Sediment Linkages in a Small Catchment in the Mount Fletcher Southern Drakensberg Region, South Africa
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SEDIMENT LINKAGES IN A SMALL CATCHMENT IN THE MOUNT FLETCHER SOUTHERN DRAKENSBERG REGION, SOUTH AFRICA A thesis submitted in fulfilment of the requirements for the degree of MASTERS IN SCIENCE of RHODES UNIVERSITY by PEARL NONJABULO MZOBE September 2013 i ABSTRACT Soil erosion is a persistent problem that requires continued control efforts as agricultural land loses productivity and communities dependent on the land become increasingly vulnerable to decreased food security. The negative effects of soil erosion in Khamopele River catchment, in the Mount Fletcher southern Drakensberg region of South Africa, are manifest in extensive gullying and wetland loss. Soil erosion has resulted in siltation in a recently constructed dam and the alteration of aquatic habitats. This research was undertaken to identify the sources of eroded sediment in the small upper catchments of the Mzimvubu River catchment to inform broader catchment management strategies. The scale of erosion was quantified using field surveys of gully extent and form. Environmental magnetic tracing techniques were used to determine the sources of eroded sediment in Khamopele River and upper Tina River catchments. The radionuclide 137Cs was used to determine soil loss over a 55 year period in Khamopele River catchment. The Landscape Connectivity framework was used to describe the sediment source, pathway and sink interactions at sample area level. Results indicated that historical and contemporary land management practices such as uncontrolled grazing, grassland burning and furrows promoted soil erosion in the catchment. Soil erosion was most pronounced in the Taung sample area where there was extensive gullying, tunnelling and subsurface erosion. Environmental magnetic tracing results indicated that there were clear ii differences in source areas. Despite its prevalence in the area, gully erosion was not shown to be a major source of sediment to downstream sinks. Topsoil and hillslope derived sediment were shown to be mobile in the catchment, suggesting that sheet erosion processes were dominant in the catchment. Radionuclide tracing studies showed that at least 20 cm of soil had been eroded from the Khamopele River catchment surface since 1956. This research has shown that it is possible to distinguish source areas of erosion in the catchment by matching catchment mineral magnetic signatures to those in sink areas. This means that rehabilitation projects can use resources efficiently as the areas needing the most attention can be identified. 341 words Keywords: degradation, soil erosion, environmental tracing, magnetic susceptibility, sediment connectivity iii DECLARATION I have read an understood the Rhodes University plagiarism policy. This study presents original work by the author and has not plagiarized the works of others. Where use has been made of the work of other authors it is acknowledged in the text accordingly. «««««««««««««««««««« iv ACKNOWLEDGEMENTS I would like to thank the Gamtoos Irrigation Board (GIB) for providing me with the financial resources to conduct this research. Thank you to the Eastern Cape Restoration Group for your work in the province which led to the selection of the study site. I am grateful to the NRF for the provision of funding to attend the 2011 International Union of Geodesy and Geophysics (IUGG) General Assembly in Melbourne Australia. To my supervisor, Professor Kate Rowntree, I thank you for your tireless encouragement and support even when it seemed as though the curtain had closed. I would like to thank Professor Foster for his course on environmental tracing studies. Together with Professor Rowntree, he has provided the springboard for such studies South Africa. My sincerest gratitude to the following people: Professor Ian Rutherford, Dr -RODQWDĝZLĊFKRZLF]'U9LQFHQW1DNLQ Dr Ramroop, Ms Glyn Armstrong, Mr John Landman, Mr Gawie Naude, Mr Charles Coetser, Ms Rebecca Joubert and Mr Benjamin van der Waal. My time in Makhoaseng village has been one of the most treasured times in my OLIH , WKDQN +HDGODG\ 0DP¶ 6HWORERNR -RQDV 1HR DQG 7huso for their hospitality. I would like to thank everybody who housed and fed me during the research period. To my friends and colleagues: Sthembiso Mbatha, Breanne Robb, Anna Schoch, Brigitte Melly, Kerry Bobbins, Rumbidzai Ziyambi, Pumelela Mabeka, Smriti Ravjee, Zamanguni Gumede and many more, thank you. Finally, a big thank you to my parents and family for your understanding and positive words during this time. v TABLE OF CONTENTS ABSTRACT ......................................................................................................................... ii DECLARATION .................................................................................................................. iv ACKNOWLEDGEMENTS ................................................................................................... v LIST OF FIGURES .............................................................................................................. x LIST OF TABLES ............................................................................................................. xiii CHAPTER 1 .........................................................................................................................2 1.1. BACKGROUND ....................................................................................................3 1.2. MOTIVATION ........................................................................................................4 1.3. RESEARCH AIM...................................................................................................6 1.4. RESEARCH OBJECTIVES ..................................................................................7 1.7. RESEARCH APPROACH ....................................................................................7 1.8. CHAPTER OUTLINE ............................................................................................8 1.9. CONCLUSION ......................................................................................................9 CHAPTER 2 ....................................................................................................................... 10 2.1. INTRODUCTION ..................................................................................................... 11 2.2. PHYSICAL CHARACTERISTICS .......................................................................... 13 2.2.1. GEOMORPHOLOGY ....................................................................................... 13 2.2.2. RAINFALL AND CLIMATE ............................................................................. 14 2.2.3. GEOLOGY ....................................................................................................... 19 2.2.4. SOILS ............................................................................................................... 21 2.2.5. VEGETATION .................................................................................................. 23 2.3. SOCIO-ECONOMIC BACKGROUND .................................................................... 24 2.3.1. CONTEXT ........................................................................................................ 24 2.3.2. BETTERMENT PLANNING ............................................................................. 26 2.3.3. LOCATION ....................................................................................................... 27 2.3.4. ECONOMIC ACTIVITIES ................................................................................. 28 2.4. SURVEY FINDINGS: LAND USER PERSESPECTIVES ON SOIL DEGRADATION IN KHAMOPELE RIVER CATCHMENT ........................................... 30 2.4.1. INTRODUCTION ......................................................................................... 30 vi 2.4.2. RESULTS .................................................................................................... 31 2.5. CONCLUSION .................................................................................................... 36 CHAPTER 3 ....................................................................................................................... 37 3.1. INTRODUCTION ..................................................................................................... 38 3. 2. LITERATURE REVIEW ......................................................................................... 39 3.2.1. INTRODUCTION TO GULLIES ....................................................................... 39 3.2.2. SURFACE FLOW PROCESSES IN GULLY FORMATION ........................... 40 3.2.3. SUB-SURFACE FLOW PROCESSES IN GULLY EROSION ........................ 40 3.2.4. GULLY CLASSIFICATION .............................................................................. 43 3.2.5. GULLIES IN THE LANDSCAPE ..................................................................... 49 3.2.6. GULLIES AS SOURCES, PATHWAYS AND SINKS ..................................... 52 3.2.7. GULLY EROSION IN THE KHAMOPELE RIVER CATCHMENT .................. 53 3.3. METHODS .............................................................................................................. 55 3.3.1. DESKTOP MAPPING ...................................................................................... 55 3.3.2. FIELD MEASUREMENTS ............................................................................... 57 3.3.3. LABORATORY METHODS ............................................................................