HYDRODYNAMIC DRIVERS OF SEDIMENT TRANSPORT ACROSS A FRINGING REEF

Abstract

Coral reefs are highly valuable ecosystems, which are under an increasing number of environmental pressures. Sedimentation and sediment transport patterns are among key physical drivers of coral reefs, so it is important to improve our understanding of these poorly studied dynamics on reefs. To this purpose, flume experiments were performed on a scaled fringing reef in the laboratory facilities of Deltares in Delft. The objective was to improve the understanding of hydrodynamic and sediment transport processes across fringing reefs. 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 over the short waves at the back of the reef flat. The flow velocities in the rough cases were lower than those in the smooth cases as a result of the bed friction. Analysis of the third order velocity moment and the bed level changes indicates that both the short and the long waves play a role, but that the long waves appear to be the dominant factor in sediment transport and bed profile development - especially close to the beach. Bed roughness affected the shape of a swash bar, which was more pronounced for the smooth than for the rough cases. This demonstrates that the dominance of long waves in a fringing reef lagoon results in different sediment dynamics than, for example, on a regular sandy beach.