## 3 Results and Discussion
### Is the Free Access GIS Data Sufficient for Urban 2D-1D Stormwater Modeling? As presented in the above sections, by assessing the performance of the newly developed 2D/1D TRENOE platform on an urban catchment at the city district scale, detailed land-use information is required for accurate simulations of water flows at sewer outlets. On the contrary, it is not recommended for urban stormwater managers to apply ad hoc RS techniques for measuring high-resolution altimetry data. Using only easily achieved GIS topographic data with artificially dug streets seems to be a reasonable compromise to obtain acceptable simulations. Nevertheless, the finding of this paper does not mean that the high-resolution topographic data is useless. In fact, detailed RS data can be used for developing and analyzing physically-based urban stormwater quality models. As the descriptions of spatially-variable processes can be supported by spatial measurements, researchers will be able to test new theories and gain a new understanding of the physical mechanisms with higher-resolution data, which will help improve the modeling techniques [19; 27]. On the contrary, in the aspect of operational applications, lower-resolution images have the advantages of greater spatial and temporal coverage, lower costs, and a lack of copyright restrictions, hence, they are preferred by urban stormwater practitioners [1]. Besides, in order to achieve acceptable model performance with the free access GIS data, certain adaptations could be considered for further modeling applications. In addition, based on the results of this paper, it can be noted that the precise description of manhole locations (sewer inlets, sewer drain grills, etc.) is also an important factor for urban 2D-1D \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Tested Scenarios vs.** & \multicolumn{2}{c}{**DTM + DL vs.**} & \multicolumn{2}{c}{**DTM\_dug + DL vs.**} & \multicolumn{2}{c}{**DTM\_LiDAR + CL vs.**} \\
**Reference Scenario** & \multicolumn{2}{c}{**DTM\_LiDAR + DL**} & \multicolumn{2}{c}{**DTM\_LiDAR + DL**} & \multicolumn{2}{c}{**DTM\_LiDAR + DL**} \\ \hline Performance indicator & MD (L-s\({}^{-1}\)) & RMD (\%) & MD (L-s\({}^{-1}\)) & RMD (\%) & MD (L-s\({}^{-1}\)) & RMD (\%) \\ \hline
8 October 2014 & 1.96 & 14.7 & 1.31 & 9.33 & 6.32 & 47.6 \\
12 October 2014 & 1.72 & 16.5 & 1.70 & 16.3 & 5.34 & 51.3 \\
3 November 2014 & 3.76 & 23.3 & 1.99 & 12.3 & 7.54 & 46.9 \\
15 November 2014 & 2.80 & 10.5 & 1.94 & 7.30 & 12.5 & 47.1 \\
25 November 2014 & 0.94 & 17.4 & 0.86 & 15.9 & 2.76 & 51.1 \\
12 December 2014 & 2.32 & 11.3 & 2.31 & 11.2 & 9.58 & 46.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean deviation (_MD_) and relative mean deviation (_RMD_) between the reference simulations and simulations using low-resolution topographic data and coarse land-use information, respectively. modeling. As the roads are the major water pathways for stormwater runoffs at the city district scale, the main benefit of using high-resolution road LiDAR data is to accurately simulate the surface runoffs over roads, particularly for representing flow directions and velocities. However, this high-resolution data is no longer a key factor when manholes are close to one another, which implies that the runoff time from the road surface to the nearest manhole can be correctly simulated even with lower-resolution topography. Therefore, using the free access GIS data that accompanies the detailed urban sewer network design might be an acceptable low-cost solution for accurate urban 2D-1D stormwater modeling during moderate rainfall events. Moreover, perspectives of using advanced RS technologies on urban 2D-1D modeling can focus on (i) the identification of the manhole locations, in which an automatic manhole detection algorithm [28] can be an promising next step, and (ii) the detailed characterizations of land-uses close to manholes as well as on the adjoining urban surfaces.