Alfeite, the evolution is episodically forced by high-energy/low-frequency events related with wave generation by northerly strong winds. Cross-shore sediment transport between the upper and the lower beach face, without significant change in profile slope is the main source of morphologic changes. Sediment transferences between the beach face and the tidal flat can be considered negligible. Tidal currents do not affect beach stability, i.e., do not cause sediment transferences within beach face, due to the wide tidal flat that separates the beach from the tidal channel. Also with no seasonality, the evolution of Ancão beach is driven by generated waves, at short-term, and by tidal currents, at long-term, with different magnitude changes. At short-term (between months) changes are of low magnitude, and occur mainly at the beach face as result of alterations in local wave conditions. However, the associated volumetric variability is low, and the local wave climate is just responsible for second order readjustments in the beach profile. The induced cross-shore transport is restricted to the morphology limits (e.g. displacement of small sand waves at the beach face, Figure 3), without exchanges between the main morphologic profile features. Significant changes and persistent variations in the beach shoreline do not seem to have a direct relation with wave conditions, and can only be seen, when considering a larger scale of analysis (years to decades). The overall time of beach response to waves is slow and continuous, and the main morphological evolution is probably more related with changes on the tidal currents regime (CARRASCO et al., 2008). Beach profile is affected by current oscillations induced by ebb/flood fluctuations, with possible stimulation of longshore transport (mainly at the sand spit). Due to the difficulty of interpretation, further analysis is needed to confirm beach dependence on tidal currents. In particular, it is necessary to quantify the magnitude of the transport and associated profile response, and relate them to the tidal cycle fluctuations. While at both systems river influence is negligible at Alfeite beach, located in the inner estuary, waves are the main responsible for beach evolution; at Ancão backbarrier, in addition to waves, tidal currents play an important role in beach evolution. Obtained results suggest the need of a simple beach classification system that can be useful to characterize the beach dominant forcing mechanism which, in turn, controls the beach morphodynamic behaviour. DAVIS and HAYES (1984) have approached this subject quantifying the relative effect of wave and tidal forcing. However, the influence of wave action was based on qualitative approach that, even though can be useful for ocean beach systems, seems to be inadequate in fetch-limited systems where wave heights are considerable smaller. In the scope of this study a new classification scheme is proposed, also based on the relative tide and wave effects, but where both influences are quantified through a similar physical quantity: tidal current at the ebb/flood peak and the magnitude of maximum breaking wave near bottom orbital velocity, respectively. Assuming the linear wave theory (KOMAR, 1998) the orbital velocity at breaking (H b ) is given by 0.5(.g.H b ) 0.5, which can be simplified to 1.4 H b 0.5, since is the ratio of wave height to water depth ( 0.78) and g is the acceleration of gravity (9.81 m.s -2 ). Considering that a system is a wave dominated environment when wave orbital velocity excess tidal current, a simple classification scheme can be developed (Figure 4). In this classification both studied beaches fall within fetch-limited domain (mean wave height < 0.25 m, as defined by JACKSON et al., 2002) but showing different energy levels and differences in the relative effect of wave and tide. Alfeite beach shows higher energy than Ancão and in the latter the relative importance of wave effect is smaller. CONCLUSIONS The morphodynamics of two fetch-limited beaches within different geomorphologic settings (Alfeite-estuary and Ancão-backbarrier) are compatible with the morphotypes of these beaches presented in the literature. In a medium-term analysis (months to years), both beaches present a relatively morphologic-sedimentary stability and no seasonal evolutionary pattern is observed in opposition to beaches affected by ocean waves. Site specific factors, related with the geomorphologic settings of the study systems (e.g. fetch; distance between the beach and the tidal channel), are responsible for the mechanisms that promote beach evolution and how this evolution takes place. At Alfeite beach, waves are the main driven factor of beach evolution promoting cross-shore sediment transferences within the beach face; beach stability is episodically interrupted by storm wave action causing significant erosion in the upper part of the beach face; non-storm waves slowly promote profile recuperation. At Ancão, even if the waves promote short-term/low-magnitude changes at the beach face, tidal currents seem to be responsible for significant and persistent variations at the beach at a large time frame. A simple beach classification system for fetch-limited beaches, based on the relative wave and tide effects is proposed. Studied beaches present differences in energy levels and in the relative importance of wave/tide effects. Further developments are needed to achieve a better quantitative specification of the wave/tide relative importance in beach morphodynamics and to improve and confirm the beach classification suggested here. Future work will include testing this classification in other fetch-limited systems. 186

