Another internal boundary equation is the balance of hydraulic heads at a junction. At each junction, the user may optionally model headloss with a user specified and static value, or by calculating the loss through one of several methods:

• Standard loss method - a user-defined loss coefficient is used to calculate the head loss based on the velocity head of the exit conduit. The standard method calculates structure headloss based on the exit pipe's velocity. The exit velocity head is multiplied by a user-entered coefficient to determine the loss.

  For numerical stability reason an empirical velocity filter is used when the velocity is larger than 5.0 ft/s as follows:

  v’ = 5.0 + 0.1*(v -5.0)

  in which v’ is the velocity applied to the head loss equation and v is the original velocity.

• Absolute loss method - a user-defined loss amount (relative change in elevation) is used as the head loss.
• HEC-22 Energy (Second Edition) method - a procedure of calculating the junction head loss specified in Hydraulic Engineering Circular No. 22 (HEC-22) Second Edition manual is used to calculate the head loss. See "HEC-22 Junction Energy Loss Method"-354.
• HEC-22 Energy (Third Edition) method - a procedure of calculating the junction head loss specified in Hydraulic Engineering Circular No. 22, (HEC-22), Third Edition manual is used to calculate the head loss. See "HEC-22 Junction Energy Loss Method"-354.
• Generic loss method - a user defined loss coefficient is used to calculate the head loss based on the velocity head difference between entry and exit conduits. The loss will be set as zero if the value given by the equation is negative.
• The AASHTO method - (as defined in the AASHTO Model Drainage Manual) for structure headloss is based on power-loss methodologies.
• The HEC-22 Minor Loss method - (as defined in the HEC-22 Manual) computes a loss based on the conservation of momentum. It is best suited for modeling the loss at the joining of a lateral pipe into a larger pipe, and that connection is not an access hole structure.

All of the available head loss methods can be specified for the GVF solvers. However, because of the complexity of the dynamic wave solvers (implicit and explicit), only certain head loss methods can be specified for a given solver. . Those methods are listed in the tables below.

The default method is Absolute with a head loss set to 0 which works across all solvers.

For those methods that can be applied based on EGL or HGL, the user selects the mode globally in the Calculation Options under the property "Structure Loss Mode".