An evolutionary categorisation model for backbarrier environments

Marine Geology 251 (2008) 156–166

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

An evolutionary categorisation model for backbarrier environments A.R. Carrasco a,⁎, Ó. Ferreira b, M. Davidson c, A. Matias a, J.A. Dias b a b c

CIACOMAR/CIMA, Universidade do Algarve, Av. 16 de Junho s/n, 8700-311 Olhão, Portugal FCMA/CIACOMAR/CIMA, Universidade do Algarve, Campus de Gambelas, 8005-139, Faro, Portugal School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, United Kingdom

a r t i c l e

i n f o

Article history: Received 24 November 2006 Received in revised form 29 January 2008 Accepted 5 February 2008 Keywords: backbarrier categorization evolution maturation Ria Formosa

a b s t r a c t Shorelines occurring along restricted fetch environments, such as the backsides of barrier islands, are extremely diverse with respect to their morphologic characteristics and evolution. In order to better understand the morphodynamic evolution of backbarriers and the associated implications for entire barrier systems, this study proposes a backbarrier evolutionary categorisation model based on the development of two types of index: backbarrier evolution tendency and backbarrier maturation condition. The proposed characterisation system is applied to the Ria Formosa backbarrier (located in southern Portugal) for the period 1947 to 2001. Cross-shore and longshore backbarrier trends in Ria Formosa suggest a shrinking of the lagoon system as a consequence of a decrease in the coastal length of the backbarrier coastline and a landward displacement of it. Even though some of the backbarriers examined were found to be in an immature state, the results obtained illustrate a maturing trend for the system overall. Barriers in Ria Formosa fall into two main evolutionary categories: backbarrier reduction and backbarrier growth. This means that neither smoothing nor branching has been significant and therefore that backbarrier recent evolution is closely related to barrier coast length. Application of the proposed characterisation to the Ria Formosa case study has helped reveal backbarrier evolutionary trends and therefore should be of use in the management of backbarrier systems. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Low energy beaches are located in sheltered environments and fetch-limited environments. Sheltered environments occur in the lee of islands, reefs, or submarine ridges (Hegge et al., 1996), being protected to varying degrees from higher energy (deep-water) ocean waves. Fetch-limited environments occur in lakes (e.g. Roy et al., 2001), bays (Goodfellow and Stephenson, 2005), estuaries (e.g. Jackson and Nordstrom, 1992; Jackson et al., 2002a), and lagoons (e.g. Jackson et al., 2002b). Limited fetch conditions produce small, steep waves that are erosive due to short wave periods (Battjes, 1974; Jackson et al., 2002a). Tidal range affects the vertical distribution of wave energy, determining the width of the beach and the duration of wave breaking at any elevation (Nordstrom, 1992). These environments are dependent principally on wind conditions and basin dimensions (Nordstrom, 1992; Jackson et al., 2002a). Great variability exists in the scale, morphology, composition and coastal behaviour of fetch-limited beaches, even when located in the same estuary or along the same backside. Morphological variations over small areas result from local differences in fetch, wind direction, stratigraphy, inherited topography, type of vegetation growth, and

⁎ Corresponding author. Tel.: +351 289707087; fax: +351 289706982. E-mail addresses: [email protected] (A.R. Carrasco), [email protected] (Ó. Ferreira), [email protected] (M. Davidson), [email protected] (A. Matias), [email protected] (J.A. Dias). 0025-3227/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2008.02.009

human activities (Nordstrom, 1992). Many low sediment banks and marsh platforms with elevation close to the Mean Sea Level (MSL) support ephemeral strand plain beaches and abundant fringing marshes, which not only diminish wave energy, but also build backbarrier platforms critical for barrier island migration processes in response to rising sea level (Nordstrom et al., 1996). In the case of beaches located in backbarrier environments, the proximity to inlets represents another of the main coastline controls, where locally generated waves rework sediment delivered by tidal currents from the oceanside. As flood-tidal deltas stabilise and inlets are filled, the inlet sequence migrates and the older flood-tidal deltas might be incorporated into the barrier. Backbarrier marshes develop upon the tidal flats between tidal channels of the abandoned inlet (Kraft et al., 1979). Low, narrow barriers can be dominated by other processes that may cause major sediment input to the backbarrier area in response to overwash events (e.g. Dillon, 1970; Leatherman, 1976; Dingler and Reiss, 1990) or migrating dune sands (Jackson et al., 2002a). Overwash occurrence is considered to be important in limiting backbarrier morphological evolution, contributing to the increase in width and height of barrier system islands and peninsulas, as well as in the development of aeolian features (Dingler and Reiss, 1990). Storm overwash processes lead to the deposition of washover fans over the backbarrier marshes which extend into adjacent coastal lagoons (Kraft et al., 1979). Overwash can be considered a sink in the littoral system, where the resulting washover (i.e., the morphology generated by overwash) is a source in the barrier island sediment budget that

