Are deltaic islands stable landscape features?

Abstract:

Islands are a common and seemingly stable feature in deltaic landscapes. However, the possibility of islands that maintain their shape while building at pace with relative sea level (RSL) rise is incompatible with sedimentation rates set by sediment advection and settling, which produces an exponential decline in sedimentation rate away from channels. Here, we present results from a 1D morphodynamic model of a deltaic island. The island hydrodynamics are set by the backwater equation and water level boundary conditions at both the upstream and downstream end of the domain. We use the Rouse profile of the primary deltaic channel to set the upstream sediment concentration in the model domain. We then model sediment concentration and sedimentation rate within the island domain according to advection, settling, and entrainment. Finally, the island elevation profile changes through time according to mass conservation. On one hand, when entrainment is not included in the model, the model does not produce islands with a stable shape: although levee sedimentation may keep with RSL, the model inevitably predicts drowning in the island interior. Here, the threshold for levee drowning is purely a function of the Rouse profile and the dimensionless sea level rise rate. On the other hand, entrainment allows for two possible stable states: the first is a true steady state, where the concentration and entrainment profiles are linear, and sedimentation matches RSL rise everywhere. The second is a dynamic steady state, where oscillations in the island discharge and shape allow the island to maintain a form that remains stable on average. We interpret the true steady state condition as island bisection by a new deltaic channel, and the oscillating condition as the repeated formation and healing of a secondary channel/crevasse. We identify the key dimensionless parameters controlling the regions of phase space under which the two different conditions occur, and compare the parameter values to natural deltas. From the phase space, we find that increasing RSL rise rate can cause a transition from the oscillating to steady regime, suggesting that deltas will respond to 21^st century sea level rise by increasing network density and reducing island size.