Presets
Playback
Flow
Discharge, crest width and water density.
Geometry
Fall height and the local gravity.
Layers
Model checks
The science
Flow over the crest
Water leaves a broad crest at critical depth. With discharge per metre of crest width q = Q/L the critical depth and speed are hc = (q2/g)1/3 and vc = √(g hc), the Froude number is 1, and the head over the crest is H = 1.5 hc. This reproduces the broad-crested weir law Q = 1.705 L H3/2 (in SI units).
Free-falling nappe
Once over the edge each parcel is a projectile with horizontal speed vc: x = vc t, y = ½ g t2, landing at xL = vc√(2H/g) after t = √(2H/g). The speed is v = √(vc2 + 2 g y) (Bernoulli), and continuity makes the sheet thinner as it accelerates: b = q / v.
Energy in the pool
The potential energy lost is dissipated in the plunge pool as turbulence, spray and heat: P = ρ g Q H. For example 25 m3/s over 28 m is about 6.9 MW.
Honest simplifications: the sheet is treated as a smooth nappe, with no air resistance, no breakup into drops, no aeration, no pressure at the crest and no erosion of the pool (scour depth is not modelled). Mist and ripples are decorative. Real falls may have a ledge, cascading steps or a free jet that spreads.
What to try
- Compare the creek with the flood: a larger Q gives a thicker, faster nappe that lands farther out.
- Pick the Moon preset: lower g gives a slower, much longer arc and a gentler impact (compare impact speeds).
- Raise the fall height to 150 m and read the power and the thickness of the sheet at impact.
- Check that the yellow vector speeds agree with the speed graph.