8th International Coastal Symposium (Instructions for Authors of papers)

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Differential short- and medium-term behavior of two sections of an urban beach

Differential short- and medium-term behavior of two sections of an urban beach Javier Benavente †, Theocharis A. Plomaritis †, Laura del Río †, María Puig †, Cristina Valenzuela †, Bruno Minuzzi † www.cerf-jcr.org

† Dept. of Earth Sciences, Faculty of Marine and Environmental Sciences, University of Cádiz, 11510, Puerto Real, Cádiz, Spain [email protected]

ABSTRACT

www.JCRonline.org

Benavente, J., Plomaritis, T.A., del Río, L., Puig, M., Valenzuela, C., Minuzzi, B. 2014. Differential short- and medium-term behavior of two sections of an urban beach. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 621626, ISSN 0749-0208. The present study aims to identify factors that control the different morphological evolution of two sections of an urban beach. The study area is the Victoria Beach in Cadiz city (SW Spain). The beach comprises two sectors: Cortadura zone (CZ) and Victoria zone (VZ), both subject to the same offshore wave conditions. The area is a typical low energy coast with dissipative beach morphology, where storm conditions are generally recorded in the winter season. Variability of beach profiles was analyzed through the use of empirical orthogonal functions (EOF). For short-scale processes, nearshore wave and current measurements were collected simultaneously at both zones during a tidal cycle. Additionally, medium term beach evolution from the last 50 years was evaluated using aerial photos. Results of medium term beach changes show a clear retreat that in fact led to several artificial nourishments of the beach over the last years. Erosion observed in CZ was greater than in VZ. In the short term, VZ presented greater morphological variability than CZ, which only experienced some changes in beach volume. Wave and current measurements showed a higher wave height and longshore current velocity in CZ under the same offshore wave conditions, which might account for the higher erosive trend in this area and development of a more dissipative profile. The reason for this differential behavior is related to the different characteristics of wave propagation in the outer surf and shoaling zone of both sections. ADDITIONAL INDEX WORDS: Beach morphodynamics, swash bars, geological framework, coastal erosion, Gulf of Cadiz.

INTRODUCTION Seasonal changes in beach profiles constitute an important aspect of the variability of the coastal environment and reach utmost importance in the case of urban beaches. Since the late 1940’s and the early work of Shepard (1950) on southern California beaches, it is known that seasonal beaches generally experience sand transport towards the beach during spring and summer, resulting in a steeper beach face and a well-developed berm at the end of the summer, with a wider dry beach. During fall and winter, storm-generated waves that cut away the berm cause an offshore sediment motion which results in the accumulation of sediment on a winter bar located offshore beyond the surf zone. The ability of a given beach type to shift between different morphodynamic states shows the health of that beach. This capacity allows the beach to adapt to changes in energy conditions, so its absence could be an indicator of erosion trends (Benavente et al., 2000, Benavente et al., 2002). These morphological and morphodynamic changes are usually altered on urban beaches strongly modified by human interventions, especially on nourished beaches. As a consequence, ____________________ DOI: 10.2112/SI70-105.1 received 30 November 2013; accepted 21 February 2014. © Coastal Education & Research Foundation 2014

these type of beaches commonly lack the ability to adapt to changes in incident wave energy. In fact, nourishment works have often had limited durability, due to factors like the type of artificial beach profile, sediment grain size, contouring conditions (i.e. nearshore bathymetry), etc. (Anfuso et al., 2001). Apart from that, it is frequent that diverse sectors of a single beach show contrasting behavior, with a different response during storm conditions and, mainly, during the recovery period. This constitutes a problem from the management perspective for two reasons. On one hand, sometimes interventions are undertaken in the entire beach but with a correct knowledge of differential beach behavior, they could be done only where they are really necessary, with fewer expenses. On the other hand, this different behavior is usually related to the presence of artificial coastal engineering structures, like groins or breakwaters. According to this general belief, coastal managers often act over these structures several times instead of analyzing the beach as a natural environment probably controlled by natural factors. Among these natural factors, geological framework is commonly one of the most important (Jackson et al., 2005; Lentz & Hapke, 2011) because it controls the energy that arrives to the beach, by determining nearshore bathymetry and other boundary conditions.

