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West Palm Beach, Florida. November 2005. Morphodynamic Characteristics and Short-Term Evolution of a Coastal Sector in SW Spain: Implications for Coastal.
Journal of Coastal Research

21

6

1139–1153

West Palm Beach, Florida

November 2005

Morphodynamic Characteristics and Short-Term Evolution of a Coastal Sector in SW Spain: Implications for Coastal Erosion Management Giorgio Anfuso and Francisco-Javier Gracia Departamento de Geologı´a Facultad de Ciencias del Mar y Ambientales Polı´gono Rı´o San Pedro s/n 11510 Puerto Real, Ca´diz, Espan˜a [email protected]

ABSTRACT ANFUSO, G., and GRACIA, F.J., 2005. Morphodynamic characteristics and short-term evolution of a coastal sector in SW Spain: implications for coastal erosion management. Journal of Coastal Research, 21(6), 1139–1153. West Palm Beach (Florida), ISSN 0749-0208. In SW Spain, numerous tourist beaches have been nourished over recent years to counteract coastal retreat. Nourishment projects have not produced great endurance, mainly because of the lack of basic information on coastal dynamics and longshore sediment transport paths. In order to cover these aspects in a sandy coastal sector south of the Guadalquivir River mouth, a beach monitoring program was carried out between 1996 and 1998, with a secondary program between 2000 and 2002. Four main beach types were identified, each one with different volumetric trends during the monitoring periods. Most beaches recorded accretion because of the prevalence of fair weather conditions, which emphasized differences among beaches. In a broad sense, beaches located on intertidal rock platforms recorded erosion or no change. Those located updrift of rock platforms or human-made constructions (groins) recorded accretion. These structures divide the coast into several littoral cells, which control longshore sediment transport between beaches. All this information is very useful for planning replenishment projects on eroding beaches. The distribution of cells, the sediment transport paths associated with them, and the volumetric trends obtained for every beach type were used for evaluating nourishment procedures, sand volume needs, and expected life time of the resulting artificial beaches. ADDITIONAL INDEX WORDS: Beach volumetric trends, littoral cells, nourishment, Cadiz Gulf.

INTRODUCTION Demand for the recreational use of beaches has been growing during recent decades, and human pressure on the coastal area has substantially increased along the world’s coastline; as a result, many man-made structures have been built too close to the shoreline and are now threatened by coastal retreat (SHORT, 1999). In Spain, weather conditions make the coastal environment very attractive to national and international tourism during several months per year. Since 1983, more than 600 fills and refills have been performed along the Spanish coast in order to prevent coastal erosion, especially on the Mediterranean coast (HANSON et al., 2002). In Cadiz province (South Atlantic coast), numerous tourist beaches were nourished during the last decade with the aim of balancing coastal retreat trends and, especially, of making beaches more attractive by enlarging the dry beach width. Sometimes these works were accompanied by the construction of small groins for stabilizing artificial fills (M UN˜OZ and GUTIERREZ, 1999). Investment in nourishment works in Cadiz province during the 1990s, excluding the cost of the construction of hard structures, was about US$18,000,000 (MUN˜OZ et al., 2001). DOI: 10.2112/03-0075.1 received 15 July 2003; accepted in revision 28 May 2004.

This paper presents the results of a beach study carried out in a coastal sector of Cadiz Gulf (SW Spain). The study zone consists of a retreating linear coast formed by urban and natural beaches between Chipiona and Rota (Figure 1). The first are found predominantly in Chipiona and Rota, whereas the second predominate in the central sector and are suffering an increasing degradation caused by urbanization. Several nourishment works were carried out in the zone: two in La Costilla beach (Rota), one in Regla beach (Chipiona), and another one in Punta Candor (Figure 1). In general, nourished beaches have not shown great endurance. The limited success of the works had a number of causes (ANFUSO et al., 2001), including the lack of basic information on the coastal morphodynamic of the zone. In the present study, topographic and granulometric data were taken monthly during two periods (1996–1998 and 2000–2002) in order to ascertain the general morphodynamic behavior of this coastal area. Data were also applied to the differentiation of littoral cells and to the characterization of short-term erosive/aggrading trends in each cell. The results obtained allow the estimation of the main causes that explain the differing dynamic behavior and evolutive trend of each cell. This methodologic approach is very important for knowing the regional extent of coastal erosion processes. The resulting information is quite useful for planning where, when, and how a nourishment project should be done in a given beach.