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contributes to the vertical accretion of the backside (Leatherman, 1976). Complete overwashes are observed when overwash water reaches the lagoon, leading to the existence of extensive lenticular sheet-like washovers, laid down over the backbarrier (complete washover; Carter, 1988). Research dedicated to low energy environments, particularly concerning fetch-limited areas and backbarrier systems, is limited. In contrast, studies on beach morphodynamics in backbarrier environments, on the morphology of open-ocean beaches, and on the relevant forcing mechanisms are more widely represented in the coastal literature. There is a need, therefore, for the development of broad-scale characterizations of backbarrier morphodynamics in order to better understand the occurrence and duration of the inherent morphological features and the mechanisms of hydrodynamic controls. This study proposes a backbarrier evolutionary categorisation model that aims to provide a basis for backbarrier typology classification, for use in the definition of past and current coastal behaviour and in the prediction of future behaviour. The proposed model incorporates different backbarrier coastline behaviours and stages of maturation, with potential application to similar environments (Fig. 1). In particular, observations of the recent evolution (54 years) of a backbarrier environment located on the Ria Formosa barrier system, Portugal, are used to test the model application. The classification provides a common categorisation mode for backbarrier systems, and enhances our current understanding of barrier systems. The categorisation model should also be useful for coastal management purposes given that it provides insights into the behaviour of backbarriers. 2. Method This section describes a methodology for categorising the morphological behaviour of backbarrier systems, and outlines the characteristics of the case study area. The proposed categorization model is based on measurements of backbarrier coastline change, including cross-shore and longshore behaviour, for a given period of interest. The method involves: (1) determination of backbarrier evo-

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lution; and (2) development of backbarrier evolutionary categorisation model. 2.1. Determination of backbarrier evolution The determination of backbarrier evolution involves the processing and interpretation of aerial photographs to define the main morphological units present in the backbarrier environment. A minimum of three sets of photographs defining two periods of analysis need to be evaluated in order to identify major trends. The aerial photographs need to be georectified prior to the identification and mapping of morphological units. Three dominant morphological units composing the backbarrier coastline can be distinguished and identified from aerial photographs: inner beach next to a barrier, salt marsh, and inner beach next to inlet (Fig. 1). The criteria for the definition of each morphologic unit and identification on aerial photographs are provided in Table 1. Morphological limits are defined based on sediment texture and vegetation type (floral limits), being easy of identification on aerial photograph and on the field. For the inner beach next to a barrier coastline, the dune/inner bluff edge represents the upper limit of the morphology (transition from vegetated to non vegetation zone). This limit indicates changes in the dune field, such as retreat or progradation into the lagoon, which cause movements in the position of the inner beach. When vegetation does not exist, for example in the presence of washover fans, the last spring tide marks are chosen as the limit, or when present, the inner bluff edge. This mark can be identified on the field by an evident sediment textural difference. In aerial photographs, occurs a tone transition and a dark band can be observed (e.g. washover fans). Careful should be taken when considering the last spring tide mark in order to avoiding tide effect. This same procedure is used for the delimitation of the inner beach next to inlet coastal limit (the inner bluff edge presents a darker colour). The salt marsh coastline is defined by changes in vegetation type and sediment properties, as the transition between salt marsh vegetation and dune vegetation or between salt marsh vegetation and the tidal mark (Table 1). Changes in vegetation type and sediment properties on aerial photographs can be

Fig. 1. Schematic representation of the main morphological units existing at backbarriers. IBB corresponds to the inner beach next to a barrier; SM corresponds to salt marsh; IBI corresponds to the inner beach next to an inlet.

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Table 1 Definition of each morphological unit considered for backbarriers (see example in Fig. 1) Morphology

Definition

Inner beach next to a Sandy beach on the backbarrier sometimes cut by the presence barrier of salt marsh areas, and episodically dominated by washover fans (Andrade, 1990; Jackson et al., 2002a). Hydrodynamically dominated by waves of short period, with restricted fetch, and by low to medium velocity tidal currents (Andrade, 1990). The dune/inner bluff edge is represented by a sharp transition in colour and texture. In black and white aerial photos this interface registers a difference from dark gray (presence of vegetation) to a bright gray/white colour; in coloured photos this interface registers a transition from green (vegetated) to light brown or burnt yellow (sand). In the presence of washover fans, the spring tide mark is chosen, and a tone transition and a dark band can be observed. Salt marsh Corresponds to the salt marsh and tidal flat areas along the backbarrier coastline, dominated by the presence of fine sediments (silt and clay), being hydrodynamically controlled by low velocity tidal currents (Andrade, 1990; French and Spencer, 1993; French 1997). Salt marsh limits are defined by different type of vegetation and sediment properties. Transition between salt marsh and dune is traduced by the transition from dark gray/black to bright gray in black and white photos from green to bright brown in coloured photos. Inner beach next to Sandy beach on the backbarrier, located next to inlets. an inlet Hydrodynamically dominated by strong tidal currents and some oceanic waves that cross the inlet (Andrade, 1990). There is no dune field between these beaches and the oceanic beaches, the spring tide mark is chosen, and a tone transition can be observed.

