Country Specific – Great Britain

Our elevation model of Great Britain is based primarily on open LIDAR composite DTM models made available by the Environment Agency, Natural Resources Wales, and the Scottish Environment Protection Agency. The Environment Agency National LIDAR Programme started in November 2016 and aims to have coverage of all of England by 2021. We strive to keep our model up to date with the latest sources.

In order to use an elevation model for hydrological analysis such as watershed and flow accumulation computations, two primary conditions need to be met:

• The upstream area of any river should be covered by the elevation model.
• Structures on top of the terrain should only be present in case they actually block water from flowing under or through them.

Below, we discuss how we process the model to fulfill these conditions as well as possible.

Extensions

In order to cover all of England, Wales and Scotland, we have extended the open LIDAR data from the UK government in the following areas:

• For the parts of England not yet covered by the National LIDAR Programme, we have used the older Environment Agency 1m DTM, and for parts not covered by the 1m DTM we have used the 2m DTM.
• For the parts of Wales not covered by the Wales 1m DTM, we have used the Wales 2m DTM.
• For the parts of Scotland not covered by the LiDAR for Scotland Phase 4 DTM, we have used the Phase 3, Phase 2 or Phase 1 DTM, as well as Environment Agency's "LIDAR Composite DTM for Scotland".
• For the parts not covered by open LIDAR datasets mentioned above, we use the 30-meter EU-DEM dataset, which in turn is based on SRTM and ASTER GDEM data.

A full overview of which data source is used for which part of the model is available clicking the gear icon next to an elevation layer, selecting the "Source" tab, and "Show source information". Use the point query tool to highlight a coverage area and see the name of the source.

Bridges and underpasses

Major bridges have generally been removed from the models delivered by the Environment Agency, making most bigger rivers able to flow freely to the sea. However, for many smaller bridges and underpasses, additional "hydrological corrections" may be necessary. These can e.g. be made through workspaces in SCALGO Live.

Apart from vegetation and major bridges, also buildings have been removed from the terrain model during construction. When computing water flow paths, more realistic results are generally obtained when the elevation model does include buildings so water can be simulated to flow around them. In SCALGO Live, we accomplish this by adding buildings back into the model using a dataset of building footprints, where we raise all grid cells covered by a building to a height 10 meters above the highest terrain point within the building footprint. This model is called "Terrain/Buildings" and is the basis for all nationwide hydrological computations.

The building footprints are taken from the Ordnance Survey OpenMapLocal Building dataset.

Rivers

Major rivers have been lowered in the elevation model to ensure that water can flow freely to the coast. The river lines are taken from the Ordnance Survey Open Rivers dataset.

Coastline

We have removed parts of the LIDAR models that are off the coast, as the data is generally unreliable. The coastline is taken from the Ordnance Survey Open ZoomStack vector dataset, layer named "Land".

Land Cover

The SCALGO land cover map is available in the UK. For more information, please consult the land cover section of this documentation.

Base map and aerial photography

The default map view when you go to SCALGO Live shows the Ordnance Survey Open ZoomStack map, rendered in the "Outdoor" style. Other map styles are available through the gear menu on the "Base Map" layer. The street names and place labels are sourced from OpenStreetMap. You also have the option of viewing Aerial photography provided by Mapbox.

Rain events

Design rain events in the United Kingdom are based on the PXR-2 dataset (Parametrized eXtreme Rain), a global gridded product developed by Courty et al. (2019). PXR-2 provides Generalized Extreme Value distribution parameters for deriving intensity–duration–frequency (IDF) curves at a spatial resolution of approximately 31 km. For design rainfall estimation in the UK, we extract rainfall depths for a 4-hour event duration and return periods from 2 to 500 years.

We generate design hyetographs using a storm profile developed specifically for the UK by CEH (Center for Ecology and Hydrology) as part of the ReFH2 (Revitalised Flood Hydrograph) model framework. We use the summer storm profile, which represents convective rainfall patterns observed in the UK during summer months. The profile defines how the total rainfall depth is distributed over the event, with the peak intensity occurring at the midpoint. 

Please note that the PXR dataset generally underestimates rainfall intensities compared with the intensities prescribed by ReFH2, especially for shorter durations (<24 hr). 

Soil type

The soil type map used in Scalgo Live UK is based on the Soil Parent Material Model from BGS. Specifically, we use the soil texture layer, and map the classes found in that layer to our USDA-based topsoil typology according to the table below. For more information about the Scalgo soil type classes and how we enhance the topsoil map, see the soil type documentation.

Urban and sewered areas

To determine whether an area is to be considered sewered (affecting the fate of all water on artificial surfaces in DynamicFlood) we use the map Built Up Areas from Ordnanace Survey (can be found in the Library). We assume that all areas that fall within a polygon in this map have a sewer system, and that all areas that fall outside these polygons do not have any sewer system.

We set the maximum capacity of the sewer system in DynamicFlood in Great Britain to 20 mm/hr. This value was chosen to reflect common historical dimensioning practice in Great Britain.

To determine whether an area is to be considered urban (affecting some soil types in the Topsoil map), we use the same map as for sewered areas.