# The Influence of Hard Protection Structures on Shoreline Evolution in Riohacha, Colombia
## 3 Methodology  
The methodological workflow followed in this research includes the following: (1) image acquisition; (2) georeferencing (using ArcGIS Pro); (3) shoreline profiling (using ArcGIS Pro); (4) data extraction with the use of Digital Shoreline Analysis System (DSAS) [48] and change in the surface area; and (5) future trends in shoreline change. To conduct this research, twelve images were obtained from two sources: (i) aerial photographs from 1987 and 1997 provided by the Instituto Geografico Agustin Codazzi (IGAC) [49] and (ii) ten satellite images from Google Earth [50]. Table 1 details the images and their acquisition dates. For consistency in the analysis, all images were georeferenced using common control points, with the November 2022 image serving as the reference. The 1987 photographs were individually rectified, while the 1997 images were processed into an orthomosaic using Agisoft Metashape [51] and IGAC-provided projection center coordinates. The reference system used for georeferencing was \"MAGNA-SIRGAS/Riohacha urban grid EPSG:6267.\"  
Figure 2: Aerial image year 2018. Source: Google Earth Pro. (**A**) Zone 1 of the study area with the 6 groins numbered from north to south. (**B**) Zone 2 with the seawall. (**C**) Zone 3 of the local (indigenous) communities. After uploading the 12 orthophoto mosaics covering all study years into the GIS, the next step involved manually digitizing the shorelines and cliff tops [20]. Manual delineation was chosen over automated remote sensing methods [52], as the relatively small study area ensures greater accuracy through manual tracing. Although manual digitization improves precision in heterogeneous environments and allows for expert interpretation of complex features, it is not free from limitations. This approach may introduce a degree of subjectivity, and reproducibility can be affected by inter-operator variability. To maintain consistency in the identification of geomorphological features, all delineation tasks were carried out by a single operator. In this study, although no repeated digitization or formal cross-validation was conducted, the operator applied expert visual criteria, and each delineation was carefully reviewed within the GIS environment to ensure topological coherence and alignment with identifiable geomorphological features. Shoreline mapping was based on the most recent wet tide mark visible in the orthophotos, marking the boundary with the backshore [3; 53], a method well-suited for sedimentary coasts exposed to open-sea dynamics. Similarly, the cliff top was delineated by identifying the visually distinguishable crest of the slope [54]. The DSAS v5.1 was used to statistically analyze the historical evolution of both the shoreline and the cliff [48]. The DSAS workflow consists of three key steps: defining a baseline, generating transects (profiles), and calculating change rates [55]. The shorelines and cliffs for each zone were stored in a shapefile within a personal geodatabase, with each feature attributed to a date in MM/DD/YYYY format. The oldest available shoreline (IGAC1987 in Table 1) was selected as the baseline. Transects were generated perpendicular to the coastline at regular intervals, allowing DSAS to detect intersections with shorelines from different years. These intersections provided a series of position-date points, forming the basis for analyzing coastal evolution. Using DSAS, the primary metric obtained is the Net Shoreline Movement (NSM), which represents the maximum displacement along each transect between the earliest and most recent shoreline or cliff position, measured in meters. Additionally, DSAS applies to a linear regression model fitted by least squares to analyze shoreline or cliff evolution over time, providing key parameters for predictive modeling. The Linear Regression Rate (LRR) quantifies the historical rate of change in meters per year, where negative values indicate erosion and positive values signify accretion.