Measurement of pocket beach morphology using geographic information technology: the MAPBeach toolbox

Transcription

1 Measurement of pocket beach morphology using geographic information technology: the MAPBeach toolbox 1397 Measurement of pocket beach morphology using geographic information technology: the MAPBeach toolbox João Horta, Delminda Moura, Selma Gabriel, Óscar Ferreira Marine and nvironmental Research Centre (CIMA), University of Algarve, Faro, Portugal ABSTRACT Horta, J., Moura, D., Gabriel, S., Ferreira, Ó., Measurement of pocket beach morphology using geographic information technology: the MAPBeach toolbox In: Conley, D.C., Masselink, G., Russell, P.. and O Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, ngland), Journal of Coastal Research, Special Issue No. 65, pp , ISSN The measurement and evaluation of the morphological response of a beach to changes on wave conditions has been the subject of several studies over the past years. This paper presents and discusses a methodology to compute variations in pocket beaches after morphological monitoring performed trough topographical surveys. The GIS toolbox MAPBeach GIS tools for Morphological Analysis at Pocket Beaches was developed to simplify the morphological analysis of pocket beaches. The methodology was applied and tested at two beaches (Galé and Olhos de Água, Algarve, Portugal) with different exposition to the predominant swell. The morphological monitoring used the Global Navigation Satellite System (GNSS) and was performed with relative positioning in real time kinematic (RTK) mode. The obtained data were compiled and used to produce digital terrain models in a Geographic Information System (GIS) environment. The developed GIS toolbox enabled a quick and efficient evaluation of the morphological changes occurred at the selected beaches, associated to different forcing conditions, through the determination and further analysis of a set of pre-defined morphological parameters, including beach profiles, beach slope, elevation, volumes, beach curvature, beach rotation, among others. The developed tools proved to be robust and easy to operate, facilitating both the scientific analysis as the determination of useful parameters for coastal management, being therefore of broad application. ADDITIONAL INDX WORDS: Morphology, beach parameters, GIS processing models, surface modeling INTRODUCTION The alongshore variations on waves approaching the shoreline, determined by the offshore conditions, bathymetry, and coastal orientation are determinant in the amount of energy transferred to the beaches and in the amount of sediment exchanged between adjacent littoral cells. These variations on energy and sediment exchanges result on morphological shifts, at several time scales, that are particularly conspicuous in pocket beaches. Beaches in a headland-beach system as are pocket beaches evidence some particular morphometric problems due to their small dimensions in planform and very rapid response to morphodynamic processes. The alongshore drift is strongly constrained by headlands and, as a consequence, the sedimentary accretion and erosion occurring between two consecutive headlands provoke the beach rotation sometimes at a daily scale. For authors such as Cooper and Pontee (2006), Cappucci et al. (2011) and Marchand et al. (2011) the knowledge of sediment dynamics reveals to be the main concern for all stakeholders in the use and management of coastal areas. The interest on such knowledge has being evidenced in the dynamics of pocket beaches by authors like Masselink and Pattiaratchi (2001), Soomere et al. (2008), Dehouck et al. (2009) and Thomas et al. (2010). The dynamics of the associated pocket beaches, although recognized to be high, was never studied in detail. To better understand the transformations at this coastal DOI: /SI received 07 December 2012; accepted 06 March Coastal ducation & Research Foundation 2013 environment it is important to easily and efficiently characterize the morphology in order to afterwards associate it with the hydrodynamic forcing. This work aims to present a Geographic Information System toolbox (MAPBeach - GIS tools for Morphological Analysis at Pocket Beaches) to facilitate the study of morphological changes at pocket beaches. MAPBeach includes a graphical interface and uses a simple approach that combines the capabilities of data processing and spatial modelling. The main focus of such toolbox is on geomorphometry applied to coastal dynamics that allows the study of beach surfaces and selected morphologies obtained from digital elevation models. MAPBeach was applied and tested at two coastal stretches from South Portugal facing different wave exposition. STUDY ARA The study beaches are located at the Algarve, Southern Portuguese coast, a crenulated coast lithologically and tectonically controlled (Figure 1). In this area, pocket beaches bounded by headlands and shore platforms occur in small bays adjacent to rivulet mouths, bounded by rocky headlands. In front of the headlands intertidal shore platforms are well developed (up to 80 m wide). Cliffs expose Miocene carbonate rocks strongly karstified with the karst exhumation being an important process on coastal evolution (Dias 1988; Moura et al., 2006). Two pocket beaches 15 km apart were selected and analysed concerning morphological changes (Galé beach and Olhos de Água beach). The Galé beach, about 100 m long, is completely

