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National flood mapping, Scotland

The Scottish Environment Protection Agency (SEPA) has used the Flood Modeller 2D FAST solver to produce, for the first time, a national surface water flood map to cover the whole of Scotland (total area: 78,000 km2). This is referred to as the national pluvial flood map for Scotland.

The creation of this mapping dataset is one component of a programme of work by SEPA to respond to the 2009 Flood Risk Management (Scotland) Act.

The national pluvial flood map complements the fluvial and coastal national maps already available to SEPA to inform the preliminary Flood Risk Assessment, which will in turn enable determination of the areas at significant flood risk, and inform the requirements for future detailed flood risk mapping.

The project was delivered in two phases. The first phase involved data collection and method development together with trial application to a pilot area. The second phase involved application of the method to the whole of Scotland and reporting on this.

During the first phase of the project, a range of calculation methods and data options were assessed for a pilot area which covered Glasgow and the Clyde and Loch Lomond catchments (total area: 3917 km2) in western and central Scotland.

The NextMap SAR (Synthetic Aperture Radar) DTM was the most accurate readily available national terrain data set and therefore was used. The data provides a representation of the ‘bare earth’ (i.e. without buildings and vegetation canopy) ground levels on a 5m grid resolution. During the DTM preparation the dataset was modified based on OS Vector Map to include an approximate representation of building thresholds by increasing ground levels by 0.3m within building footprints.

Representation of rainfall is based on the Depth-Duration-Frequency (DDF) model of the Flood Estimation Handbook (FEH). Rainfall was processed using a 100m resolution urban/rural classification of the country to generate a 100m grid of effective rainfall. Based on trials on the pilot region and literature survey, rainfall data was prepared for three rainfall scenarios: 30 year rainfall return period, a 200 year rainfall return period, and a 200 year rainfall return period with 20% increase for climate change. Under each of these scenarios, a further distinction was made for urban and rural areas taking into account representative percentage runoff, storm duration and sewer capacity for each land use.

The Flood Modeller 2D FAST solver was used to simulate pluvial flood spreading. Our FAST solver uses simplified hydraulics to spread water over a DTM. Its high computational efficiency, robustness (numerical stability), and volume conservative approach (which focuses on identifying ponded areas) makes it suitable to identify risk of flooding from pluvial sources.

Flood spreading was carried out using a set of about 4,000 2D FAST models per rainfall scenario (each model representing a hydrological catchment). The total simulation run time for the whole of Scotland is under 48hours per rainfall scenario.

The 2D FAST run for each rainfall scenario produced a GIS file (raster grid) of pluvial flood depths for each catchment at a 5m grid resolution. These results were then processed to remove small ponded areas (<200 m2), apply flood depth thresholds of 0.1m and 0.3m, and generate polygon GIS files of flooded areas for the two depth thresholds aggregated for the whole of Scotland.

The project deliverables consist of six sets of GIS polygon ‘shapefiles’ identifying the areas predicted to be susceptible to pluvial flooding for the three rainfall scenarios with depth thresholds set at 0.1m and 0.3m. In addition, GIS raster grids of simulated pluvial flood depth at a resolution of the 5m x 5m cells (consistent with the DTM data) were delivered thresholded at 0.1m and 0.3m.

Validation of the simulated pluvial flood outputs was undertaken using a comparison with historical observed pluvial flooding data and with simulated results generated by other, more detailed, methods. ‘Sensibility’ checks to identify non-physical behaviour was also carried out.

The comparison data sets identified during the project that were relevant, reliable and available for use included the following information: (a) post flood event report on Glasgow East End flooding of 30 July 2002, (b) selected interim results (March 2011) from the modelling for the Glasgow Surface Water Management Strategy which used a detailed integrated surface-drainage model and LiDAR data. The qualitative assessment undertaken confirmed that the method provided satisfactory results for identification of areas susceptible to pluvial flooding influenced by topography (as represented in the DTM) at national and regional scales.

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