Systems Resilience to floods: a categorisation of approaches
Authors: Alamanos Angelos , Linnane Suzanne 
Published: March 2022

&lt;p&gt;Floods occur when the capacity of the drainage system (natural or man-made) cannot safely drain-channel the volume of water produced by excess rainfall. The natural phenomena causing floods cannot be controlled, but the technical, geological, geomorphological and soil conditions of the river basin can be optimised by human intervention. The goal is to have resilient systems capable of satisfactorily responding to flood events. This work attempts to answer two research questions arising:&lt;/p&gt;&lt;p&gt;Firstly, how the different definitions of resilience are reflected in the proposed approaches from the international literature to date. &amp;#160;Following a review of 57 studies, the perspective of each one in relation to resilience was analysed (general definitions, engineering, ecological resilience, static or dynamic, adaptive and withstanding capacity, transformability, and hazard-based definitions were examined). Diverse definitions of resilience lead to different measures-indices-approaches to assess it quantitatively or qualitatively, always depending on the system considered by each study. These cases are analysed in terms of data availability, simplicity and applicability &amp;#8211;for the examined system and purpose.&lt;/p&gt;&lt;p&gt;Secondly, to categorise these approaches into general groups based on the structure of each perspective analysed above. Key criteria for this categorisation were the type/s of impact(s) studied, the combination (or not) with a hydraulic model (spatial information), the methods, and the context of the study (awareness, typology-definitions, model-quantification of resilience or policy-oriented)&lt;em&gt;.&lt;/em&gt;&lt;/p&gt;&lt;p&gt;The main categories referred to: i) theoretical-qualitative approaches, focused on conceptualisation and communication (covering social aspects, awareness, and behavioural science to provide a solid basis for any resilience assessment); ii) general and hazard-based approaches (examining flood resilience as a system&amp;#8217;s feature-capacity or evaluated by other types of response to flood events); iii) resilience as a function of the system&amp;#8217;s performance (modelling a mathematical relation, a function of resilience expressing its dynamic behaviour subject to the system&amp;#8217;s performance during disturbance); iv) other resilience metrics, e.g. indices and indicators (more customised approaches, where the analysts define the system&amp;#8217;s satisfactory and failure states according to functionality thresholds depending on the system&amp;#8217;s features and observed behaviour); v) combination of hydraulic/hydrodynamic models to resilience estimates (flood simulation results containing spatial information are used as evaluation factors for other parameters that affect resilience, e.g. different types of failures, damage or exposure). The literature is also vast regarding examples of measures (protection, mitigation, adaptation) and strategies, which, in essence, enhance the systems resilience to floods.&lt;/p&gt;&lt;p&gt;Resilience is a versatile concept, so are the examined approaches so far. Using the proper definition each time is challenging, case-specific, and a very important stage because it will define the analysis of the resilience of the selected system (quantitative or qualitative). This work aimed to facilitate future studies by providing a useful clarification of definitions and categorisation of resilience approaches. Thus, one can chose the most suitable approach depending on the studied problem&amp;#8217;s conditions and purpose.&lt;/p&gt;