High-resolution hydraulic modelling as an approach to planning rehabilitation interventions in unchanneled valley-bottom palmiet wetlands: a case study of the Kromme River
- Authors: Langner, Wiebke
- Date: 2023-10-13
- Subjects: Hydraulic models , Prionium serratum , Wetland conservation South Africa Kromme River (Eastern Cape) , LiDAR , Fluvial geomorphology
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424610 , vital:72168
- Description: This study employs high-resolution hydraulic modelling techniques to inform the planning of rehabilitation interventions in unchanneled valley-bottom palmiet wetlands, using the upper Kromme River wetlands as a case study. It investigates the impact of geomorphic processes on the morphology of the valley, how changes in valley morphology affect the flow characteristics (velocity, stream power, depth) of the river, and how these changes affect the geomorphic dynamics of the wetlands. An aerial LiDAR survey was conducted for a 23km-long reach of the upper Kromme River where the wetlands are situated. A high-resolution (5 m) DTM was created from the LiDAR data to examine the valley morphology. Focusing on three major wetland basins, the relationship between valley morphology and geomorphic processes was examined using high-resolution imagery that accompanied the LiDAR survey and Google Satellite imagery. The hydraulic modelling software HEC-RAS was used to investigate the spatial variation in velocity, stream power, and water depth down the surveyed length of the river. The model outputs provide insight into the effect of valley morphology on flow characteristics. The river appears to have a graded longitudinal profile, such that there is a systematic reduction in slope down its length. Water flowing down the river works, through the processes of erosion and deposition, to control the longitudinal slope, channel planform, and geometry to create a valley with a gentle longitudinal slope (approximately 1%) and a broad, near-horizontal valley-bottom in the mountainous landscape of the Cape Fold Mountains. The overall form of the Kromme River valley and wetlands is primarily a consequence of repeated cycles of cutting and filling. Tributary alluvial fans control the initiation of gully erosion in the wetlands, but their effect is diminished in a downstream direction. Despite a 10-fold increase in discharge down the 23km length of river for a given flood magnitude, there is no significant increase in flow velocity, stream power, or depth in a downstream direction. Consequently, the kinetic energy of the water in the lower wetland basin is surprisingly low. These conditions favour the establishment of palmiet. Flows in eroded reaches are much higher than in non-eroded reaches where discharge is spread across a broad valley bottom. In terms of palmiet establishment and regeneration, this means that areas dominated by depositional processes are best suited to the establishment of palmiet. Based on this information, optimal sites to trial new wetland rehabilitation strategies that employ palmiet were selected. This work supports the importance of understanding the role of geomorphology in wetland structure and dynamics when approaching wetland rehabilitation and is likely to be more sympathetic to natural processes than current interventions. , Thesis (MSc) -- Faculty of Science, Geography, 2023
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- Date Issued: 2023-10-13
Relating vegetation distribution to cycles of erosion and deposition in the Kromme River wetlands
- Authors: Jarvis, Samuel Cameron
- Date: 2023-10-13
- Subjects: Biogeomorphology South Africa Kromme Estuary (Eastern Cape) , Earth observation , Remote sensing , Niche construction , Wetland ecology , Geomorphology , Ecological succession , Optical radar , Prionium serratum
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424582 , vital:72166
- Description: The role of geomorphic disturbance has been increasingly recognized as fundamental in the creation and functioning of wetlands. This is true of the Kromme River wetland which has been formed through repeated cycles of erosion and deposition. However, the response – and influence – of wetland plants to these sorts of disturbance has not been investigated. This study sought to fill this knowledge gap by classifying vegetation communities over a range of hydrological and geomorphic disturbance regimes that have happened over the last few decades, and relating those vegetation communities to environmental factors. The study identified seven vegetation communities based on their species composition and abundance, which were related to geomorphic disturbance events. A conceptual model that accounts for vegetation distribution in the Kromme wetland was developed. Soil saturation was the most important factor explaining vegetation community distribution, which, in turn, is influenced by cycles of erosion and deposition. Following an erosional event on the valley floor, Prionium serratum dominated wetland is converted to a number of other vegetation communities. On the floodplain surface adjacent to the eroded gully, the Prionium serratum dominated wetland is transformed over time to Cynodon dactylon and Sporobolus fimbriatus communities. Prionium serratum clumps immediately adjacent to the recently incised gullies are able to persist, having sufficient access to water. Within the newly formed gullies, Juncus lomatophyllus colonizes the gully beds flooded to a shallow depth, Miscanthus capensis colonizes the gully bars and Setaria incrassata colonizes the exposed gully banks. Localised depositional features close to the thalweg in the gully are colonized by Prionium serratum seedlings and vegetative propagules. These plants represent the regenerating phase of Prionium serratum wetland, which also colonizes depositional floodouts downstream of the newly-formed gully. The Stenotaphrum secundatum community dominates drier, more elevated areas of the floodout. Over time, as the gully fills, Prionium serratum expands beyond the gully onto the valley floor, to replace the floodplain communities Cynodon dactylon and Sporobolus fimbriatus. Over time, Prionium serratum is thought to colonize the valley floor as the gully fills, stabilising it and promoting diffuse flow. Many restoration efforts in damaged palmiet wetlands have been focused on the preservation of intact palmiet communities upstream of erosional headcuts, with limited understanding of vegetation dynamics associated with the cut-and-fill cycles that naturally occur in these wetlands. Understanding the regeneration of Prionium serratum following erosional events is thus important for wetland restoration, as it should focus more attention on promoting palmiet restoration on depositional floodouts downstream of eroded gullies. A secondary aim of this study was to explore the possibility of mapping palmiet communities in Kromme River wetland using remote sensing techniques. Using a combination of ground-truthed data from this and previous studies in the Kromme River wetland, together with raster layers derived from a LiDAR survey, an overlay analysis was developed to effectively map the distribution of the Prionium serratum dominated community. The overlay was created using a machine learning library in RStudios known as Rpart. The results found that the model were 91% effective in classifying the distribution of the Prionium serratum community. A secondary finding was that the inclusion of a Relative Elevation Model in the overlay analysis allowed for the identification of Prionium serratum communities vulnerable to degradation following previous geomorphic disturbance events and those Prionium serratum communities that are likely to persist following a geomorphic disturbance event. , Thesis (MSc) -- Faculty of Science, Geography, 2023