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Figure 1. Location map of the study area and distribution of topographic transepts. Bathymetric contours modified from Achab (2000).

STUDY AREA The study littoral includes about 14 km of sandy beaches between Chipiona and Rota, respectively South of the Guadalquivir river mouth and North of Cadiz Bay (Figure 1). Beach sediments are composed of fine to medium quartz sands. Beaches are backed by dune ridges and low cliffs (Figure 1), cut into Quaternary clays and siltstones. During the last decades beach erosion presents an average rate greater than 1 m/year (MUN˜OZ and ENRı´QUEZ, 1998). The scarcity of sediments gives rise to the outcropping of a wide and smooth intertidal rock platform, which is sometimes well developed in the nearshore (Figure 1). According to MUN˜OZ and ENR´ıQUEZ (1998), at present the platform prevents the arrival of sediments to the beach from the nearshore zone as well as from the Guadalquivir River, the main sedimentary supply

of this coast in the past. At the same time, gullies reaching the coast along this area present very small catchments, and they supply virtually no sediment to these beaches. Because sediments eroded from cliffs are too fine and unstable in the beach, the study zone can be considered as an independent sector with almost no sedimentary input. According to these assumptions, most sediment present on the beaches between Chipiona and Rota could hence be considered as relict deposits. Three sectors can be identified along this zone. The northern one, from Chipiona village to Punta Camaro´n (Figure 1), has an almost N–S strike and is formed by wide beaches and dunes. The central sector has a broadly homogeneous straight outline (with the exception of Trayuelas embayment) that is NNW-SSE oriented and represented by narrow beach-

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Figure 2. (a) Aerial view of Regla beach to the South at low tide. The beach is limited by a groin in its northern edge (photo taken in 1994). (b) Aerial view of La Costilla and Piedras Gordas beaches (Rota), separated by a small inflexion in beach plan form. The large Punta Candor rock platform can be observed in the upper-left side, and a terminal groin is visible in the low side (photo taken in 1995). Images obtained from the ‘‘Direccio´n General de Costas,’’ Spanish Ministry of Environment.

es backed by low cliffs. Finally, the southern sector, from Punta Candor headland to Rota village, has a broad WNWESE direction and is formed by beaches backed by well-developed dune ridges. The irregular distribution of rock platforms in the zone is related to morphostructural and litho-

logical controls, giving rise to small local headlands in the nearshore and low foreshore zones, as in Punta Camaro´n and Punta Candor (Figures 1 and 2). Two groins are present in the northern and southern ends of the study area (Figure 2). The study zone is a mesotidal coast with 3.2 m and 1.1 m

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Figure 3. Significant wave directions, frequencies, and heights for the period June 1997 to December 2000. Values recorded by the directional buoy ‘‘Ca´diz’’ (EMOD network, Wave Climate Service, Spanish Ministry of Environment), located south of the study zone, over a water depth of about 20 m (see Figure 1 for location).

