The Fixed Head Node represents a point at which pressure (head) does not change with respect to the demands in the system, although it might change over time. The Transfer Node or demand node is a point at which a fixed amount of water is added or taken out of the system.
A third type of boundary condition is provided where the correlation between flow and pressure is known. Examples of these boundary conditions are wells, where the level in a well decreases as pumping increases, and pressure-correlated demand nodes.
However, there is another type of boundary condition that is not directly accounted for in any of these three types. This kind of boundary condition, called a spring, can behave intermittently as either a Fixed Head Node or Transfer Node depending on the balance between flows in the fixed head and system demands. In a spring, there is a fixed flow coming from the ground which is typically collected in a small reservoir chamber that feeds into the network.
The user normally knows the flows at the spring and the elevation of the overflow from the chamber. Depending on the balance between the spring flows and system demands, spring objects can behave as Fixed Heads or Transfer Nodes.
If supply exceeds demand, the excess water will spill through the overflow and the spring will act as a Fixed Head Node, with the elevation set to the overflow level. When demand is greater than the spring flows can meet, the spring will supply all the water available straight into the system, acting as a Transfer Node. This paper discusses how to accurately model the change from Fixed Head to Transfer Node.
Depending on the situation, InfoWorks WS offers four alternative ways to model spring objects.
1. Fixed Head Node
If the supply at the spring is consistently higher than the demand in the system, the spring should be modeled as a Fixed Head Node with the elevation set at the overflow level. This approach ensures proper distribution of the pressures in the system and provides all the water required.
However, if demand in the system exceeds the supply at the spring, this approach would produce more water than the spring could supply.
2. Transfer Node
If the supply at the spring is consistently smaller than the demand in the system, the spring should be modeled as a Transfer Node with flows set to the spring flow, ignoring the head.
This approach will limit the amount of the water that is supplied by the spring and set the appropriate pressures to push this water into the system. However, if the amount of water supplied exceeds the demand in the system, this approach will ignore the overflow and push extra water and pressure into the system.
3. Overflowing Transfer Node
If the user wishes to model local storage - typically when the local reservoir volume is not negligible - and the overflow at the spring, such cases can be modeled using a Transfer Node connected to a Reservoir. The Transfer Node would define the flow at the spring, the Reservoir would define the local storage and the overflow elevation, and the connecting pipe would define the local losses at the spring.
This approach has the advantage of accounting for the local storage in the spring and the amount of water lost through the overflow. However, since local storage at springs is often small, there is a risk that the reservoir will empty. Also small reservoirs can create instability in models and can be hard to model.
4. Intermittent Spring
Finally, a typical approach to modeling springs would be as an intermittent spring that acts as a Fixed Head Node when there is sufficient supply and as a Transfer Node when the supply is limited. To model this scenario, the user should create a Fixed Head Node with the elevation at the overflow level. The Fixed Head should be connected to the system through a Flow Regulating Valve (FRV), which would limit the flows to the level that can be supplied by the spring.
The setting of the FRV should be equal to the spring flows. This setup will behave as a Fixed Head when the supply is sufficient to meet demand. When the demand is smaller then the spring flows, the FRV will be fully open and the system will be directly connected to the Fixed Head Node, ignoring the open valve, and will take as much flow as is required. Flows will be equal to the system demand and the pressures will be determined by the elevation of the Fixed Head Node.