Impact of tidal currents on delta-channel deepening, stratigraphic architecture, and sediment bypass beyond the shoreline

摘要

Deltas are sensitive indicators of coastal processes (e.g., waves and tides) and show dynamic changes in shoreline morphology, distributary channel network, and stratigraphic architecture in response to coastal forcing. Numerical modeling has long been used to show delta evolution associated with a single dominant coastal process, but rarely to examine the sensitivity of deltas to mixed processes. Physics-based morphodynamic simulations (Delft3D) are used to investigate the influence of tidal currents on deltas. Tidal amplitude and the sand: mud ratio of subsurface sediment have been varied in the model. The results show that increasing tidal amplitude causes deeper and more stable distributary channels and more rugose plan-form shoreline patterns. A new metric for channel geometry quantifies tidal influence on the distributary channel network. Stable distributary channels act as an efficient mechanism for ebb-enhanced currents to (1) bypass sediment across the delta plain, and (2) extend channel tips seaward through mouth bar erosion. The basinward channel extension leads to sandier deposits in the tide-influenced deltas than in their river-dominated counterparts. The delta-front bathymetry also reflects sediment redistribution, changing the delta-front profile from concave to convex with compound geometries as tidal amplitude increases. These results suggest that channel overdeepening is a possible tidal signature that should be considered when interpreting ancient systems, and that sand may be bypassed much farther basinward in tide-influenced than in purely river-dominated deltas.