CONCEPTS - FLOOD RISK INDEX
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Disaster case – A disaster case is a unique hydro- geomorphologic occurrence (flood or landslide), which fulfils the DISASTER database criteria and is related to a unique spatial location and a specific period of time people (Zêzere et al., 2014). The DISASTER database provides detailed information about each disaster case, such as its type (flood or landslide), date of occurrence, publishing date of the newspapers, location, and number of human damages (casualties, injured, missing, evacuated and displaced people).
Flood hazard – Flood hazard is the probability of occurrence of a flood with a given severity in a defined area and with a defined recurrence interval. In this research, flood hazard considers the spatial propensity to flooding or flood susceptibility (SUSCF) and the weather and climate events index (WCE) as an indicator of the temporal frequency of rainfall events, which may generate flash floods or slow-onset peak flows.
Flood hazard index (H) results from the sum of weighted WCE and SUSCF: H= (SUSCFx0.9)+(WCEx0.1)
Weights assigned to SUSCF and WCE were based on expert opinion and validated with flood disaster cases (for further information see Santos et al., 2020). A flood hazard value was computed per each municipality, and further normalized to the interval [0-1].
Exposure – refers to the elements at risk (people, property, systems) in hazardous zones that are subject to potential losses (UNISDR 2009). The assessment of municipal exposure was based on three variables: (i) population density (inhabitants per km2) calculated from the population census of 2011 (INE, 2011); (ii) road density (km of road per km2) calculated from data extracted from the OpenStreetMap© Geofabrik mirror; (iii) average degree of imperviousness calculated from the layer “imperviousness degree 2012”, available at the portal of the Copernicus’ Land Monitoring Service (CLMS, 2012), representing the percentage of sealed area in 2012 (See Santos et al., 2020 for further details). Exposure results per municipality were further normalized to the range [0, 1].
Social vulnerability – is a multidimensional and complex concept related to the characteristics of the individual, but also his social and economic relations, as well as the physical and social environment where the individual is inserted (Tapsell et al. 2002, 2010). The concept also involves a collective dimension, which considers the pre-existing characteristics that influence the preparation, response and recovery from disasters (Chen et al. 2013).
The SV assessment considers two dimensions – Criticality and Support Capability (Mendes et al. 2011a). The Criticality index for the municipalities of Portugal was carried out using 22 variables, grouped into seven groups, representing distinct dimensions: social support, demography, housing conditions, economy, education, housing and health. The calculation of Support Capability at the municipal level was performed using 12 variables grouped into four distinct groups: economy; building characteristics; civil protection resources and health facilities. After calculating Criticality and Support Capability, the Social Vulnerability (SV) composite index was calculated using the following equation: Social Vulnerability = Criticality × (1- Support Capability) (Mendes et al. 2011, Tavares et al., 2018).The SV results per municipality were normalized to the interval [0-1].
The vulnerability to floods was based on the social vulnerability per municipality because it is assumed that social variables are more important during the disaster recovery, namely to disaster caused by more extensive hazards like those caused by flooding.
Flood Risk Index (FRI) – Flood risk is the probability of occurrence of a flood that cause direct and indirect impacts on people, properties and infrastructure. Expressing flood risk at the municipal level as a flood risk index (FRI) implies losing a direct probabilistic relation between the physical processes of flooding in a given exposed element with a given vulnerability. The FRI is a dimensionless and comparable measure of flood risk, calculated to the municipal level and valid for the entire municipality’s territory. FRI is the product of Hazard, expressed by the weather and climatic forcer, and the flood susceptibility (H), Exposure of population, assets and roads to flood hazard (E) and Social Vulnerability (SV) expressed in the following equation:
FRI = (Hazardâ…“) x (Exposureâ…“) x (Social Vulnerabilityâ…“)
This enunciation of FRI – considering the due differences in regard to scale, risk components and input data – is based on the INFORM risk index (De Groeve et al. 2014). The flood risk index (FRI) enables to compare hazard, exposure and social vulnerability between municipalities and to rank them according to each component of flood risk.
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REFERENCES
Santos, P.P.; Pereira, S.; Zêzere, J.L.; Tavares, A.O.; Reis, E.; Garcia, R.A.C.; Oliveira, S.C. (2020). A comprehensive approach to understanding flood risk drivers at the municipal level. Journal of Environmental Management, Volume 260, 15 April 2020, 110127.https://doi.org/10.1016/j.jenvman.2020.110127.
Tavares, A.O., Barros, J.L., Mendes, J.M., Santos, P.P., Pereira, S., 2018. Decennial comparison of changes in social vulnerability: a municipal analysis in support of risk management. Int. J. Disaster Risk Reduct. 31, 679–690. https://www.sciencedirect.com/science/article/pii/S2212420918303728
Zêzere, J. L., Pereira, S., Tavares, A.O., Bateira, C., Trigo, R.M., Quaresma, I., Santos, P.P., Santos, M., Verde, J., 2014. Disaster: a GIS database on hydrogeomorphologic disasters in Portugal. Nat. Hazards 72, 503–532. https://doi.org/10.1007/s11069-013-1018-y.
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