An investigation in vegetation changes and their effects on urban flooding by RS, case of Behshahr flood event

Urban flooding has many damaging effects on human society. According to the evidence both human population and intensity of precipitation and urban flooding are increasing in the recent decades. The flooding event in October 14th, 2012, has caused enormous financial damages and losses of lives in Behshahr City, Mazandaran, Iran. Up to 235 residential buildings, 90 vehicles, and 50% of agricultural farms have been reported that were destroyed. It has also left behind 8 people dead. It is required to investigate about the factors that exacerbated the effects of this flooding to neutralize such intensifying factors. A huge amount of precipitation has occurred in a short period of time that caused the huge flooding. A decrease in vegetation can decline infiltration and lead to greater and faster peak discharge in river. The objective of this study is to examine the influence of vegetation changes in the watershed upon the intensification of this catastrophe. We have used Landsat TM and ETM+ Images, in 1987, 2001 and 2006, to measure vegetation densities by NDVI. After preprocessing and needed corrections, the images were prepared for analyses. The difference between three raster layers, as the recent image has been subtracted from the previous, has indicated the vegetation changes during the two periods. For an easy interpretation, the results have been classified in three classes of increase in vegetation, no considerable changes and decrease in vegetation. The results have indicated that vegetation has been decreased from 1987 to 2001 for an area up to 2.87 km2. Up to 186 km2 had no considerable changes. About 19 km2 have experienced a decrease in vegetation, mainly in upstream areas. From 2001 to 2006 the classes of increase, no change and decrease in vegetation were less than 1 km2, 202 km2 and about 4.5 km2, respectively. This can be concluded that along with the increase in urbanization and in the intensity of precipitation and consequently higher probability of urban flooding, the vegetation as a factor preventing flooding is decreasing progressively. The high precipitation in short time interval has played the main role in this event and the decrease in vegetation was greatly as exacerbation.

کلیدواژه ها:

Urban Flooding ، Vegetation Changes ، NDVI ، Behshahr