Junction Element

The Junction element is a simple network node that combines flows from multiple upstream elements. It has no configurable parameters; it purely aggregates incoming hydrographs or peak flows.

When to Use

  • Where two or more drainage paths converge (e.g., two tributaries meeting).
  • As a collection point before routing to a pond or outlet.
  • Whenever you need to combine flows from multiple upstream elements before further analysis.

How Junctions Work

Unit Hydrograph method: Junction performs hydrograph superposition, where the outflow hydrograph at each time step is the sum of all inflow hydrographs at that time step. This preserves the timing and shape of each contributing hydrograph.

Rational Method: Junction sums the peak flows from all upstream elements.

A Modified Rational basin is a detention sizing, not a conveyance — its flow is not summed at a junction. If it is linked to a pond, the pond's routed outflow is what continues downstream.

The Rational Method summation is a simplified approach. In reality, peak flows from different basins may not occur simultaneously. For more accurate analysis of combined flows, use the Unit Hydrograph method.

Input Parameters

No hydrologic parameters — only a name, a Drains To selector, and the optional Share across plans setting. It is purely a network routing node.

Results

Output Units Description
Peak Outflow cfs Combined peak flow from all upstream elements
Outflow Hydrograph cfs vs. time Superposed hydrograph (UH method only)

Tips & Best Practices

  • Junctions do not introduce any delay or attenuation. Use a Reach downstream if routing effects are needed.
  • When using the Rational Method with multiple basins at a junction, consider whether the combined peak is conservative. The Rational Method sum assumes all peaks coincide, which may overestimate the combined flow.
  • For complex confluences, consider using the Unit Hydrograph method, which handles the timing of individual hydrographs.
  • A Junction must have at least one upstream element to produce results.

When using the Rational Method with multiple basins at a junction, the combined peak assumes all peaks coincide. This may overestimate the combined flow. Use the Unit Hydrograph method for more accurate timing-based superposition.