There are actually three flows computed or used for a link in SWMM 5:
1. The St. Venant Flow equation flow
2. The Upstream Normal Flow Manning’s equation based on the link roughness, slope, upstream cross sectional area and upstream hydraulic Radius,
3. The flow actually used in the model which is either the flow computed from St. Venant or Manning’s equation
The following three links shows how this works in a real model:
· Link 8040 almost always uses the St. Venant Equation because it is dominated by backwater and surcharge
· Link 8100 almost always uses Manning’s equation except at the beginning and end of the simulation,
· Link 1600 is an adverse slope link and it mainly uses the St. Venant equation.
· Flow = the flow actually used during the simulation
· Qdynamic = the flow computed from the St. Venant Equation
· QNormUp = Flow based on Manning's equation at the upstream end of the link.
· QNormDown = Flow based on Manning's equation at the downstream end of the link.
Link 8100 almost always uses Manning’s equation except at the beginning and end of the simulation. The beginning and end of the simulation is when the non linear terms dominant.