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Sediment Connectivity in the Upper Thina Catchment, Eastern Cape, South Africa

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Sediment Connectivity in the Upper Thina Catchment, Eastern Cape, South Africa Sediment Connectivity in the Upper Thina Catchment, Eastern Cape, South Africa THESIS Submitted in fulfilment of the requirements for the Degree of DOCTOR OF PHILOSOPHY of RHODES UNIVERSITY by Benjamin Wentsel van der Waal December 2014 Abstract Sediment dynamics are influenced by transformed landscape connectivity in catchments worldwide. The upper Thina catchment, an important high rainfall resource in the northern Eastern Cape, South Africa, is an example of where ongoing subsistence farming on communal land has led to overgrazing and trampling that has initiated large erosive features (e.g. gullies) and river incision. The formation of gullies led to increased hillslope-channel connectivity and the resultant river incision decreased the channel-valley fill connectivity. These two changes in connectivity led to increased sediment export from the catchment that has various down-stream ecological and socio-economic impacts. This study investigates how the change in hillslope-channel and channel-valley fill connectivity has altered the sediment dynamics in the Vuvu catchment, a headwater tributary of the Thina River. A combination of methods were used to assess the changes in hillslope-channel and channel-valley fill connectivity. High resolution aerial images were used to map source features, such as fields, gullies, sheet erosion, landslides, roads and livestock tracks. Topographic and geological characteristics of the source features were extracted using a Geographic Information System. Furthermore, hillslope-channel pathways, such as the natural drainage network, continuous gullies, discontinuous gullies, roads and livestock tracks were mapped and analysed in terms of topographic and geological characteristics. Historic aerial images were assessed to calculate the date the larger gullies began forming. Recent aerial photos and cross sectional surveys of the valley fill were combined to map the various sediment sinks. Particle size and organic content were analysed for flood bench cores and terrace samples. The chronology of the flood benches was determined using unsupported Pb-210 and Cs-137 dating, and determined for the terraces using Optically Stimulated Luminescence dating. Quantitative and qualitative sediment tracing approaches, using mineral magnetic properties, were used to trace the origin of suspended sediment (collected during flood events), sediment stored in the flood benches and sediment stored in the terraces. Hydrological monitoring was used to assess the potential to store sediment on flood benches along the valley fill through flood bench inundation frequency. Hydrological and hydraulic modelling extended the measured inundation frequencies to a 73 year period and other cross sections along the valley fill. Furthermore, a future scenario of an increased vegetation cover and reduced hillslope-channel connectivity was assessed in terms of channel-valley fill inundation frequency. In the Vuvu catchment, 9% of the land is severely eroded and half of the eroded areas are directly connected to the drainage network. The average soil loss rate for the catchment, based on the mapping of erosional features, was 13 t ha-1 y-1. Fields, gullies and areas of sheet erosion were calculated to be the main sediment sources that could be mapped, producing ~4 t ha-1 y-1 of sediment per source type for the catchment. The areas characterised by mudstones had the highest density of erosion features, with soil loss estimates as high as 9 t ha-1 y-1 for fields, 5 ha-1 y-1 for gullies and 5 t ha-1 y-1 for sheet erosion, i confirming the sensitivity of this geology to land use pressures. Furthermore, erosional features were more prevalent on moderate (5–200) and north-facing slopes. Historic mapping showed that the gullies are still active sediment sources and that the larger gullies were initiated in the 1920s; smaller gullies however were initiated in the 1950s. Gullies increased the natural drainage network by 21% in a down- slope direction, whereas livestock tracks and roads have increased the natural drainage network by 178% in an across slope direction. Continuous gullies were the most effective hillslope-channel linkages as they were directly linked to the channel, while discontinuous gullies, given their location, were buffered from the channel. Across slope drainage features intercepted and concentrated hillslope runoff, routing water directly to the drainage network, reducing chances of water infiltration as well as sediment deposition. Sediment storage along the valley fill was limited due to the valley confinement, narrow flood benches and terraces that are active sediment sinks. Lower flood benches