Analysis of Shoreline Evolution in Support of Marine and Coastal Management in The Gambia

Abstract

Beaches worldwide are plagued with problems associated with coastal erosion and degradation. In the Gambia, coastal erosion is a significant challenge exacerbated by the increasing pressures of climate change and sea-level rise, natural wave action, and human activities leading to coastal degradation and biodiversity loss. This study analyzes shoreline evolution in The Gambia over 32 years, 1986-2018, using remote sensing and GIS techiques. It investigates the effect of wave action and human activities on the coastal stretch along Banjul to Bald Cape (Cells 3 to 6). Multi-temporal Landsat imagery was used for the extraction of shorelines using the Digital Shoreline Analysis System (DSAS), an extension of ArcGIS. The spatio-temporal shoreline movements and the rate of statistical changes were analyzed for two periods, 1986-2002 and 2002-2018. The period 1986-2002 recorded an average rate of change of -1.76 m/yr with a mean significant wave height of 0.45m, while an average rate of change of 2.79 m/yr was recorded for the 2002-2018 period with a mean significant wave height of 0.46m. The cumulative DSAS statistics computed for these periods reveal that most shorelines have actively accreted over time. However, a continuous erosive trend was prevalent along cells 4 and 6 with erosional rates of -5.2 m/yr and -1.33 m/, yr respectively. Seasonal and monthly variations of the mean significant wave height have produced asymmetric shoreline erosion by changing the trends of sediment movement in the nearshore area. The shoreline changes in these areas are mainly due to unregulated coastal mining, longshore sediment transport, and coastal defense structures. Artificial structures such as groynes, revetments, and breakwaters along the coast alter the natural coastal processes, causing shoreline changes by significantly reducing the sediment material from the beach. Therefore, frequent monitoring of shorelines is essential to understand coastal dynamism that will direct effective marine and coastal zone management plans to curb coastal hazards and degradation, biodiversity loss, and land and economic resources loss in the coastal area.