of spring and neap tidal ranges. Dominant winds blow from the ESE (19.6% of annual occurrence) and WNW (12.8%). Significant wave height is usually lower than 1 m and, during storms, is about 3 m (REYES et al., 1999), which classifies the area as a low- energy coast (BENAVENTE et al., 2000). According to RODR´ıGUEZ et al. (2003), storm generation in Cadiz Gulf is related to the North Atlantic Oscillation (NAO), which represents the difference of atmospheric pressures at sea level between the Azores and Iceland. In Southern Europe, positive NAO values are associated with low cyclonic activity, and vice versa (RODWELL et al., 1999). The littoral is mainly affected by winds and waves (both ‘‘sea’’ and ‘‘swell’’ wave conditions) approaching from the SW to WNW (Figure 3). Atlantic winds blowing from the W and WSW are responsible for the most important winter storms in the area. Because of the coastline orientation, prevailing littoral drift currents in the zone flow to the SE. A secondary, opposite drift is also occasionally recorded, associated with wind-driven waves generated by strong winds blowing from the S and SE.

METHODS Offshore wave height (H) and period (T) for the study period were obtained from the offshore nondirectional buoy ‘‘Sevilla 1,’’ located in front of Chipiona over a depth of about 15 m and belonging to the Remote Radar Operator (REMRO) network (Spanish Wave Climate Service, Ministry of Environment). Data on wave approach directions were obtained both from the offshore directional buoy ‘‘Ca´diz’’ (which belongs to the EMOD network, Spanish Wave Climate Service) and from visual estimations carried out in the days before every field survey. Previous research on other similar nearby beaches (BENAVENTE et al., 2000) showed that the beach-changing rate in this region is usually slow, resulting in negligible daily and even weekly morphological changes. In general, beaches recorded significant changes only after periods of time long

enough for the beach systems to adapt to the prevailing wave conditions. Hence, beach morphological changes in the zone were studied through a topographic monitoring program carried out with a monthly periodicity from March 1996 to May 1998. Thirteen shore normal transepts were distributed along the study littoral (Figure 1). A theodolite was used for obtaining the topographic profiles, from the dry beach to a depth equivalent to the mean spring tide low water level. Beach width, beachface slope, and erosion/accretion volumes of sand per unit of beach length were calculated afterward. Each beach profile was considered as broadly representative of a coastal sector according to detailed field observations (ANFUSO et al., 2000). This assumption allows a first approach to the beach morphodynamic characteristics of the whole littoral. Between April 2000 and October 2002, a secondary monitoring program was performed by topographical profiling of only five transepts (P.I, P.IV, P.VI, P.XI, and P.XII) every 2 months. Samples of beach sediment were collected and sieved in a laboratory with a Ro-tap machine using a nest of sieves at 1phi intervals. Granulometric parameters were calculated according to FOLK and WARD (1957). The Surf Similarity Index (BATTJES, 1974) was applied to the obtained data:

j 5 tan b/(Hb/L0)0.5

(1)

where b is the intertidal beach slope, Hb is the significant breaking wave height, and L0 represents the deep water wave length. This index predicts the wave breaking type, from surging (j . 2) to plunging (0.4 , j , 2) to spilling breakers (j , 0.4) (FREDSOE and DEIGAARD, 1992), and is strictly related to the beach morphodynamic state. Significant breaking wave height (Hb) was calculated following KOMAR and GAUGHAN (1972) by considering the mean value recorded at the buoy during the month before the beach profiling.

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Figure 4. Significant (a) wave height and (b) peak period recorded during the study period (January 1996 to May 1998) by the scalar buoy ‘‘Sevilla 1’’ (REMRO network, Spanish Wave Climate Service, Ministry of Environment), located north of the study zone, over a water depth of about 15 m (see Figure 1 for location).

RESULTS

winds and waves coming from the SSE and SSW (compare Figures 3 and 5).

