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Flood Risk Analysis of the Limpopo River Basin Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Open Access Nat. Hazards Earth Syst. Sci. Discuss., 2, 1367–1404, 2014 Natural Hazards www.nat-hazards-earth-syst-sci-discuss.net/2/1367/2014/ and Earth System doi:10.5194/nhessd-2-1367-2014 NHESSD © Author(s) 2014. CC Attribution 3.0 License. Sciences Discussions 2, 1367–1404, 2014 This discussion paper is/has been under review for the journal Natural Hazards and Earth Flood risk analysis of System Sciences (NHESS). Please refer to the corresponding final paper in NHESS if available. the Limpopo River basin Flood risk analysis of the Limpopo River M. Spaliviero et al. basin through past evolution reconstruction and geomorphological Title Page approach Abstract Introduction Conclusions References 1 2 3 M. Spaliviero , M. De Dapper , and S. Maló Tables Figures 1United Nations Human Settlements Programme, Nairobi, Kenya 2 Ghent University, Ghent, Belgium J I 3Eduardo Mondlane University, Maputo, Mozambique J I Received: 31 December 2013 – Accepted: 17 January 2014 – Published: 10 February 2014 Back Close Correspondence to: M. Spaliviero ([email protected]) Full Screen / Esc Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion 1367 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract NHESSD This research reconstructs the past evolution of the Limpopo River, a transboundary system located in south-eastern Africa, and describes its geomorphological settings 2, 1367–1404, 2014 through literature review and field work activities, with aim to analyse the risk of floods 5 in the basin. Major changes have occurred since the late Jurassic – early Cretaceous Flood risk analysis of period due to successive tectonic events. The paper demonstrates that the apparently the Limpopo River abandoned drainage conformation of the palaeo-Limpopo in the upper and middle basin stretches of the river constitutes today preferential flood-prone areas in case of ma- jor rainfall events. An important palaeo-delta is identified in the lower Limpopo, which M. Spaliviero et al. 10 imposes a particular drainage pattern to the floodplain in Mozambique and influences the floods dynamics at present. The adopted method is helpful in determining flood risk in a data-scarce area showing complex fluvial dynamics, and allows identifying Title Page unsuitable locations for human settlements. Abstract Introduction Conclusions References 1 Introduction Tables Figures 15 The Limpopo River belongs to a transboundary basin located in the south-eastern Africa with its outlet in the Indian Ocean (see Fig. 1). Spaliviero et al. (2011) noted J I that human settlements tend to concentrate closer to river streams due to the semi- arid or sub-humid conditions of the basin. These authors indicate that a demographic J I increase was registered in recent decades in the Botswana and South Africa sections Back Close 20 of the basin, in the delta area and along the main river channel in Mozambique, as well as in the upper reaches of the basin in Zimbabwe, often in areas prone to flooding. The Full Screen / Esc greater human density in these areas increases the level of vulnerability. Therefore, it becomes important to propose scientific methodologies that allow identifying unsuitable Printer-friendly Version areas for developing human settlements with a fair level of accuracy, using the limited Interactive Discussion 25 data available, and which can take into account the complex flood dynamics of the river. 1368 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Spaliviero (2003), while studying the Tagliamento River in northern Italy, concluded that flood hazards tend to concentrate in the same locations where past fluvial changes NHESSD occurred, allowing the identification of areas under major risk. Similarly, this paper aims 2, 1367–1404, 2014 to perform a flood risk analysis in the Limpopo River basin based on a good under- 5 standing of its past and geomorphological characteristics. As stated by Goudie (2005), a detailed and systematic understanding of the complex Flood risk analysis of African drainage system still requires further investigation. This statement particularly the Limpopo River applies to the southern Africa region. Specifically, we will try to answer the following basin question: do clear relationships exist between past fluvial changes and flood risk in the M. Spaliviero et al. 10 Limpopo River basin? From a methodological perspective, the description of the river’s past evolution is combined with a geomorphological approach. In particular, the paper investigates the Title Page role played by tectonics (as underlined by Blum and Tornqvist, 2000) in determining major changes of the