The map provides web-based flood outlines indicating the flood hazard that exists at any particular location (for instance, whether an area may be inundated by a fluvial flood event of one in 100, 200 or 1000 year return period in the absence of flood defences). It is designed to show the flood hazard in any particular area, and has a postcode and place search facility.
The map is the result of a three-year project, undertaken by a joint HR Wallingford and Wallingford Software team. InfoWorks RS was a logical choice for the task as SEPA’s requirements for the software were for it to be robust and defensible, consistent and minimize user intervention. It also had to be capable of being updated with new or improved data; extended to include coastal floodplains, asset data and flood risk management, and supporting regional and national scenario analysis.
SEPA required flood outlines for all rivers with a watershed area greater than 3km2. The country was divided into three model areas, covering some 170,000km of river over a 79,000km² land area. The project was split into six work packages - the first involving the use of a novel suite of custom GIS tools to transform a handful of nationally available datasets into detailed fully functional river model networks. The second providing InfoWorks RS data processing and importing to create models with the data required for normal depth calculations.
The third package involved normal depth model simulation to provide corrected normal depth flood outlines, and the fourth focused on backwater model simulation, to provide backwater model flood outlines. The fifth package aggregated the results to enable merged flood outlines and the final package, internet delivery, enabled the flood outlines to be accessible in the public domain via the SEPA website.
The project has been hailed for its innovative approach, which combined four national datasets into a logical river network - the Digital River Network, CEH Flow Grid, National Digital Terrain Model and Land Cover Map (LCM) 2000. Hydraulic calculation of water levels involved using topography and slope information from the DTM and the latest Environment Agency/Defra roughness advisor to assign roughness values to the LCM. The InfoWorks RS modeling suite was used to calculate normal depth and flood spreading.
The processed flood outlines were added to a MultiMap base map (that is, overlaid on the National Ordnance Survey (OS) maps) to show their location. MultiMap provided the specified functionality, lay-out and ’look and feel’ specified in SEPA’s web style sheet.
The project took a simple approach to assessing and representing uncertainty: uncertainty flags are based on topography, hydrology, the hydraulic model/methodology and output - that is, how the results compare to local knowledge and detailed models.
The approach also provides the potential for more detailed localised models and information on flood defences to be incorporated later. Elements such as backwater reaches or structures such as bridges, weirs and flood defences can be readily included. Tyrone Parkinson, Sales Manager River Products said: ”The indicative flood outlines provide an effective tool for the public to understand the flood risk attached to any particular area in an age of increasing urban flood risk, as recent events prove.”