Whirlpools, drains, springs and currents

Hydraulic features you place and tune in the details panel instead of writing a Blueprint that fires an impulse every tick. Three typed components, Source, Force and Wave, each with a matching placeable actor, describe a persistent contribution to a shallow water simulation: how much water enters or leaves, which way it is pushed, and what shape the surface should be driven toward. None of them tick. A world coordinator gathers them on the solver's own cadence, so a hundred placed features cost nothing per rendered frame.

The three lanes

• Source moves water mass. Add Water Depth is a spring or an inflow; Remove Water Depth is a drain or a leak; Relax To Rest Surface holds a level, converging on the authored rest surface plus an offset rather than accumulating. Added water carries the momentum you author through Injected Velocity, and removing water removes foam in the same proportion as mass, so a drain does not leave a foam stain hanging over nothing.
• Force moves momentum. Directional is a current. Radial pushes outward for a positive acceleration and inward for a negative one. Vortex is the whirlpool, rotating counter-clockwise for a positive acceleration. Target Velocity converges the water toward a chosen velocity at a chosen response rate instead of accelerating it forever, which is what you want for a channel that should settle at a speed.
• Wave drives a bounded analytic target surface from a direction, amplitude, wavelength, frequency, phase and response rate, either continuously or as a one-shot pulse. Because it targets a surface rather than adding height, repeating it cannot pile up.

How they reach the water

Each contributor resolves exactly one target simulation: automatically from its own location, or explicitly by water body, water zone or simulation actor. The coordinator builds one immutable, deterministic snapshot per solver dispatch and then advances lifetimes only for the records that were actually accepted into it. That distinction matters for timing. Duration counts simulated seconds this contributor genuinely influenced, never wall clock, so a timed feature does not burn its life during a load hitch, while the game is paused, or while an editor preview is stopped, and slow motion stretches it correctly instead of desynchronising it. Continuous entries are re-evaluated on every numerical substep; one-shot entries are applied during the first substep only and acknowledged by event sequence, so nudging the actor in the editor cannot re-fire them.

Shape, orientation and order

Footprints are an ellipse, a box, or the entire simulation, oriented by the component's own yaw, with half extents in centimeters and a world-space edge feather. Sort priority decides evaluation order, which genuinely matters now that the operations are not all commutative, and stable identity breaks ties so the same scene produces the same result frame after frame.

Recipes

• Whirlpool: a Force in Vortex mode over an ellipse. Pair it with a Source in Remove Water Depth mode at the same center and it drains while it spins.
• Drain or sinkhole: a Source in Remove Water Depth mode. Depth rate is centimeters of column per second, so it is a flow rate you can reason about, not a magic hole.
• Spring, spout or outflow pipe: a Source in Add Water Depth mode with an injected velocity, so the new water arrives already moving in the pipe's direction.
• Current, millrace or tidal race: a Force in Directional or Target Velocity mode with a box footprint aligned to the channel.
• Weir or held pool: a Source in Relax To Rest Surface mode with a positive or negative offset. It settles on a level and stays there, and the water returns on its own when you disable it.
• Ship wash, explosion swell or a scripted surge: a Wave in One Shot Pulse mode, re-armed from Blueprint with Restart Contributor.