Delimitation of floodplains from modified normalized diference water index (mndwi) in the City of Esteio (RS, Brasil)

Abstract

One of the possibilities of analysis from satellite images is the spatial and temporal variation of flodplains. Among the indices developed for analysis of water resources highlights the Modified Normalized Difference Water Index (MNDWI), prepared in order to highlight flood areas with a higher power of discrimination of targets. This work aims to verify the spatial and temporal dynamics of floods, using Landsat images to observe the overflow of water from the Sinos River, from MNDWI index, as support for the delimitation of flooding in the city of Esteio, located in the Metropolitan Region of Porto Alegre, Rio Grande do Sul, Brazil. Therefore, we used four dates of selected images from the reading of accumulated rainfall of ten and thirty days: 08/30/1984 and 03/30/2004 (low rainfall), 11/19/1990 and 10/01/2007 (highest accumulated rainfall). The first two dates showed average accumulated rainfall of 113.6 mm and 61.5 mm, within 30 days and 19.7 mm and 11.9 mm, within 10 days preceding ten image acquisition. The dates of 11/19/1990 and 10/01/2007 presented, respectively, 201.7 mm and 200.1 mm accumulated in the period of 30 days and 28.1 and 126.6 mm accumulated in the previous 10 days of the image acquisition. Concerning the analysis of flood spots in images with low accumulation of precipitation, average MNDWI stood at -0.33, that is lower in relation to the date 10/01/2007, with a mean of -0.11. This data can be seen mainly in points where flood spot was separated from MNDWI composition and overlaps on the city’s image, allowing the study of the influence of this flood in the flow of the streams that cutting through the city of Esteio.