Rapid flash flood impact assessments at different spatial scales
Authors: Ritter Josias , Berenguer Marc , Park Shinju , Sempere-Torres Daniel 
Published: March 2021

&lt;p&gt;In September 2019, a weather phenomenon known in Spain as &amp;#8220;DANA&amp;#8221; brought rainfall accumulations of up to 452&amp;#160;mm in 48&amp;#160;h to the south-eastern part of Spain, triggering numerous flash floods and a severe fluvial flood in the Segura river. As a consequence, seven people died, over 5000 were evacuated, and the economic losses exceeded 2.2&amp;#160;billion Euros.&lt;/p&gt;&lt;p&gt;During such devastating events, early warning systems (EWSs) are a key element for the effective mitigation of impacts. They provide emergency responders (e.g. civil protection authorities) with essential information for the coordination of the flood response.&lt;/p&gt;&lt;p&gt;In Europe, emergency responders co-operate on different spatial scales: National and regional civil protection authorities collaborate in monitoring and applying specific actions, such as evacuations, road closures, or the installation of mobile flood barriers. For this task, they require location-specific information in high spatiotemporal resolution. At a larger scale, the Emergency Response Coordination Centre of the European Union (ERCC) monitors the entire continent for upcoming emergencies and supports the regional and national authorities with information and resources. Such international actors prefer order-of-magnitude statements over large spatial domains to make informed decisions. The different requirements of end-users operating at different spatial scales need to be taken into account for the development of EWSs.&lt;/p&gt;&lt;p&gt;Traditionally, flood EWSs are designed to predict the hazard component of the flood (e.g. in terms of river discharge). In recent years, however, a number of methods were developed that automatically translate the flood hazard into the corresponding socio-economic impacts (e.g. the number of people affected). Such impact-based EWSs enhance the decision support for the emergency responders and thus facilitate an effective flood response.&lt;/p&gt;&lt;p&gt;In this work, we analyse the DANA event of 2019 from the perspective of impact-based early warning. We present, validate, and compare rapid flash flood impact assessments from the following two methods:&lt;/p&gt;&lt;p&gt;Firstly, the ReAFFIRM method (Ritter et al., 2020) generating quantitative flash flood impact estimates in high resolution to support decisions at local and regional scales. Secondly, a newly developed method (named ReAFFINE) that qualitatively estimates flash flood impacts with pan-European coverage, as decision support for end-users operating over large spatial domains.&lt;/p&gt;&lt;p&gt;Simulation results for the DANA event show that the flash flood impact assessments from the pan-European method (ReAFFINE) correspond well to reported impacts and to the results from the regional method (ReAFFIRM) while providing more context-specific information for end-users operating at the international level.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;Ritter, J., Berenguer, M., Corral, C., Park, S., Sempere-Torres, D., 2020. ReAFFIRM: Real-time Assessment of Flash Flood Impacts &amp;#8211; a Regional high-resolution Method. Environ. Int. 136, 105375. https://doi.org/10.1016/j.envint.2019.105375&lt;/p&gt;