Flood Analysis and Socio-economic Impacts on Households in Northern Bahr El Ghazal State of South Sudan Using Gis and Swat Model

Abstract

Flood is one of the facets of hydrometeorological cycle that tend to affect human activities disproportionately and the impacts tends to be more in the under-developed countries as they are relatively more vulnerable to flood events than the developed countries. One of the main elements in flood analysis is precipitation especially when estimated rainfall data from model distribution are verified by existing rain gauge stations data. Modeling of rainfall area-average distribution and assessing the slopes of the flood prone areas to analyze flood events in areas with no observation stations is crucial for disaster warning because rainfall data often obtained from rain gauge networks sometimes do not represent the area of study. In areas lacking meteorological records such as northern Bahr El Gazaal in South Sudan, the use the SWAT Model rainfall output data to investigate flood events after verifying the SWAT rainfall output consistency with the available rain gauges as used in this study is recommended. The main objective of this study was to determine the flood situation in Northern Bahr El Ghazal State of South Sudan using SWAT model and statistical data analysis techniques. The SWAT model outputs accuracies are determined by the input data set quality such as the spatial and temporal resolution of the input data. In this study, the influence of precipitation data on the hydrological modeling was considered as most significant input data set. The globally gridded high resolution precipitation datasets from NCEP/NCAR, the 30m DEM, 1 km spatial resolution of land use shapefile and 10 km spatial resolution of soil shapefiles were the main input in the SWAT model simulation for the period 1980 to 2013. The double mass curve method was used to verify the SWAT model rainfall data output with the available five rain gauge stations. The Standard Precipitation Index (SPI) was used to determine flood episodes while statistical tests of differences and associations were used to measure impacts at household level. The SPI values indicated that there occurred excessively humid periods with seven wet cycles starting from 2002 and at least four flooding events occurring in 2007 (2.2 SPI index), 2008 (1.82 SPI index), 2010 (1.62 SPI index) and 2012 (1.49 SPI index) respectively. The abundance of water for flooding were related to trends of increased rainfall events in the last decades and this was in line with what had been reported in other literature that there were indications of increase in the annual rainfall in the area. The surface characteristics of the study area were found to be one of the major factors in floods of Northern Bar El Ghazal. On the socio-economic impacts, it was found that most household had experienced flood and that the most severe flood condition in the last seven years was reported in 2007. Damages to housing units due to flooding were generally the results of poor quality of housing structures. Other flood damages included displacement, health problem, damage to crops and loss of properties. In conclusion, the study established that flooding in Northern Bahr El Gazal tended to cause more damages largely due to topography, soils and location interactions with hydro-climatic inputs and that SWAT model could be a very useful tool in flood management. The recommendation of the study was that simple activities like widening the water channel to accelerate the release of the inundated water be regularly conducted to reduce the flood impacts.