Evaluating the Efficiency of a Nature-Based Solution on Flood Risk Reduction under climate change scenarios
Authors: Debele Sisay , Bowyer Paul , Sahani Jeetendra , Alfieri Silvia Maria , Menenti Massimo , Zieher Thomas , Kumar Prashant 
Published: March 2022

&lt;p&gt;Climate change is increasing the probability of extreme precipitation in many regions, which will lead to an increased risk of major flooding events. Recent years have seen an interest in the use of so-called nature-based solutions (NbS) to help respond and reduce the risk posed by such extreme events. This paper provides an analysis of the use of NbS to help reduce flood risk at the open-air laboratory Germany (OAL-Germany), which is part of the EU Horizon 2020 project OPERANDUM. OAL-Germany is located in the Biosphere Reserve Lower Saxony Elbe Valley. Following major flooding events which occurred in OAL-Germany in 2002 and 2013, a cooperative flood management NbS was implemented over the period 2014-2015 and has been in place since then. The NbS consisted of cutting back woody vegetation in certain locations along the riverbank which impeded overbanking during flood events, and the use of various grazing animals to try and prevent the regrowth of such woody vegetation. The objective of this study is to evaluate the efficiency of NBS against flood risk under present-day climate change scenarios and assess future flood inundations and velocities in OAL-Germany. The daily precipitation data obtained from the EURO-CORDEX project dataset for 1971&amp;#8211;2000 and 2051&amp;#8211;2080 represented historical and future simulations, respectively. The hydrologic model HEC-HMS was integrated with the hydraulic model HEC-RAS to simulate discharge, flood velocity, and water depth/inundations of past and future events. For HEC-RAS model boundary conditions, daily flow data with long-term quality-controlled data, obtained from the Global Runoff Data Centre were used. The model was simulated for two scenarios: (1) pre-NBS implementation, considering the landcover of mixed forest; and (2) post-NBS implementation using pastureland, which is the current NBS/landcover in place. The results of the simulation show that the pastureland released the floodwater from the main river system faster than the previous landcover. Overall, the floodwater velocity of pastureland increased by 21%, while flood depth showed a decrease of 2% compared with mixed forest. Therefore, if the modelled NBS had actually been in place in 2012, then it is reasonable to expect that they would have contributed to a reduction in flood risk further downstream from the modelled NBS areas, in the June 2013 flood event. This study can help to improve NBS uptake and upscaling, which is critical to improve NBS planning, implementation, and effectiveness assessment.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Keywords&lt;/strong&gt;: Nature-based solutions; HEC-RAS Model; Flood depth; Flood Velocity; Roughness coefficients; Climate Change&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Acknowledgments&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;This work has been carried out under the framework of OPERANDUM (OPEn-air laboRAtories for Nature baseD solUtions to Manage hydro-meteo risks) project, which is funded by the European Union's Horizon 2020 research and innovation programme under the Grant Agreement No: 776848.&lt;/p&gt;