2 1398 Horta, et al. Figure 1. Location of the study beaches, with their relative position and shoreline orientation. enclosed by cliffs and shore platforms (see Figure 1), having a mean direction of 127º, which makes this beach more exposed to the dominant W-SW swell and more protected from the -S waves. The Olhos de Água beach, with about 150 m long and a higher radius of curvature, has a mean direction of 77º, which makes it more exposed to the -S waves and more protected from the W-SW waves. The offshore waves at the Algarve coast present two predominant directions, with the W-SW being more energetic and occurring 71% of the time, while the -S accounts for just 23% of the occurrences (Costa et al., 2001). These pocket beaches intermittently suffer erosion, resulting in occasional loss of recreational area, cliff retreat, and damage to buildings (e.g., bars, houses) on the waterfront. Prior works showed that these coastal cliffs retreat very rapidly and classified them as having high or very high susceptibility to erosion (Marques, 1997; Nunes et al., 2009; Bezerra et al., 2011) presenting mass movements that can reach a maximum width (inland penetration) of near 50 m for a return period of 50 years (Teixeira, 2006). MTHODOLOGY Field Data Topographic surveys were performed in two different months during low spring tides. For each field campaign around 1000 points were surveyed for a total area of about 1500 m 2. The surveys covered the entire beach, varying between approximately -2 m and 6 m relative to MSL (mean sea level). During surveys two GNSS (Global Navigation Satellite System) receptors (GPS Trimble R6 and GPS Trimble 5800) were used in real-time kinematic mode (RTK) with differential global positioning system (DGPS), providing centimetric accuracy. The beach topographic data were collected along profiles perpendicular to the shoreline and split apart approximately 15 m. For monitoring the topography of the beach near and along the cliff, points were obtained using a total station, due to the difficulty in having GNSS signal. The surveys were performed during low spring tides to maximize the surveyed areas. The survey dates were chosen in order to test the morphological analysis tools for different wave exposures at both beaches. The first surveys were conducted in February 2011, under conditions of moderate W-SW wave climate when the significant wave height (H S ) was about 1.4 m. The last surveys were conducted in March 2011 coinciding with the beginning of an -S more energetic event with H S = 2 m (Figure 2). GIS Toolbox - MAPBeach The developed GIS toolbox requires information concerning the beach topography as input data and performs three main types of geomorphometric analyses using the surveyed data. Geomorphometry allows the analysis of the arth surface based on Digital Terrain Model (DTM) (Pike, 2000). A DTM is created for each survey and is the main entrance to the morphometric analysis that aims to extract the parameters and objects. It is a picture or a map that shows the height of the land surface above mean sea level or another level of reference.

3 Measurement of pocket beach morphology using geographic information technology: the MAPBeach toolbox 1399 Figure 2. Offshore wave characteristics for February and March H s refer to the offshore significant wave height while the expressed direction is the peak direction. The survey days are marked by dash lines. The metric scale inside the rectangle refers to the wave directional range. In this work the morphometric analysis is implemented in six key steps (Figure 3): levation data gathered in the field surveys. Generating points on a surface model of quota sampled. Triangulation of points for generating a triangulated surface. Creating a surface model using TIN (triangular irregular network). Gridding the TIN for generating a surface model raster. Applying the concepts of geomorphometry for obtaining parameters and surface objects (using MAPBeach). The automatic determination of the spatial characteristics of the beach morphology results from the combination of several calculation methods. The toolbox MAPBeach results from the junction of three models (tools) created in ModelBuilder, which after running originate a window for entering all the necessary information for data processing. The ModelBuilder is a commercial application created by SRI and it is included in the ArcGIS software package. It provides a graphical interface prepared for geoprocessing flows that allows the use of tools available in the ArcToolbox library and contains a visual language, based on shapes and colors of figures that represent the flow elements. The beach morphological analysis is performed along three phases as follows. The first one called beach profile is used to characterize the beach profiles through the extraction of points and lines. Initially the tool performs the first steps as represented in Figure 3 until obtaining the DTM level curves, according to a user-defined resolution. Through the interception of a user defined line with the DTM generated level curves, profile points, with a given altimetry and distance relative to the reference profile, are calculated. The profile data are used to compute the average beach slope. The tool computes the area above a given level (user defined) using the trapezoidal method and the associated beach volume per beach length unit, expressed in m 3 /m. The data generated by the beach profile tool includes beach volumes above the reference line (MSL for the shown cases) and volumetric balances can be estimated per profile. The second tool, named single beach, permits to obtain the shoreline orientation, the radius of beach curvature, and the spatial distribution of the beach slope and associated slope directions. After the first five steps discriminated in Figure 3 the user defines the border limits of the beach face or the beach area to be analyzed. This area includes the MSL contour line, which is used by the tool to define the mean beach direction. The shoreline radius of curvature is then obtained by using the length of the chord defined between the mean direction line and the shoreline, Figure 3. MAPBeach toolbox working flow.