5 Freire et al. tidal current (m.s -1 ) Ancão tide dominated wave dominated 0.00 Alfeite fetch-limited beaches ocean beaches wave height (m) mixed energy v w =v c v w =2v c Figure 4. Classification of fetch-limited beaches based on wave height and tidal current (vw-wave velocity; vc-current velocity). LITERATURE CITED CARRASCO, A.R.; FERREIRA, Ó.; DAVIDSON, M.; MATIAS A. and DIAS, J., An evolutionary categorisation model for backbarrier environments. Marine Geology, 251, DAVIS, R. A. and HAYES, M. O., What is a Wave-Dominated Coast? Marine Geology, 60, FREIRE P., Morphological and sedimentary evolution of estuarine banks (Tagus Estuary, Portugal). University of Lisbon, LNEC, Ph.D. thesis, 380p. (in Portuguese). FREIRE, P. and ANDRADE, C., Wind-induced sand transport in Tagus estuarine beaches. First results. Aquatic Ecology, 33, FREIRE, P. and ANDRADE, C., Short-term Morphological Evolution of the Alfeite Estuarine Beach. Proceedings of the 3º Simpósio sobre a Margem Continental Ibérica Atlântica (Faro, Portugal), pp FREIRE, P.; TABORDA, R. and SILVA, A. M., Sedimentary Characterization of Tagus Estuarine Beaches (Portugal). A contribution to the sediment budget assessment. Journal of Soils and Sediments, 7 (5), FRIEDMAN, G., Distinction Between Dune, Beach and River Sands from their Textural Characteristics. Journal of Sedimentary Petrology, 31 (4), FRIEDMAN, G., Dynamic Processes and Statistical Parameters Compared for Size Frequency Distribution of Beach and River Sands. Journal Sedimentary Petrology, 37 (2), HEGGE, B.I.; ELIOT, I.; and HSU, J., Sheltered Sandy beaches of Southwestern Australia. Journal of Coastal Research, 12(3), JACKSON, N., Wind and waves: Influence of local and non-local waves on mesoscale beach behaviour in estuarine environments. Annals of the Assoc of American Geographers, 85 (1), JACKSON, N. and NORDSTROM, K., Site specific controls on wind and wave processes and beach mobility on estuarine beaches in New Jersey, USA. Journal of Coastal Research, 8(1), JACKSON, N.; NORDSTROM, K.; ELIOT, I. and MASSELINK, G., Low energy sandy beaches in marine and estuarine environments: A review. Geomorphology, 48, KOMAR, P. D., Beach Processes and Sedimentation. Upper Saddle River, New Jersey: Prentice-Hall, Inc., 544p. MARETEC, [On line]: Tagus estuary. Hydrodynamics on the inlet and adjacent platform [accessed on 09th November 2008]. Available at: Inicio/Introducao.htm (in Portuguese). NORDSTROM, K., The Use of Grain Size Statistics to Distinguish Between High-and-Moderate-Energy Beach Environments. Journal of Sedimentary Petrology, 47 (3), NORDSTROM, K., Cyclic and Seasonal Beach Response: A Comparison of Oceanside and Bayside Beaches. Physical Geography, 1 (2), NORDSTROM, K., Estuarine beaches. An introduction to the physical and human factors affecting use and management of beaches in estuaries, lagoons, bays and fjords. New York: Elsevier Science Publishers Ltd, 225p. NORDSTROM, K. and JACKSON, N., Two-dimensional Change on Sandy Beaches in Meso-tidal Estuaries. Zeitschrift für Geomorphologie, N. F., 34 (4), NORDSTROM, K.F.; JACKSON, N.; ALLEN, J.R. and SHERMAN, D.J., Wave and current processes and beach changes on a microtidal lagoonal beach at Fire Island, New York, USA. In: Nordstrom, K., Roman, C. (eds.), Estuarine Shores: Evolution, Environments and human alterations: John Wiley & Sons, USA, pp OLIVEIRA, F.S.B.F.; VARGAS I.C.C.; and COLI, A.B., Characterisation of the alongshore dynamics of an estuarine beach. Thalassas, 25(1), PORTELA, L. and NEVES, R., Numerical Modelling of Suspended Sediment Transport in Tidal Estuaries: a Comparison Between the Tagus and the Scheldt. Netherlands Journal of Aquatic Ecology, 28 (3-4), TRAVERS, A., Low-energy beach morphology with respect to physical setting: a case study from Cockburn Sound, Southwestern Australia. Journal of Coastal Research, 23 (2), VARGAS, C.; OLIVEIRA, F.S.B.F.; OLIVEIRA, A. and CHARNECA, N., Vulnerability analysis of an estuarine beach to inundation: application to Alfeite spit (Tagus estuary). Gestão Costeira Integrada, 8(1): (in Portuguese). ACKNOWLEDGEMENTS This paper is a contribution from project BERNA - Beach Evolution in Areas of Restricted fetch: experimental and numerical analysis (POCTI/CTA/45431/2002), supported by FCT (Foundation for Science and Technology). 187