Wave Climate Wave height presented important seasonal variations (Figure 4a): higher values characterized winter months (November–January) and springtime (April–May); lower values prevailed during summer and February–March. These data coincide with the regional wave conditions prevailing in the eastern Ca´diz Gulf, which were compiled by RODRı´GUEZ et al. (2003): high-energy events concentrate in December–January, although isolated storms can also take place in springtime. Regarding wave period, data from the ‘‘Ca´diz’’ buoy did not show clear differences between ‘‘sea’’ and swell conditions, probably because of refraction processes that affected long waves during storms: extreme values were shortened, but intermediate values remained more or less unchanged (BENAVENTE et al., 2000). Instead, wave peak period data from the ‘‘Sevilla 1’’ buoy clearly recorded three main situations (Figure 4b): high energy during November–March (with values of 8–12 s), intermediate energy during April–May and September–October (6–8 s), and fair weather conditions during summer months, with low values (5–6 s). In general, these wave data are consistent with the regional trends compiled by SA´NCHEZ (1988) and MUN˜OZ (1996). However, average wave height during 1997 and 1998 winters were lower than those recorded in the preceding years (MUN˜OZ and ENRIQUEZ, 1998). Both winters (especially the one of 1997) were characterized by a higher frequency of

Morphodynamic and Sedimentological Characteristics The morphodynamic classification of the studied beaches was carried out after analysis of their morphological characteristics and by applying the Surf Similarity Index. This methodology made it possible to distinguish four beach types: intermediate, intermediate with rocky platform, dissipative, and low tide terrace beaches. Intermediate beaches (Figure 6a), visually close to the ‘‘reflective’’ beaches of W RIGHT and SHORT (1984), recorded important seasonal variations: during fair weather conditions they presented a berm with a relatively high foreshore slope (tan b ù 0.06) and a smoother profile during erosive conditions (tan b ù 0.04). Surf Similarity Index reflected these seasonal variations with plunging breakers in summer (j 5 0.40) and spilling breakers in winter (j 5 0.27), associated with the smoother foreshore slope. Intermediate with rock platform beaches (Figure 6b) presented a rock-shore platform well developed in the low foreshore (Figure 7). These beaches showed small morphological changes, normally consisting of a beach narrowing during wintertime from sediment loss, with almost no beachface slope variation. Average slope showed a value of tan b ù 0.05, and Surf Similarity recorded small variations with values close to the limit between plunging and spilling breakers (j 5 0.36–0.46).

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Figure 5. Significant wave directions, frequencies, and height during 1997 and 1998. Data recorded by the offshore directional buoy ‘‘Ca´diz’’ (EMOD network, Wave Climate Service, Spanish Ministry of Environment).

Dissipative beaches (Figure 6c), visually close to the ‘‘dissipative’’ beaches of WRIGHT and SHORT (1984), presented a flat profile (tan b ù 0.03), and their morphodynamic state (clearly dissipative, j 5 0.21) did not present seasonal changes. Low-tide terrace beaches (Figure 6d) presented a dissipative, large, and smooth tidal flat ( j 5 0.08) and a reflective (j 5 0.4) upper foreshore. This beach type is similar to the ‘‘low tide terrace beach’’ of MASSELINK and SHORT (1993). Spatial beach-type distribution is presented in Figure 8. A wide and apparently random distribution can be observed along the study zone.

Concerning sediment characteristics, they are constituted of moderately well sorted fine and medium sands. Medium grain size presented little spatial and temporal variations: 0.28 mm in intermediate beaches, 0.25 mm in dissipative beaches, and 0.26 mm in intermediate beaches with rock platform. Low-tide terrace beaches presented very fine sands in the low foreshore and medium sands around the mean and high water levels. Seasonal variations in grain size were about 0.06 mm and showed very different trends depending on the zones, even with opposite behavior in neighboring beaches (Figure 9, c and e or f and g).

Figure 6. Morphodynamic characterization of main beach states observed at the study area (following classifications proposed by Wright and Short, 1984, and Masselink and Short, 1993): (a) intermediate; (b) intermediate with rocky platform; (c) dissipative; (d) low tide terrace.

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Figure 7. Rock shore platform at Piedras Gordas beach (P. X). Photo taken during low tide, in winter 1996.

