3 Types of Subcatchment Flow in SWMM 5

 3 Types of Subcatchment Flow in SWMM 5

The three fundamental types of subcatchment flow routing in SWMM 5, which are essential for accurately representing how rainfall transforms into runoff in urban drainage models. Here's a breakdown of each type with some extra context:

1. Impervious Area with Depression Storage

• Characteristics: This represents surfaces like roads, parking lots, and rooftops where water initially accumulates in depressions (puddles, small dips) before flowing over the surface.
• Depression Storage: This initial storage delays the onset of runoff and reduces the peak runoff rate.
• Evaporation: Water stored in depressions is subject to evaporation, which further reduces runoff volume.
• SWMM Representation: SWMM models this using the %Imperv parameter in the [SUBCATCHMENTS] section and the Simp (impervious depression storage) parameter in the [SUBAREAS] section.

2. Impervious Area without Depression Storage

• Characteristics: This represents impervious surfaces where water flows directly to the drainage system without significant ponding. Think of smooth, sloped roofs or well-drained paved areas.
• No Delay: Runoff generation is immediate, leading to faster and potentially higher peak flows.
• Evaporation: Even without depression storage, some evaporation can occur from the thin sheet flow over the surface.
• SWMM Representation: This is modeled by setting the %Zero parameter in the [SUBAREAS] section to represent the portion of the impervious area with no depression storage.

3. Pervious Area with Depression Storage

• Characteristics: This represents areas like lawns, gardens, and parks where water infiltrates into the soil.
• Depression Storage: Similar to impervious areas, depressions in the terrain can temporarily store water before it infiltrates or runs off.
• Evaporation: Evaporation occurs from both the depressions and the soil surface.
• Infiltration: The key process here, where water soaks into the ground, reducing runoff volume and delaying peak flows. SWMM offers several infiltration models (e.g., Horton, Green-Ampt) to simulate this.
• SWMM Representation: Modeled using the Nperv (Manning's n for pervious areas), Sperv (pervious depression storage), and infiltration parameters in the [SUBAREAS] and [INFILTRATION] sections.

Why These Distinctions Matter

Accurately representing these different flow types is crucial for:

• Flood Prediction: Understanding how quickly and how much water runs off different surfaces is essential for predicting flood risk and designing drainage infrastructure.
• Water Quality Modeling: Different surfaces contribute different pollutants to runoff. Knowing the flow paths helps in assessing and managing water quality.
• Sustainable Drainage Design: Simulating pervious areas and infiltration is vital for designing green infrastructure like rain gardens and bioswales, which promote natural water management.

Figure 2: Subcatchment SubArea Types

This is a helpful guide for determining the appropriate time step and conduit lengthening settings in SWMM 5 ICM SWMM and InfoSWMM.

This is a helpful guide for determining the appropriate time step and conduit lengthening settings in SWMM 5 ICM SWMM and InfoSWMM. Here's a breakdown of why this is important and how to use the information provided:

Why Time Step Matters

• Accuracy: In hydrodynamic modeling, the time step influences the accuracy of flow routing and water level calculations. A smaller time step generally leads to higher accuracy but longer simulation times.
• Stability: If the time step is too large, the model can become unstable, leading to unrealistic results or even simulation crashes.
• Efficiency: Finding the right balance between accuracy and simulation time is crucial.

Conduit Lengthening

• Purpose: Conduit lengthening is a technique used in SWMM to artificially increase the length of conduits (pipes). This allows you to use a larger time step while maintaining model stability.
• Challenge: Determining the optimal amount of lengthening can be tricky. Too much lengthening can distort the hydraulic behavior of the system, while too little might not provide enough stability.

The Time Step Guide

The formula provided offers a practical approach to estimate a suitable time step:

Time Step Guide (seconds) = Link Length / [Velocity + sqrt(g * Maximum Depth)]

• Link Length: The length of the shortest conduit in your model.
• Velocity: The flow velocity in the conduit.
• g: Acceleration due to gravity (approximately 9.81 m/s² or 32.2 ft/s²).
• Maximum Depth: The maximum depth of flow in the conduit.

Key Takeaways

• Wave Celerity: The formula approximates the wave celerity in the conduit, which is a critical factor in determining the appropriate time step.
• Average Time Step: The average time step used during a preliminary simulation run can be a good starting point for refining the time step and conduit lengthening settings.
• Adjustment Factor: An adjustment factor (like the 0.75 mentioned) might be necessary to fine-tune the time step for optimal performance.

How to Use This Information

1. Initial Simulation: Run a preliminary simulation with an estimated time step.
2. Analyze Results: Observe the average time step used in the simulation.
3. Apply Formula: Use the formula to calculate the "Time Step Guide."
4. Adjust and Refine: Compare the calculated value with the average time step from the simulation. Adjust the time step and conduit lengthening settings iteratively until you achieve a stable and accurate model with a reasonable simulation time.

Important Notes

• This is a guideline, and some adjustments might be needed depending on the specific characteristics of your model.
• Consider factors like the complexity of the network, the presence of pumps or other hydraulic structures, and the desired level of accuracy when fine-tuning your time step and conduit lengthening.
• Always validate your model results to ensure they are reasonable and reflect the real-world system behavior.