Flood inundation mapping using 2-d streamflow hydraulic modeling and land subsidence data from InSAR observations in the Alto Guadalentin valley, Spain.

Floods are natural extreme events that occur after heavy rains, having a great impact on human settlements developed along flood risk areas (such as floodplains, valleys, etc.). Alto Guadalentin Valley is an orogenic tectonic depression affected by extreme flash floods. Additionaly, this area is affected by the fastest subsidence in Europe with a rate up to -10 cm/year due to groundwater withdrawal. In this study we present two flood event 2-D models comparison between different time land subsidence scenarios (1992 and 2016). The flood inundation modelling was performed in the Alto Guadalentin River and their tributaries using the Hydrologic Engineering Center River Analysis System 2D (HEC-RAS 2D) model, for the purpose of determining the flooded area extent and the depth water variations produced by the effect of land subsidence over time. To recreate both scenarios, different sets of synthetic aperture radar (SAR) images acquired by ERS (1992-2000), ENVISAT (2003-2010) and Cosmo-Skymed (2011-2016) satellites were used to calculate the magnitude and  spatial distribution of land subsidence using SAR Interferometry (InSAR) technique. The subsidence accumulated between 1992 and 2009 and between 2009 and 2016 derived from InSAR was substracted and added, respectively, to a Digital Surface Model (DSM) with 2.5 m spatial resolution from 2009 obtained using Light Detection and Ranging (LiDAR) to obtain the actual topography of the valley before (i.e. 1992) and after (i.e. 2016) the subsidence period covered by InSAR. These DEMs were used to generate the two 2D hydraulic models that ran in an unsteady mode. The results revealed significant changes in the water surface elevation with an increase of 3,073,200 m² in the areas with depth water greater than 0.8 m over 24 years. From these simulation a flood risk map was performed. The resulting flood hazard data provides useful information to understand the inundation risk taking into account land subsidence contribution in the Alto Guadalentin Valley. This information can be of paramount importance for emergency management and civil protection against future potential floodings.