To determine if there is sufficient tractive stress in a gravity conduit or channel, it is necessary to compare the actual tractive stress calculated by the model with a target tractive stress required to move particles. Typical target values are 0.018 lb/sq ft or 0.87 Newtons/sq m (Pascals). Target tractive stress values can be specified globally in the calculation options or locally for the individual conduit or channel. The model calculates that available tractive stress for each link element and reports if the target value is exceeded for the current time step or at any time step.

While the term “Tractive Force” is commonly used, the quantity of interest is actually tractive stress which has units of force per unit area parallel to the direction of flow. When the tractive stress is sufficient sewers are essentially self-cleaning in that particles will either not settle or they be re-suspended if they do. The tractive stress does not need to continuously exceed the target tractive stress to be self-cleaning but should exceed the target at least some time during each day.

Tractive stress is calculated according to the equation:

The user can specify the global target for tractive stress to achieve self-cleaning in the Calculation options under Gravity hydraulics > Tractive stress (Global minimum).

In addition, the user can override the global target for any conduit by setting the conduit or channel property "Use local minimum tractive stress" to True and specifying a target value for that individual element.

The solver calculates four properties:

• Hydraulic radius = Area/Wetted perimeter
• Tractive stress target ever exceeded
• Tractive stress target exceeded at the current time step
• Tractive stress calculated (at current time step)

Tractive stress calculations are only performed for the GVF Convex solver but may be extended to other solvers.