identified by differences in colour, tone and texture. In the field, salt marsh limits are easily identified by the presence of halophytes (e.g. Spartina sp.) and very fine sediment (Table 1). Backbarrier marsh sites typically exhibit a stratigraphic succession from intertidal muds through alternating salt marsh and brackish peat facies (French and Spencer, 1993). Two types of quantification of backbarrier coastline are proposed: cross-shore and longshore. Cross-shore evolution is determined using cross-shore transects located along salt marsh stretches and sandy beaches, by measuring the distance between each transect origin and the coastline. Cross-shore changes (in metres/year) define two coastline behaviours, either accretion (landward displacement = positive) or

erosion (shoreward displacement= negative). Transects are distributed along the entire backbarrier longshore extent, with a maximum spacing of 500 m. In the case of extremely branched backbarriers, which may contain several embayed sectors, additional transects with different spacings should be considered. Landward/shoreward displacements in areas of complete washover are also analysed (involving additional cross-shore transects). Changes caused by complete overwash events are assessed by measuring the longshore extent of complete washovers along the backbarrier, and computing the percentage of backbarrier coastline affected by such washovers. Longshore evaluation of the backbarrier coastline takes into account the determination of inner beach next to a barrier, salt marsh, and inner beach next to inlet coastlines for each set of photographs (Fig. 1). The total extent of the backbarrier coastline (BC) represents the sum of these three morphological coastlines for each coastal stretch in each year of analysis (in metres). Longshore changes also include the decrease or increase in extent of an island or peninsula. Therefore, in addition to BC, the rectilinear length of each island/peninsula (coastal length, CL) is also considered in the longshore evaluation (Fig. 1). The CL of each barrier is always smaller than BC, due to the influence of barrier shape; only in limiting conditions may CL approach BC. 2.2. Development of backbarrier evolutionary categorisation model Based upon the longshore measurements defined above, an evolutionary categorisation is proposed for identifying different modes of backbarrier evolution and maturation. The evolutionary tendency is defined by the relation between the backbarrier coastline variation (ΔBC) and the coastal length variation (ΔCL) in a given period (Fig. 2). The backbarrier coastline variation (ΔBC) is given by: DBCðkÞ ¼ ðBCfinal  BCinitial Þ⁎100=ðBCinitial Þ

ð1Þ

where BC is the extent of the backbarrier coastline taking into account prominent and embayed parts of the coastline, measured in metres (see Fig. 1). The coastal length variation (ΔCL) is given by: DCLðkÞ ¼ ðCLfinal  CLinitial Þ⁎100=ðCLinitial Þ

ð2Þ

where CL is the rectilinear length of each barrier, measured in metres (see Fig. 1). Backbarrier growth is defined by increases in both BC and

Fig. 2. Conceptual scheme defining the different modes of backbarrier evolutionary tendency.

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Fig. 3. Conceptual scheme defining the different modes of backbarrier maturation condition.

CL (Fig. 2). In contrast, a backbarrier reduction exhibits decreases in both BC and CL. For intermediate states, backbarrier branching and backbarrier smoothing correspond to an increase in BC and a decrease in CL, and a decrease in BC and an increase in CL, respectively (Fig. 2). The maturation condition is given by the relation between the backbarrier maturation state (Bmat) and the backbarrier maturation evolution (ΔBmat). Backbarriers are considered mature if salt marsh development occurs during a particular year of analysis; backbarriers are maturating if the observed development of the salt marsh segments occurs during a given period of analysis. The backbarrier maturation state (Bmat) is given by: Bmat ¼ SMCfinal =TIBCfinal

ð3Þ

and corresponds to the ratio of the salt marsh coastline extent (SMC) to the sum of inner beach next to a barrier and inner beach next to inlet coastline extents (TIBC, Fig. 1), for the most recent set of photographs analyzed (Fig. 3). Higher levels of maturation are reflected by higher values of SMC:TIBC (i.e., a greater extent of salt marshes in relation to

sandy beaches). The evolution of Bmat (ΔBmat) provides an indication of the maturation process between the initial and final years of analysis: DBmat ¼ ½ðSMCfinal =TIBCfinal Þ=ðSMCinitial =TIBCinitial Þ

ð4Þ

Backbarrier categorisation using the obtained values of ΔBmat can be related to changes in local hydrodynamic conditions. After a decrease in hydrodynamic intensity it is expected a construction/maintenance of salt marsh (Pethick, 1998), in which case ΔBmat N 1 (maturating backbarrier, Fig. 3). Greater magnitude conditions, generated either by overwash deposition or inlet dynamics, result in an increase in sandy inner beaches. Thus, salt marsh destruction is associated with rejuvenating backbarriers, in which case ΔBmatb 1 (Fig. 3). 2.3. Case study The method developed was applied to the Ria Formosa multi-inlet barrier island system, located in southern Portugal (Fig. 4). The system's present configuration consists of two peninsulas and five

Fig. 4. Location and major features of the case study area: the Ria Formosa barrier system, southern Portugal.

