The "Flow (Local Surface)" attribute represents all surface flows to the inlet of a catch basin. This includes runoff flow from catchments, flow derived from the Inflow (Wet Weather) collection on the catch basin, and upstream carry over flow in gutters.

Sanitary Loads on Catch Basins should be added directly into the system, and should not be applied as surface flow.

SWMM Engine

Sanitary Loads will be applied to a catch basin when using the SWMM engine in a different manner depending on which routing method (Dynamic, Kinematic, Uniform) is used.

• Sanitary Loading at Catch Basins when using Kinematic, Uniform:
• When using these routing methods the sanitary loading will be applied to the Catch Basin differently depending on the inlet type, which determines whether or not the catch basin is split into two nodes upon export to SWMM.
• HEC-22 in Sag, Inflow-Capture Flow Curve, Percent Capture with Non Dynamic:
• For these inlet types, catch basins are split into two nodes upon export; a surface node, and a subsurface node. In other words the sanitary loads on the catch basin will be applied to the subsurface node.
• Full Capture:
• If the inlet type is full capture, then the catch basin will be exported as two nodes. All wet weather flows will be applied to the main node, whereas the sanitary loads will be applied to a "dummy" node connected to the main node by a virtual link. This allows surface flow to be differentiated from internal flow.
• Sanitary Loading at Catch Basins when using Dynamic Routing:
• When using the dynamic routing method, and a sanitary load exists on the catch basin, the load will be applied to the node differently depending on the inlet type that is used.
• Full Capture, HEC-22 on Grade, Inflow-Capture Flow Curve, Percent Capture:
• For these inlet types, internal SWMM engine modifications handle the inlet/flow split calculations. A separate injection node is created and connected to the catch basin node with a dummy/virtual conduit.
• For example, in the following system with two catch basins. CB-1 has an applied sanitary load, whereas CB-2 does not. CB-1 and CB-2 have HEC-22 style inlets.
• Upon export to SWMM a junction is created connected to CB-1 with the name CB-1-SAN, where all sanitary loads are applied.

Dynamic Wave (Implicit) Solver

The Dynamic Wave (Implicit) solver treats the dry weather flow (sanitary load) the same for all type of sanitary loads; it creates a separate inflow for the load and the solver knows that the inflow is a dry weather flow (a new inflow type is added in the input file).

For the calculation, the solver will not include the dry weather flow when calculating the inlet capture/bypass flows, so the dry weather flow is always added to the pipe system and there is no bypass flow for them.

All other calculations are unaffected.