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The objective of this paper is to analyze the medium and short term evolution of two different sectors of the same urban beach, and to identify the reasons for their differential behavior.

STUDY SITE The study area, La Victoria beach, is located in the Gulf of Cadiz (SW Spain) facing the Atlantic Ocean (Figure 1). It is a mesotidal coast with 3.2 m and 1.1 m of spring and neap tidal ranges, respectively. Dominant winds blow from ESE (19.6% of annual occurrence) and WNW (12.8%), although the first is not significant in wave generation due to its short fetch. These conditions together with coastline orientation make sea and swell waves approach generally from the third and fourth quadrants (Benavente et al., 2000). According to this, prevailing longshore drift is directed southeastwards. Significant wave height is usually lower than 1 m, with waves over 4 m high being uncommon and occurring only during the most significant storms, which usually take place between November and March and approach from the third quadrant (Del Río et al., 2012). In fact, waves greater than 1.5 m are considered storm waves by the National Ports Authority, so the area can be classified as a low-energy one. Concerning the geological setting, this area is located in the southern part of Cádiz Bay. It belongs to the end of the Guadalquivir Neogene depression, characterized by soft, subhorizontal sedimentary deposits which give rise to a linear NNWSSE oriented, low-lying coastline. The most important river courses in this area are the Guadalquivir and Guadalete rivers, which flow North of the study site. From a geomorphological point of view, La Victoria beach is located on a sandy tombolo attached to the rocky outcrop of Cádiz city (Figure 1). Bathymetric contours in the study site are broadly parallel to the coastline and the nearshore zone shows a generally gentle slope, interrupted by several shore-parallel rocky outcrops. In detail, La Victoria is an urban beach around 3000 m long, located in Cádiz city and backed by a seafront. Outside the limits of the city, on its southernmost sector, it is backed by foredunes and a low, mostly non-vegetated dune ridge artificially stabilized by fences. It is a flat beach, with slope values ranging from 0.025 to 0.02, composed by medium to fine quartz-rich sands (D50=201μm). It is between 5-90 m wide, with a narrower north and south ends and a wider central sector (Plomaritis et al., 2009). From the morphodynamic point of view, it is an intermediatedissipative beach where flat bars are often observed on the foreshore. According to this classification the beach shows a wide surf zone with prevailing spilling breakers.

METHODS Medium-term shoreline changes were determined on six sets of aerial photographs and orthophotographs dating from 1956 to 2005, at scales between 1:18,000 and 1:33,000. The images were processed in GIS environment in order to digitize the shorelines and calculate rates of shoreline change, by means of DSAS extension for ArcGIS™ (Thieler et al., 2009). Due to the absence of dunes or beach vegetation in the study area, the HWL was used as a valid shoreline proxy (Del Río & Gracia, 2013). Short-term variability was assessed through beach topographic monitoring carried out from February 2012 to May 2013. Surveys were conducted almost every spring tide (every 15 days) using a DGPS-RTK (Leica GPS 900) and a Total Station (Leyca Geosystem TC 407). Profiles were surveyed in two sections of La Victoria beach with a different behavior (Muñoz-Pérez et al., 2001): Cortadura zone (CZ) and Victoria zone (VZ). The processing of the topographic data led to the calculation of the erosion/accretion volumes of sand per unit of beach length up

