Author(s): Jonathan Kemp; Arash Bakhtiari; Eleftheria Kragiopoulou
Keywords: Historical; Beach; Morphology; Modelling
Abstract: The existing Elmina port, Ghana suffers from sedimentation at the port entrance and is a danger for navigation. As part of the Elmina fishing port rehabilitation and expansion project the coastal morphology and sedimentation risks have been assessed. The use of historical resources, such as old admiralty charts, sketches, drawings, and old photographs, as a source of evidence to assist in the understanding of the long term changes to coastlines has been well proven, as exemplified in the 2011 Publication “A coastal historical resources guide for England.” These historical resources have been collected and analyzed for Elmina Bay to infer how the beaches have responded to various constructions over the centuries and how this can be applied to guide the future development of the fishing port. Through examination of the available historical resources, four key construction activities have been identified that led to significant changes to the beaches. These consist of the construction of two masonry river training walls between 1786 and 1799 (and can partially still be seen today); a rock breakwater in 1958-1959; a lee breakwater in 2008; and a rock revetment in 2017. The construction of the two masonry training walls in the 18th Century by the Portuguese to improve access to the river led to the creation of a beach in front of the Castle, prior to which historical drawings indicated there were only rocks. Adding the new breakwater in 1958-1959 caused a rapid accretion of around 50 m to the adjacent beaches, compared to the 1937 shoreline. By 2005 the northern training wall extension had been completely buried under sand and the man breakwater had come to the end of its life and fallen into disrepair. The 2008 repair works and construction of the lee breakwater led to further changes in the morphology including a reorientation of the river. Sedimentation at the entrance of the port continued, which was due to bypassing of the sand around the seaward end of the main breakwater and partly being pumped through the whole length of it, from a lack of the appropriate core material. This led to navigation issues. Following the construction of the rock revetement the beach in front of the castle reoriented itself and further accreted. The information gleaned from examination of historical resources has illustrated how the shoreline has evolved over the centuries. The use of this resource provides coastal engineers insight on long-term changes and can be applied for the further development of the fishing port at Elmina. This information was used to create a sediment transport pathway map and supplemented the numerical modelling of sediment transport to ensure the new port design does not suffer from the same sedimentation risk.