This is the same top-down nearshore circulation cell as the 3D version, viewed from directly above so the whole flow pattern reads as a map: shore at the bottom of the frame, open ocean toward the top. Waves breaking over a sandbar pump water shoreward; that water escapes back to sea wherever the bar is weakest — a gap. This uses a simplified, pedagogically-scaled version of the classic radiation-stress / bar-gap mechanism (Bowen 1969; see also MacMahan et al. 2006 for the real field picture):
Wave-driven velocity scale: U₀ = 0.18·√(g·H_b)
Bar constriction: κ = (S − G) / S (S = gap spacing, G = gap width)
Feeder (alongshore) current: V_f = U₀·κ / √(40·C_f)
Rip-neck jet (continuity): V_r = min( 0.6·V_f·(S/G), 4.5·U₀ )
Physically: κ is the fraction of the bar that is solid — more solid bar (small gaps, wide spacing) means a stronger alongshore pressure gradient feeding each gap. Bottom friction C_f damps the feeder current. Once that flow is forced through a narrow gap, mass continuity (same volume flux through a smaller cross-section) speeds it up into the rip jet — the narrower the gap relative to the spacing, the faster and narrower the rip.
- Hb slider — breaking wave height; sets the whole velocity scale U₀.
- Gap width / spacing sliders — control the bar's constriction, i.e. how "leaky" it is and how far feeder currents must travel.
- Cf slider — bottom friction/roughness; higher values damp both currents.
- The cyan dye tracers are advected by this analytic velocity field: they drift alongshore over the bar toward the nearest gap, accelerate through the neck, jet seaward and fan out in the rip head, while a slow diffuse return flow shoreward elsewhere closes the circulation cell.
- Drag the map to pan and scroll (or pinch) to zoom — useful for following a single rip channel closely.
Real-world relevance: this is the same mechanism that makes rip currents the leading cause of beach lifeguard rescues worldwide — they are strongest exactly where a bar has a gap or channel, which is precisely what this model reproduces.