4 1400 Horta, et al. and the maximum shoreline indentation relative to the chord. Regarding the characterization of the slopes, the model generates two rasters containing the user defined area, one with slope values and the other with slope directions. ach pixel has therefore an associated value of slope and slope direction. The single beach tool analyses the shoreline curvature and azimuth allowing to define the beach rotation angle and differences on the radius of curvature. The third tool, called multi beach, permits extracting parameters to distinguish beach morphological changes. Additionally, multi beach enables the extraction of raster data with altimetric variations and tables showing changes on volumetric budgets. After running the first 4 steps of Figure 3, for each given DTM, the model calculates the surface resulting from the difference between two generated TINs. To extract the volumetric difference, the second (newer) surface is subtracted from the first and the calculation is performed in the vector domain. The tool defines an "above" or "below" status for each area as well as the volumetric and vertical difference between the surfaces. The multi beach calculates volumes of erosion and accretion, and from these results it is possible to define volumetric balances and vertical changes. These results were obtained by processes implemented on each tool and enable a general analysis with multiple scenarios, all data were compiled into a single table. RSULTS The surveys conducted in 21th/22th February and 21th/22th March 2011 provided data for the GIS toolbox application and validation and helped to understand the morphological reaction occurred on the beaches of Galé and Olhos de Água to the incident wave conditions. The tools were tested for two surveys at two consecutive months where both directional and wave energy changes occurred. During the surveys of February 2011 the offshore wave energy direction was moderate from WSW while in March the survey was performed during an energetic event from SS (Figure 2). Wave conditions represented in Figure 2 were obtained by the Portuguese Hydrographic Institute (IH) at Faro buoy, located 50 km to the southeast of the study beaches (Figure 1). According to the results obtained with the beach profile tool major modifications occurred between surveys in the beach profile at Galé beach when compared to Olhos de Água (Figure 4). Galé showed a plane beach profile after being acted by shore parallel wave crests with H s = 1.4 m, whereas a concave profile with a berm developed in response to the oblique waves from -S with H s = 2 m (Figures 2 and 4). When compared to Galé, minor modifications where observed in the central profile of the wider beach of Olhos de Água in a sheltered position for W-SW waves. The average beach slope value determined along the central profile slightly reduced after the more energetic conditions (March) for both beaches. The slope value was twice higher in Galé than in Olhos de Água for both observed conditions (Figure 4; Table 1). The most noticeable difference between the studied sites was on the beach volume change, which had opposite signs. rosional processes dominated at Galé for -S waves with 21.5 m 3 /m of lost sand. On the other hand in Olhos de Água occurred an accretion process with similar magnitude (Figure 4; Table 1). The single beach tool allowed the comparison of results obtained with the generated rasters of beach slopes and their directional domain for the two analyzed surveys. On the first survey (WSW waves), the beach surface slopes were mainly oriented towards SW (75%) and S (90%) at Galé and Olhos de Água (Figure 5), respectively. The slope orientation distribution after the action of waves approaching from S shows the same trend and therefore the beach slopes did not realigned according to the wave direction. The adjacent directions (S and W) had frequencies of less than 10% for both surveys (Figure 5). In terms of beach rotation, we observed a similar response in both beaches, changing from W to, with values of 13.4 and 4.4 at Galé and Olhos de Água respectively (Table 1). The radius of curvature analysis demonstrated a decrease in Galé and an increase in Olhos de Água (Table 1). In March, Olhos de Água presents a larger radius of curvature, which turns the beach more exposed and aligned with the crest of the incident waves. The multi beach tool permitted to analyze the overlapped