Extreme weather events in the Sneeuberg, Karoo, South Africa: a case study of the floods of 9 and 12 February 2011

Two destructive flood events occurred in rapid succession in the semi-arid Sneeuberg Mountains of the Karoo, South Africa in February 2011. The temporal and spatial characteristics of these two extreme events are examined in this paper through analysis of data from an unusually dense, and reliable, network of farm rain gauges. These analyses add to our understanding derived from existing rain gauge information. Comparisons are then made with patterns from a range of modeled products derived from remote sensed information: the Modern-Era Retrospective Analysis for Research and Applications (MERRA), the Tropical Rainfall Measuring Mission (TRMM) and the Global Land Data Assimilation System (GLDAS). We found that the first flood event was widespread and precipitation was related strongly to altitude. The second was highly localised, with no relationship to altitude. Both had very sharply peaked rainfall intensities. These findings are of significance to the studies of flooding and landscape change in the area as such events have become more pronounced over the past 50 yr and it is likely that this trend will accelerate. The modeled patterns are derived largely from remote sensing and we found that they are reliable for drawing out monthly and annual variations but they make noticeable underestimates. They are poor estimates, however, both for the spatial distribution of precipitation, and the short term trends as they struggle to estimate the impact of topography and other local forcing factors. This finding corroborates information derived from other analyses at broader spatial scales using more widely spread, established rain gauge stations. Ten percent of southern Africa has been classified as mountainous and these areas provide much of our water resources so our findings are significant to water managers throughout this and similar mountainous regions

Vegetation controls on channel stability in the Bell River, Eastern Cape, South Africa

Channel instability has occurred in the Bell River in the form of meander cutoffs, a number of which have occurred since 1952. Increased sediment loading from widespread gully erosion in the catchment has been proposed as the trigger for this instability. Willow species of the Salix family, in particular S. caprea, have been planted along the banks in an effort to prevent further channel shifting. This study reports the results of an investigation into the effect of vegetation on channel form and stability over a 17 km stretch of channel. Results indicate that riparian vegetation has significant effects on channel form which have implications for channel stability. Riparian vegetation increases bank stability and reduces channel cross-sectional area, thereby inducing stability at flows less than bankfull. Evidence indicates that narrow stable stretches are associated with relatively high levels of riparian vegetation. Wider, unstable channels are associated with relatively less riparian vegetation. The effectiveness of riparian vegetation relative to bank sediments was investigated. A dense growth of willows was found to have an equivalent effect to banks with a silt-clay ratio of about 70 per cent. The channel narrowing induced by vegetation may contribute to channel shifting at high flows. The reduced channel capacity is thought to result in more frequent overbank flooding which may ultimately lead to channel avulsion. Thus where increased sediment loading is pushing the channel towards instability, vegetation may be effective in imparting local stability, but it is unable to prevent long-term channel shifts, and may rather help to push the system towards more frequent avulsions

Topographic thresholds in gully development on the hillslopes of communal areas in Ngqushwa Local Municipality, Eastern Cape, South Africa

The relationships between the spatial distribution of gully erosion and topographic thresholds in the form of slope angle, position and configuration, as well as land use change in the form of abandoned lands were examined in several affected catchments of the Eastern Cape Province, South Africa. Land use and permanent gullies were mapped, digitized from orthophoto maps in Arc/info 3.5.2 GIS and converted to shapefiles using ArcView 3.2 GIS. Relationships between the mapped phenomena and topographic variables were sought using a Digital Elevation Model (DEM) in Idrisi Kilimanjaro GIS. A comparison between areas with a high potential for gullying and actual gully erosion was made using the Stream Power Index (SPI) as a surrogate for critical flow shear stress. Field surveys were also conducted to assess the present condition of the gullied sites as well as to validate DEM derivations. Seventy five percent of the gullied area was noted to lie on abandoned lands. A predominance of gullying in concave bottom lands was also identified. The SPI values highlighted a distinct preferential topographic zone for gully location. A conceptual model depicting the interaction between land use and topographic parameters to induce gully erosion was developed. This should assist local authorities to develop a policy regarding management of abandoned lands

Futures studies for the southern African region : ‘from Africa’ not ‘on Africa’

