Lateral bank connections for coupling 1D-2D models

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

 

Linear Coupling:

Bank lines are used to connect 1D models to 2D zones across multiple element faces.

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

Bank lines can be created from cross-section ends, a ground model, or survey data.

For 1D-2D river model, cross sections narrower than with 1D river model—no longer need to contain floodplain.

Ideally, cut cross sections back to top of bank for good match between cross section and ground model.

Flow Over Bank Lines:

Calculated using irregular weir equation.

Linkage based on depth in elements and calculation node points within link.

Important to ensure elements adjacent to bank faces are large enough to handle exchange of water, per simulation timestep, to avoid oscillations and instability.

 

Inline Bank:

Allows coupling in direction of flow.

Bank line drawn where coupling will take place; inline bank link drawn in direction of flow, intersecting bank line.

Used at river reach ends to pour flow out onto 2D mesh, or around boundary of storage areas to allow exchange of flow.

Ensure downstream node is set as Outfall, NOT Outfall 2D.

 

Bank Lines Build Checks:

Important to undertake before running simulation.

• Transferring substantial flow between 1D and 2D engines with bank lines potentially introduces volume balance issues. As such, reduce simulation timestep to something more appropriate—typically, less than 5 seconds.
• Look for spiky or poorly aligned bank lines with large discrepancies between 1D bank levels and 2D element levels.
• Ensure alignment of bank lines appropriate to ground model used, and good agreement between bank levels and element levels.

More bank lines build checks:

 

Cross Sections:

Cross sections that extend beyond the bank lines can lead to double counting of conveyance and storage areas.

2D engine - interprets size of river reach as boundary polygon. Flow can exist within 2D mesh where it has not been voided.

1D engine - interprets size of river reach at length of cross sections.

To rectify this:

• Move banks out to cross-section ends and rebuild river polygon, OR
• Cut cross-sections down to bank lines.

 

Poorly Schematized Cross-Sections:

When bringing existing 1D river models into ICM, common problem is river cross-sections not adjusted appropriately for 2D connection.

River sections simulate horizontal 1D water surface, so a small wall or embankment in section where ground level behind it is represented will effectively be ignored.

Two options:

• Cut section back, so last section point is top height of wall, then set this level as bank level.
• If defense will likely be overtopped, cut cross section back to before wall, and then build wall or embankment into 2D domain.

 

Bank Line 2D Element Differences:

Significant differences in levels between 1D and 2D calculation points can lead to flow oscillations—especially for ground elevation in 2D zone higher than bank line.

Result is too much flow calculated flowing from 2D to 1D model, due to exaggerated head above bank line crest.

Ensure banks lines run along top of bank and match ground model.

 

High Resolution Bank Lines:

Can lead to generation of small elements along banks.

May result in large oscillations in depth, and therefore, calculated minimum 2D timestep may be very small.

Primarily an issue with classic meshing—mostly overcome using clip meshing method.

 

Unsnapped River Reach Vertices:

Generation of very small elements is also a problem at river reach section ends.

May happen between two rivers, or due to slight conflict between river boundary polygon and another object.

Simple fix is to move and snap vertices, but problem is difficult to identify if mesh does not generate.

Always make sure snap tool is active when editing or creating objects.

 

Areas of Small Triangles Between Reaches and Structures:

Areas of small triangles between reaches and structures are a necessary schematization tool due to links representing structures.

By leaving small gap, link is visible.

Unlike above image, area representing structure should be voided from mesh. To do this:

• Extend river reach polygon, OR
• Use a void/storage polygon.

Allowing flow to pass between river reaches in 2D domain can create circulating flow, leading to flow generation and volume balance issues.

 

Bank Line Summary:

Represent critical link between 1D and 2D in river modelling.

Control exchange of flow and represent flooding mechanism in model.

1D bank levels and 2D element levels must be well aligned, or calculations will not be representative.