Figure 1. Study area in SW Spain showing two different sectors of the same beach: La Victoria (VZ) and Cortadura (CZ). to mean low-water level, as well as beach gradient, width of the dry beach and intertidal beach gradient between mean high and low water levels. During the surveys surface sediment samples were collected with a seasonal periodicity and the granulometric parameters were calculated. In order to separate the spatial and temporal variability of the beach profile the method of the Empirical Orthogonal Functions (EOFs), also known as Principal Component Analysis (Lorenz, 1956) was used. EOF is one of the most widely and extensively used methods for decomposing a space-time field into spatial patterns and associated time indices. Winant et al. (1975) were among the first to apply it for studying the seasonal changes in cross-shore beach profiles. Since then it has been used in beach morphodynamics (review in Larson et al., 2003). The method is purely statistical and tries to represent the complex field of spatiotemporal variability through a number of basic spatial patterns coupled with time-dependent function (Kroon et al., 2008). Although the resulting patterns lack any direct physical meaning they are often linked with coastal processes and behavior (Harley et al., 2011). This method was already applied on La Victoria beach in order to identify profile variations (Muñoz-Pérez et al., 2000) and different longshore evolution patterns (MuñozPérez et al., 2001). Hourly wave height and period data were obtained from the offshore Wave Rider buoy “Cádiz”, which belongs to the Spanish Sea Wave Recording Network (REMRO). Energy values were calculated, as well as the erosivity parameter proposed by Benavente et al. (2000): Er = EΩ = ρgH³/8WsT

(1)

where Ω is the Dean Parameter with Ws being the sediment fall velocity parameter, ρ is the density of water, H is the mean significant offshore wave height of the period prior to the beach profiling and T is its associated period. Finally, an intensive field experiment was undertaken on 10th and 11th October 2012. Measurements of nearshore waves and currents were collected at the two zones of La Victoria beach simultaneously during this tidal cycle. An ADV (acoustic Doppler velocimeter) and a pressure sensor with an electromagnetic current meter were deployed in the VZ and in the CZ respectively. They



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Figure 2. Energy and erosivity conditions between February 2012 and May 2013. were programmed in a burst of ten minutes, with a frequency of 2 Hz in the case of the electromagnetic current meter, 4 Hz in the pressure sensor and 32 Hz in the ADV.

RESULTS Medium-term evolution The aerial photographs reveal a moderate advance of the HWL along the whole beach over the last decades. However, shoreline trends vary in space and time. Accretion rates are higher in the central sector of the beach (VZ), reaching up to 1,2 m/yr for the whole analysed period, but it is remarkable that shoreline advance occurred only after the massive nourishment works performed in 1991, when 2,000,000 m3 of sand were poured on the beach (Muñoz-Pérez et al., 2001). As a consequence of the nourishment, beach width was increased about 80 m on average, especially in the central sector of the beach. Shoreline accretion decreases towards the southern end of La Victoria, and in fact no net changes are recorded in CZ across the studied period. Considering that three nourishment works have been carried out in La Victoria since 1991 (Muñoz-Pérez et al., 2013), the contrast between the aforementioned HWL advance in VZ and the apparent stability recorded in CZ reveals a clear trend of sand loss in CZ.

Energy conditions During 2012 significant wave heights were around 1 m and the maximum wave heights rarely were higher than 2 m. Therefore, along 2012 wave conditions never crossed the threshold values proposed for the storm conditions in this area (Ribera et al., 2011; Del Río et al., 2012). This situation changed at the beginning of 2013, so during this year four storms were recorded, in January,

February, March and April. Approach direction was WSW in all cases, typical for storms in this area (Del Río et al., 2012). According to this, the most energetic periods were the winter and the beginning of spring 2013, and the less energetic were the first winter and summer 2012 (Figure 2). Regarding the erosivity parameter, the most erosive periods were January and March 2013. These results indicate that 2012 was a year when beach accretion would be expected, and erosive conditions occurred only during short periods of 2013. It is also interesting to remark that the intensity of the January 2013 storm was very high but the duration was extremely short (Figure 2).

Profile evolution Profiles on CZ (Figure 3) show a clearly dissipative state without any important features along the entire profile; however, a small berm is developed during the summer till the beginning of November. Along the spring season, a very low bar system can be observed in the lower part of the beach. In the central sector of the beach (VZ), profile morphology is steeper and more variable according to Muñoz-Pérez et al. (2001). A well developed berm can be seen during the whole year 2012, with a width around 90 m. Only between January and April 2013 this morphology disappeared, with part of the sediment being deposited in the lower portion of the intertidal zone building a well developed bar system. Regarding the volumetric evolution, both areas showed a similar behavior, with remarkable variations only from the beginning of 2013. Values showed significant erosion in winter 2013, most important in CZ, and small recovery in May, most important in VZ. Intertidal slopes showed the same behavior as the sediment volume.


