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