Volumetric Variations The quantification of volumetric changes of a beach is of great importance to understand longshore and cross-shore variations and beach type distribution. Volumetric changes (m3/m) along the studied littoral have been represented in Figure 10 by considering the volumetric differences recorded at each transept between consecutive field surveys. Two different main evolution trends can be identified: alongshore homogeneous variations (erosion or accretion) and opposite behavior between adjacent transepts. Despite the fact that few beaches showed a soft contrary behavior, a general accretion was recorded in February 1998 (Figure 10a), March 1997 (Figure 10b), and May 1997 (Figure 10c), whereas erosion was observed in December 1996 (Figure 10a) and November 1997 (Figure 10c). Sometimes opposite behaviors were recorded between adjacent beaches, as in April 1997 (Figure 10b), October and November 1997 (Figure 10d), or groups of them, as in May 1997 (Figure 10c) and in March and May 1998 (Figure 10e). According to field observations, waves approaching from S and SSW prevailed before December 1996, May, October, and November 1997, and waves from W and NW predominated in July 1997 and May 1998. Homogeneous variations along the littoral were related mainly to cross-shore transport, by which beaches passed from a constructive state to a dissipative one as a result of erosion exerted by winter storms. Contrasting behavior of adjacent profiles reveals the predominance of longshore over cross-shore transport and the subdivision of the coast into littoral cells. Volumetric changes were calculated for different parts of all studied beaches in the 1996–1998 period (Figure 11) and for the total beach volume in the 2000–2002 period (Figure 12). Both sets of results represent a first approach to the

short-term volumetric evolution of this littoral. Despite the occurrence of some erosive episodes, most of the studied beaches recorded accretion trends during both periods. Regarding the record between 1996 and 1998, volumetric variations reflected morphological changes; in this sense, intermediate beaches faced important changes, especially in dry beach volumes (Figure 11a, f, h, and k), and other beaches had smaller variations. Between 1996 and 1998 the greatest accretionary trends were observed in Tres Piedras (P. IV), Aguadulce (P. VI), and La Costilla (P. XI). The largest erosional episode was recorded in the southernmost part of this beach (P. XII) during autumn 1996, when beach volume passed from 239 m3/m (nourished volume) to 85 m3/m after several storms. Total volumetric budgets were calculated for each profile during both study periods by considering the initial and final volume values indicated by the regression lines of Figures 11 and 12. Total volumetric changes recorded in profiles P.I, P.IV, P.VI, and P.XI between 2000 and 2002 showed the maintenance of former trends. P.XII recorded an opposite (accretionary) behavior in comparison with the former one. Finally, volumetric results obtained from the 1996–1998 period were classified into different intervals of erosion/accretion rates for the study period (Figure 13).

DISCUSSION It is important to know the longshore distribution of beach morphodynamic states, their relationships, and the shortterm littoral evolution for a proper management of coastal erosion.

Littoral Cell Distribution Volumetric changes recorded on the beach profiles (Figure 10) suggest the existence of several littoral cells along the

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Figure 8. Beach type distribution along the study coast. Topographic base obtained from Mun˜oz and Enrı´quez (1998), modified by Anfuso et al. (2003).

study coast. Taking into account results obtained from the 1996–1998 period, a longshore distribution of six major littoral cells has been proposed for the study coast (Figure 14). Cells are separated by both human and natural structures. The former ones are represented by two groins at Chipiona (P.I) and La Costilla (P. XII) beaches (Figure 14). The second ones are constituted by rock platforms well developed between Punta Camaro´n and Punta Cuba Promontories, Punta Candor, and Piedras Gordas and close to P. XI at La Costilla (Figures 7 and 14). The definition of littoral cells is strongly dependent on profile spacing and, hence, the cell distribution proposed in this work is a simple scheme. However, granulometric variations recorded along the coast (Figure 9) confirm the observed littoral cells: beaches belonging to a same cell presented similar granulometric behaviors, while the passage from one cell to another was marked by opposite trends. Sediment transport paths within every cell and between adjacent cells strongly depend on the prevailing wave approach direction. During 1997 and 1998, approaching waves (Figure 5) broadly reproduced the average trends and directions recorded in this coast during longer periods (Figure 3). However, WSW, SW, SSW, and SSE directions were more persistent and with more energetic waves, especially during

