Morphological analysis of the Sudd region using land survey and remote sensing data

Abstract

An analysis of the vast wetland of the Sudd swamps in southern Sudan was designed to assess and describe interdependencies between morphology and hydrology. Findings based on field survey and remote sensing data include an assessment of the effect of ground slopes and morphological features on spill and flooding of the seasonally flooded grasslands. Through bathymetric surveys and analysis of Landsat images, depth profiles and cross-sectional depth and flow distributions were established. Data from remote sensing, field survey and historical sources were correlated and correction factors established; the SRTM was found to be partly unsuitable for further assessments. Further analysis of remote sensing data was used to investigate the Sudd's inland delta as well as the question of spill into the Bahr el Ghazal basin. Figure 1. Overview of project area in southern Sudan with detailed core area of interest.Figure 3. Water level (WL) time series of Burakok (within the Swamp) and at the tails of Khor Adwar and Khor Baidit, showing surface level fluctuations in correlation to the wet and dry season. (Note that the station at Khor Adwar is not cut off but located just to the west of the road dam.) Morphological analysis of the Sudd region 1715 Figure 5. SRTM DEM of the area between Bor and Shambe, showing an inland delta formation. The transition from incised trough in the south to elevated channels in the north is clearly visible. Morphological analysis from SRTM Digital Elevation Model (DEM)Evaluating the DEM in Figures 5 and 6 on a coarse scale, varying slope directions can be identified along the course of the Bahr el Jebel. From Bor northwards, where the incised and restricting trough disappears in the east, the ground slopes in a north-northwesterly direction, like the flow direction of the river. After reaching Shambe, the ground slope changes into a westerly direction towards the Bahr el Ghazal basin, while the river, bound by alluvial banks, is heading oblique to this slope towards the north until it reaches Lake No. Slope gradients can be estimated as 0·1 m/ km, with very limited local undulations, the only pronounced features being the incised river trough south of Bor with banks disappearing further north. The system changes to another formation visible from the DEM, the pronounced inland delta between Baidit/Padak and Shambe, as shown in Figure 5. Here the channel flows at higher elevation than the surrounding plains, with level differences between the river and the surrounding plains about 2-3 m. Further north, the inland delta becomes less defined, but can still be observed running obliquely to the general westerly slope of the area towards the north. A series of cross-sections derived from the DEM at locations perpendicular to the river is shown in Figure 7, illustrating the transition from incised trough to elevated inland delta.Cross-sections from JIT (1954) have been digitized and compared with SRTM DEM extracted cross-sections, as shown in Figure 8. While qualitatively the cross-sections are very sim...

Mapeamento de áreas aluvionares no semiárido brasileiro por meio de dados colaterais e imagens orbitais

“…Chagas et al (2009) O comportamento visto nas Figuras 9 e 10 também foi observado em estudos realizados por Petersen et al (2008) por meio de perfil de seção perpendicular à linha de drenagem, os quais observaram discrepâncias de 6-8 m em relação aos dados de referência de nível. Ludwig & Schneider (2006) verificaram, em análise de dados SRTM para aplicação em modelagem hidrológica, por meio de visualização de perfil que as altitudes apresentavam maior ruído que os dados de validação, o que pode estar associado à vegetação e a outros objetos não topográficos.…”

Flooding and drying mechanisms of the seasonal Sudd flood plains along the Bahr el Jebel in southern Sudan

“…Next to the area morphology with its very gentle gradients (Petersen et al, 2008) and so limited overland flows, soil characteristics and vegetation are, in combination with rainfall and evapotranspiration, the most important factors controlling the flooding and drying processes of the seasonal flooded grasslands of the Sudd area. For a better understanding of the progress and retreat of the flood waters, the individual factors of influence are described in detail followed by a detailed assessment of their interrelations and effects on the water budget of the flood plains.…”

“…The area has a flat topography, as described by Petersen et al (2008), and, in its hydrological regime, is dominated by the annual cycle of flooding and drying caused by river flooding and heavy rainfall. The soils of the Sudd region are typical African flood-plain black cotton soils with high clay content, classified as vertisols (IUSS Working Group WRB, 2006).…”

“…While the influence of the region's morphology on large-scale flooding was discussed by Petersen et al (2008), local flooding mechanisms of the flood plains are, next to the water levels in the permanent swamp, influenced by different parameters. These include the soil, the vegetation and the meteorological conditions influencing evapotranspiration.…”

“…Sutcliffe & Parks (1999) considered evapotranspiration with a value of 2150 mm/year. The influence of the area morphology on flooding was described by Petersen et al (2008) evaluating flooding conditions in local depressions.…”