A river of fresh water (ρ≈1000 kg/m³) meets the sea (denser, salty) at the estuary mouth. The dense salt water intrudes upstream along the bed as a wedge while lighter river water flows out on top. How far the wedge pushes inland, and how sharp the interface between the two layers is, is controlled by two real dimensionless numbers used in estuarine oceanography:
g' = g · β · S_ocean (reduced gravity, β≈0.78×10⁻³ per ppt)
Fr = u_river / √(g'·H) (densimetric Froude number of the outflow)
Ri = g'·H / u_tide² (estuarine/layer Richardson number)
A small river Froude number (weak outflow relative to buoyancy) lets the wedge intrude far upstream; a large one flushes it back toward the mouth — this simulation scales the wedge tip position directly from Fr each frame, then the tide phase modulates it between its flood and ebb extremes.
- Ri > 20 — strong stratification, sharp interface: a classic salt-wedge estuary.
- 2 < Ri < 20 — tidal turbulence starts eroding the interface: partially mixed.
- Ri < 2 — tidal currents dominate buoyancy and mix the water column top to bottom: well-mixed.
The mixing-regime selector sets how diffuse the drawn interface is (for a clean visual comparison of the three textbook cases); the Froude/Richardson readouts are computed live from the river-discharge and salinity sliders you control, so you can check whether they land in the regime's real classification band.