- 05 Aug 2022
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# Link elements

- Updated on 05 Aug 2022
- 6 Minutes to read

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Link-elements are used to connect 1D urban networks and 2D modelling components. They are set via the **1D urban network panel** when in **2D linking active** mode.

Technical details about link-elements are provided below.

#### Advanced Settings

In some model scenarios the default elements defining links to 2D may not be applicable. In this case you can try changing some of the default link element properties. These are located on the 'Advanced' tab of the 2D linking tool pop-up window.

Options offered here are:

Shapefile type – each element can be a single point, a line or a square (i.e. polygon). Shape, coupled with size and orientation, define how many 2D grid cells will exchange data with each linked 1D Urban node.

Dimension – defines size of shapefile element. Only applies to line and square shape options; defines length of each link line or side length of each link square. Default is 3 map units (i.e. metres or feet)

Orientation – defines orientation angle (relative to horizontal) of shapefile element. Only applies to line and square shape options.

Discharge coefficient (applies to Weir and Flow links) – coefficient of weir equation used in the weir equation for Flow links (free discharge to Urban only) and Weir links; equivalent to the k in the free weir flow representation Q=kbh

^{1.5}, where Q is the discharge, b is the effective breadth, and h is the water level above effective weir crest.Modular limit (applies to Weir links only) – ratio of respective downstream to upstream water levels above effective weir crest, above which weir flow is considered “drowned” (i.e. not free/modular), and incorporates the drowning factor in the weir equation.

Multiplier – applies a multiplier to the 1D urban data passed to the 2D domain, e.g. set to less than 1 to only exchange a proportion of urban flow to the 2D domain link. Default is 1.0.

Note: This factor was originally intended in order to be able to reverse the flow sense for 2D links to

**1D river networks**. It should therefore not normally be changed for 2D links to**1D urban networks**.

#### Editing a link-elements shapefile

As the 2D linking file is a shapefile format it can also be edited using the Flood Modeller shapefile editor, e.g. if the location of an element needs to be moved. Right-click on the shapefile entry in the Layers panel and select Start Edit from the displayed menu.

The Layer Editor tab will be activated in the main toolbar. This provides tools to move, edit or delete existing shapes. New features can be drawn and existing attributes can be edited. If editing a 2D linking shapefile with the Layer Editor, take care to ensure the required format is maintained and valid attribute values are entered. For most cases, it is recommended to use the 1D Urban – 2D linking tool to create a new file instead of manually editing an existing linking shapefile.

#### Using a weir-type link

When integrating your 1D urban network and 2D components, a weir-type link-element can be specified over the more standard flow-type link-element. In this situation, the water level (rather than the flow) at the connected urban node is used to ascertain the water entering the 2D aspects of the system. This cannot be used for connecting Outfall nodes, however it can be used for connecting Storage nodes (which are not compatible via a standard flow-type link-element).

If wanting to use a weir-type link-element, you will need to manually edit your link-element shapefile (see above) and adjust the flow-type attribute from a Q to a W. It is also necessary to ensure inflows are on for the connected nodes, as using this link-type for connecting to 1D river networks.

#### Operation modes

There are six modes of operation as follows, which are also reported to the 1D Link Flows summary (csv) file:

Free discharge into the urban solver (Mode 1). This occurs when surface water exists in the 2D domain at the location of the connection AND the ponded node is not surcharged. In this case, the 2D solver applies a weir equation to determine the discharge rate. It applies this rate as a sink term in the 2D domain and as an [additional] Lateral Inflow to the urban node.

Surcharged discharge into the urban solver (Mode 2). This occurs when surface water exists in the 2D domain at the location of the connection AND the ponded node is surcharged and the flow régime (as determined by the urban solver) is inflow into the urban node. In this case, the 2D solver calculates the decrease in ponded volume and applies it as a sink term; again, the 2D solver returns the revised ponded volume after routing for one timestep.

Outflow from the urban solver (Modes 3 and 4). This is essentially when Ponding (Mode 3) or Flooding (Mode 4) occurs within and determined by the urban solver. The 1D river or 2D solver (as applicable) intercepts the ponded volume, adding the increased ponded volume [overflow] within the timestep as a discharge to the river or 2D solver; the river or 2D solver then routes this over the surface [for one timestep] and returns the adjusted ponded volume back to the urban solver. Note that Flooding (Mode 4) is determined as flow not being able to return to the Urban solver due to the “Allow Ponding” option being switched off or the Ponded Area being set to zero.

Outfall from the urban solver (Modes 5 and 6). An Outfall node discharges water into the river or 2D (as applicable) if the calculated water level is higher at the node (Mode 5). Conversely, if the water level is higher in the river or 2D domain at the location of the connection (and there is no flapped gate), the water will re-enter the urban system (Mode 6).

The effective breadth, b, used in the weir equation (Q = c_{d}bh^{3/2}) in Free Discharge mode [mode 3 above] is determined as follows:

For a polygon, its perimeter

For a polyline, its length

For a point, the width of one cell (thus this is dependent on computational grid size)

In all cases, one may choose to adjust the “discharge” coefficient c_{d} as necessary (NB a c_{d}=1.7m^{1/2}/s [3.1ft^{1/2}/s in US units], for example, equates to that of a standard broad-crested weir)

#### Ponding on and ponding areas

A ponding area must be set for any Junction or Flow-divider nodes connected to a 2D domain. This area should be comparable to the area of the connection in the 2D, i.e. for a polygon, its area; for a polyline, its length multiplied by the area of a single cell; for a point, the area of a single grid cell.

The 2D solver calculates the ponded volume by summing up the individual depths in wet cells within the link-element and multiplying by the cell area. The urban solver will interpret this as an effective head by applying this ponded volume over the specified ponded area.

In the case that the “Allow Ponding” option is not selected, no volume is returned to the urban network. This means that modes 2 and 3 above do not apply, and in the case of mode 1, the flooded volume is routed in the river or 2D solver (as applicable), but no value is returned to the urban solver.

#### Elevation

When integrating 1D and 2D modelling components, care should be taken where appropriate that any ground elevation differences, e.g. between the 2D ground and the 1D element crest level, are minimised. On adding a connected urban network to a 2D simulation, these elevation differences can be automatically adjusted in the 2D component by using the “Auto-adjust DEM if lower than the 1D link elevation by amount…” option in the 2D run simulation parameters (“1D Urban Links” tab). Find out more about this in the Urban Links Tab section of this manual.

It should be noted that the link-element can contain more than one cell, potentially with differing ground levels, whereas the urban node only contains a single “flooding” elevation level.