What are Hours Above Full Normal Flow in SWMM 5?

Subject:  What is Hours Above Full Normal Flow in SWMM 5?

The Conduit Surcharge Summary Table in the Status Report or Report text file of a drainage system model provides detailed insights into the performance of various pipes (or conduits) in the system. One of the critical columns in this table is "Hours above Full Normal Flow," which represents the total number of hours the flow in a given conduit was above the reference full flow.

Hours above Full Normal Flow:

This parameter indicates the duration for which the flow in the conduit exceeded the 'Full Normal Flow.' Full Normal Flow refers to the maximum flow that can pass through a conduit under normal (gravity) conditions, as calculated by Manning's equation.

However, the flow in the conduit can surpass the Full Normal Flow even when the conduit isn't entirely full. This situation can occur when the head difference across the conduit is high enough.

Head Difference:

The head difference is the difference in water surface elevations at the upstream and downstream ends of a conduit. Essentially, it's the drop in water level from one end of the conduit to the other. This head difference is the driving force for the flow of water in gravity-fed systems. The higher the head difference, the greater the force pushing water through the conduit, which can result in flows exceeding the Full Normal Flow.

Capacity or d/D:

The capacity of the conduit, also represented by the ratio d/D, is a measure of how full the pipe is. In this ratio, 'd' stands for the depth of flow in the conduit, while 'D' denotes the diameter of the conduit. This ratio varies from 0 to 1, where 0 signifies an empty conduit, and 1 represents a fully filled conduit.

Understanding these parameters allows engineers and system designers to monitor the system's performance and identify potential issues. For instance, frequent or prolonged periods of flow above the Full Normal Flow might suggest that a conduit is undersized for the flow it is receiving, indicating a need for system upgrades or management changes.

Figure 1.  Hours above Normal Flow in SWMM 5 Links

Figure 2. Flow versus Full Flow in SWMM 5