Guidance

Represent buildings and infrastructure (receptor)

Updated 21 December 2023

Applies to England

This document is part of the flood modelling standards for river systems. There are 4 more documents that cover this topic. Read all the documents to make sure you have the information you need to start your modelling project.

Buildings: modelling flood risk to property

Many flood modelling projects focus on identifying the flood risk to residential, commercial and public service buildings. Hydraulic models are judged on their ability to predict flood risk to property.

One dimensional (1D) modelling for buildings

You should not use a 1D model to represent buildings where you can avoid it. It’s incorrect to represent a building in the cross-section profile by raising ‘ground’ levels because it would:

• represent a scenario where no flow could enter the building and therefore reduce the conveyance and storage area of the section
• interpolate the building profile upstream and downstream of the cross-section - it’s more appropriate to increase floodplain hydraulic roughness values to represent possible tortuous flow routes

You can include building profiles in cross-sections if they determine when water can spill from the 1D channel. For example, a building located on channel banks, or above a culvert, may prevent spilling into the floodplain until water levels reach window height. In these cases, the building acts as an informal defence and you could specify the spill height as the elevation of the windows.

Two dimensional (2D) modelling for buildings

There are 4 main options for representing buildings in 2D models. You could:

1. Apply an increased hydraulic roughness value to the urban area, accounting for the mixed associated land use.
2. Superimpose buildings from Ordnance Survey mapping and increase the hydraulic roughness and flow constriction over the building footprint to represent how the building can impede flow.
3. Increase building thresholds (typically by 0.25 to 0.30 metres if no threshold survey data exists) and apply an increased hydraulic roughness - commonly called a ‘stubby building’ approach.
4. Represent buildings as solid blocks, raised above flood levels.

This approach assumes the underlying ground model has been filtered to remove buildings and vegetation. Read source of topography for more details.

Options for detailed and broad scale modelling

The document, ‘LIT11327: computational modelling to assess flood and coastal risk’ concludes that option 3 is the best choice for detailed modelling and option 1 is appropriate for broad scale modelling.

Options for flood risk mapping

For flood risk mapping it might not be an advantage to raise buildings above the flood level. In large urban floodplains, raising many building thresholds will reduce floodplain storage volumes and could therefore artificially increase the extent of floods. It’s also possible that this increase in elevation would inaccurately block floodplain flow paths.

The Environment Agency (EA) Flood Zone Policy (2007) does not provide a clear statement on how to manage building thresholds, but it does imply that you should use the land surface rather than threshold levels. This aligns with the general agreement in the flood modelling community. This invalidates options 3 and 4 and most modelling follows option 2.

It’s important that you retain information about building thresholds from the project. You can still use these to estimate depth of flooding within buildings (to do this, subtract the threshold level from the modelled water surface grids). This is essential for economic damages calculations and flooded property counts.

Infrastructure and environmental receptors

You need to represent infrastructure and environmental receptors in your model, but it generally needs fewer modelling decisions than buildings.

You should find out if the model domain includes any critical national infrastructure as this may need more detailed consideration. There are 13 national infrastructures in the UK, these include:

• chemicals
• civil nuclear
• communications
• defence
• emergency services
• energy
• finance
• food
• government
• health
• space
• transport
• water

These features are often already part of ground models, or are represented using modelling for buildings. Where they are already represented, you must check their characteristics are represented appropriately. You should consider the condition of the infrastructure. For example, small raised, non-defence embankments are unlikely to be designed to withstand a significant depth of water behind it, and may collapse.

You may need to specify particular outputs at infrastructure if needed. For example, TUFLOW models are able to plot flow hydrographs across floodplain flow paths only if you specify plot observation lines.

Flood risk mapping

To assess the flood risk to receptors, raw model outputs usually need post-processing, particularly if the model is 1D. Scottish Environment Protection Agency (2016) gives a comprehensive overview of the process you need to follow. That process is summarised in this section.

Flood risk mapping with 1D models: what you need to know

The method you use to generate flood extents from 1D models will depend on the software platform you use. The general principles you should follow are the same. You need to:

1. Create a triangular irregular network (or similar) from cross-section and reservoir locations. Use the peak water level at each node to generate a water surface grid. If the model includes reservoir or storage units you need to represent them separately to include the maximum level in the feature, rather than at an adjacent cross-section.
2. Subtract ground elevations, usually from a digital terrain model (DTM), from the water surface grid and remove areas of negative depths (no flooding). The resolution of the DTM determines the level of detail in flood outlines.
3. Check outlines for interpolation of level results between model cross-sections. Remove isolated areas of flooding that are not connected to the river channel and make modifications if there are straight lines in outlines that are not associated with topography.
4. Consider building characteristics when producing 1D flood outlines (post-model simulation).

If building thresholds are available, you can process outlines to prevent the building from flooding if it’s raised above the flood levels, and large enough to maintain as a dry area of the outline.

Flood mapping with 2D models: what you need to know

Producing flood maps from 2D models is simpler than for 1D models because the software outputs flood extent grids. Some post-processing may be needed to meet criteria outlined by the client for outline complexity or the minimum size of dry areas.

Flood mapping with 1D2D models: what you need to know

For 1D2D models you need to use the correct method of outline production for each model domain. If the channel is represented in 1D with associated 2D floodplains, you should:

• add water level lines (WLL) within the 1D area representing levels of the 1D
• remove 1D outline areas that extend into the 2D domain and replace these with 2D results
• remove small gaps between 1D and 2D flood extents created as part of this process and carry out further post-processing to adhere to client criteria

Flood mapping with EA outputs: what you need to know

EA flood mapping projects will typically need model outputs, such as flood outlines, tabulated results and grid data to be formatted to allow inclusion in MapEdit systems. You should read the project scope and the document, ‘LIT18686: minimum technical requirements’. This will help you make sure you produce the correct range of outputs.

Request referenced documents

To request a copy of a document referenced in this guidance or a copy of the full PDF version of this guidance and the Microsoft Excel-based Fluvial Model Assessment Tool, email [email protected].

You should quote the reference number of the document you need, for example, LIT11327.