SWMM functions by representing a conveyance network through a series of interconnected nodes and links. These links control the flow rate between nodes and can include conduits like open channels, pipes, or other elements such as orifices, weirs, and pumps. Nodes dictate the system’s elevation and the varying hydraulic head applied at each linked link’s end. Ultimately, the flow traversing the model’s links and nodes reaches the final outfall node. In the case of Dynamic Wave modeling, outfalls can experience various hydraulic boundary conditions, such as free discharge or time-varying water surfaces. Detailed explanations about these hydraulic system elements can be found in the “Hydraulic Elements in SWMM” sidebar.
Hydraulic routing involves consolidating all incoming inflows at the upstream end of each conduit in the conveyance network and transporting them downstream over time. This process is influenced by factors like conduit storage, backwater effects, and pipe surcharging. SWMM employs three distinct hydraulic routing methods:
- Steady Flow: This method promptly transfers a hydrograph from the upstream conduit end to the downstream without temporal delay or alterations due to conduit storage. It accumulates surface runoff from all subcatchments upstream of the chosen node throughout time.
- Kinematic Wave: Kinematic Wave routing relies on the normal flow assumption for guiding flows within the conveyance system. The hydraulic grade line’s slope corresponds to the channel slope. It’s most suitable for upstream, dendritic segments of drainage systems without significant flow restrictions that could lead to backwater or surcharging. In non-dendritic systems, it can approximate flows with the inclusion of “flow divider” nodes.
- Dynamic Wave: This method is the most potent among flow routing techniques as it addresses the complete one-dimensional Saint Venant equations for the entire conveyance network. It accommodates gradually-varied flow conditions, including backwater, surcharged flow, and flooding, commonly observed in urban drainage systems. Dynamic Wave modeling accommodates looped conduit systems and junctions with multiple downstream links (bifurcated systems), enabling advanced parallel modeling of pipes and gutters, as discussed in Example 7.