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Assessing sediment connectivity at the hillslope, channel and catchment scale
Expanded Title:Connectivity is being embraced increasingly by hydrologists, geomorphologists and ecologists as a concept that allows integration of landscape structure and function at a number of time and space scales. Connectivity allows the free flow of energy and materials through the system and, as a result,mutual adjustment between system components. It is counterbalanced by storage sites, which allow material to be retained in the system. Ecologists, hydrologists and geomorphologists have all embraced the idea of connectivity as described in the literature review. The research was conducted in the catchment of Thina River (Tertiary catchment 34), lying in the headwaters of the Mzimvubu catchment, located in the former homeland of the Transkei where subsistence farming has been practiced for many years. A subcatchment of the Thina, the Vuvu, was investigated in detail. Two changes to connectivity are evident in the research area. First erosiondongas and incised cattle tracks are widespread on hillslopes in the area, increasing hillslope-channel coupling and delivery of sediment to the valley floor and channel. Second the main channel shows evidence of incision into the valley floor, resulting in a reduced connectivity between the channel and former flood zones. The research findings presented in this report indicate that there have been two phases of incision, one occurring within a geological time scale of thousands of years and one within the recent past (50 years) pointing to anthropogenically induced causes. In order to interpret sedimentary features on the valley floor in terms of catchment scale sediment dynamics it was necessary to identify sediment sources and provide a chronology of sediment deposition. Sediment fingerprinting techniques based on mineral magnetism were used to identify sources of sediment deposited in sink areas. Radionuclides (Cs 137 and Pb210) were used to date recent sediment (less than 100 years) deposited on flood benches, while Optically Stimulated Luminescence (OSL) provides a dating tool applicable over a range of time scale from 300 to 100,000 years. The research has provided insight into the geomorphic processes that influence water related ecosystems through their effect on fluvial structures and the functional relationships between the channel and adjacent valley floor. By disentangling natural processes from human induced change the research has potential to inform the assessment of the geomorphic Reference condition for similar upland rivers that have been subject to incision in the past and will provide guidance to interventions that aim to restore natural ecosystem function. It will thus contribute to the sustainable management of upland catchments that comprise the main water supply areas of the country. By focussing on the dynamics of sediment storage and reworking of stored sediment the research findings will also address the need expressed by Msadala et al. (2010) to incorporate sediment storage into sediment yield models.
Date Published:01/11/2015
Document Type:Research Report
Document Subjects:Water Resource Management/IWRM - Catchment Management
Document Keywords:Hydrology
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2260/1/15
ISBN No:978-1-4312-0715-2
Authors:van der Waal BW; Rowntree KM
Project No:K5/2260
Originator:WRC
Organizations:Rhodes University
Document Size:9 318 KB
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