Surface Flow Path (SFP) Tool
The Surface Flow Path (SFP) Tool delineates overland flow paths that represent the movement of water across the terrain under overflow conditions from stormwater inlets or manholes. This tool helps modelers develop dual-drainage system models, where both surface and subsurface (pipe) flow systems are represented within a single integrated model.
In a SWMM model, when the stormwater network becomes overloaded during heavy rain, the excess water that surcharges from manholes or inlets is treated as flood loss. This means the water is assumed to leave the system entirely, since it bypasses the underground pipes. The SFP Tool helps overcome this limitation by allowing users to build a dual-drainage model. In this type of model, the underground storm sewer system handles smaller, more frequent storms, while excess runoff from larger storms is redirected over the ground surface. This approach makes it possible to simulate how water moves along streets, where it ponds, and how it finds its way back into the drainage system through overland flow paths.
Note: Surface Flow Paths (SFPs) are not generated for nodes where storm sewer pipes discharge directly into an open channel, and the nodes are defined as outlet points. However, if a node serves as an inlet to the storm sewer system, SFPs can be generated for that node.
Input Data Requirements
The tool requires the following data as inputs:
- Digital Elevation Model (DEM): A continuous elevation raster used to identify the direction and connectivity of surface flow paths.
- Node Layer: A point feature layer representing stormwater inlets, manholes, or junctions. These nodes serve as the starting points for flow path delineation.
Workflow Overview
The workflow begins by specifying the input node layer and DEM. The user can choose to delineate flow paths for all nodes or only for overflow nodes (as identified in the model). Based on this selection, the tool automatically identifies all required nodes for processing. The DEM can be selected directly from the map contents or loaded from an external file (Fig. 6.39).
Flow Path Tool requires a successfully simulated stormwater model as input. A simulated stormwater model ensures that the storm sewer network is hydraulically connected and that overflow nodes are correctly identified for surface flow path generation.
Once the inputs are defined, the user initiates the Flow Path Generation process. The tool uses the DEM to delineate overland flow paths from each selected node toward the nearest downstream receptor—this may be another node, a channel, or the model boundary. Each generated flow path is represented as a polyline feature, following the steepest descent derived from the DEM to ensure realistic routing that reflects the terrain’s natural topography. After generating the SFPs, the user can directly import them into the model by clicking the Import to Model button. A confirmation message will appear once the processing and integration are complete. All generated surface flow paths are automatically tagged as SFP in the attribute table of the Conduit feature class within the GeoSWMM project.
