SWMM 5 and Energy Losses

SWMM 5 and Energy Losses

SWMM 5 does account for energy losses in its calculations, but it has specific ways of handling them:¹

• Friction Losses: SWMM 5 primarily focuses on friction losses within conduits (pipes and channels). These losses are calculated using Manning's equation, which considers the roughness of the conduit material and the flow velocity.
• Minor Losses: SWMM 5 also includes options for modeling minor losses, which are typically associated with changes in flow direction or velocity.² These can be represented using loss coefficients (K values) applied at the entrance, exit, or midpoint of a conduit.
  

Invert Changes and Energy Losses

Now, here's the nuance regarding invert changes:

• Direct Calculation: SWMM 5 doesn't explicitly calculate energy losses solely due to the difference in invert elevations across a node. It doesn't have a specific loss coefficient for that situation.
• Indirect Consideration: However, the change in invert elevation influences the water surface elevation at the node, which in turn affects the hydraulic gradient and flow conditions. This indirectly accounts for some of the energy losses associated with the invert change.
• Minor Loss Approximation: If you want to explicitly account for additional losses due to the invert change, you could potentially approximate it by applying a minor loss coefficient to the downstream conduit. The value of this coefficient would depend on the specific geometry of the invert change.

Key Points

• SWMM 5's primary focus is on friction losses and user-defined minor losses.
• Invert changes indirectly influence energy losses by affecting water surface elevations and flow patterns.
• You can approximate additional losses due to invert changes using minor loss coefficients, but this requires careful consideration and potentially some engineering judgment.

In summary: While SWMM 5 doesn't directly calculate energy losses solely based on invert changes, its hydraulic calculations incorporate the effects of those changes on flow and water levels, indirectly accounting for some of the associated energy losses. If more precise modeling of these losses is needed, you can use minor loss coefficients to refine the simulation.

How SWMM 5 Handles Invert Jumps

How SWMM 5 Handles Invert Jumps

An important aspect of how SWMM 5 handles flow routing at nodes where there's a difference in pipe invert elevations between the incoming and outgoing pipes. This is a common scenario in real-world drainage systems.

Understanding Invert Elevations

• Invert: The invert elevation is the lowest point inside a pipe or channel. It essentially defines the "floor" of the conduit.
• Node: In SWMM, a node represents a point where pipes connect, such as a manhole or junction.

How SWMM 5 Handles Invert Jumps

When there's a difference in invert elevations across a node, SWMM 5 uses the following logic:

1. Water Surface Elevation: The primary driver of flow in SWMM is the water surface elevation at the node. This elevation is determined by the inflow from upstream pipes and any outflow to downstream pipes.
2. Flow Initiation: Even if the invert of an outgoing pipe is higher than the invert of an incoming pipe, flow will not occur in the outgoing pipe until the water surface elevation at the node rises above the outgoing pipe's invert.
3. Gravity Flow: Once the water level is high enough, flow will occur from the higher water surface elevation in the node to the lower elevation in the downstream pipe, driven by gravity.

Example

Imagine a manhole where an incoming pipe has an invert elevation of 10 meters, and the outgoing pipe has an invert elevation of 10.5 meters.

• Initially, if the water level in the manhole is below 10.5 meters, there will be no flow in the outgoing pipe, even though water is entering from the incoming pipe.
• As inflow continues, the water level in the manhole will rise.
• Once the water level exceeds 10.5 meters, flow will begin in the outgoing pipe.

Key Considerations

• Backwater Effects: This difference in invert elevations can create backwater effects, where the higher invert of the downstream pipe can cause water to back up in the upstream pipe.
• Surcharging: If the inflow exceeds the outflow capacity, the node may surcharge, meaning the water level rises above the crown of the pipes. SWMM 5 can simulate surcharge conditions.
• Energy Losses: There are typically energy losses associated with these invert changes, which SWMM 5 accounts for in its calculations.

Practical Implications

Understanding how SWMM 5 handles invert jumps is crucial for:

• Accurate Modeling: It ensures that the model realistically represents the flow behavior in the drainage system.
• Design Considerations: Engineers can use this knowledge to design systems that avoid excessive backwater effects or surcharging.
• Troubleshooting: If unexpected flow patterns are observed in the model results, checking for invert differences at nodes can be a helpful troubleshooting step.

By correctly representing invert elevations and understanding how SWMM 5 handles these situations, you can create more reliable and accurate models for stormwater management.