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Figure 3. Profile evolution in Victoria beach in CZ (a) and VZ (b).

EOFs Results of the variance of each profile for the five first eigenfunctions are ranked in Table 1. For each profile an eigenvalue is assigned, which represents percentage of the variability which they explain, defined as the Mean Squared Value (MSV) of the data. Values from EOF4 onwards are related to very small morphological changes with a temporal scale shorter than the frequency of the surveys, hence outside the scope of this work. Table 1. Variance explains by each eigenfunction in two profiles representative from both areas. CZ1

CZ2

VZ1

VZ2

EOF1

58,9

67,4

38,5

37,1

EOF2

15,8

15,4

22,2

22

EOF3

8,9

9

20,4

20,2

EOF4

4,5

2,3

8

9,6

EOF5

3,5

1,5

4,3

4,4

The first eigenfunction (EOF1) explains most of the MSV of the data and represents the average profile of the beach. Values are lower in VZ than in CZ due to the higher variability of the first zone; therefore, the low variability of CZ allows building a mean profile significantly accurate for this area (Figure 4). The second eigenfunction (EOF2) explains the major part of the remaining MSV. Both EOF1 and EOF2 are needed in order to explain the mean profile and its variability in the case of VZ. The third eigenfunction was no so important for CZ but it explained around 20% of the variance in VZ. This eigenfuncion use to be related with sediment transport processes. In CZ, the second eigenfunction explains the presence of the berm and its variability, located at around 65 m from the profile base (Figure 4). Also, the variability that appears around 100 m would explain the presence or absence of the bar system during the period when the upper part of the beach was eroded. The third eigenfunction (EOF3) just explain these movement of sand between the berm and the bar system, large variability around 25 m from the start of the profile during accretion periods and around 60m during erosive situations. Variations of the EOFs during the study period were really low, only they increased during the beginning of 2013. It must be noted that temporal variation of EOF1 coincided with the evolution of beach volume.

Figure 4. Variation of the first three eigenfunctions across the profile in CZ. a) EOF1, b) EOF2, c) EOF3 and d) temporal variations of these. In VZ (Figure 5) the movement of the berm makes it impossible to design a mean profile from EOF1. The second eigenfunction explains this variability of the berm between 40 and 80 m, as well as the presence of a bar beyond 90 m during erosive periods. Finally, the third eigenfunction explains the high variability of the berm around 80 m. Also here temporal variation of the eigenfunctions was low until the beginning of 2013. In that case the evolution of EOF1 is mostly related to the intertidal slope of this area, and as mentioned above, EOF2 is also needed to explain volumetric evolution.

Figure 5. Variation of the first three eigenfunctions across the profile in VZ. a) EOF1, b) EOF2, c) EOF3 and d) temporal variations of these three.



Intensive survey Results of the intensive survey show that the wave energy in CZ was barely higher than in VZ and only during high tide. This way, in CZ a maximum height of 0.77 m was recorded, with an average height of 0.35 m, while in VZ the maximum wave height was 0.69 m and the average was 0.21 m. Data from current measurements were more interesting (Figure 4). It can be observed that the velocity in the cross-shore direction was higher in CZ, with a mean value around 0.7 m/s while in VZ it was around 0.4 m/s. It is also evident that the direction of the most intensive fluxes was towards SW, i.e. offshore with a small longshore component. Nevertheless, in VZ the most intense currents were directed towards the coast.

