The cause of coastal erosion on a nourished beach in Kololi, The Gambia

Abstract

The erosion problem at the Senegambia area (The Gambia) is the subject of this thesis. The Senegambia area is the largest Tourism developed area which contributes significantly to the economy of The Gambia. In November 2003 Royal Haskoning has constructed a beach nourishment of 1,000,000 m3 as an erosion buffer with a lifetime of 10 to 15 years. In 2010 this nourishment has been eroded almost completely. The processes that drive this ongoing high erosion are not known; therefore two research questions have been defined: (i) What happened to the nourished sand, where has it been transported to? and (ii) Which processes cause the high erosion rate at Kololi Beach? To gain insight in the coastal processes in front of the Senegambia area field data have been collected and analysed, the nearshore wave climate has been modelled using SWAN and sediment transport capacities and shoreline movements along the coast have been computed using DHI software. The most important results are: Nourished sediment contains 10-20% of shell fraction with a maximum of 30% for the lowest quality of sediment; these shell fractions wash out very easily and do not contribute to the total volume of the sediment in case of erosion, increasing the erosion rate. Shoreline movement for the 2004-2009 period has qualitatively the same trend as the shoreline movement for the undisturbed period 1964-1983, however a factor 5 to 8 larger. According to the computations two direction reversal points are present along the coast causing a large area of small accretion and a small area of large erosion. This was not indicated during the large-scale study in 2000 on which the design of the nourishment has been based. The computed net sediment transport is southerly directed between south of the Senegambia area, north of this area the sediment transport is northerly directed. The water level and a changing wave climate for the period 2007-2009 have a large influence on the sediment transports. These transports have been computed for the recent bathymetry and wave data, which can differ from the conditions between 1964 and 2000. These results lead to the following conclusions: The largest part of the nourishment has been transported to the stretch south of the Senegambia area. The accreted volume along the coast is approximately 50% of the total nourished volume. The cause of high erosion rate at Kololi Beach is an enumeration of multiple processes. The combination of a large amount of shell fractions in the nourished sediment, spatial placement of the nourishment, sand deficit due to sand mining and the effect of sea level rise as the water level has a high influence on sediment transport. The reason why erosion rates are 5 times higher than before 1990 is not exactly known. The modelling study on which the conclusions have been based is performed using the present bathymetry and the wave climate from 2000 to 2009. It is possible that both the bathymetry and wave climate have changed over time; this could have large influence on the sediment transport along the coast. A Combination of a “hard” structure with initial nourishment in front of the Senegambia area may be considered as a mitigation measure for the present erosion.