The Kufrah paleodrainage system in Libya: A past connection to the Mediterranean Sea?

International audiencePaillou et al. (2009) mapped a 900 km-long paleodrainage system in eastern Libya, the Kufrah River, that could have linked the southern Kufrah Basin to the Mediterranean coast through the Sirt Basin, possibly as long ago as the Middle Miocene. We study here the potential connection between the terminal part of the Kufrah River and the Mediterranean Sea through the Wadi Sahabi paleochannel, which may have constituted the northern extension of the lower Kufrah River paleodrainage system. New analysis of SRTM-derived topography combined with Synthetic Aperture Radar images from the Japanese PALSAR orbital sensor allowed the mapping of seven main paleochannels located west of the Kufrah River, each of which is likely to have formed a tributary that supplied water and sediment to the main paleodrainage system. The northernmost four paleochannels probably originated from the Al Haruj relief, a Pliocene alkaline basaltic intracontinental volcanic field, and potentially connected to the Wadi Sahabi paleochannel. The remaining three paleochannels are in the more southerly location of the Sarir Calanscio, North-East of the Tibesti mountains, and barely present a topographic signature in SRTM data. They end in the dunes of the Calanscio Sand Sea, forming alluvial fans. The most southern paleochannel, known as Wadi Behar Belama, was previously mapped by Pachur (1996) using LANDSAT-TM images, and was interpreted by Osborne et al. (2008) as representing part of an uninterrupted sediment pathway from the Tibesti mountains to the Mediterranean Sea. Processing of SRTM topographic data revealed local depressions which allow to connect the seven paleochannels and possibly the terminal alluvial fan of the Kufrah River to the Wadi Sahabi paleochannel, through a 400 km-long, south-north oriented, paleocorridor. These new findings support our previous hypothesis that proposed a connection between the lower Kufrah River in the region of the Sarir Dalmah and the Wadi Sahabi paleochannel, which connected to the Mediterranean Sea. Including the newly mapped paleochannels, the Kufrah River paleowatershed, at its maximum extent, would have covered more than 400,000 km2, representing close to a quarter of the surface area of Libya

The sky is the limit:Reconstructing physical geography from an aerial perspective

In an era of rapid geographical data acquisition, interpretations of remote sensing products are an integral part of many undergraduate geography degree schemes but there are fewer opportunities for collection and processing of primary remote sensing data. Unmanned Aerial Vehicles (UAVs) provide a relatively inexpensive opportunity to introduce the principles and practice of airborne remote sensing into fieldcourses, enabling students to learn about image acquisition, data processing and interpretation of derived products. Two case studies illustrate how a low cost “DJI Phantom Vision+” UAV can be used by students to acquire images that can be processed using Structure-from-Motion photogrammetry software. Results from a student questionnaire and analysis of assessed student reports showed that using UAVs enhanced student engagement and equipped them with data processing skills. The derivation of bespoke orthophotos and Digital Elevation Models has the potential to provide students with opportunities to gain insight into various remote sensing data quality issues, although additional training is required to maximise this potential. Recognition of the successes and limitations of this teaching intervention provides scope for improving future UAV exercises. UAVs are enabling both a reconstruction of how we measure the Earth’s surface and a reconstruction of how students do fieldwork

Morphodynamic simulation of sediment deposition patterns on a recently stripped bedrock anastomosed channel

Some mixed bedrock-alluvial dryland rivers are known to undergo cycles of alluvial building during low flow periods, punctuated by stripping events during rare high magnitude flows. We focus on the Olifants River, Kruger National Park, South Africa, and present 2-D morphodynamic simulations of hydraulics and sed-iment deposition patterns over an exposed bedrock anastomosed pavement. We examine the assumptions un-derlying a previous conceptual model, namely that sedimentation occurs preferentially on bedrock highs. Our modelling results and local field observations in fact show that sediment thicknesses are greater over bedrock lows, suggesting these are the key loci for deposition, barform initiation and island building. During peak flows, velocities in the topographic lows tend to be lower than in intermediate topographic areas. It is likely that inter-mediate topographic areas supply sediment to the topographic lows at this flow stage, which is then deposited in the lows on the falling limb of the hydrograph as velocities reduce. Subsequent vegetation establishment on de-posits in the topographic lows is likely to play a key role in additional sedimentation and vegetation succession, both through increasing the cohesive strength of alluvial units and by capturing new sediments and propagules

Cosmogenic 3HE Measurements Provide Insight into Lithologic controls on Bedrock Channel Incision:Examples from the South African Interior

Resistant bedrock outcrops can exert control on river long profile adjustment, upstream transmission of base level fall, and valley development, particularly in post-orogenic settings. To examine how variation in lithologic resistance impacts landscape development in the post-orogenic eastern South African interior, cosmogenic 3He in pyroxene from Karoo dolerite was measured in samples from valleys of the Klip and Mooi Rivers and the Schoonspruit. The denudation rates measured from cosmogenic 3He in the Klip and Mooi Rivers and Schoonspruit are widely variable, with channel bed denudation rates ranging from 14-255 m/Myr and valley side and top denudation rates ranging from 11-50 m/Myr. Various processes of channel bed erosion occurring at grain to block scales (abrasion, plucking, subaerial weathering) result in the widely ranging channel bed incision rates. In this setting, river incision rates are restricted by moderate unit stream powers (~20 to >50 W/m2) and by limited sediment supply, resulting in a lack of abrasive tools. In many dolerite valleys, channel bed incision is commonly slow enough for local base levels to remain essentially stable for extended periods of time (>10 kyr). These results suggest that in the post-orogenic eastern South African interior, resistant dolerite in channel long profiles can result in spatially variable rates of channel bed incision, with implications for the patterns and tempo of wider landscape dynamics

Quantifying and contextualising cyclone-driven, extreme flood magnitudes in bedrock-influenced dryland rivers

In many drylands worldwide, rivers are subjected to episodic, extreme flood events and associated sediment stripping. These events may trigger transformations from mixed bedrock-alluvial channels characterised by high geomorphic and ecological diversity towards more dominantly bedrock channels with lower diversity. To date, hydrological and hydraulic data has tended to be limited for these bedrock-influenced dryland rivers, but recent advances in high-resolution data capture are enabling greater integration of different investigative approaches, which is helping to inform assessment of river response to changing hydroclimatic extremes. Here, we use field and remotely sensed data along with a novel 2D hydrodynamic modelling approach to estimate, for the first time, peak discharges that occurred during cyclone-driven floods in the Kruger National Park, eastern South Africa, in January 2012. We estimate peak discharges in the range of 4470 to 5630 m3s-1 for the Sabie River (upstream catchment area 5715 km2) and 14 407 to 16 772 m3s-1 for the Olifants River (upstream catchment area 53 820 km2). These estimates place both floods in the extreme category for each river, with the Olifants peak discharge ranking among the largest recorded or estimated for any southern African river in the last couple of hundred years. On both rivers, the floods resulted in significant changes to dryland river morphology, sediment flux and vegetation communities. Our modelling approach may be transferable to other sparsely gauged or ungauged rivers, and to sites where palaeoflood evidence is preserved. Against a backdrop of mounting evidence for global increases in hydroclimatic extremes, additional studies will help to refine our understanding of the relative and synergistic impacts of high-magnitude flood events on dryland river development