stored sediment for up to 60 years, higher flood benches for 92+ years and terraces for up to 4500 years. It was estimated that the flood benches only stored 2% of the sediment that was eroded over the past 100 years, confirming the limited sediment storage potential along the valley fill. The average sediment accumulation rate was higher for lower flood benches (3.2 g cm-2 yr-1) when compared to the average accumulation rate of the higher flood benches (0.9 g cm-2 yr-1), showing that the lower benches were more active in receiving and storing sediment coupled with a shorter storage period. The flood benches acted as down-river sediment buffers, whereas both the flood benches and terraces acted as a buffer for hillslope-channel linkages in the immediate vicinity of the valley fill, unless directly linked to the channel by an incised tributary channel. Sediment tracing using mineral magnetic characteristics could discriminate between igneous and sedimentary sources. Quantitative source apportionment confirmed the results of a qualitative approach, leading to a novel semi-qualitative sediment tracing approach. Suspended sediment concentrations were high (up to ~11 g l-1) and the source depended on rainfall extent and catchment shape. Local sedimentary sources dominated most of the flood water sediment, unless high rainfall in the upper catchment contributed large quantities of igneous sediment. This was reflected in the lower flood benches that predominantly stored locally sourced sediment, whereas the higher flood benches were characterised by a greater contribution from the igneous sources of the upper catchment. This suggests that the sediment stored in higher and lower sinks reflect the stage (low stage from local events and high stage when entire catchment contributes) and the source of the water and sediment. Sediment concentration decreased throughout the wet season, indicating that sediment availability decreases as vegetation cover increases or that sediment sources are depleted. The majority of the sediment stored in the valley fill was derived from local sedimentary sources, reflecting the erodible nature and good hillslope-channel connectivity of the lower catchment and that the contribution from local sources has been relatively stable over the past 4500 years. This also confirms that degradation is a catchment-wide phenomenon ii that has maintained the contribution ratio from the different geologies (sedimentary sources being dominant). Flow discharge and inundation levels were monitored using in-situ level loggers and rating curves at two sites. Estimates based on hydraulic modelling were made at an additional 13 sites. Inundation frequencies for a 73 year period were estimated using a hydrological model for the current increased hillslope-channel connectivity and a future rehabilitated condition of reduced hillslope-channel connectivity. Intensive summer rainfall in the Vuvu catchment produced steep hydrographs of short duration (3 hours). Lower flood benches were inundated 1.5-4.5 times a year and the higher flood benches 1 in 2 years during the monitoring period (2012-2014). Where the channel was incised, inundation frequencies were lower due to the higher elevation of the flood bench in relation to the channel bed, reducing the potential to store sediment on these benches. The inundation frequency of flood benches decreased during drier periods, reducing the potential to deposit sediment on the flood benches. During wet periods the chances of sediment deposition on flood benches was increased due to the increased inundation frequency, but was dependent on sediment availability that increased during dry periods as soil is exposed to erosive rainfall due to reduced vegetation cover. Sediment storage on the valley fill could thus be greater during dry periods compared to wet periods. The potential of sediment storage on flood benches decreased for the post-rehabilitation scenario as both sediment concentration and peak flood volumes would be decreased, reducing hydrological and sediment connectivity. Sediment connectivity has been increased in the Vuvu catchment due to anthropogenic influences such as cultivation and subsequent abandonment, overgrazing and frequent burning of rangelands. Large areas of erosion are effectively connected to the drainage network, transferring sediment directly to the channel. Once in the channel, very little suspended sediment is deposited on the valley fill, due to the small size of the depositional features and the short, high energy character of the floods. Sediment transfer rates in the Vuvu catchment are thus high from source areas to the bottom of the catchment due to increased hillslope-channel connectivity and limited sediment buffers along the valley fill. This research shows how important sediment connectivity is in understanding where sediment is sourced, how
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