Total Surcharge Time vs Total Time Above Rim Elevation in InfoSWMM

Total Surcharge Time vs. Total Time Above Rim Elevation in InfoSWMM

InfoSWMM tracks flooding at nodes: Total Surcharge Time and Total Time Above Rim Elevation. These metrics help assess the severity and duration of flooding events in a drainage system.

Total Surcharge Time

• Definition: This refers to the total time that the water level at a node exceeds the crown elevation of the highest connecting pipe. Essentially, it indicates how long the node is experiencing pressure flow due to being over capacity.
• Hydraulic Significance: Surcharging can force flow back into connected pipes, potentially causing backups and overflows elsewhere in the system. It's a key indicator of potential system stress.

Total Time Above Rim Elevation

• Definition: This is a more direct measure of flooding. It represents the total time the water level at a node exceeds its rim elevation. This means water is physically spilling out of the node, whether it's a manhole overflowing onto the street or a pond exceeding its designed storage capacity.
• Practical Significance: This directly reflects visible flooding and its potential impacts on surrounding areas. It's a crucial metric for public safety and property damage assessment.

Relationship Between the Two

• Surcharge Precedes Flooding: Typically, a node will surcharge before it floods. The water level needs to rise above the crown of the highest pipe (surcharge) before it reaches the rim elevation (flooding).
• Not Always Equal: While surcharge often leads to flooding, they aren't always equivalent. Factors like the node's geometry and the presence of surface ponding options can influence the relationship.

InfoSWMM Reporting

• Junction Summary Report: You can find "Total Flood Time" (Total Time Above Rim Elevation) in InfoSWMM's Junction Summary Report. This helps identify critical nodes with the longest flooding durations.

Understanding Flooding Types

You also correctly described the different scenarios that constitute flooding in InfoSWMM:

• Flooding at Rim Elevation: Water spills out of the node when the water level reaches the rim.
• Surface Ponding: If the "Surface Ponding Option" is enabled, any water depth above the rim contributes to flooding, simulating ponding around the node.
• Flooding at Surcharge Elevation: In some cases, a node might have a defined surcharge elevation. When the water level reaches this elevation, it represents controlled flooding, such as through a designed overflow structure.

Why These Metrics Matter

• System Performance: These metrics help evaluate the overall performance of the drainage system and identify vulnerabilities to flooding.
• Design Optimization: Engineers can use this information to optimize the design of nodes, pipes, and storage structures to minimize surcharge and flooding.
• Mitigation Strategies: Understanding where and how long flooding occurs can guide the implementation of mitigation measures, such as increased capacity, flow control structures, or early warning systems.