Controls on river delta formation; insights from numerical modelling

Abstract

Primary hydraulic and sedimentary controls on river deltas were previously postulated in classification schemes in the field of sedimentary geology, based on many field observations. However, detailed mechanistic models were restricted to the river-dominated delta class. Analogous to the concept of morphological conditioning, here we show that antecedent stratigraphy controls morphometrics of prograding river-dominated delta distributary networks under steady sea level. Further, we use coupled hydrodynamic-morphodynamic-stratigraphic detailed numerical modelling to assess the influence of wind-generated waves and tides on clastic river delta formation. Our synthetic simulations show that deltas forming under mere riverine forcing prograde via sequences of mouth-bar induced flow bifurcation and upstream channel shifting. Windwave action suppresses sequestration of fine sediments on the developing delta plain, entailing relatively smooth shorelines, perturbed by a limited number of distributary channels. In contrast, tide-influenced river deltas are found to prograde mainly via lengthening of initially-formed, relatively stable distributaries, as well as being characterized by cyclicity in deposits (interbedding of sands and silts). These results provide a framework for physics-based river delta modelling under various environmental conditions. Particularly, our findings suggest that relatively low-energetic basin conditions can already significantly impact morphological and stratal patterns of prograding river deltas.