Tuesday, July 4, 2023

SWMM 5.2.3 Inlet Workflow from the Help File

SWMM 5.2.3 Inlet Workflow from the Help File

The process of conducting an inlet analysis using the Storm Water Management Model (SWMM) is quite thorough and systematic. Here, we provide a detailed expansion of the general workflow you've presented, demonstrating how to apply it to a simple street and sewer drainage system:

  1. Layout both the street and sewer networks: Begin by designing and sketching your street and sewer networks. This involves determining the locations of your roads, open channels, and sewer lines, as well as their interconnections. This layout forms the fundamental skeleton for your model. You can use GIS (Geographical Information System) data or manual drafting to aid in creating the layout.

  2. Assign subcatchment runoff and user-defined inflows to individual street nodes: Next, designate the runoff from surrounding land areas (subcatchments) and any user-defined inflows to the nodes within your street network. Runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. User-defined inflows could be point sources of water other than the subcatchment runoff. This assignment takes into account how rainfall will be distributed in the area and which roads it will likely flow down.

  3. Create a collection of Street cross-sections: Design a collection of street cross-sections. These cross-sections represent a vertical slice through the street, showing elements such as the roadway, sidewalks, and gutters. The cross-sections provide a detailed view of the street’s shape and slope, which are key factors influencing how stormwater flows.

  4. Assign specific Street cross-sections to individual street conduits: Each conduit in your street network should be assigned a specific cross-section from your collection created in the previous step. The assignment should accurately represent the actual structure of the street at the conduit's location.

  5. Create a collection of Inlet designs: After dealing with the streets, focus on creating a variety of inlet designs. These inlets are structures that allow stormwater to enter the drainage system from the street surface. They can have different designs depending on their size, shape, and the specific conditions they will be handling.

  6. Assign specific Inlet designs to selected streets: Once you have a collection of inlet designs, assign them to the specific streets within your network. Inlet placement depends on factors such as street slope, expected water flow, and city planning guidelines.

  7. Set appropriate analysis options and run a simulation: Configure the parameters for the SWMM analysis according to the objectives of your study, including the simulation duration, time steps, and any specific routing or runoff methods. Then, execute the simulation, which will use the SWMM algorithms to model how stormwater will flow through your designed network.

  8. Review the simulation results to see if flow spread and depth values are acceptable: After the simulation has run, assess the results. Examine key outputs like flow spread (the width of water flow on a street) and depth values at each inlet and node. Ensure that these values are within acceptable ranges as defined by your design standards. If the results are not satisfactory, you may need to revisit and revise your design, which may include altering the street layout, changing the assigned cross-sections, or modifying the inlet designs.

This detailed procedure offers a step-by-step guide to designing and analyzing a basic street and sewer drainage system using SWMM. It's essential to remember that real-world scenarios may require additional steps or considerations, depending on the specificities of the site, climate conditions, and regulatory requirements.

Inlets from the SWMM 5.2.3 help file

 Inlets from the SWMM 5.2.3 help file 

The Storm Water Management Model (SWMM) is a powerful hydrological simulation program, used for planning, analysis, and design related to stormwater runoff, combined sewers, sanitary sewers, and other drainage systems. This software models various components of these systems, including pipes, channels, and inlets. When modeling inlets with SWMM, you should consider the following key concepts:

  1. Inlet Assignment: Inlets are assigned to conduit links that represent either streets or open channels. In this context, conduits are used to represent the physical structures that carry flow in the drainage system, such as pipes, channels, and tunnels. Therefore, each inlet, representing a point of entry for stormwater into the system, needs to be linked to a specific conduit.

  2. Inlet Type and Conduit Cross-section: The type of inlet you choose is determined by the cross-section of the conduit it is assigned to. For instance, curb and gutter-type inlets are designated to conduits with a street cross-section. Meanwhile, drop inlets are used with conduits that have either a rectangular or trapezoidal cross-section. Alternatively, custom inlets can be placed in any type of conduit, providing greater flexibility in modeling.

  3. Node Assignment: Each inlet is assigned to a node, typically part of a sewer line, that will receive the flow captured by the inlet. A node represents a junction point in the system where the flow may divide or combine.

  4. Multiple Inlets: Multiple inlets of the same design and receptor node can be assigned to a single conduit. This is particularly useful when modeling two-sided streets. For on-grade placement, the flow captured by each inlet is determined sequentially, which means the flow approaching the next inlet in line is the bypass flow from the inlet before it.

  5. Inlet Operation: Users can determine whether an inlet operates on-grade, on-sag, or let SWMM decide based on the street layout and topography. An on-grade inlet is situated on a continuous grade, while an on-sag inlet is located at a sag or sump point. The latter is an area where all adjacent conduits slope towards the inlet, leaving no place for water to flow except into the inlet.

  6. Flow Capture: Flow capture for on-grade inlets is determined by the approach flow seen by the inlet. However, for on-sag inlets, it is a function of the depth of water at the sag point node. This distinction is crucial for correctly modeling the flow behavior.

  7. Clogging and Flow Capture Restriction: Inlets can be assigned a degree of clogging and a flow capture restriction. These factors account for potential real-world conditions that may impede the flow into the inlet, providing a more accurate representation of the system’s functionality.

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