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Assessing the impact of spatial rainfall variability on the quantification of flood hazard and exposure: a case study of the Itajaí-Açu river basin, Brazil

Bastos Moroz, Cassiano (2021) Assessing the impact of spatial rainfall variability on the quantification of flood hazard and exposure: a case study of the Itajaí-Açu river basin, Brazil.

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Abstract:The risk to flash floods and riverine floods have been accentuated by rapid urbanization, population growth, and migration, especially in low- and middle-income countries. Additionally, climate change tends to intensify this risk by increasing the magnitude and frequency of hydrometeorological hazardous events. In this context, the quantification of the three components of the risk equation is essential to support better mitigation and adaptation strategies. These components are hazard, exposure, and vulnerability. This research aimed to investigate how the spatial rainfall variability impacts the quantification of flood hazard and exposure in a large catchment area. The Itajaí-Açu river basin, in southern Brazil, was selected as a case study area. First, the study conducted a validation of multiple gauge- and satellite-derived rainfall products to select the most suitable alternative for the analysis. The modified Kling-Gupta efficiency (KGE) was adopted as a statistical parameter to compare the rainfall products against reference rain gauge measurements. For this analysis, two satellite-derived rainfall products were selected: GSMaP and IMERG. As an attempt to improve the accuracy of the satellite estimates, this study also tested two merging techniques to combine satellite and gauge rainfall: mean bias correction (MBC) and residual inverse distance weighting (RIDW). In a later stage, the selected rainfall product was adopted to generate two rainfall scenarios of spatially distributed (SD) and spatially uniform (SU) design storms. The design storms were generated from intensity-duration-frequency (IDF) curves through the alternating block method. These rainfall scenarios were then adopted as input rainfall in the OpenLISEM hydrological model, which was previously calibrated with the 2013 historical flood event, presenting a Nash-Sutcliffe efficiency (NSE) of 0.97. The simulations were performed for the return periods of 2, 5, 10, and 25 years. The results of the rainfall validation indicated a strong potential of the merging techniques to decrease the bias and improve the accuracy of satellite rainfall estimates. In this sense, RIDW proved to be the most stable technique. Regarding the satellite products, GSMaP presented the best results for the region, with higher KGE values, and it was selected as a rainfall input in the methodology. In its turn, the analysis of the design storms indicated a high spatial rainfall variability in the region, as indicated by significant differences among the IDF curves in different locations of the basin. This variability was also demonstrated through the analysis of the total flooded area and the exposed urban area, classified by flood depth, in each one of the seven sub-catchments in the basin. Overall, changes in flood depth seemed to be more sensitive to the adopted rainfall scenario when compared with changes in flood extent, with variations of more than 400% depending on the sub-catchment and return period. In its turn, the variations in flood peak discharge and total water volume were up to 23.76% and 20.74%, respectively, at the outlets of the sub-catchments. While this research provided a large-scale analysis of the impacts of spatial rainfall variability on flood hazard and exposure in the Itajaí-Açu river basin, it is necessary to conduct more site-specific studies with high-resolution hydrological models and a stronger focus on a specific urban area. The Municipality of Blumenau is recommended as a possible case study area.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
Link to this item:https://purl.utwente.nl/essays/88974
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