Hindcasting sand spit morphodynamics after groyne construction in Ghana

Groynes were constructed west of the Volta river mouth sand spit in Ghana to mitigate the local average beach retreat of 6 m/year. Even though severe swell events occurred during the construction of the first couple of groynes, the sand spit retreat of about 150 m over less than 1 year was surprising. An in-depth analysis was done by performing a hindcast over that year with the 2DH morphological model XBeach. It qualitatively reproduced the sand spit evolution as measured in situ. The governing processes of the severe erosion of the sand spit, in addition to the effect of the groynes, were identified to be a longshore sediment transport gradient and differences in overwash occurrence. A second hindcast, without groynes, has shown that the groynes were not the main cause. It also demonstrated that the beach, which was to be protected, has clearly benefitted from the construction of the groynes.