Effects of Tidal Range and Significant Wave Height on Delta Development

Only around 40% of rivers globally have deltas, but the conditions which inhibit or facilitate river delta formation are not well understood. Many studies have investigated the response of delta development to marine and river conditions. However, few have investigated the limits of such processes beyond which delta formation may be prevented, and none have done so using numerical modeling. This is in part due to ambiguity in the definition of the term “delta,” which can make identification difficult in ambiguous cases. Here we propose a systematic method for identifying deltas, based on: accumulation of sediment above the low tide water level; proximity of such deposits to the initial coastline; and the presence of active channels. We run 42 simulations with identical river $\left(1280{\text{m}}^3{\text{s}}^{-1}\right)$ and sediment $\left(0.048{\text{m}}^3{\text{s}}^{-1}\right)$ discharges, under combinations of significant wave height and tidal range typical for coasts globally, and determine if/when a delta is formed by this definition. Where deltas do form, we classify four formational regimes—river-controlled, river/tide-controlled, wave-controlled, and wave/tide-controlled—and discuss the mechanisms of delta development for each regime. Furthermore, we find that, under the discharge conditions considered, delta formation is prevented for combinations of, approximately, significant wave heights of $2.0m$ and tidal ranges $\ge 3.0m$. We hypothesize that inhibition of delta formation can be explained as a consequence of sufficient marine-driven alongshore sediment transport. This is tested by deriving a 1D alongshore sediment diffusion equation, and comparing predictions made using this formula to the cross-shore integrated sediment volumes of the simulations.