Rip Current Circulation Simulator

Wave & Bar Geometry

Breaking wave height H_b 1.2 m

Sandbar gap width 8 m

Gap spacing (alongshore) 28 m

Bottom friction C_f 0.020

Live Readouts

Rip jet speed

— m/s

Feeder current

— m/s

Constriction ratio

—

Rip channels shown

—

How it works

Waves breaking over a sandbar pump water shoreward; that water has to escape back to sea somewhere, and it prefers the deepest, easiest route — a gap in the bar. This model 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.

H_b 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.
C_f 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.

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.

Dye tracer (surface flow) Sandbar Trough / channel

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.