**4. Flags**

The solution can hit obstacles, but these are almost always the result of unachievable entry data (**Table 1**). Convergence of the model is assessed by the convergence of actual efflux concentration with that desired. If this cannot be achieved a flag is raised. The axial pipe velocity for a stationary bed is the solution of a quadratic equation (see Eqs. (37)–(40)). If an imaginary solution is obtained, a discriminant flag is set. The *SB* flag is not really an obstacle, but an indication that the


**Table 1.** *Preparatory data gathering.* value obtained for the velocity of the lower layer (*U2*) is zero or less than zero. If *U2* is calculated to a value less than zero, this does not generally indicate a reverse flow direction, but an established stationary bed. The model sets it to zero, and the *SB* flag is raised.


The outputs from the model are shown in **Table 2**.

**Table 2.** *2LM outputs.*