1997. Transport paths proposed in Figure 14 would be associated to waves broadly coming from the SW, although they could suffer modifications according to the exact wave approach angles, tide conditions, and the erosive/accretionary response of each of the studied beaches. MAY and TURNER (1973) terminology was used to distinguish among the different parts of a cell. It was not possible to identify smaller cells in Tres Piedras and La Ballena beaches, which are located along a rectilinear and quite homogeneous coastal sector (Figures 13 and 14). Cell subdivisions within this area can be characterized only through a detailed topographic and bathymetric monitoring program of the nearshore zone. Rock platforms act as natural submerged groins that block littoral drift. These structures constitute fixed limits that work as impermeable or semipermeable limits according to wave characteristics (height and approaching angle) and tidal range (rock platform is a permeable structure at high tide and impermeable during low tide). In a broad sense, the limits between adjacent cells were considered as transit limits, although with different permeability degrees depending on their nature (natural, shore platforms/artificial, groins). Sometimes they work as convergent limits because of refraction and diffraction processes on rock platforms. Field surveys carried out with waves approaching from the SW re-

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Figure 9. Granulometric variations recorded in some selected profiles during the study period.

corded accretion updrift of these structures (denoted ‘‘d’’ in Figure 14), in Regla (P. I), Tres Piedras (P. IV), La Ballena (P. VI), Peginas (P. VIII), Piedras Gordas (P. X), and La Costilla (P. XI). During the less common conditions of wave fronts approaching from the NW, an opposite pattern was observed. Despite the historical erosive trend (MUN˜OZ and ENRIQUEZ, 1998) and the observed seasonal behavior, most beaches accreted during the study periods. A similar trend, i.e., accretion at a short to medium term and erosion at a long, decadal one, was also observed by LEE et al. (1995) and CORBAU et al. (1999) in beaches of North Carolina and northern France, respectively. These authors emphasized the role played by the hydrodynamic and meteorologic conditions prevailing during the period of data acquisition, which in the

first case was defined by L EE et al. (1995) as ‘‘fair-weather conditions’’—the time between extreme storm events, lasting years to decades. According to MUN˜OZ and ENRIQUEZ (1998) and RODRı´GUEZ et al. (2003), in Cadiz region storms have two main recurrence periods, 2–3 years and 6–7 years. RODR´ıGUEZ et al. (2003) identified eight main winter storm periods between 1956 and 1996, the latter (1995–1996) being the one that occurred just before the monitoring program presented in this paper. In the study case, beach evolution can be clearly ascribed to ‘‘fair-weather conditions,’’ characterized by the prevalence of winds and waves of low intensity from the SW, S, and SE (Figures 4 and 5) that did not greatly affect the coast and gave rise to a North-Westward longshore drift. This is also quite evident by comparing Figures 13 and 14: beaches that

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Beach-Type Distribution Beach-type distribution (Figure 8) appears to be strongly dependent on the interaction between wave climate and natural or human structures that divide the coast into littoral cells (Figure 14). Prevailing sediment transport between cells under the most common wave approach angles gives rise to eroding and accreting zones with different rates of sediment gain and loss. Taking into account Figures 8, 11, 13, and 14, some general trends can be observed. When considering waves from the SW, accumulation zones mostly appear updrift of natural or human structures. In such cases sand deposition, related to wind or marine processes, increases dry beach width and intertidal slope, giving rise to intermediate and low-tide terrace beaches (P. I, VI, VIII, XI, and XIII). The resulting profile type is a consequence of their accumulative trends. Under fair weather conditions, these beaches grow at an average rate of 10–20 m3/ m/year. Intermediate beaches with rock platforms recorded erosion (P. II, III, and XII) or no changes (P. VII, X) and seem to coincide with zones of important longshore transport, representing transitional areas between adjacent cells (Punta Camaro´n—Punta Cuba, Punta Candor, Figure 14) or erosive zones within a cell (‘‘b’’ in Figure 14). The general erosion of this beach type is related to the presence of the rock platform, which prevents beach recovery (MUN˜OZ and ENRIQUEZ, 1998), dealing with an average annual loss of about 10 m3/m. Finally, no evident relationship was found between dissipative beaches (P. IV and P.V) and littoral cell distribution.