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Table 2 Periods of analysis considered for each island/peninsula in Ria Formosa barrier system Island/peninsula

Period of analysis

Ancão P. Barreta I. Culatra I. Armona I. Tavira I. Cabanas I. Cacela P.

1947–1976 1947–1976 1947–1972 1969–1989 1947–1976 1989–1996 1976–1989

1976–2001 1976–2001 1972–2001 1989–2001 1976–2001 1996–2001 1989–2001

islands that extend over 56 km. The entire backbarrier covers an area of 8.4 × 107 m2 (Andrade, 1990), being characterised by: i) large salt marsh areas with a dense distribution of shallow meanders, composed of silt and fine sand (Bettencourt, 1994); ii) large sand flats partially

flooded and reworked during spring tides (Pilkey et al., 1989); and iii) a complex network of natural and partially-dredged channels, which narrow and shoal in upper regions of the system (Salles, 2001). The inner coastline, along the backbarrier islands and peninsulas, is characterised by low, narrow sandy beaches alternating with portions of salt marsh, and washover plains (Andrade et al., 1998). Tides in the area are semi-diurnal, with average ranges of 2.8 m and 1.3 m for spring and neap tides, respectively. However, maximum ranges of 3.5 m can be reached during spring tides. Wave climate in the area is moderate to high (Ciavola et al., 1997), with an average offshore significant wave height of 0.92 m (Costa et al., 2001). Incident waves are normally from the W–SW, representing 71% of occurrences (Costa et al., 2001), although “Levante” (SE Mediterranean wind) conditions represent 23% of the total (Costa et al., 2001). Storms have been defined for this area as events with significant wave heights greater

Fig. 5. (A) Distribution of the cross-shore transects; and (B) backbarrier coastline mapping, for Ancão Peninsula (photomosaic from 2001). SM represents the salt marsh coastline; IBB represents the inner beach next to a barrier coastline; and IBI represents the inner beach next to an inlet coastline.

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than 3 m (Pessanha and Pires, 1981). Pires (1998) established return periods for the main incident wave directions and concluded that SW storms are more energetic than SE storms for the same return period. The cuspate shape of the Ria Formosa system presents two different areas in terms of exposure to wave action. The west flank (Ancão Peninsula and Barreta Island) is more energetic, being under the direct influence of the dominant wave conditions (W–SW), while the east flank (Culatra Island, Armona Island, Tavira Island, Cabanas Island and Cacela Peninsula) is directly exposed only to the “Levante” conditions (E–SE wind and waves). Sand supply can be a critical factor affecting rates of island retreat. At present, almost the entire shoreline is retreating at an annual rate of b1 m/yr (Pilkey et al., 1989). Recent evolution of the Ria Formosa barrier system is strongly dominated by hydrodynamic changes induced by the natural inlets displacement (see inlets location in Fig. 4), mainly associated to the Ancão, Fuzeta and Lacém inlets (Salles, 2001; Vila-Concejo et al., 2006) and to the width decrease of Armona Inlet (Andrade, 1990; VilaConcejo et al., 2006). Ancão Inlet and Fuzeta Inlet relocations occurred in 1997 and 1999 respectively, with both inlets being left to evolve naturally after their relocation (Vila-Concejo et al., 2004). Both FaroOlhão Inlet and Tavira Inlet are currently stabilised. The opening and

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stabilisation of Faro-Olhão Inlet took place between 1929 and 1955 (Esaguy, 1986a), and for Tavira Inlet occurred between 1927 and 1985 (Esaguy, 1986b). The stabilization of Faro-Olhão Inlet resulted in the interruption and diversion of the littoral drift, leading to significant changes in the hydrodynamics, of the whole barrier system, particularly in the vicinity of the downdrift Armona Inlet (Andrade, 1990; Salles, 2001; Pacheco et al., 2006). Further anthropogenic intervention occurred in 1996 and 1999–2000 when about 2 650 000 m3 of sediment was dredged from lagoon channels and used for beach renourishment and dune recovery (Ramos and Dias, 2000). 3. Results 3.1. Determination of backbarrier evolution The period of analysis, 1947 to 2001, comprised two distinct periods each of about 25 years for each island/peninsula (Table 2). The exact periods for each island/peninsula differed due to the variously available aerial photograph coverages and observations about the recent evolution of the islands. Moreover, the inner beach next to inlet coastline was determined only for the four non-stabilised inlets (Ancão, Armona,

Fig. 6. Cross-shore evolution of the backbarrier coastline for the period 1947–2001. The lines represent the cross-shore variations obtained for each island/peninsula for the initial and final periods of analysis (for periods see Table 2). Positive values represent landward displacement (accretion), and negative values represent shoreward displacement (erosion).

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Fig. 7. Identification of complete washovers at Ria Formosa for the period 1947–2001, showing the number of washover occurrences (bars) and the percentage of backbarrier coastline with complete washovers (line).

