Why doesn't the known flow add up in the downstream structure?
Known flows are used to represent measured or metered subsurface (bypasses the inlet) flow from the field and are not additive.
For example, if you have a known flow of 10 cfs empty into an inlet with a Known Flow set to 5 cfs, the flow in the downstream pipe will not be 15 cfs. It will be 5 cfs, as it overwrites the upstream flow value. That means, if the value of the local user defined known flow at a node is greater than 0, then this local value will overwrite the upstream known flow rather than adding directly to the upstream known flow. If the local user-defined known flow is equal to 0, total known flow will be equal to the known flow in upstream pipes. Flow (Known) remains constant as they travel downstream and combine with other flows, similar to additional loads.
If you wish for the flows to be additive, use Additional Flows or pattern based loads (sanitary loads) instead of known Flows.
The behavior of known flows is different for steady state & extended period simulation modeling and it applies to GVF- Convex solver and the GVF- Rational solver.
During a steady state analysis known flows are not additive except at network junctions. The Known Flow component will remain constant until it encounters a downstream Known Flow of a different value. In other words, flows are being reset to the known flow values downstream of the pipe.
During an extended period simulation known flows are treated as a constant inflow hydrograph and lumped in with other hydrographs entering at the same point. Known Flows are additive during Extended Period Simulations. In other words, known flows get to combine with flows downstream and do not reset flow values.
The main purpose of Known Flows are to provide the user a way of "forcing" the flow in a pipe based on flows that were already calculated or measured externally, instead of the user having to figure out all of the node inflows necessary to generate those pipe flows.
Understanding Additional Subsurface Flows and Additional Carryover flows
Entering catchment data directly to a catch basin
Types of flows that can be applied at a catch basin for the GVF-Rational solver