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About coupling 1D-2D models

Describe the two methods of coupling 1D and 2D river models and the role of initial conditions 2D objects.

When combining 1D and 2D river models, exchange of flow between 1D and 2D systems can only take place at specific objects.

Important to understand methodology for different approaches, to accurately represent flow exchange within models.

Could be two separate existing models, or you may be building new integrated model—same mechanism, but easier to plan with new model build.



Can take place in two ways:

  • Link between 2D elements and 1D nodes made up of point connections
  • Link between 2D elements and 1D river reaches made up of lateral bank connections.

By default, flow exchanged between engines at major timesteps.

In the case of large flows, complex interactions, or need for accuracy, reduce simulation timestep to be more appropriate.

Typical simulation timestep for 1D-2D model is between 1 and 5 seconds.

To accurately represent exchange of flow, need to select approach appropriate for mechanism and scale of model.


Point/Linear Coupling:

Between single element and node object.

Typically used in locations with manholes, gullies, or outfalls.

Where structures far exceed single face length, employ linear coupling approach, so flow is exchanged across multiple elements.

A meshed 2D river model with an example of linear coupling.

Less flexible methodology—only have irregular weir linkage.

Typically used along river banks and at river ends.

Linear coupling can occur at either river reach banks or inline bank links—both built from bank line data.

Inline bank allows coupling in direction of flow.


Initial Conditions:

When linking 1D and 2D model, may need to consider starting condition of simulation in detail, and how levels are aligned in 1D and 2D domains.

Only 1D model undertakes initialization, so need to set starting conditions within 2D model via initial conditions 2D object—can be used to assign hydraulic, infiltration, and water quality conditions to mesh elements at start of 2D simulation.

Can use one initial conditions 2D object per run setup.

An Initial Conditions 2D grid window, with initial values defined for a 2D hydraulic IC zone, including Level type and Elevation (m AD).


2D Initial Conditions Zones:

Used as part of mesh generation.

Can be imported or generated manually.

Initial conditions are defined in initial conditions 2D object.

Values defined in initial conditions 2D object supersede values from initial state simulation.


Initial State File (simulation):

Aligning 1D and 2D levels at start of simulation unlikely to be simple process in large or complex 1D-2D models.

If using initial conditions 2D objects to set all conditions is too time-consuming, can use initial state file in place of initialization, which represents both 1D and 2D in steady state.

An area of the Run dialog box with options configured to use an initial state file instead of initialization.

With extreme starting conditions, creating state file can be more involved— likely requires starting initial state simulation at low or dry condition, and then increasing flows over reasonable period.