Erosion and shoreline retreat indicators in the Rabat-Salé littoral and their impact on coastal planning

Abstract

In-depth field investigations have allowed us to assess the coastal sector of Rabat-Salé by describing various forms of erosion, the condition of historical monuments located by the ocean, and the ineffectiveness of hard solutions implemented against erosion. This emphasizes the primary anthropogenic causes of the degradation of the studied coastal environments, including former calcarenite quarries and old sewage drainage canals. The interaction of these anthropogenic factors with natural processes has led to a significant retreat of the coastal cliff, which is clearly visible on-site, posing a threat to certain constructions, notably historical monuments of great cultural value to the Kingdom. To quantify the retreat in the study area between 1993 and 2022, we utilized the Digital Shoreline Analysis System (DSAS), version 5 integrated into ArcGIS 10.4, which revealed relatively high rates with an average rate of 1.2 m per year. We also adopted another method by using threatened historical monuments to calculate average retreat rates of the shoreline using two old plans, one dating back to 1935 (1/1000 scale) for Fort Rottembourg and another from 1922 (1/2000 scale) containing two historical monuments: Borj Sirat and Borj Eddar. The calculations yielded average rates that align with those obtained through DSAS. Despite the risks associated with erosion in the study area, a regulatory framework for the protection and enhancement of the coastal environment (Law 81-12) has been in place since 2015, prohibiting any construction within 100 m of the shoreline. This distance can be extended in cases of significant threats, such as severe erosive activity. However, some projects are still located in high-risk erosion areas, where erosion may be further exacerbated by karstic phenomena affecting the substrate. The results of this study align well with the provisions of Law 81-12 (Article 12).