Futures studies is well established in the Nordic region and its history can be readily charted, but in Africa it barely exists in an institutional form and its evolution and impact is little known or understood. The first two sections of our paper briefly examine the history of futures studies, spending most attention on the African experience. We go on to show that the Higher Education landscape in the Southern African Development Community (SADC) region is very different to that in the Nordic region. Recent futures reports present forecasts and scenarios that show a differentiated Higher Education landscape in the SADC; there are few Higher Education Institutions (HEIs) and even the most optimistic forecasts show that the region as a whole will not meet the international enrollment norm of 30% by 2050. The last part of the paper examines our experience of collaboration with Finland and its well developed linkages between state and Universities. One outcome of three years of collaboration from 2007 to 2009 between two SANORD members, the Finland Futures Research Centre (now a part of the University of Turku) and Rhodes University, was a proposal to develop a multi-disciplinary, inter-institutional futures studies program intended to help Africa find its own voice in futures studies. The final part of our presentation reflects on the unsuccessful experiences that we have had to date in finding funding. We conclude by asking whether our experience can be seen as highlighting some of the challenges SANORD may be positioned to overcome if the SADC region’s HEIs are to achieve the Knowledge Village scenario and begin to match their Nordic counterparts

Sediment yield changes in the semi-arid Karoo: a palaeoenvironmental reconstruction of sediments accumulating in Cranemere Reservoir, Eastern Cape, South Africa

The history of sediment yield from a 57 km 2 catchment in the semi-arid Karoo of South Africa was reconstructed from the sediment stored in a reservoir at Cranemere dating back to 1843. The volume of sediment accumulating in the reservoir between different dates was estimated firstly by developing a core chronology based on Cs-137 and Pb-210 using gamma spectrometry and secondly by estimating volumetric changes from the depth of 17 cores located across the dry reservoir surface. The variation in sediment yield was interpreted in relation to historical changes in livestock management, rainfall patterns and catchment connectivity. Daily rainfall data were available from 1891. The results indicated low sediment yields until the 1950s, after which they increased rapidly up to the 1970s before declining gradually to the present day. There is historical evidence that accelerated erosion was initiated before the turn of the 19th Century due to high stock numbers and poor stock management until soil conservation practices were put in place during the 1950s. However, there is little evidence of this increase in erosion in the sedimentary record indicating that there were considerable lags between the onset of erosion and the sediment reaching the reservoir. The rapid increase in sedimentation after the 1950s is explained as a result of increased connectivity between the local sediment sinks and the reservoir following the construction of new culverts under a major road less than a kilometre upstream of the reservoir in the 1950s. Peak sediment yields of the 1970s coincide with high daily rainfalls and widespread flooding. The rainfall record indicates that daily rainfall maxima have increased since 1950, leading to potentially more erodible conditions. Our explanation of the increased sediment yields suggests that we need to examine more closely the links between catchment sources, local sinks within drainage lines and the final sink, the reservoir

A reconstruction of historical changes in sediment sources, sediment transfer and sediment yield for a small, semi-arid Karoo catchment, South Africa

Land degradation is widespread in South Africa but few long-term data sets are available to help identify the timing of this degradation and its impact on catchment sediment yields. We report a case study reconstructing the erosion history of a small upland catchment in the Sneeuberg mountains of South Africa based on the dating of reservoir sediments using 210Pb and 137Cs. Sediment sources have been identified using mineral magnetic measurements. Additional information has been derived from the analysis of aerial photographs and satellite images and the measurement of badland erosion using erosion pins. Results suggest that sediment yields increased dramatically from < 30 t km-2 yr-1 in the early part of the 20th Century to ∼1600 t km-2 yr-1 in the 1970s, subsequently falling to ∼850 t km-2 yr-1 in the last decade. Contemporary erosion pin measurements suggest that badlands are still extremely active and help maintain current high yields. Analysis of aerial photographs and satellite images indicate that badlands and extensive gullies existed in the catchment before 1945. An analysis of mineral magnetic signatures suggested that badlands made some contribution to the reservoir sediment in the 1930s, but did not significantly increase sediment yields. Photographic evidence suggests that a major badland became connected to the main channel network (and to the reservoir) from the early 1960s onwards. This interpretation was confirmed by significant and sustained changes in the mineral magnetic signatures of the reservoir sediments. The results indicated that a delayed increase in sediment yield, despite extensive badland development much earlier, appears to have been caused by changes in catchment connectivity rather than being a direct response to contemporary changes in land degradation within the catchment. These results add additional complications to our understanding of the relationship between the response to, and recovery from, periods of overgrazing in the 1930s that probably initiated the badland development