Derive flow segments along a spline: the subdivided points are grouped
into equal-arc-length runs (one per available slot) and each run becomes
a straight flow chord whose speed scales with its slope — gentle drift
on the flats, fast water down the cascade steps. The spline is authored
source → mouth, so every chord already points downstream.
Derive flow segments along a spline: the subdivided points are grouped into equal-arc-length runs (one per available slot) and each run becomes a straight flow chord whose speed scales with its slope — gentle drift on the flats, fast water down the cascade steps. The spline is authored source → mouth, so every chord already points downstream.