## 3 Results
### Case 1: Results Using Rural SAR Data Only  
Case 1 only used rural SAR WLOs in the method. This meant that the waterline height threshold map used in Figure 4 was derived solely from the SAR waterline map, and the effective height map was simply the DSM. The precomputed FRP maps and dynamic Flood Foresight model flood extents were not used. Table 2 gives the flood detection and false-alarm rates for the six SAR scenes. For Wraysbury, the 3 \(\times\) 3 km domain was divided into upper and lower windows (each 3 \(\times\) 1.5 km) along the reach, to take account of the fall-off down the reach. Figure 5 shows the correspondence between the SAR and aerial photo flood extents in the Wraysbury validation area for the three SAR scenes of 12-14 February 2014, together with an extract from each SAR image for comparison. An average flood detection rate of 92% was achieved for the Wraysbury images, together with an average precision of 98%. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Image**} & **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 & 90 & 99 & 89 \\ \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 2: Urban flood detection accuracy using rural SAR WLOs only. Figure 5: (**a**) Correspondence between the SAR image of 12 February 2014 and aerial photograph flood extents in urban area of Wraysbury, superimposed on the LiDAR image (lighter grey = higher), and (**b**) extract from SAR image of 12 February 2014; (**c,d**) as (**a,b**) for SAR image of 13 February 2014; and (**e,f**) as (**a,b**) for SAR image of 14 February 2014. For Blackett Close, the 3 \(\times\) 3 km domain was divided into upper and lower windows as for Wraysbury. Figure 6 shows the aerial photo used for validation, the SAR subimage for 13 February 2014 covering this, and a return-period map for the Staines area. Figure 7 shows the correspondence between the SAR and aerial photo flood extents in the Blackett Close validation area for the SAR scenes of 13-14 February 2014 together with an extract from each SAR image for comparison. Very high flood-detection accuracies were obtained in this validation area, with an average flood detection rate of 99.5%, though with a slightly higher average false positive rate leading to a lower average precision of 92%. However, it should be noted that the validation areas for Blackett Close and Wraysbury were limited and contained housing that was not particularly dense. For Tewkesbury, the 2.6 \(\times\) 2 km domain was divided into four windows, each of 1.3 \(\times\) 1 km, to reflect the fact that Tewkesbury lies on the confluence of the Severn and the Avon. Figure 8 shows the SAR subimage of 25 July 2007 covering the urban areas together with the associated return-period map. The extensive aerial photography used for validation on that date is shown in Figure 3 of [39], which also describes the steps used to process the validation data. Figure 9 shows the correspondence between the SAR and aerial photo flood extents in the Tewkesbury urban area. A flood detection rate of 88% was achieved, though the false positive rate was rather high, leading to a precision of 77%. Figure 6: (a) Aerial photo of flooding in Blackett Close, Staines (51.4\({}^{\circ}\) N, 0.5\({}^{\circ}\) W; about 150 \(\times\) 150 m) (@ Getty Images 2014) (after [42]), (b) CSK subimage (3 \(\times\) 3 km) of Thames flood in Staines, West London on 13 February 2014 (pixel intensities are DN backscatter values, and dark areas are water;red outline shows the arecovered by aerial photo), and (c) flood return-period map (black areas masked out).