Problem

Does StormCAD (or the GVF Rational solver) consider the special "junction loss" (equation 7-10 of HEC-22) for junctions of two pipes where there is no structure (or the structure size is negligible)?

Solution

The "junction loss" method (equation 7-10 of HEC-22) is not something that we currently support in the GVF-Rational (StormCAD) solver.  Our existing HEC-22 (2nd Ed) or HEC-22 (3rd Ed) choices refer to the access hole or junctions with significant size to the structure.  In some cases, you may want to model headloss through a junction of two links such as a lateral saddle connection where the size of the node is negligible, and therefore a separate calculation of loss is justified (see above highlighted equation).

This equation incorporates partially-filled cross sectional flow area of both the inflow pipes and outflow pipe.  That makes it a unique equation compared to the existing available loss method choices.  This situation may be similar to the angled inflow loss coefficient term C_theta within the HEC-22 (Third Edition) loss method that we offer, but that calculation choice will also incorporate several other component loss terms in solving the junction loss that are undesirable in this configuration.

Starting with the CONNECT Edition release of StormCAD and the storm-sewer products with the GVF-Rational Solver, a new headloss method called HEC-22 Minor Loss is included to model this using the equation above. Note the units in this equation. If you do hand calculations in Excel for this, keep in mind the angle units will be radians.