Sediment transport in a fringing reef environment

Abstract

Coral reefs are highly valuable structures in many respects and it is increasingly important to understand hydrodynamics and morphodynamics around reefs to be able to better protect them and their hinterland. Flume experiments were performed on a scaled fringing reef in the laboratory facilities of Deltares in Delft. The objective of this thesis was to analyse the measurements and to model the experiments using the numerical model XBeach. The water depth and bed roughness were shown to have influence on many processes such as short wave breaking, infragravity (IG) wave generation, IG wave transformation, reef flat seiching, wave-induced setup and wave reflection. The measurements showed that long waves dominate the short waves in the lagoon. The flow velocities in the rough cases were lower than that in the smooth cases as a result of the bed friction. The result of the sediment data analysis paints a very consistent picture in which both the short and the long waves play a role, but the long waves appear to be the dominant factor in sediment transport and bed profile development especially close to the beach. Also the effect of the bed roughness becomes visible, mainly in the shape of a swash bar, which is more pronounced for the smooth than for the rough cases. This shows that the dominance of long waves in a fringing reef lagoon results in different sediment dynamics than for example on a regular sandy beach. The XBeach modelling reproduced the short wave height, long wave height, flow velocity and reef flat seiching rather well. The effect of bed roughness on wave-induced set-up was found to be complex. In the experiments the roughness had no influence on the set-up while in XBeach the set-up increased significantly when using a high bed roughness value. This can possibly be solved by adding an extra force according to Dean and Bender (2006) to account for a shear stress exerted by the roughness elements on the water column counter to the direction of wave propagation. This could increase the application range of XBeach. The use of XBeach and its default sediment transport formulation resulted in very little sediment suspension in the sandy lagoon, which was in disagreement with the data. Also the development of the beach profile was not correctly predicted by XBeach. Further research is recommended to increase the ability of XBeach to model these processes.