Monitoring Flood Extent and Area Through Multisensor, Multi-temporal Remote Sensing: The Strymonas (Greece) River Flood

Satellite monitoring of flood events at high spatial and temporal resolution is considered a difficult problem, mainly due to the lack of data with sufficient acquisition frequency and timeliness. Typically, cloudy weather conditions associated with floods obstacle the propagation of e.m. waves in the optical spectral range, forbidding acquisitions by optical sensors. This problem is not present for longer wavelengths, so that radar imaging sensors are recognized as viable solutions for long-term flood event monitoring. In selected cases, however, weather conditions may remain clear for sufficient amounts of time, enabling monitoring of the evolution of flood events through long time series of satellite images, both optical and radar. In this contribution, we present a case study of long-term integrated monitoring of a flood event which affected part of the Strymonas river basin, a transboundary river with source in Bulgaria, which flows then through Greece up to the Aegean Sea. The event started at the beginning of April 2015, due to heavy rain, and the flooded areas lasted up to the beginning of September. Due to the arid climate characterizing the area in this period of the year, weather conditions were cloud-free for most of the time interval covering the event. We collected remotely sensed data, including one high-resolution, X-band, COSMO-SkyMed and several C-band, Sentinel-1 SAR, and optical Landsat 8 images of the area. The SAR backscatter and optical NDVI maps were thresholded to obtain binary flood maps for each day. Threshold values for microwave and optical data were calibrated by comparing one SAR and one optical image acquired on the same date. Results allow to draw a multi-temporal map of the flood evolution with high temporal resolution. The extension of flooded area can also be tracked in time, allowing post-flood recovery monitoring, as well as to envisage future testing of evapotranspiration/absorption models.