Fuzeta, and Lacém, Fig. 4). Backbarrier shoreline limits were defined based on field verification performed in some parts of the system (Ancão Peninsula, Barreta Island, Culatra Island and Cacela Peninsula). The photographs were georectified using ERMapper software, which enabled the images to be resampled into the Portuguese Melriça Coordinate System (Datum 73). For each analysed period (Table 2), rectification errors were estimated by measuring the distance between identical points on each photograph set. Most of the errors were lower than 10 m, with the average error being 8 m. Cross-shore evolution was determined for the three morphological units present in the backbarrier (Table 1). Cross-shore transects were defined for each island/peninsula, at an average spacing of 500 m (see example in Fig. 5). However, in some islands such as Culatra and Armona that possess extremely branched backbarrier coastlines, additional cross-shore transects with smaller spacing (~100 m) were included. The results show a landward displacement of the backbarrier (accretion) for almost all islands/peninsulas (Fig. 6). Average crossshore movements of the inner sandy beach (TIBC) were 0.6 m/yr and

0.7 m/yr for the initial and final periods, respectively. Accretion of the inner sandy beach (TIBC) was greatest at Cabanas and Armona islands in the initial period, and at Cacela Peninsula during the final period of analysis. For the salt marshes, average cross-shore movements were 0.5 m/yr and 0.4 m/yr for the initial and final periods, respectively. Negative rates (i.e., erosion) were observed mainly for the inner sandy beaches of Barreta and Tavira islands during the final period of analysis. The salt marshes accreted significantly at Barreta and Cabanas islands during the initial period of analysis, and at Armona Island in the final period (Fig. 6). Negative cross-shore movement of the salt marsh (shoreward displacement) was observed only for Armona Island in the initial period of analysis. A total of 62 complete washovers were identified for Ria Formosa between 1947 and 2001. The aerial photograph sets of 1969, 1972, 1985, 1989 and 1996 were used within the two periods of cross-shore evaluation to confirm washover maintenance/recovery. The 62 complete washovers represent a total longshore extent of about 4000 m (7% of the system's coastal length), 3616 m in the western flank (Ancão Peninsula and Barreta Island), and 388 m in the eastern (Culatra Island, Armona Island, Tavira Island, Cabanas Island, and Cacela Peninsula). Two main periods of overwash occurrence leading to the formation of complete washovers were identified: prior to 1947 (19 occurrences) and 1947–1976 (20 occurrences) (Fig. 7). Cabanas Island had the highest number of occurrences: 21 complete washovers, with major occurrence between 1976 and 1989. The percentage of backbarrier coastline with complete washovers was more significant prior to 1947, and between 1989 and 1996. Due to restrictions in the coverage of the aerial photograph sets, it was possible to estimate landward/shoreward displacement, resulting from complete overwash, only for Ancão and Cacela peninsulas and for Barreta Island. The backbarrier coastlines of Ancão and Cacela peninsulas were displaced landward at rates of 1 m/yr between 1947 and 1976, and 2 m/yr respectively between 1976 and 1989, while the backbarrier coastline of Barreta Island was displaced shoreward at a rate of 4 m/yr between 1947 and 2001.

Fig. 8. Longshore changes (in m) for the three evaluated morphologies: (A) Inner beach next to a barrier; (B) Salt marsh; (C) Inner beach next to an inlet; and for the total backbarrier coastline (D). Negative values represent coastline extent decreases and positive values represent coastline extent increases.

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The entire system experienced a decrease in the longshore backbarrier coastline (BC) of about 5000 m between 1947 and 2001. Longshore increases (of almost 50%) in BC were locally significant for Barreta and Culatra islands in the initial period of analysis. Tavira Island registered the highest value of salt marsh coastline decrease for the initial period (Fig. 8). Over the entire barrier system, salt marsh evolution exhibited the largest changes, and inner beach next to an inlet coastline the smallest. Considering the overall evolution, different trends can be distinguished through the barrier system. Only Barreta Island experienced an increase in BC in both periods of analysis and for all the morphologies (Fig. 8). Cabanas Island experienced an increase in BC, but underwent small decreases in inner beach next to a barrier during the first period of analysis. In contrast, an overall decrease in BC (inner beach and salt marsh coastlines) was observed for Tavira Island between 1947 and 2001. In addition, Ancão Peninsula experienced an overall decrease in BC, even with small increases of salt marsh coastline (Fig. 8B and D). Culatra and Armona islands both experienced increases in inner beach next to a barrier coastline in the first period of analysis, but decreases in the final one; salt marsh coastline increases were also more significant in the first period of analysis for these two islands. These two islands, and Cacela Peninsula, all exhibited overall decreases in BC. Two distinct periods are thus representative of cross- and longshore trends: an accretion-dominated behaviour for the period 1947– 1976 (except Cabanas Island); and an erosion-dominated behaviour for the period 1976–2001. In general terms, Ancão Peninsula, Barreta Island, Culatra Island and Armona Island are the barriers that have undergone the largest changes. Landward displacement and coastline increase (cross-shore and longshore) are dominant in the central part of the Ria Formosa system (Culatra, Armona and Tavira islands), while

