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

3 Types of Subcatchment Flow in SWMM 5

Subject:   3 Types of Subcatchment Flow in SWMM 5

There are three types of Subcatchment flow in SWMM 5

 1.   Impervious area with depression storage in which the runoff from the precipitation is delayed due to the depression storage.  Evaporation occurs based on the depth of water in the subarea of the Subcatchment.

2.   Impervious area without depression storage in which the runoff from precipitation is NOT delayed.  Evaporation does occur based on the depth of water in the subarea of the Subcatchment.
3.   Pervious area with depression storage in which the runoff from the precipitation is delayed due to the depression storage.  Evaporation and Infiltration occurs based on the depth of water in the subarea of theSubcatchment.