Limpopo River. First, it reconstructs the past river’s development Abstract Introduction 15 through an in-depth literature review regarding the geological evolution of southern Africa since the late Jurassic – early Cretaceous period. Then, it collates data resulting Conclusions References from participatory land use planning exercises undertaken in the upper-middle Limpopo Tables Figures and geomorphological observations made in the lower Limpopo during a field visit, to analyse how palaeo drainage patterns are re-activated by the floods at present. For J I 20 the lower Limpopo, the fairness of this analysis is tested by referring to the actual geographical extent of the flood events that occurred in the years 2000 and 2013. J I Finally, the paper discusses the relevance of the approach proposed and derives some Back Close conclusions. Full Screen / Esc 2 The geological evolution of southern Africa since the Jurassic period and its Printer-friendly Version 25 influence on the Limpopo River’s development Interactive Discussion For a better understanding of the current fluvial geomorphological settings of the Limpopo River basin, one needs to revert to the late Jurassic – early Cretaceous period 1369 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | corresponding broadly to the Gondwana disruption (separation of the supercontinent and formation of the existing continents due to plate tectonics). The sequence of events NHESSD which have successively changed the territorial morphology of southern Africa from 2, 1367–1404, 2014 the Jurassic to present consisted in cycles of erosion (Maufe, 1935; King, 1963; Lis- 5 ter, 1987; Partridge and Maud, 1987). According to the pioneering interpretation of Du Toit (1933), which has then been substantiated by the findings of Moore (1999), Moore Flood risk analysis of and Blenkinsop (2002), Cotterill (2003) and Moore et al. (2007, 2008, 2009a, 2009b), the Limpopo River the referred erosion cycles were primarily triggered by crustal movements determin- basin ing periodically new drainage patterns. These movements were derived from complex M. Spaliviero et al. 10 plate tectonic dynamics associated with rifting processes and deep-originating mantle plumes, resulting in crustal flexuring or warping. Volcanic eruptions of the Karoo, which Moore and Blenkinsop (2002) estimated to Title Page have occurred from the Permo-Carboniferous to the lower Cretaceous period in as- sociation with the Gondwana fragmentation, have provoked a major reorganisation of Abstract Introduction 15 the drainage system in southern Africa. By that time the Karoo plume (see location in Fig. 2), which is linked to the opening of the Indian Ocean, initially imposed a westwards Conclusions References flowing drainage pattern due to its doming effect. This pattern was reverted 40 to 50 Tables Figures million of years later (approximately during the Early Cretaceous) to a dominantly east- wards flowing system, due to the opening of the Atlantic Ocean related to the Paraná J I 20 plume (see Fig. 2). While referring to White (1997), Moore and Blenkinsop (2002) ex- plain that such change of flowing direction was also magnified by the subsidence of the J I Mozambique plain during the same period as a result of sediment loading. The East Back Close still represents the dominant flowing direction of the fluvial system in southern Africa today, with Mozambique, located downstream, receiving nine international rivers in its Full Screen / Esc 25 territory. Figure 2 is derived from the drainage reconstructions of Moore (1999), who based Printer-friendly Version some of his deductions from early intuitions of Du Toit (1933), later confirmed by De Wit (1999), Moore and Larkin (2001) and Goudie (2005). It shows clearly that the palaeo- Interactive Discussion Limpopo River during the early Cretaceous period was by far the largest river of south- 1370 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ern Africa, as it was also including the southeast-flowing Cuando, Okavango and Upper Zambezi Rivers as main tributaries. The latter, which was most probably linked to the NHESSD Limpopo through the Shashe River (Moore and Larkin, 2001), was receiving waters 2, 1367–1404, 2014 from the Kafue and Luangwa Rivers originally flowing to a southwest direction (Moore 5 and Blenkinsop, 2002). Moore and Blenkinsop (2002) and Moore et al. (2007) indicate that the palaeo- Flood risk analysis of Limpopo River entered the coastal plain of Mozambique by exploiting the corridor the Limpopo River traced by the Botswana dyke swarm oriented along a east-northwest direction (see basin Fig. 2). This important geological feature, related to the disruption of Gondwana and M. Spaliviero
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