 Two-layer sedimentation with a compression shock

With upper concentration $a$ and lower concentration $2a$, the clearing, internal and deposit fronts have speeds $-u_s(1-a)$, $-u_s(1-3a)$ and $2u_{s}a$. If the lower two merge, the resulting deposit/upper-suspension front has speed $u_{s}a$. Final deposit thickness is $a(H+z_m)$ by mass conservation.