How is Capillary Suction Head Used in SWMM 5 Green-Ampt?

 How Capillary Suction Head is Used in SWMM 5 Green-Ampt

Role of Capillary Suction Head in Green-Ampt Method:

In SWMM 5, the Green-Ampt infiltration method uses the capillary suction head as a crucial parameter to estimate infiltration rates in pervious areas. Here's how it works and its impact on infiltration:

Sensitivity of Infiltration to Capillary Suction Head:

• Functionality: The capillary suction head reflects the soil's ability to draw water downward into the unsaturated zone by capillary forces. In the Green-Ampt model, this parameter is combined with the depth of ponded water to calculate the driving force for infiltration.
• Equation in SWMM 5:
  • The parameter C1 in the infiltration module (infil.c) is calculated as
  • 
    
  • C1=(Suction Head+Depth of Ponded Water)×IMD:

    1 = (\text{Suction Head} + \text{Depth of Ponded Water}) \times \text{IMD}

    Where:
    • Suction Head is the capillary suction head in units of length (e.g., inches).
    • Depth of Ponded Water is the current depth of standing water on the surface.
    • IMD (Initial Moisture Deficit) represents how much water can be absorbed by the soil before it reaches saturation.

Impact of Changing Suction Head:

• Figure 1: This figure would typically illustrate:
  
  
  • Total Infiltration Loss: As the suction head decreases from 12 to 6 to 3 inches, you might observe:
    • Higher total infiltration loss with lower suction heads because a smaller suction head reduces the resistance to water infiltration, allowing more water to penetrate the soil for the same amount of ponded water.
  • Total Loss Rate: Similarly, the rate at which water infiltrates (loss rate) would generally increase as the suction head decreases:
    • A lower suction head means the soil can pull water down more quickly, increasing the initial infiltration rate until other factors like soil saturation become limiting.

Sensitivity Analysis:

• Infiltration Loss Sensitivity: The model is quite sensitive to the capillary suction head, especially in the early stages of an infiltration event. A lower suction head can lead to significantly higher infiltration rates and volumes, particularly in conditions where the soil is far from saturated.
• Real-World Implications:
  • Design Considerations: When designing systems like stormwater management or green infrastructure, accurately choosing or calibrating the suction head is crucial. An overestimation could lead to under-designed systems for handling runoff, while underestimation might overestimate the ground's capacity to manage water, potentially leading to flooding or ineffective use of space.
• Calibration: In practice, this parameter might need calibration based on local soil conditions or through field experiments to match observed infiltration behaviors.

By adjusting the capillary suction head, users can model how different soil types or conditions would respond to rainfall, aiding in more accurate urban water management and runoff prediction.

 

Figure 1.  The sensitivity of the total infiltration loss to the capillary suction head in a continuous simulation