5.11 Virtual Pipes

Virtual pipes can be used to simulate flows in drainage networks without full linking to a 1D model network. A depth flow relationship is applied at inlet locations and the flow is instantaneously transferred to the outlet location or out of the model (eg. to the ocean). No pipe details are entered, all model inputs are defined using points defining the pit locations (refer to Section 5.10.2) and references to depth flow curves:

• The inlet depth discharge capacity curves are specified in a Pit Inlet Database (see Section 5.10.3) as per a “VPI” or “Q” type pit in Table 5.26.
  
• Using virtual pipes, the flows at the inlet and outlet can be limited. If the outlet becomes limited, the inlet capacity is reduced and pits can surcharge. The order in which pits are limited / surcharge is discussed in Section 5.10.2.2).
  
• The flows and water levels for each inlet and outlet are output to .csv files and _TS GIS layers. Note that the downstream pit level is not known nor is needed for the calculations as the pit is always assumed to have no downstream influence, and the downstream levels are output as 10m below the upstream level.

Virtual Pipes were introduced to TUFLOW Classic and HPC in the 2018-03-AA release. Prior to build 2018-03-AA virtual pipes were only available in TUFLOW HPC / GPU.

From build 2023-03-AB or later it is possible to lag virtual pipe flows from the inlet to the outlet. Two additional attributes are required, and these are output in empty files written with the 2023-03-AB or later build of TUFLOW. These additional 1d_pit attributes are described in Table 5.26. These data are input on the inlets (VPI).

Two approaches are available for lagging flows, these are “Shift” and “Decay”. For the shift method, the outflow is lagged (shifted) by the lag value time, but is otherwise unchanged.

The decay method is based on exponential filtering of the inflow. A “current internal volume” is tracked:

\[\begin{equation} Q(T) = \int_{0}^{T} [\dot{Q}_{in}\,(t)-\dot{Q}_{out}\,(t)]dt \tag{5.31} \end{equation}\]

The outflow rate is computed from the internal volume and the time constant (the Lag_Value attribute):

\[\begin{equation} \dot{Q}_{out}\,(t) = \frac{Q(t)}{Lag\_Value} \tag{5.32} \end{equation}\]

Figure 5.20 below shows the lag approaches applied with a lag value of 60 seconds which is input as 0.0167 hours. All pits with the same VP Network ID (i.e. form a local network) must use the same approach and Lag_Value.

Figure 5.20: Virtual Pipe Lag Methods

Models using virtual pipes are provided in the 1D Pipe Network / 2D Floodplain Modelling Example Model Dataset on the TUFLOW Wiki.