2D culverts can be used in a 2D simulation to introduce unidimensional hydraulic structures directly in the 2D engine, allowing flow to be transferred between two areas of a 2D zone in an InfoWorks network.
A 2D culvert is defined as a conduit whose Conduit type is set to Culvert (2D). When used in 2D simulations, flow is calculated using the empirical equations, outlined in the Representation of Culverts in InfoWorks, that use tabulated data to replicate the head losses at culvert inlets and outlets.
A 2D culvert will only be included in a 2D simulation if it is connected to a Connect 2D type of node within the 2D zone. The type of connection for the Connect 2D node defines the way the 2D culvert exchanges flow at the upstream/downstream end vertex. See Edge linkage below for further information.
Only one 2D culvert can be connected to a Connect 2D node, and two 2D culverts should not be connected to the same node, regardless of its type. You can however have one 2D culvert connected to a 2D Connect node (whose Connection type is set to Break) which is, in turn, connected to a 2D conduit. This set up could be used, for example, to model changes of cross sections in culverts.
The properties required to model a 2D culvert are similar to those required for modelling a 1D culvert (a conduit whose Conduit type is set to Culvert), which makes it very easy to switch between using these type of culverts in your models. One of the differences is that, unlike 1D culverts where you can choose a reverse flow model, the 2D simulation engine automatically models reverse flow as outlet.
2D culverts are sub-divided in a series of computational cells, which are generated following the existing procedure for standard conduits. However, an extra check has been added so that a computational cell cannot be smaller than the specified Min Space Step.
The time step in the 2D culvert is calculated using the standard Courant-Friedrichs-Lewy condition (described in 2D Conduits) which has to be fulfilled at the computational cells. The minimum between the time step calculated in the 2D culverts and the 2D time step is used as the global time step by the 2D engine.
Edge linkage
When a 2D culvert is connected to a Connect 2D type of node, flow calculation depends on the type of connection:
- Closed - no flow through the edge. Used, for example, at the upstream edge of the 2D culvert.
- Lost - any flow through the edge is lost. The 2D engine assumes uniform flow conditions at the edge.
- 2D - indicates that there is a connection to an element in the 2D surface. The hydraulic variables in the 2D element are used as a boundary condition in the 2D culvert. For subgrid 2D elements, these hydraulic variables are calculated at the 2D triangle where the Connect 2D node is located. The flow exchange between the 2D element and the 2D culvert includes momentum.
- Break - used to connect two 2D culverts. Flow is calculated as if the two 2D conduits are joined together and that the break type of Connect 2D node does not have any storage. Momentum is preserved.
- 2D line - indicates that there is a connection to multiple elements in the 2D surface. For subgrid 2D elements, hydraulic variables are calculated at the 2D triangles intersected by the 2D line connect. The specified 2D line connect object represents the flow exchange boundary.
Boundary conditions at Connect 2D nodes
Connect 2D nodes linked to 2D culverts now support boundary inflow (qin) and level (lev) inputs where appropriate. Inflow is used for Closed and Lost connection types, while level boundaries apply to Lost connection type and override inflow if both are present. The inflow to the Closed connection type doesn't conserve momentum, but an inflow to the Lost connection type does. If a boundary is assigned to an unsupported connection type, it is ignored and a warning is issued.
Oscillatory flow behaviour
Flow results, when using 2D culvert type of conduit, are more likely to experience oscillatory behaviour than when using a conduit with its type set to 2D conduit. This is because the 2D culvert flow calculations are based on empirical equations (outlined in Representation of Culverts in InfoWorks) not shallow water equations (as used by 2D conduits), so flow reversals may occur at the calculated 2D time step. Although numerical solutions are included in the software to minimise this issue, there may be some situations where oscillations may still occur.
- Ensuring that you use a Connect 2D node with a Connection type set to 2D line (not 2D). As there will be more 2D elements connected the culvert, the 2D model should generate lower head changes upstream/downstream of the culvert, which in turn will produce smaller changes in flow.
- Reducing the value of the Time stability control parameter used for the 2D simulation. By default, this is 0.95, but it could be reduced, for example, to 0.7 or 0.5.
Results
The results of a simulation that includes 2D culverts can be displayed on the results grids for links and nodes, or on the applicable property sheet while viewing a replay of a simulation.
Meshing
2D culverts do not act as a break line for triangulation during meshing and are therefore not included in the 2D mesh.