Tracer studies on the updrift margin of a complex inlet system

Three sediment transport studies using tracers were performed at Ancão Inlet (southern Portugal). The objectives of the experiments were to understand the sediment transport pathways and to determine their magnitudes on the updrift margin of an inlet. In order to apply the traditionally used Space Integration Methodology to the tracer experiments, adaptations were required. The study area was divided into four morphologically defined sectors and this was found to be a key factor for the applicability of tracers in a complex area. The four sectors are as follows: sector A is the straight part of the updrift beach; sector B is the upper area of the swash platform; sector C is the lower area of the swash platform; and sector D represents the inner parts of the inlet margin. The integrated analysis of all collected data (forcing mechanisms, tracer distribution and topographic evolution) led to the determination of the sediment pathways. A semi-quantitative conceptual model was developed in order to explain the sediment transport pathways and magnitudes that a known mass of sediment would follow after arriving at the swash platform. It was found that the areas with the largest sediment accumulation were sectors B and C, while almost no sediment was retained in sector D, which experienced significant erosion. According to the model, 53% of the initial mass of sediments remain in the system after two tidal cycles. It is hypothesised that sediment losses are caused by sediment transport towards the ebb delta and by sediment bypassing occurring from the ebb delta to the downdrift beach through swash bar processes. The herein defined conceptual model represents a useful tool that could be applied to other tidal inlets under similar conditions, facilitating sediment budget studies around tidal inlets.