Adding building polygons

When creating a 1D-2D runoff model of an urban area, one of the main challenges is how best to represent buildings, both from a hydrological and a hydraulic perspective.

Buildings are polygon objects in InfoWorks networks that are used to represent rain falling onto a roof of a building and entering the drainage system and/or remaining on the surface. They can also be used to define roughness, levels and porosity within a 2D mesh. This allows for a single object in place of many.

Buildings can be imported via the Open Data Import Centre or digitized directly on the GeoPlan.

In the GeoPlan, an example model, with polygons representing buildings displayed as yellow rectangles on a 2D mesh.

Building parameters are edited in the Building tab of the Polygons grid, or in the building property sheet.

A polygons grid showing example building parameters, with the active Building tab called out.

Each building will drain to one of the following:

A single inflow node
A single lateral link
A single 2D point source (allows the hydrology aspects of a building to be applied directly to a 2D mesh)

SUDS controls can also be assigned to buildings, allowing for modeling the impact of SUDS structures, such as green roofs, or storm runoff entering the drainage system.

A building selected in the GeoPlan, and in the Properties window, the Drains to and SUDS control settings highlighted in red.

A Capacity limit can be set, where any flow above this limit is capped from entering the 1D network; and based on the Exceedance flow type chosen, the excess is either lost from the system or passed to the 2D mesh. This allows the hydrological representation of the buildings to be improved, and better represents extreme rainfall conditions where the drainage system would not have the inlet capacity to cope with the flows.

In the Properties window, the Drain limiting settings highlighted in red, and on the GeoPlan, a running rainfall simulation, with drain limiting settings on a building label highlighted in red.

Buildings can be included as part of the mesh generation process when undertaking 2D modelling. Use the Single element box to choose whether the building is to be represented as a single element in the mesh. Enabling this option can simplify the process of calculating the flood depth within a building and reduce the total number of elements. It also tends to look more aesthetically pleasing for results presentation.

In the Building Properties window, Single element enabled and highlighted in red.

Roughness can also be modelled for a building. A specific roughness value can be set, or a roughness definition assigned to the building.

Buildings can be used to define specific areas where ground levels are to be modified in the 2D mesh. The same options are available for a building as for a mesh level zone.

Typically, a model will take one of two main approaches. First, the elements of the building are simply raised to the height of the roof. This approach looks more visually impressive in 3D view alongside aerial photography, but prohibits any flow passing through the building.

In the Properties window, the Mesh level settings highlighted in red, and on the GeoPlan, a running rainfall simulation.

Alternatively, the elements are raised to the flood threshold of the building, and a low porosity is applied up to the roof height. This approach gives a more accurate hydraulic representation of the building and allows for the production of more accurate flood maps and damage calculations.

If changes are made to the mesh level within a building after meshing of the 2D zone has been carried out, it will be necessary to recalculate mesh elevations. This can be done by resampling elevations for selected 2D zones.

Adding building polygons

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