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at the extremities of the system (Ancão and Cacela peninsulas), coastline decrease and shoreward displacement assumed special importance in the inner beach next to a barrier. Cross-shore and longshore backbarrier trends obtained for the Ria Formosa system point towards a shrinking of the lagoon system as a consequence of backbarrier coastline landward displacement, and a simultaneous decrease in backbarrier coastline extent. 3.2. Backbarrier evolutionary categorisation model The backbarrier evolutionary categorisation model was applied to the Ria Formosa system for the period 1947–2001 (Fig. 9). The categorisation showed that the assessed islands and peninsulas were distributed mainly between the backbarrier growth and backbarrier reduction categories (Fig. 9A). An intermediate state was observed only for Armona Island (smoothing backbarrier, Fig. 9A). The variability in the evolutionary trend of each island/peninsula did not allow the definition of a tendency for the entire barrier system. However, there was a backbarrier reduction trend in the extremities of the system (Ancão and Cacela peninsulas) and for Tavira Island, together with a backbarrier enlargement at Barreta, Culatra and Cabanas islands. Determinations of the backbarrier maturation condition were also performed for the period 1947–2001. Almost all the analysed backbarrier shorelines presented positive values of ΔBmat (maturating backbarriers) between 1947 and 2001 (Fig. 9B). The extremities of the system (Ancão and Cacela peninsulas) were categorised as immature backbarriers, while in the central part of the system barriers tended toward maturation, with mature backbarriers at Armona and Tavira islands (Bmat N 1) and an immature backbarrier at Culatra Island (see Fig. 4 for location). Barreta Island was considered as immature but

Fig. 9. Classification of backbarrier evolution and maturation based on the measured values of BC, CL, Bmat, and ΔBmat, for the period 1947–2001: (A) results from the backbarrier evolutionary tendency; and (B) results from the backbarrier maturation condition.

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moving towards maturation (Bmat b 1), while Cabanas Island was categorised as already mature and maturating (Fig. 9B). 4. Discussion 4.1. Evolutionary categorisation model The proposed model of backbarrier evolutionary categorisation is based on longshore measurements. The categorisation relies on the definition of the backbarrier evolution tendency and maturation condition (Figs. 2 and 3). The evolutionary tendency relates the changes in backbarrier coastline extent (BC) over time to the changes in coastal length (CL), which allows individual backbarrier behaviours to be integrated and combined to reveal the evolution of the entire barrier system. BC is determined based on longshore measurements performed on the main morphologic units present along the backbarrier. Positive variations in BC (increases in longshore extent) may result from landward displacement of the backbarrier, as well as from significant increases in CL. Cross-shore evaluation is aimed at determining the landward or shoreward displacement of the backbarrier coastline. Backbarrier landward displacement results from a natural increase in barrier width as a result of sediment availability, and may lead to backbarrier growth or backbarrier branching categories. Marsh progradation over the backbarrier is interpreted as a result of abundant sediment, which permits marsh expansion (French, 1997; Pethick, 1998). One of the main sources of sediment to the backbarriers is the infilling/incorporation of old flood-tidal deltas into islands following inlet closure (Godfrey and Godfrey, 1974; Armon, 1979; Kraft et al., 1979). Continued migration of an inlet leads to development of additional islands and the subsequent preservation of existing features as sand supply and wave approach is reduced. The major building process is related to the proximity of the flood-tidal sand sheet. These structures can evolve to vegetated flood-tidal deltas, with development occurring where sand from the flood-tidal shield overtops the adjacent tidal marsh (Cleary and Hosier, 1979). In such cases the salt marshes begin to become well developed, the marsh fringing behind the barrier expands, and on the barrier itself new dune lines and vegetation may appear (Godfrey and Godfrey, 1974; Godfrey et al., 1979). Another major sediment source for the backbarrier is the incidence of complete overwash events. Usually these events are related to beach and dune erosion, and consequent accumulation at the backbarrier, not only over the inner sandy beach but also over the existent salt marsh (Schwartz, 1975). Whether or not overwash plays a significant role in a barrier island's dynamics depends on numerous factors such as marine conditions (Fisher et al., 1974), nearshore bathymetry (e.g. Ritchie and Penland, 1988), beach topography (e.g. Leatherman, 1976) back-beach elevations (e.g. Morton and Sallenger, 2003), dune morphology (e.g. Donnelly and Sallenger, 2007) and engineering structures (e.g. Hayden and Dolan, 1977). The complete washovers observed in the Ria Formosa system for the period 1947–2001 were generated in areas of particular vulnerability, related to tidal inlet dynamics (e.g. Ancão Peninsula, Barreta Island and Cacela Peninsula; Matias et al., 2007). In contrast to backbarriers that are smoothing or those that are in reduction, backbarriers in branching or in growth modes are representative of an alternative evolutionary tendency reflecting different environmental conditions leading to maturation. Overwash is recognised as a process that rejuvenates the backbarrier by creating new marsh fringes (Godfrey and Godfrey, 1974), although continuous excessive washover deposition can result in the destruction of the marshes by exceeding their capacity to recover and benefit from the input of new substrate (Godfrey et al., 1979). Washover deposition on the backbarrier also results in a vertical accretion of sediment in addition to the lateral accretion that occurs at the marsh or lagoon fringe (Leatherman, 1976; Godfrey et al., 1979).

