## 4 Discussion  
Using the CCD technique applied to Sentinel-1 SAR datasets, we mapped Houston city and its surrounding areas that are vulnerable to flooding from possible future severe weather events. To offset the possible limitations of C-band SAR in detecting floodwaters underneath vegetation and to validate the accuracy of the adopted method, L-band data-based unsupervised classification of the UAVSAR data from Hurricane Harvey was undertaken. The quality and reliability of the SAR data-based product was further tested using the NOAA aerial imagery acquired during Hurricane Harvey. Although the validation of Sentinel-1-based flood mapping was based on datasets from a single severe weather event (i.e., Hurricane Harvey), the notion behind the validation exercise was to show the reliability of Sentinel-1-based floodwater detection in similar situations. Our analysis to identify areas that are susceptible to future floods based on past flood occurrences showed that most of the urban centers of the city of Houston, with the exception of a few localities (e.g., Bear Creek area; Figures 3, 4 and 6), are not believed to be impacted by recurrent flooding hazards that are persistent (standing water) and significant (spatially) enough to be detected by SAR remote sensors and validated through other data-gathering approaches. However, the disparity in the rate and intensity of the rainfall from the severe weather events, along with the other factors outlined above, might affect the delineation of the flood-prone areas described in this assessment. On the other hand, the major urban centers surrounding Harris county (e.g., Fort Bend and Galveston counties) have experienced recurrent floods and are highly susceptible to flood hazards (Figures 3, 4 and 6). Furthermore, our findings showed that stream networks (Figures 3, 4 and 6) have a significant contribution to constraining the impact of flooding following severe weather events. The assessment report by FEMA (Mitigation Assessment Team Report [84]) on the impacts of Hurricane Harvey supports our evaluation of the role and significance of stream networks in controlling the spatial distribution of floodwaters in Harris county [84]. Overall, our assessment revealed that nearly 44% of the flood-prone areas mapped in all of the events lie within a 1 km radius of the drainage networks of the study area, including along the two major rivers in the northeastern and southwestern areas (Figures 3, 4 and 6). With the projected population increases and the accelerated anthropogenic land use/land cover (LULC) alterations in the counties (i.e., Harris, Fort Bend, and Montgomery counties) dissected by these two rivers [85; 86], the impact from riverine flooding from future events could potentially be greater in the absence of proper mitigation efforts. Land subsidence has been credited for aggravating the impact of flooding following severe weather events in parts of the study area [14; 87]. For instance, the Galveston Bay area (Figure 1) has a history of being affected by recurrent flooding from the floodwaters of freshwater (San Jacinto River) following major severe weather events or tidal sources [14; 87; 88]. Land subsidence attributed to anthropogenic-led processes, such as high fluid extraction rates, has reportedly modified the slope and direction of flow, further exacerbating the problem [89; 90]. Using the method discussed earlier, our assessment showed that parts of the coastal Galveston Bay area (e.g., city of Baytown) have repeatedly been affected by severe weather-induced flooding (Figure 3). Several studies have estimated higher rates of subsidence for the Galveston Bay area, including Baytown city and its surrounding areas (\(>\)5 mm/year; [14; 91]).