VZ

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part of the winter and the beginning of the spring. Despite these low energy conditions, erosion was observed in CZ but not in VZ. Nevertheless, important morphological changes were recorded in VZ but not in CZ. Several studies used volumetric variations as indicator of the state of the beach (e.g. Allen, 1981; Carr et al., 1982; Oyegun, 1991; Thom and Hall, 1991) instead of morphological changes. In this study, it has been obtained that morphological and volumetric changes in VZ occur in the same sense, but this is not the case in CZ. In this area morphological changes were really low compared to the volumetric ones. In spite of the similar behavior of both parameters in VZ, the correlation between them was also poor. This result, according to Benavente et al. (2000), would mean that the beach could adapt its morphology to the energetic conditions, but there is not enough sediment to increase beach volume during fair weather conditions. When comparing changes in beach profiles and wave conditions, a good agreement was observed between profile slope and erosivity parameter in VZ (Figure 7) but not in CZ. Conversely, in CZ, due to the little morphological changes recorded, a good agreement was found between volumetric changes and erosivity parameter (Figure 7). Therefore, an increase in energetic conditions generates volumetric changes in CZ, while in VZ it generates morphological changes not linked to beach erosion. This differential behavior reveals a higher adaptability of VZ to changes in wave energy, and a lower adaptability of CZ, hence the latter being more vulnerable to erosion (Benavente et al., 2002).

CZ

Figure 6. Current roses obtained during the intensive survey for the both areas: VZ (upper) and CZ (lower). Coastal orientation is NNW-SSE.

DISCUSSION In the medium term, the apparent advance of the HWL in VZ can be directly attributed to the diverse artificial nourishments performed on the beach, aimed at keeping an adequately wide dry beach. In CZ the apparent stability of the shoreline is also a result of these replenishments, and therefore the natural trend of the beach is clearly erosive in the medium term. Regarding short-term beach changes and their relation with wave conditions, it must be pointed out that according to Del Río et al. (2012), the most intensive storm episodes generally occur in December and the greatest erosion in January, due to the high seasonality of the storminess. However, during 2012 no storms were recorded and during 2013 they occurred during the second

Figure 7. Relationship between erosivity parameter and normalized volume in CZ and relationship between the same parameter and normalized slope in VZ. Correlation made by an exponential decay equation with a significance level higher than 0.9 in both cases.


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It is important to note that although energetic conditions along the studied period were quite low, erosion was recorded in CZ and beach recovery was limited. This could be observed during the intensive survey when under a mild wave situation, for the same energetic conditions, in VZ there would be accretion, while in CZ there would be erosion due to the stronger rip currents and a higher longshore transport transferring sediment out of this area. The contrasting energy levels measured in the intensive survey supports the data from López-Dóriga et al. (2010). These authors observed, through wave propagation, the wave refraction and diffraction processes on the rocky shoal located in front of CZ, which generates energy concentration on this area of the beach. Regarding the EOFs, they reveal that the first eigenfunction is related to enegetic changes in both study areas, although in CZ is linked to volumetric variations while in VZ is linked to slope changes. This method has also been useful for quantifying the higher variability of VZ, where sediment exchange between the berm and the bars, together with berm changes, reveal the sedimentary stability of this area. In a similar study performed also in la Victoria beach, MuñozPérez et al. (2001) related the first eigenfunction (EOF1) to shoreline recession and dry beach reduction, highlighting the seasonal behavior as responsible for changes in beach slope. However, in the present work the first eigenfunction is not strictly related to beach retreat. These differences might be due to two main reasons. The first one is related to differences in the energetic regime considered in each study. The above authors assume a dominant seasonal behavior, with a nearly complete recovery of the beach profile at the beginning of the spring, but this situation did not occur during the present study due to the special conditions recorded in 2012. The second reason would be related to changes in beach characteristics, as the field work by Muñoz-Pérez et al. (2001) was performed shortly after the massive beach nourishment in 1991 and they assumed that the whole beach showed the same behavior. In the present work most of the principal components are defined in a similar way to that proposed by Winant et al. (1975), where EOF1 represents the average level of beach profile, while EOF2 represents the seasonal changes in slope according to the position of the bar and the berm.

CONCLUSIONS In this study the differential behavior of two sectors of the same urban beach has been demonstrated and analysed using diverse methods in the short and medium term. It has been found that the southern area of the beach (CZ) shows a clearly erosive trend at both time scales. The use of Principal Component Analysis on topographic beach profiles has revealed that the limited morphodynamic response of CZ indicates a higher vulnerability to storm conditions, compared to the central part of the beach (VZ). This differential behavior can be attributed to geological controls. As pointed out by previous works, the presence of a rocky shoal in the nearshore area determines differential conditions of incident energy in both areas. This study supports this conclusion through in-situ surveys of waves and currents in the breaker zone. From the methodological perspective, this work shows that PCA on beach profiles can be considered a useful tool to identify the kind of beach response to storms.

group RNM-328 of the Andalusian Research Plan (PAI). The authors would like to acknowledge Puertos del Estado for providing wave data.

