
ID	0392
Class	BUG
Severity	2
Status	ready

Models: RbCable/Cable

Found: 2026-10-16, feature seabed-pins (plan D1 phase 0, step 4, defect candidate D-d)

Decision needed: Ruled 2026-10-16 (ruling 7): FlatSeafloor gets real nonzero sigma/tau/rho defaults like the other bathymetries. Fix with the ME switches of D1 phase 2; baselines that use FlatSeafloor move.

Evidence

src/RMCable/Cable.cpp:578-602 (CableRM): below the seabed the density is (1 - blend) * kWaterDensity + rho0 * blend with rho0 from GetSeabedRho, and buoyancy and drag are rho times the element's volume and area. The initial double rho = 1.025; at :576 is dead.
marenv src/seafloor/FlatSeafloor.cpp:38-43 GetSeabedRho gives 0 (and sigma, tau 0 as well, :24-36).
So below a FlatSeafloor the blend reaches rho = 0: no buoyancy and no drag, only the weight. The element sinks through the seabed. The pin SeabedContactPin.L6_CableRMElementBelowTheSeabed in tests/SeabedContact_Pin_Test.cpp records rho 0 and 2000.

Effect

A cable (RbCable/Cable) that touches a flat seabed falls through it without resistance. Other bathymetries (default 2000 kg/m3) hold it.

Possible fix

Ruling 7: give FlatSeafloor nonzero sigma, tau and rho (marenv); then L6 holds the cable. Alternatively make L6 use a minimum density. Moves every baseline that uses FlatSeafloor and bed contact.

Test that would prove it

Test first: CableRM element 0.3 m below a FlatSeafloor, at rest: its acceleration must be bounded by the soil buoyancy; today it is the free fall of the weight. The pin of that case moves.

Risk

Moves baselines on a FlatSeafloor. The numbers of sigma, tau and rho are a physics choice of the owner (Q5h rulings).