Overwash processes and inlet dynamics are vital components of the low barrier island environment, and in early stages may rejuvenate backbarriers by increasing sandy beach portions. Later, the backbarriers undergo a maturation process. The definition of backbarrier maturation condition (Fig. 3) provides an assessment of the maturation state of backbarrier morphological units, including an evolutionary perspective on the maturating/rejuvenating processes. A decrease in local hydrodynamic intensity leads to a backbarrier sustained by higher proportions of salt marsh coastline (potentially more mature); while an increase in local hydrodynamic intensity is more likely to be associated with higher proportions of sandy beaches along the backbarrier (rejuvenating backbarrier; French and Spencer, 1993; Pethick, 1998). Currently there exists only limited information related to backbarrier morphodynamics, and the precise ways in which processes interact in the development of backbarriers are as yet poorly understood. Therefore, because each type of restricted fetch environment has a particular process-signature and morphodynamic behaviour, the main benefit of the categorisation model is that it enables a first-order assessment to be made of the major evolution/maturation states of backbarrier environments for a defined period of analysis. The prediction of future barrier changes using the model would require a detailed understanding of all the factors (natural and anthropogenic) responsible for alteration in the barrier system. Some issues should be considered regarding the indices developed in this study and regarding the application of the categorisation model. First, the measurement of the indices is dependent upon data resolution and accuracy. Constraints and sources of error include aerial photograph availability and resolution, and inaccuracies in the georectification process (estimated error) caused by the number and distribution of the existent ground control points (Coyne et al., 1999). Second, the definition and identification of the morphologic units used affect the indices. For example, backbarrier coastline limits are dependent on local conditions, photograph quality, and ground knowledge. Special attention needs be taken during the morphological identification process and shoreline mapping. Third, application of the model to other barrier settings would allow the indices to be validated for other types of backbarriers that have different sediment supply, wave climate, and/or tidal conditions. In addition, because backbarrier evolution tendency and maturation condition are physically-based representations of backbarriers, the model has the potential to be applied to other backbarrier environments. This would be of benefit in order to identify other aspects that may relate to the recent evolution of this type of environment under different hydro/ morphodynamic settings and at different temporal and spatial scales. Finally, the categorisation model is useful for understanding and integrating different backbarrier evolutionary trends, upon which may be based an indication of future backbarrier behaviour. Such information should be used to guide coastal management practices for backbarrier environments. 4.2. Application of the evolutionary categorisation model to the case study The overall evolutionary trend for the Ria Formosa backbarrier indicated a shrinking of the lagoon due to the landward displacement of the backbarrier together with a decrease in the extent of the backbarrier coastline. In general, cross-shore accretion and longshore coastline increase were more dominant in the central part of Ria Formosa system (Culatra and Armona islands), while at the extremities of the system (both peninsulas), shortening was of special importance in the inner beach next to a barrier coastline (Figs. 6 and 8). The system exhibited an erosion-dominated behaviour between 1976 and 2001 and an accretion-dominated behaviour between 1947 and 1976. The variability in the backbarrier categorisation obtained after application of the model did not indicate that there is a generic trend