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- Date Issued: 2023-10-13
An investigation into the origin and evolution of the Tierkloof Wetland, a peatland dominated by Prionium serratum, in the Western Cape
- Authors: Bekker, Debra Jane
- Date: 2017
- Subjects: Prionium serratum , Tierkloof Wetland (Western Cape, South Africa) , Wetlands -- South Africa -- Western Cape , Aquatic plants -- South Africa -- Western Cape Cape , Peatland ecology -- South Africa -- Western Cape , Kromme River (Eastern Cape, South Africa)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/53689 , vital:26311
- Description: The Tierkloof Wetland in the Western Cape has been damaged through erosion down to bedrock with gullies that are up to 50 m wide and 7 m deep. The purpose of this study was to investigate the origin and evolution of the Tierkloof Wetland and consider the role both natural and human-induced processes have played in its degradation. The main methods used were digital mapping using GIS software, satellite and aerial photographic interpretation of past and current land use, detailed topographic surveying, coring to bedrock, stratigraphic analysis of valley-fill sediments, and radio carbon dating of peat samples. Nine GIS-produced topographic profiles across the valley were plotted to analyse downstream changes in valley width and morphology along the length of the Tierkloof valley. Detailed cross sections of the stream channel were produced using topographic survey data. Depth to bedrock analysis indicated that there is evidence of a degree of planing of bedrock over geological time periods. Survey data indicates that the longitudinal slope of the former wetland surface is remarkably uniform over most of its length and there is an overall thinning of the valley fill from the head to the toe of the wetland. The stratigraphy and associated organic matter content of the wetland soils showed that the sedimentary fill varies considerably. There are appreciable accumulations of organic sediment, including sandy peat and peat layers with organic content of up to 50 %. The two radio carbon dates obtained from the gully wall midway down the Tierkloof Wetland suggest that peat accumulation started following the last glacial maximum. The peat has accumulated consistently and is mostly associated with the growth of the palmiet, Prionium serratum. Evidence from past and current land-use activities from aerial photographic interpretation show that natural vegetation in the Tierkloof Wetland has been severely degraded, alien plants are extremely common, and road-building and artificial drainage in the wetland have influenced fluvial processes. It is likely that these activities, exacerbated by the high rainfall events in the early 2000s, have triggered the erosional phase that was initiated a few decades ago. However, longer term processes (thousands to tens-of-thousands of years), such as the planing of the bedrock, also indicate that erosional processes have been a natural part of the Tierkloof Wetland development. A conceptual model of wetland development is thus proposed, based on long-term erosional and depositional processes that are linked with climate change and the current aggradation phase of peat formation that is being driven by the ecosystem engineering processes of palmiet plant growth. It is further suggested that relatively recent land-use activities have resulted in an already vulnerable wetland, in geomorphological terms, to cross a threshold to become a severely degraded, gulley- dominated wetland.
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- Date Issued: 2017
Ecosystem engineering by the wetland plant palmiet: does it control fluvial form and promote diffuse flow in steep-sided valleys of the Cape Fold Mountains
- Authors: Barclay, Amy
- Date: 2017
- Subjects: Prionium serratum , Wetlands -- South Africa -- Eastern Cape , Aquatic plants -- South Africa -- Eastern Cape , Peatland ecology -- South Africa -- Eastern Cape , Kromme River (Eastern Cape, South Africa)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/4646 , vital:20708
- Description: Ecosystem engineering refers to the way that organisms control the structure and function of ecosystems. It has been suggested that palmiet (Prionium serratum, Thurniaceae) works as an ecosystem engineer, shaping peat wetlands in South Africa. However, there is currently a paucity of evidence supporting this claim. Palmiet has a dense root, rhizome and stem system that forms dense stands, growing from channel banks into fast flowing river channels. This slows river flows, traps sediment, which builds up riverbeds and ultimately blocks river channels, turning the river into a wetland. The aim of this study was to determine if palmiet is an ecosystem engineer and to document its pattern of colonisation and the nature of its control of a fluvial system. This was achieved by undertaking vegetation surveys in the Kromrivier Wetland in the Eastern Cape. The data was analyzed using vegetation classification and ordination, where vegetation communities were linked to environmental factors. It was found that palmiet occupied three distinctive habitats; 1) on near-horizontal valley- bottom habitats filled with sediments that are a mixture of autochthonous organic sediment and allochthonous clastic fines, 2) the bed of gullies that have recently filled with coarse grained clastic sediment, and 3) open water bodies. Three conceptual models were developed, one that accounts for the process of gully bed colonisation, sediment trapping and gully filling, another involving rapid colonisation of sedimentary fill from tributary sediment sources that block a gully, and the third involving colonisation of open-water areas that form in former gullies upstream of the blockage. The study suggests that the wetland has been characterised by repeated cutting and filling cycles, despite which, palmiet has repeatedly reinstated diffuse flow conditions across the valley floor. Palmiet was indicated to exert a key control on fluvial form and dynamics of the wetlands in the Kromrivier valley.
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- Date Issued: 2017