Estuary Cross-Section: Longitudinal Salt-Wedge Model

This is the 2D companion to the 3D Estuarine Stratification–Mixing Balance simulator. Where the 3D model tracks a single vertical salinity column and repeats it uniformly along an imagined channel, this version solves a genuine two-dimensional advection-diffusion field spanning both depth and distance from river to sea. The same Munk–Anderson Richardson-number closure suppresses turbulent mixing wherever the water column is strongly stratified — exactly as in the 3D model — but the horizontal transport is now computed from the real longitudinal salinity gradient at every point along the channel, driving genuine Hansen–Rattray gravitational circulation (fresh water out at the surface, salt water creeping in along the bottom) instead of substituting a single global stratification number for it. Push river discharge up and watch the salt wedge retreat toward the mouth; push tidal energy up and watch the whole channel homogenize toward uniform marine salinity — tracked live by the mean vertical ΔS and by the salt intrusion length, the fraction of the channel the wedge actually occupies.