Wave reworking of a delta

Process-based modelling of sediment reworking under wave conditions in the deltaic environment

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A process-based numerical model was applied to investigate wave reworking in the deltaic environment. The two main objectives were (1) to develop this model and (2) consequently apply it to study the effects of wave reworking on the morphology and stratigraphy of a delta. A depth-average Delft3D model with two sediment fractions (fine silt and fine sand) was developed. The initial condition was the morphology and stratigraphy of a pre-defined fluvial-dominated delta. This initial condition was first subjected to gentle perpendicular waves for a period of 44 months, for a situation with no active river discharge, to resemble a degrading delta. Next, the reference model was subjected to waves for the same period and varying riverine water and sediment discharges were added to the model. The results of these simulations gave a realistic representation of sediment reworking by waves in the deltaic environment. The deltaic environment rapidly adjusted to changes in the forcing. The base case showed the effects of delta front erosion, channel infill and sediment sorting. Due to the difference in energy required for stirring up and transport of sediments, sand sediments remained in the deltaic environment while silt sediments were transported. This process of sediment sorting is dominant in wave reworking and is adequately represented by the model. Sand sediments are deposited on the edges of the delta front and thereby shield the underlying fine sediments. The results for the fluvial input case showed similar realistic behaviour and exhibited a switch towards wave-influenced delta morphology and behaviour, as defined by classical delta classifications. Also deposition of sand-ridges around the river mouth was observed. Sand deposits prevented further erosion of fine sediments of the delta front and sand-ridges shield the deltaic environment behind the ridge. This study also demonstrated that the influence of riverine sediment discharge is a steering factor for delta development. The model proved to be robust in the sensitivity analysis and provides greater insight in the coupling of morphology and stratigraphy of deltas and delta behaviour. The model therefore contributes to the understanding of the response to changes of processes in the deltaic environment, which is of increasing importance to help to sustain deltas for future generations.