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for the entire system. Almost all the backbarriers analysed presented a maturating trend between 1947 and 2001 (Fig. 9B). The extremities of the system (Ancão and Cacela peninsulas) were categorised for 2001 as immature backbarriers, while in the central part of the system, Culatra Island was categorised as immature, and both Armona and Tavira islands were mature backbarriers (Fig. 9A). Increases in hydrodynamic intensity, and thus lower values of ΔBmat, were attained only at Cacela Peninsula; none of the evaluated islands/peninsulas was categorised as mature and rejuvenating. The two main evolutionary trends of the Ria Formosa barriers are backbarrier reduction and backbarrier growth (Fig. 9A). This means that neither smoothing nor branching has been significant and therefore backbarrier recent evolution is strongly related with barrier coastal length. In Ria Formosa, major barrier length changes are directly related to tidal migration patterns (Esaguy, 1985, 1986a,b; Andrade, 1990; Salles, 2001; VilaConcejo et al., 2003). Barriers dominated by updrift spit formation generally experience backbarrier growth, while barriers where inlet downdrift erosion leads to barrier shortening tend towards backbarrier reduction. For example, the relocation of Ancão Inlet to its western position in 1997 (Vila-Concejo et al., 2004) was responsible for the reduction of CL and BC in Ancão Peninsula, with a consequent increase of BC and CL at the adjacent downdrift Barreta Island between 1997 and 2001 (Fig. 9A). The updrift relocation promoted an increase in tidal channel flow velocities that led to backbarrier reduction and salt marsh destruction, and therefore enhanced immaturity in Ancão Peninsula (Fig. 9B). In contrast, Barreta Island showed backbarrier growth (increase in BC) due to sediment availability related to flood delta deposition (Fig. 9A). The decreases in the inner beach next to a barrier coastline and the inner beach next to an inlet coastline for Tavira Island between 1947 and 1976 were associated with the eastern migration of Fuzeta Inlet (Esaguy, 1986b; Vila-Concejo et al., 2004). During the study period, migration of the Fuzeta Inlet induced a decrease in backbarrier tidal channel velocity and thus promoted salt marsh development. Cabanas Island was entirely eroded by a major storm in 1961 and has been reconstructed by natural processes since then (Weinholtz, 1964; Esaguy, 1986b). Overwash occurrence was a major process for backbarrier growth providing sediment availability. Subsequently, salt marsh development occurred over the sand substrate and the island became mature and maturating (Fig. 9B). Alterations in Cacela Peninsula BC were caused not only by the landward displacement of the peninsula but also by changes in the position of Lacém Inlet (VilaConcejo et al., 2006). The observed landward displacement was related mainly to overwash occurrence and structural erosion (Matias et al., 2007) that led to a rejuvenating backbarrier. The dredge disposal operations between 1996 and 2001 had a minor influence, since most of the sediment was placed along the barrier's oceanside. Results regarding the complete overwash processes at Ria Formosa confirm the relationship between complete washover presence and an increase in sediment supply to the backbarrier. The period with the higher number of washovers (1947–1976, Fig. 7) coincided with the period of predominant landward displacement along the Ria Formosa barriers. Barreta Island shoreward displacement (Section 3.1) resulted mainly from the migration of Ancão Inlet (Andrade, 1990; Vila-Concejo et al., 2006), while at Ancão and Cacela peninsulas, a landward displacement was observed in washover areas. Complete overwash occurrences have previously been identified as a primary mechanism of landward barrier migration (e.g. Leatherman, 1979; Dingler and Reiss, 1990; Bray and Carter, 1992; Morton and Sallenger, 2003). Barrier migration influences lagoon evolution by reducing the size of the tidal prism and by generating a series of changes in the pattern of inlets (Cooper, 1994). It seems likely that the overall evolutionary trend for the Ria Formosa backbarrier (a decrease in the backbarrier coastline extent due to landward displacement of the system), substantiates such a migration process. Although a categorisation was made for each barrier, some intra-barrier variability is evident but which was not

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extensively demonstrated in this study. For example, at the eastern end of Cacela Peninsula the backbarrier is not immature and rejuvenating (contrary to its general categorisation, Fig. 9B) as it constitutes the mainland attachment of the barrier system. In that location, lagoon currents have low velocities, and silting has been observed with consequent salt marsh development during the study period. 5. Conclusion Investigations of backbarrier morphodynamics are scarce. Therefore, the main objective of this study was to develop a backbarrier classification based on variations in evolution and maturation: the backbarrier evolutionary categorisation model. The proposed model represents the first methodology of categorisation for backbarriers. The model assesses the recent evolution of the backbarrier coastline for a given time period by quantifying the coastline length and the extent of different morphological units along the barrier. Differences in hydrodynamic conditions and consequent sediment supply define different backbarrier categories. Evolutionary categories obtained for the Ria Formosa barrier system (southern Portugal) show a predominant distribution between backbarrier growth and reduction categories; the extremities of the system (both peninsulas) are considered immature, while the remainder of the system is characterised by a predominance of mature backbarriers. The main natural processes identified as affecting the Ria Formosa barrier system, and responsible for the categorisations obtained, were inlet dynamics, overwash occurrence, and the development of salt marshes. For backbarrier environments it is paramount to develop coastal management practices based on their predicted evolution. The proposed categorisation model is useful not only in understanding different backbarrier evolutionary trends but also in indicating future behaviour. The model should be applied to other case study areas, integrating all factors (natural and anthropogenic) responsible for changes in the barrier system. Such application should enable the model to be refined and improved, and its general applicability to be tested. Acknowledgments This work was partially funded by the BERNA project (Beach Evolution in Areas of Restricted Fetch: Experimental and Numerical Analysis), funded by the Fundação para a Ciência e Tecnologia (POCTI/ CTA/45431/2002). A. Matias was supported by CIMA – Universidade do Algarve, grant CIMA/GT1-01/2005 and Função para a Ciência e Tecnologia, Grant reference SFRH/BDP/18476/2004. We thank Tiago Garcia for assistance during aerial photograph analysis. Appendix Notation BC CL ΔBC ΔCL Bmat ΔBmat SMC TIBC

Extent of the backbarrier coastline Rectilinear coastal length Backbarrier coastline extent variation Coastal length variation Backbarrier maturation state Backbarrier maturation evolution/change Coastline extent of salt marsh Sum of inner beach next to a barrier and inner beach next to inlet coastlines

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