Whole Watershed-based Estimation of FEWS Installation Site Using UAV Photogrammetry

For small rivers and reservoirs, a flood early warning system (FEWS) is generally used to prepare for sudden flood events whose impact is augmented from homonized environment and climate change. Currently, a FEWS requires the installation of a water gauge, typically on a bridge. This is a limitation of the FEWS owing to the possibility of existence of areas that are more vulnerable than the bridge. Therefore, to analyze the flood risk over the entire target basin, a procedure to install a FEWS was proposed. First, unmanned aerial vehicle (UAV)-based photogrammetry information is analyzed to preselect flood-prone areas. Second, further investigation including the ground surveying is followed for the preselected areas. Finally, the flood amount is estimated and the UAV photogrammetry information for issuing the alarm levels of the FEWS. For this purpose, Malgol Reservoir, a reservoir with high-risk level from floods, containing residential areas was selected as the current study area. Eleven flood-prone points were identified by analyzing the UAV photogrammetry information. Then, the flooding point with the lowest flood volume among the 11 points was selected as the most vulnerable point; this section was located at 464 m from the Malgol Reservoir spillway in the downstream direction. The warning alarm for the basin-wide FEWS could be issued according to the embankment of the most vulnerable section. This ensured the safety over the entire basin including the reservoir and its downstream area. The proposed procedure can overcome the limitations of the current bridge water level-based analysis by employing UAV-photogrammetry information of the entire basin and considering the entire flood-prone cross-sections.