Implications for Beach Nourishment Projects

Figure 10. Examples of volumetric variations between successive field surveys recorded at the different studied profiles.

recorded accretion during the study period broadly coincide with beaches that experienced progradation with wave fronts approaching from the SW. In fact, accretion was mostly recorded in beaches located at zones of cell deposition (denoted ‘‘d’’ in Figure 14), whereas erosion prevailed at cell points marked ‘‘b’’ (transepts II, III, V, IX, and XII). In P.IV accumulation took place especially in the lower foreshore (Figure 11d), probably because the beach is confined to the North by a rock platform. In transept P. VI, the cliff line forms a local small embayment that favors aeolian accumulation in the backshore and hence beach progradation. Accumulation in P. XIII is caused by the strong beach constraints between groins and rock platforms, conditions that facilitate sediment accumulation with waves coming from the SE and SW.

All these trends and relationships can be used for the correct management of eroding beaches. In the last few decades, the most common procedure for coastal protection and recovery used in this region consisted of beach replenishment, although most works were carried out without taking into account natural beach behavior or the existence of littoral cells. The typical problem of where to find suitable sand for beach nourishment is sometimes solved by taking sediment from nearby accreting beaches. However, the usual missknowledge of beach trends and behavior reduces this procedure to only several isolated cases. Sand removal from an accreting point to an eroding one should always be made within a single cell, in order not to substantially change the sedimentary budget of each cell. At the same time, the amount of removed sediment should be adapted to the natural behavior of every beach type. The nourishment project should always be carried out during the spring months in order to benefit from the seasonal behavior of all these beaches (Figure 11). Based on the seasonal variations, short-term beach erosion, and cell distribution of the studied zone, some sand bypassing works can be suggested: A first sand bypass could be made from the accretionary P.VI (Figures 11 and 12), to the retreating P.III and P.V points. An extended urban area for recreational use has recently been constructed behind La Ballena beach (Figure 8). Strong investments in this tourist complex require the maintenance of a beach sand volume big enough for its sustainable

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Figure 11. Volumetric variations for different parts of each studied beach, from March 1996 to May 1998. Dry beach: from the upper backshore (profile origin) to the high water level; upper foreshore: between high and mean water levels; lower foreshore: between mean and low water levels; total: the complete beach volume, i.e., the sum of three former parts. Regression line represents the volumetric trend of the beach.

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Figure 12. Volumetric variations (total beach volume) and trends of the profiles studied between April 2000 and October 2002.

exploitation. We suggest the bypass of about 10 m3/m/year, which would later be distributed between the eroding P.III and P.V beaches. However, these eroding beaches have different behaviors and trends. The beach at P. III records sudden retreat episodes and rapid sand losses within a single year (Figure 11c) because of the role exerted by the rock platform. Annual replenishments should be made for its maintenance. Instead, the P. V dissipative beach presents fewer and lower oscillations (Figure 11e), and hence, it can be expected to have a lifetime of several years (2 years or more) for the beach after the nourishment. A moderate and easy bypass could be made between Peginas beach (P. VIII, accretionary) and Punta Candor beach (P. IX, erosive). Both beaches belong to the same cell, and it can be supposed that most of the sediment pumped on to P. IX would be afterwards transported by longshore currents to the NW and finally accumulate at P. VIII. A periodic nourishment should then be designed through the removal of small amounts of sand (of about 5–10 m3/m during spring months). The softly oscillatory behavior of P. IX (Figure 11i) indicates a probable lifetime of more than 2 years before the next sand replenishment.