LITERATURE CITED Benavente, J., Gracia, F.J. and López-Aguayo, F., 2000. Empirical model of morphodynamic beachface behaviour for low energy mesotidal environments. Marine Geology. 167, 375-390. Benavente, J., Del Río, L., Anfuso, G., Gracia, F.J. and Reyes, J.L., 2002. Utility of morphodynamic characterization in the prediction of beach damage by storms. Journal of Coastal Research, SI 36, 56-64. Del Río, L., Plomaritis, T.A., Benavente, J., Valladares, M. and Ribera, P., 2012. Establishing storm thresholds for the Spanish Gulf of Cádiz coast. Geomorphology, 143-144, 13-23. Del Río, L. and Gracia, F.J., 2013. Error determination in the photogrammetric assessment of shoreline changes. Natural Hazards, 65(3), 2385-2397. Jackson, D.W.T., Cooper, J.A.G. and Del Río, L., 2005. Geological control of beach morphodynamic state. Marine Geology, 216, 297-314. Larson, M., Capobianco, M., Jansen, H., Rózyński, G., Southgate, H.N., Stive, M., Wijnberg, K.M. and Hulscher, S., 2003. Analysis and modeling of field data on coastal morphological evolution over yearly and decadal time scales. Part 1: background and linear techniques. Journal of Coastal Research, 19-4, 760-775. Lentz, E.E. and Hapke, C.J., 2011. Geologic framework influences on the geomorphology of an anthropogenically modified barrier island: assessment of dune/beach changes at Fire Island, New York. Geomorphology, 126, 82-96. López-Doriga, U., Benavente, J. and Plomaritis, T.A., 2010. Natural recovery processes in an urban beach, La Victoria (Cádiz, SW Spain). In: Mimsa,H., Sedrati, M., El Moumi, B. and Menier, D. (eds.), Proceedings 1er Colloque International Littoraux Méditerranéens: états passés, actuels et futurs (Asilah, Morocco), p. 9. Lorenz, E.N., 1956. Empirical orthogonal functions and Statistical Weather Prediction. Technical report, Statistical Forecast Project Report 1, Dep. of Meteor., MIT, 49p. Muñoz-Pérez, J.J. and Medina, R., 2000. Profile changes due to a fortnightly tidal cycle. International Conference on Coastal Engineering (ASCE), Sydney, pp. 3063-3075. Muñoz-Pérez, J.J. and Tejedor, L., 2001. Las funciones empíricas ortogonales y los cambios en el perfil de playa a corto, medio, y largo plazo. Física de la Tierra, 13, 139-166. Muñoz-Pérez, J.J., Román-Sierra, J., Navarro-Pons, M., Neves, M.G. and Del Campo, J.M. (in press). Comments on “Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SW Spain” by E. Poizot et al. (2013) Geo-Marine Letters, 33(4). DOI 10.1007/s00367-013-0344-0 Plomaritis, T., Anfuso, G., Benavente, J. and Del Río, L., 2009. Storm impact and recovery patterns in natural and urbanised beaches in Cadiz (SW Spain): Geophysical Research Abstracts, 11. EGU2009-1409. Thieler, E., Himmelstoss, E.A., Zichichi, J.L. and Ergul, A., 2009. The Digital Shoreline Analysis System (DSAS) Version 4.0- An ArcGIS Extension for Calculating Shoreline Change. U.S. Geological Survey open-file report, 2008-1278. Winant, C.D., Inman, D.L. and Nordstrom, C.E., 1975. Description of seasonal beach changes using empirical eigefunctions. Journal of Geophysical Research, 80(15), 1979-1986.

ACKNOWLEDGEMENT This work is a contribution to the project GERICO (CGL201125438) (Spanish National R & D Programme), project RNM-6547 (Andalusian Excellence Research Program) and to the research
