Select Link Lines
- 11 Aug 2022
- 3 Minutes to read
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Select Link Lines
- Updated on 11 Aug 2022
- 3 Minutes to read
- Print
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You can set up a link-line from nodes in your river network to 2D components using a specified link-line type.
3 types of link-line are available:
- Level - water levels from the river network are passed to the 2D components, and these are treated as a water level boundary condition.
- Flow - flows are calculated by the river network (e.g. from a spill unit) and passed to the 2D components, and these are treated as a water level boundary condition.
- Weir - water levels from the river network are passed to the 2D components, and weir equations are used to calculate a flow into the 2D domain. Weir coefficient and modular limit parameters must also be specified within the 2D domain.
The following table will help you decide which link type to choose.
Link type | How it works | Typical usage |
|---|---|---|
Level link (H link) | 1D solver calculates water level (at river node and next node downstream). 2D solver calculates discharge. Water level applied as a H-boundary condition to the 2D components (interpolated from the two values) along the link line. Discharge extracted from / applied to the river network as a lateral outflow/inflow. | Flood flows discharging laterally into the floodplain over natural / non-walled banks (i.e. those without a linear defence) |
Flow link (Q link) | 1D solver calculates discharge at a [dummy] Head-Time boundary (HTBDY) node. 2D solver calculates water level. Discharge applied as a source/sink flow condition to the 2D components. 2D water level applied to the HTBDY node. Discharge calculated using this as the river network boundary condition. | Culvert/orifice flow through an embankment Overtopping being applied to a short (relative to the number of grid cells) bank River discharging into a 2D area downstream (e.g. river modelled in 1D; estuary modelled in 2D). Discharge into 2D area controlled by logical rules. |
Weir link (W link) | 1D solver calculates water level (at river node and next node downstream). 2D solver calculates discharge. A weir equation is used to calculate the discharge from the river network and 2D component water levels either side of the link. This is applied as a discharge into the 2D domain. Discharge extracted from / applied to the river network as a lateral outflow/inflow. | Flood flows discharging laterally into the flood area over a linear defence The calculations can cope with flows onto and off the floodplain and drowned weir effects as well (i.e. where water levels on either side of the link are higher than the weir crest level). A weir link can replace the need for your 1D model to have a spill unit to pass water to your floodplain. |
Comments
- The level link has been found to be more stable than the weir link when water levels either side of the link are similar. This would typically apply to natural banks
- The weir link would be more appropriate for a defence wall (or similar) with lower ground behind, where flow from the river network to the 2D domain is better described as weir-type flow. Using a level link here would lead to a large jump between the boundary water level and potential problems in solving the shallow water equations.
- The flow link is more appropriate where the inflow to/outflow from the 2D area covers one or a small number of grid cells and is therefore more suitable for point discharges into the 2D domain. The assumptions of where it distributes the flow to or obtains a representative water level from (in the 2D domain) increases with the number of cells. It is also more suited to situations where the hydraulic behaviour is described accurately by the node immediately upstream of the Head-Time boundary (HTBDY) unit.
- Setting up the level and weir links requires no/little extra effort on the 1D side and a small amount amount of effort on the 2D side (especially if the 1D nodes are already georeferenced and if using the link-line generator). Setting up the flow link requires the hydraulic unit defining the flow exchange to be schematized in the 1D model, attached to a dummy HT boundary; the effort required in the 2D model is similar to that for a level/weir link.
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