Nourishments carried out in September 1996 and March 1997 at La Costilla beach transformed the natural dissipative profile into an intermediate one. In those cases 197,000 and 94,000 m3 of sand were respectively poured onto the beach, although not along the whole cell length but only on its southern part, at P. XII. As a consequence, the artificial fill modified the southern beach plan form. As observed during both monitoring programs, eroded sand was accumulated at P. XI by waves from the SW, upstream of a submerged rock platform. In recent years, periodic bypassing has been performed from the eastern side of the Rota groin to the western retreating side (Figure 13), which involved the removal of sand from the accumulative beach at P. XIII to the eroding one at P. XII, leading to a clear change in its volumetric trend (Figure 12).

CONCLUSIONS This study represents a first approach to the knowledge of littoral dynamics in a coastal zone with no previous information. A beach-monitoring program was performed between 1996 and 1998 in order to draw beach type distribution and

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Figure 13. Beach volumetric trends recorded in the 1996–1998 period.

prevailing volumetric trends. A secondary monitoring program between 2000 and 2002 was applied only to one third of the previously studied beaches and revealed the maintenance of the volumetric trends obtained during the first period. Four main beach types were identified in this mesotidal coast. Intermediate and low-tide terrace beaches developed updrift of natural or human structures and experienced accretion. Intermediate beaches with rock platforms recorded erosion or no changes because of the role played by platforms in preventing beach recovery after storms. Finally, dissipative beaches did not show substantial changes. Hence, the

spatial distribution of different beach types appeared to be dependent on the interaction between wave climate and natural or human structures. Prevailing sediment transport under the most common wave approach angles gave rise to eroding and accreting zones with different rates of sediment gain and loss, leading to different volumetric trends. Consequently, several littoral cells were identified along the coast, which control longshore sediment transport between beaches. Rock platforms act as natural submerged groins that block littoral drift and impose typical limits between cells. Despite the historical erosive trends recorded in this coast-

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Figure 14. Littoral cell distribution according to waves approaching from the SW. Limits between adjacent cells, ‘‘e/a,’’ are transit areas. Within a cell, ‘‘b’’ is a zone of erosion, ‘‘c’’ represents a zone of no change, and ‘‘d’’ is a zone of accumulation. Zones named ‘‘b’’ in Trayuelas and Piedras Gordas are presumed.

al zone during the last decades, most beaches accreted during the study periods. This short-term trend resulted from the prevalence of fair weather conditions with low-energy waves that did not greatly affect the coast and mainly produced longshore sediment transport. Such low-energy conditions

emphasized the differential volumetric evolution between beaches and facilitated the identification of cells. The distribution of cells and the sediment transport paths associated with them can be used for a correct management of eroding beaches. Coastal protective works in the zone con-

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sist mainly of beach nourishment, often through small sand transfers from accreting beaches to eroding ones. This procedure should always be made within a single cell in order to prevent disequilibrium in the sedimentary balance of each cell. At the same time, the amount of removed sediment should be adapted to the natural trends of every beach type, and transfers should be performed during naturally accreting periods (spring months in this case). Eroding beaches with recreational interest could be maintained by periodic replenishment with sediment taken from nearby accreting beaches. The amount of sand needed and the periodicity of the works should be adapted to the volumetric trend and seasonal behavior of every beach type, which can also be used as a natural reference for estimating average lifetime and design profile of nourished beaches.

ACKNOWLEDGMENTS This work is a contribution to the DIVULGA Research Project (BTE2003-05706, supported by the Spanish Ministry of Science & Technology and by European Funds for Regional Development, F.E.D.E.R.) and to the Andalusian P.A.I. Research Group no. RNM-328.

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Journal of Coastal Research, Vol. 21, No. 6, 2005