surface DTMs obtained for both surveys at each beach (Figure 6). The Galé beach presented a bigger radius of curvature in February than in March, associated to a sediment transfer from S to NW (beach rotation; Figure 6a). The topographic variations exceeds the 0.5 m in most of the studied area (Figure 6c), changing between positive variation (accretion) at the NW part of the beach, to a negative values (erosion) at the S part. At Olhos de Água the contour lines also show rotation, although not so marked (Figure 6b). From the surface comparison, it is noticed a small decrease of sand thickness (< 0.5 m) in the N region of the beach opposed by an increase at the remaining areas. In most cases the vertical change did not exceed 0.5 m for both erosion and accretion areas (Figure 6d). The results obtained for volumetric balances in each Figure 4. Beach profiles, beach volume and average slopes at: a) Galé beach and b) Olhos de Água beach. Figure 5. Graphical representation of the slopes obtained from the raster with the surface slopes, in degrees, for each of the octants and frequencies. The area occupied by each slope direction at the beach surface is expressed by the linear frequency graph: a) Galé in February 2011, b) Olhos de Água in February 2011, c) Galé in March 2011 and d) Olhos de Água in March 2011.

5 Measurement of pocket beach morphology using geographic information technology: the MAPBeach toolbox 1401 Figure 6. DTMs generated for Galé (a) and Olhos de Água (b) beaches for February and March 2011 and the respective elevation difference maps for Galé (c) and Olhos de Água (d) beaches. beaches showed an opposite behavior. In Galé the balance proved to be low and negative (-53.9 m 3 ), whereas in Olhos de Água was high and positive ( m 3 ) (Table 1). DISCUSSION AND CONCLUSIONS The developed toolbox MAPBeach enables the quick acquisition of morphological parameters and associated changes trough time which can be jointly analyzed with other information, namely wave data. By being integrated in a GIS program, visualization and processing of data is easy and fast. The developed tools enable a rapid review process and make it easier for someone with a basic training in ArcGIS to obtain results from beach morphology and morphodynamics. It is however important to know and to respect the input formats in order to easily understand the corresponding outputs. Moreover, the toolbox can be adapted and improved with new procedures in order to extend its applicability in the study of beach morphodynamics. The beach profile is a tool that performs the extraction of a profile from a grid and characterizes the obtained profile. Using user defined coordinates it guarantees the extraction of the profile at the exact same location for different surveys. All extracted profiles will be referred to the user defined origin point. Thus, it is guaranteed a correct positioning of the profile, allowing a correct vertical comparison. For the moment the tool do not allows the definition of the slope between two points defined by the user and it only calculates the average beach slope. Ideally the user should be able to select the desired points and calculate the slope between them (e.g, beach face slope). Other possible functions to be implemented in the future are the determination of the vertical variation between profiles, and the maximum vertical variations (erosion and accretion). The single beach tool allows a simple characterization of the beach face along the entire area and not just through profiles giving a planar perspective of the beach evolution. It determines the dominant directions (beach orientation, beach slope and associated directions) and the radius of curvature in an easy way even for more complex beach surfaces. This tool allows an intuitive surface analysis and a comparison between surfaces and their directions trough time, determining beach rotation processes and sediment transference. This tool can be improved in the future by imposing beach segmentation according to the user. That will allow to split a beach in several different areas with distinct orientations and behaviors. Other indentation parameters used at the analysis of headland-bay beaches can also be included in this tool in the future. Multi beach is the tool that presents a greater complexity in implementation. The great advantage of this tool is to enable direct analysis and visualization of spatial and volumetric changes through altimetric comparison. The tests performed using the MAPBeach tool were not designed to understand the beach behavior at the selected beaches but mainly to evaluate the tool performance for different beach exposure and wave conditions. For the understanding of beach behavior the tool will need to be applied in a systematic way to beach surveys at the selected areas. The MAPBeach toolbox revealed to be a tool with great potential in helping the scientific and coastal management communities to characterize and understand beach morphodynamic changes, with particular focus on embayed/pocket beaches, being to our knowledge the first

