What is Hours Above Full Normal Flow in SWMM 5?

 

What is Hours Above Full Normal Flow in SWMM 5?

Let's break down what "Hours Above Full Normal Flow" means and why it matters.

Normal Full Flow

• Manning's Equation: In SWMM 5, the normal full flow of a conduit (pipe or channel) is calculated using Manning's equation. This equation considers the conduit's geometry (shape, size, slope) and roughness to determine the flow rate when the conduit is flowing completely full under gravity conditions.¹
• Reference Capacity: This normal full flow serves as a reference point or baseline capacity for the conduit.

Exceeding Normal Full Flow

• Head Difference: The flow in a conduit can exceed its normal full flow if there's a sufficient head difference across the conduit. The head difference is the difference in water surface elevation between the upstream and downstream ends of the conduit. A larger head difference creates a greater driving force for flow.
• Surcharge: When the flow exceeds the normal full flow, the conduit may become surcharged. This means the water level rises above the crown of the conduit, and the conduit is under pressure.

Hours Above Full Normal Flow

• SWMM 5 Reporting: In the SWMM 5 output, the "Hours Above Full Normal Flow" statistic in the Conduit Surcharge Summary Table tells you the total number of hours during the simulation that a particular conduit experienced flow rates exceeding its normal full flow capacity.
• Significance: This is a valuable indicator of potential surcharge conditions and high flow stresses in the system. It helps identify conduits that might be undersized or experiencing backwater effects.

Why It Matters

• Identifying Bottlenecks: Consistently high values for "Hours Above Full Normal Flow" can pinpoint bottlenecks in the drainage system where capacity is insufficient.
• Assessing Surcharge Risk: It highlights areas at risk of surcharging, which can lead to flooding, manhole overflows, and other problems.
• Design and Rehabilitation: This information is crucial for designing new drainage systems or rehabilitating existing ones. Engineers can use it to size conduits appropriately or implement flow control measures to mitigate surcharge risks.
• Model Calibration and Validation: Comparing observed surcharge conditions in the real system with the "Hours Above Full Normal Flow" in the model can help with calibration and validation efforts.

Key Takeaways

"Hours Above Full Normal Flow" is a critical metric in SWMM 5 for understanding how the model handles flow conditions that exceed the normal capacity of conduits. By tracking this statistic, you gain insights into potential surcharge problems, system bottlenecks, and areas that may require design modifications or further investigation.

Figure 1. Hours above Normal Flow in SWMM 5 Links

 

Figure 2. Flow versus Full Flow in SWMM 5