Two-Stage Transition From Gilbert to Hyperpycnal Delta in Reservoir

Deltas are a most common form of reservoir sedimentation. When the river inflows switch between homopycnal and hyperpycnal, morphological transitions between Gilbert and hyperpycnal deltas take place. So far, however, detailed studies on how such transitions occur and quantitative descriptions of the processes have been rare. Here, we study experimentally the deltaic transitions in response to the switch of flow type. Our results show that transition from hyperpycnal to Gilbert delta is purely depositional. This contrasts the transition from Gilbert to hyperpycnal delta, which is a two-stage process involving both erosion and deposition. During the first stage, hyperpycnal flows modify the existing Gilbert delta into a subaqueous foundation, over which hyperpycnal delta develops and progrades into the basin during the second stage. After transition, hyperpycnal deltas migrate at higher speeds to recover the non-transitioned trajectories. Diffusion-based models are found well suited to describing the self-similar morphodynamics of the two-stage process.