## 3 Results
### Case 2: Results Using Rural SAR Data and Pre-Computed FRP Maps  
In this case, the waterline height threshold map of Figure 4 was again derived solely from the rural SAR waterline map (so that the dynamic Flood Foresight model flood extents were again not used), but the effective height map was derived from the model's FRP maps together with the DSM. Case 2 is the one most similar to that of [43]. The domains were divided as for Case 1. The results for all SAR scenes are shown in Table 3 and are very similar to those of Table 2, wherein only SAR data were employed to detect urban flooding. Figure 9: Correspondence between the SAR and aerial photograph flood extents in the urban area of Tewkesbury, superimposed on the LiDAR image (lighter grey – higher). An advantage of the flood return-period maps should be that they contain information on defended regions in the urban areas. On a practical note, the Flood Foresight system provides data on flood defences in a separate layer to the FRP maps, though the layers can be combined when forming the effective height map. However, there were no examples of defended regions in any of the test areas, and as a result, the effective height map for each area was very similar to its DSM, which explains the similarity of the results for Case 1 and Case 2. To illustrate the potential advantage of using FRP maps, it was necessary to simulate a defended region. Figure 9(a) shows the flood return map for the Staines area when the aerial photo validation area (including Blackett Close) was set to a return period of 1000 years, to simulate a wall being built around it. Figure 9(b) shows that the validation area was then classified as unflooded. Case 3: Results Using Rural SAR Data, Precomputed FRP Maps, and Dynamic Flood Foresight Model Flood Extents  
In this case the SAR waterline height map and the model waterline height map derived from the Flood Foresight modelled flood extent were combined, and the resulting threshold map was compared to the effective height map derived from the FRP maps (see Figure 4). The domains were again divided as for Case 1. The Flood Foresight model was driven in its monitoring mode using telemetered streamflows from EA river gauges. Flood extents were estimated over the Staines and Tewkesbury domains every 3 h during the period 21 February 2014-29 February 2014. The  
Figure 10: (**a**) Return-period map for Staines with protective ‘wall’ around the validation area including Blackett Close (c/f Figure 5(c), black areas masked out), and (**b**) urban flood classification using the SAR image of 13 February 2014, showing the unflooded validation area (white — flooded). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Image**} & \multicolumn{2}{c}{**Flood Detection Rate**} & \multirow{2}{*}{**Precision (\%)**} & **Critical Success** \\  & **(Recall) (\%)** & & **Index (CSI) (\%)** \\ \hline Wraysbury 12 February 2014 & 91 & 98 & 89 \\ \hline Wraysbury 13 February 2014 & 95 & 96 & 91 \\ \hline Wraysbury 14 February 2014 & 89 & 99 & 88 \\ \hline Blackett 13 February 2014 & 100 & 85 & 85 \\ \hline Blackett 14 February 2014 & 99 & 99 & 99 \\ \hline Tewkesbury 25 July 2007 & 88 & 77 & 70 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Urban flood detection accuracy using rural SAR WLOs and model FRP maps. flooding at Wraysbury was not modelled because insufficient gauge data were available there during the event. For Staines, Figure 11 shows an example of the Flood Foresight flood extents both early (12 February 2014 18:00) and late (21 February 2014 18:00) in the flood centred on Blackett Close in Staines. The waterline height threshold maps were combined using the measured values for the SAR WLO standard deviation of 0.3 m and model WLO standard deviation of 0.4 m. For the model, the flood return period for both 13 February 2014 and 14 February 2014 corresponded to 1-in-20 years.