Vegetation controls on channel stability in the Bell River, Eastern Cape, South Africa
- Rowntree, Kate M, Dollar, Evan S J
- Authors: Rowntree, Kate M , Dollar, Evan S J
- Date: 1999
- Language: English
- Type: Article
- Identifier: vital:6720 , http://hdl.handle.net/10962/d1006796
- Description: 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.
- Full Text:
- Date Issued: 1999
- Authors: Rowntree, Kate M , Dollar, Evan S J
- Date: 1999
- Language: English
- Type: Article
- Identifier: vital:6720 , http://hdl.handle.net/10962/d1006796
- Description: 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.
- Full Text:
- Date Issued: 1999
Controls on channel form and channel change in the Bell River, Eastern Cape, South Africa
- Rowntree, Kate M, Dollar, Evan S J
- Authors: Rowntree, Kate M , Dollar, Evan S J
- Date: 1996
- Language: English
- Type: Article
- Identifier: vital:6723 , http://hdl.handle.net/10962/d1006801
- Description: Channel instability has occurred in the Bell River in the form of meander cutoffs, incipient meander cutoffs, and general channel instability. This paper examines the incidence of channel instability in relation to the spatial variability of factors controlling channel form. The locations of channel change were mapped from aerial photographs for a 40 year period. Channel form characteristics were measured over a 17 km stretch of channel. Data collected from this stretch included pattern, width, depth, form ratio, channel gradient, particle-size distribution of bed and bank materials and riparian vegetation. Results indicate that the two primary spatial controls on channel form and pattern are riparian vegetation and bed-material size. Evidence indicates that narrow stable stretches are associated with finer bed-material and relatively high levels of riparian vegetation. Riparian vegetation increases bank stability and reduces channel cross-section, thereby inducing stability at flows less than bankfull. However, at flows greater than bankfull, reduced channel capacity results in more frequent flooding which may ultimately lead to channel avulsion. Wider, unstable channels are associated with larger calibre bed material and relatively less riparian vegetation. Predicted threshold slopes for braiding have indicated that most reaches in the Bell river are close to or above the threshold slope for braiding. Since riparian vegetation serves to increase bank stability, steeper threshold slopes are required for braiding in channels with high vegetation density Recent incidences of channel instability have been identified as occurring in the transitional zone between the upper, coarser reach and lower, finer channel reach.
- Full Text: false
- Date Issued: 1996
- Authors: Rowntree, Kate M , Dollar, Evan S J
- Date: 1996
- Language: English
- Type: Article
- Identifier: vital:6723 , http://hdl.handle.net/10962/d1006801
- Description: Channel instability has occurred in the Bell River in the form of meander cutoffs, incipient meander cutoffs, and general channel instability. This paper examines the incidence of channel instability in relation to the spatial variability of factors controlling channel form. The locations of channel change were mapped from aerial photographs for a 40 year period. Channel form characteristics were measured over a 17 km stretch of channel. Data collected from this stretch included pattern, width, depth, form ratio, channel gradient, particle-size distribution of bed and bank materials and riparian vegetation. Results indicate that the two primary spatial controls on channel form and pattern are riparian vegetation and bed-material size. Evidence indicates that narrow stable stretches are associated with finer bed-material and relatively high levels of riparian vegetation. Riparian vegetation increases bank stability and reduces channel cross-section, thereby inducing stability at flows less than bankfull. However, at flows greater than bankfull, reduced channel capacity results in more frequent flooding which may ultimately lead to channel avulsion. Wider, unstable channels are associated with larger calibre bed material and relatively less riparian vegetation. Predicted threshold slopes for braiding have indicated that most reaches in the Bell river are close to or above the threshold slope for braiding. Since riparian vegetation serves to increase bank stability, steeper threshold slopes are required for braiding in channels with high vegetation density Recent incidences of channel instability have been identified as occurring in the transitional zone between the upper, coarser reach and lower, finer channel reach.
- Full Text: false
- Date Issued: 1996
Hydroclimatic trends, sediment sources and geomorphic response in the Bell River catchment, eastern Cape Drakensberg, South Africa
- Dollar, Evan S J, Rowntree, Kate M
- Authors: Dollar, Evan S J , Rowntree, Kate M
- Date: 1995
- Language: English
- Type: Article
- Identifier: vital:6677 , http://hdl.handle.net/10962/d1006655
- Description: The Bell River in the Eastern Cape Drakensberg of South Africa has shown recent signs of channel instability as evidenced by avulsion, sinuosity reductions, gradient steepening and general channel instability. Analysis of rainfall data did not provide evidence for progressive long term change in rainfall pattern for the catchment. However, annual and seasonal rainfall cycles with variance peaks every 16-19 years were found, from which it can be inferred that flood events following years of below average rainfall may cross the threshold limit for channel stability. Channel instability is often followed by a major flood event after sustained low rainfall periods, a sequence referred to as the Flood and Drought Dominated Regimes by Warner (1987). Evidence presented also indicates an increase in catchment erosion in the past century with attendant sediment production increases to the channel. This may be due to catchment land use and management practices (following settlement by commercial farmers) superimposed on long term climatic changes. The conclusions are that catchment and channel processes are inseparable, and that disequilibrium in the catchment will be transferred to the channel.
- Full Text:
- Date Issued: 1995
- Authors: Dollar, Evan S J , Rowntree, Kate M
- Date: 1995
- Language: English
- Type: Article
- Identifier: vital:6677 , http://hdl.handle.net/10962/d1006655
- Description: The Bell River in the Eastern Cape Drakensberg of South Africa has shown recent signs of channel instability as evidenced by avulsion, sinuosity reductions, gradient steepening and general channel instability. Analysis of rainfall data did not provide evidence for progressive long term change in rainfall pattern for the catchment. However, annual and seasonal rainfall cycles with variance peaks every 16-19 years were found, from which it can be inferred that flood events following years of below average rainfall may cross the threshold limit for channel stability. Channel instability is often followed by a major flood event after sustained low rainfall periods, a sequence referred to as the Flood and Drought Dominated Regimes by Warner (1987). Evidence presented also indicates an increase in catchment erosion in the past century with attendant sediment production increases to the channel. This may be due to catchment land use and management practices (following settlement by commercial farmers) superimposed on long term climatic changes. The conclusions are that catchment and channel processes are inseparable, and that disequilibrium in the catchment will be transferred to the channel.
- Full Text:
- Date Issued: 1995
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