Dissolved Oxygen Mixing Zone — 2D Transverse-Mixing Streeter–Phelps Model

This is the 2D counterpart to the 3D Dissolved Oxygen Sag Curve simulator, and it models a genuinely different piece of the same river-quality physics: how a BOD discharge spreads across the channel, not just how it decays along it. A point-source pipe releases waste at one position across the river's width, and turbulent transverse mixing spreads it toward the far bank as the current carries it downstream — while bacteria consume oxygen locally and reaeration works to replace it. The result is a 2D BOD and dissolved-oxygen field solved by an implicit finite-difference marching scheme, so a bank-side discharge produces a real, computed low-oxygen corridor hugging the shore that only blends into the river-wide average past a calculable mixing length. Adjust the outfall's position, the river's width, and the same deoxygenation/reaeration parameters as the 3D model to see how each reshapes both the downstream sag and the cross-channel mixing pattern.