6 1402 Horta, et al. Table 1. Table summarizing the numerical results of the three applied tools. B Feb. Mar. Feb. Mar. Galé Galé OA OA A Mean slope (º) C H Volume above P R O F I L S I N G L B A C H M U L T I -2(MSL) (m 3 /m) Slope variation (º) Budget (m 3 /m) Beach azimuth (º) Radius of the beach curvature (m) Beach rotation (º) Radius difference (m) Feb. Galé Mar Galé Feb. OA Mar. OA Galé Olhos de Água Deposited volume (m 3 ) Deposited Area (m 2 ) Deposited V/A ( m 3 /m 2 ) roded volume (m 3 ) B roded Area (m 2 ) A roded V/A C ( m 3 /m 2 ) H Volumetric budget (m 3 ) V/A budget ( m 3 /m 2 ) devoted GIS based tool to determine the beach rotation process. The toolbox still has a great potential for further development in order to be of even broader application on the morphodynamic analyses of beaches. ACKNOWLDGMNTS The authors thank to the two anonymous reviewers the significant contributions to improving the paper. This work was supported by the project ROS rosion of Rocky Shores differences in protection promoted by sandy beaches and shore platforms, funded by Fundação para a Ciência e Tecnologia (PTDC/CT-GIX/111230/2009). Wave data was obtained from Instituto Hidrográfico de Portugal. LITRATUR CITD Bezerra, M.M., Moura, D., Ferreira, Ó. and Taborda, R.,2011. Influence of Wave Action and Lithology on Sea Cliff Mass Movements in Central Algarve Coast, Portugal. Journal of Coastal Research, 27(6), Cappucci, S., Scarcella, D., Rossi, L. and Taramelli, A Integrated coastal zone management at Marina di Carrara Harbor: sediment management and policy making. Ocean & Coastal Management, 54(4), Cooper, N.J. and Pontee, N.I., Appraisal and evolution of the littoral sediment cell concept in applied coastal management: xperiences from ngland and Wales. Ocean & Coastal Management, 49(7-8), Costa, M., Silva, R. and Vitorino, J., Contribuição para o estudo do clima de agitação marítima na costa portuguesa. 2as Jornadas Portuguesas de ngenharia Costeira e Portuária. PIANC. In CD-ROM, 20pp. Dehouck, A., Dupuis, H. and Sénéchal, N., Pocket beach hydrodynamics: The example of four macrotidal beaches, Brittany, France. Marine Geology, 266(1-4), pp Dias, J.M.A.,1988. Aspectos geológicos do Litoral Algarvio. Geonovas, 10(1), Marchand, M., Sanchez-Arcilla, A., Ferreira, M., Gault, J., Jiménez, J.A., Markovic, M. and Mulder, J., Concepts and science for coastal erosion management An introduction to the Conscience framework. Ocean & Coastal Management, 54(12), Marques, F.M.S.F., As Arribas do Litoral do Algarve: Dinâmica, Processos e Mecanismos. Lisboa, Portugal: Universidade de Lisboa, PhD thesis, 549p Masselink, G. and Pattiaratchi, C.B., Seasonal changes in beach morphology along the sheltered coastline of Perth, Western Australia. Marine Geology, 172(3-4), Moura, D., Albardeiro, L., Veiga-Pires, C., Boski, T. and Tigano,., Morphological features and processes in the central Algarve rocky coast (South Portugal). Geomorphology, 81(3-4), Nunes, M., Ferreira, Ó., Schaefer, M., Clifton, J., Baily, B., Moura, D. and Loureiro, C., Hazard assessment in rock cliffs at Central Algarve (Portugal): A tool for coastal management. Ocean & Coastal Management, 52(10), Pike, R., (2000). Geomorphometry-diversity in quantitative surface analysis. Progress in Physical Geography, 1, pp Soomere, T., Kask, A., Kask, J. and Healy, T., Modelling of wave climate and sediment transport patterns at a tideless embayed beach, Pirita Beach, stonia. Journal of Marine Systems, 74, S133 S146. Teixeira, S., Slope mass movements on rocky sea-cliffs: A power-law distributed natural hazard on the Barlavento Coast, Algarve, Portugal. Continental Shelf Research, 26, Thomas, T., Phillips, M.R. and Williams, A.T., Mesoscale evolution of a headland bay: Beach rotation processes. Geomorphology, 123(1-2),