## 2 Materials and Methods
### Flood Foresight System
#### 2.1.2 Flood Foresight Model  
The Flood Foresight system provides spatial data representing flood inundation spatial extents and depths with which users can measure their current or predicted impact from an event, an omission and major limitation of many current flood forecasting models [52]. Flood Foresight includes modules that provide both forecast (flood forecasting module) and real-time (flood monitoring module) flood inundation extents and depth data, driven by a range of forecast or telemetered streamflow data. The flood forecasting module provides daily forecasts of flooding up to 10 days in advance by linking forecast river flow data from European-scale and global hydrological models to an enhanced'simulation library' of precomputed FRP maps (Section 2.1.1). The flood monitoring module (the component that generated the data used in this study) provides similar output of flood inundation data but in near real-time -- every 3 h as an event unfolds -- by combining the enhanced FRP map library with observed river gauge telemetry for England, Scotland and Wales from the Environment Agency (EA), Scottish Environment Protection Agency (SEPA) and Natural Resources Wales (NRW), respectively.  
A simulation library containing precomputed FRP maps is employed for flood mapping activity (Figure 1). The simulation library approach used within Flood Foresight provides a solution to the problem of generating national-scale flood footprints from hydraulic models in near real-time or within a reasonable timescale in order for the data to be of use to flood forecasters and decision-makers. The simulation library method was one of two methods identified by the EA to provide national-scale flood inundation mapping capability [52]. Using these methods, Flood Foresight is able to generate a national-scale estimate of flood inundation across Great Britain (GB) in less than 10 min, thus giving the performance required for a national-scale strategic flood warning system.  
The Flood Foresight system uses a set of downscaled (30 m) FRP maps, based on those data described in Section 2.1, as the basis for its simulation library. For each pair of contiguous maps, an interpolation technique is applied to derive 5 intermediate maps equally spaced in the return period. The interpolation approach used computes five intermediate depth grids using a depth-slicing algorithm to achieve approximate interpolation: the volume of water on the floodplain is first computed for each modelled RP, building a volume versus RP (VRP) curve for every 1 km\({}^{2}\) cell of floodplain. The algorithm then slices the vertical depth differences between consecutive modelled RP depth grids into depth intervals, generating intermediate depth grids.  
To generate contiguous flood maps for each timestep, Flood Foresight calculates the current RP from a flow prediction (flood forecasting module) or gauge (flood monitoring module) and uses the VRP curve to estimate what volume would be expected. The interpolated depth grid with the closest volume to this is then selected for associated floodplain cells. Of course, the optimal method for developing intermediate FRP maps would be to run additional scenarios in the JFlow hydrological model. However, running this would be expensive at a national or global scale.  
Thus, the simulation library used within Flood Foresight contains 30 return periods. A similar interpolation procedure was used in [43], where it was adopted because only 3-5 FRP maps are usually made publicly available, though it was admitted that using only a limited set of maps could lead to less reliable RP estimates. In Figure 1, the river gauge telemetry from the flood monitoring module of Flood Foresight is linked to the simulation library lookup tables, from which the flood return map with the closest match for the observed streamflow is selected.