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Channel Reach (channel + reach)
Selected AbstractsSediment budget for an eroding peat-moorland catchment in northern EnglandEARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2005Martin Evans Abstract This paper describes a detailed contemporary sediment budget from a small peat-covered, upland catchment in Upper Teesdale, northern England. The sediment budget was constructed by measuring: (1) sediment transfers on slopes, (2) sediment flux on the floodplain and through the main stream channel and (3) sediment yield at the catchment outlet. Measurements were taken over a four-year monitoring period between July 1997 and October 2001 when interannual variations in runoff were relatively small. Three sites were selected to represent the major erosion subsystems within the catchment: an area of bare peat flats, a pair of peat gullies, and a 300 m channel reach. Collectively the sites allow detailed characterization of the main patterns of sediment flux within the catchment and can be scaled up to provide an estimate of the sediment budget for the catchment as a whole. This constitutes the first attempt to provide a complete description of the functioning of the sediment system in eroding blanket peatlands. Results demonstrate that fluvial suspended sediment flux is controlled to a large degree by channel processes. Gully erosion rates are high but coupling between the slopes and channels is poor and therefore the role of hillslope sediment supply to catchment output is reduced. Consequently contemporary sediment export from the catchment is controlled primarily by in-channel processes. Error analysis of the sediment budgets is used to discuss the limitations of this approach for assessing upland sediment dynamics. A 60 per cent reduction in fluvial suspended sediment yield from Rough Sike over the last 40 years correlates with photographic evidence of significant re-vegetation of gullies over a similar period. This strongly suggests that the reduced sediment yields are a function of increased sediment storage at the slope,channel interface, associated with re-vegetation. Copyright © 2005 John Wiley & Sons, Ltd. [source] Numerical simulation of overbank processes in topographically complex floodplain environmentsHYDROLOGICAL PROCESSES, Issue 4 2003A. P. Nicholas Abstract This article presents results from an investigation of the hydraulic characteristics of overbank flows on topographically-complex natural river floodplains. A two-dimensional hydraulic model that solves the depth-averaged shallow water form of the Navier,Stokes equations is used to simulate an overbank flow event within a multiple channel reach of the River Culm, Devon, UK. Parameterization of channel and floodplain roughness by the model is evaluated using monitored records of main channel water level and point measurements of floodplain flow depth and unit discharge. Modelled inundation extents and sequences are assessed using maps of actual inundation patterns obtained using a Global Positioning System, observational evidence and ground photographs. Simulation results suggest a two-phase model of flooding at the site, which seems likely to be representative of natural floodplains in general. Comparison of these results with previous research demonstrates the complexity of overbank flows on natural river floodplains and highlights the limitations of laboratory flumes as an analogue for these environments. Despite this complexity, frequency distributions of simulated depth, velocity and unit discharge data closely follow a simple gamma distribution model, and are described by a shape parameter (,) that exhibits clear systematic trends with changing discharge and floodplain roughness. Such statistical approaches have the potential to provide the basis for computationally efficient flood routing and overbank sedimentation models. Copyright © 2002 John Wiley & Sons, Ltd. [source] A method for improving predictions of bed-load discharges to reservoirsLAKES & RESERVOIRS: RESEARCH AND MANAGEMENT, Issue 2 2007Vicente L. Lopes Abstract Effective management options for mitigating the loss of reservoir water storage capacity to sedimentation depend on improved predictions of bed-load discharges into the reservoirs. Most predictions of bed-load discharges, however, are based on the assumption that the rates of bed-load sediment availability equal the transport capacity of the flow, ignoring the spatio-temporal variability of the sediment supply. This paper develops a semiquantitative method to characterize bed-load sediment transport in alluvial channels, assuming a channel reach is non-supply limited when the bed-load discharge of a given sediment particle-size class is functionally related to the energy that is available to transport that fraction of the total bed-load. The method was applied to 22 alluvial stream channels in the USA to determine whether a channel reach had a supply-limited or non-supply-limited bed-load transport regime. The non-supply-limited transport regime was further subdivided into two groups on the basis of statistical tests. The results indicated the pattern of bed-load sediment transport in alluvial channels depends on the complete spectrum of sediment particle sizes available for transport rather than individual particle-size fractions represented by one characteristic particle size. The application of the method developed in this paper should assist reservoir managers in selecting bed-load sediment transport equations to improve predictions of bed-load discharge in alluvial streams, thereby significantly increasing the efficiency of management options for maintaining the storage capacity of waterbodies. [source] Discussion of landslide self-organized criticality and the initiation of debris flowEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2007Chen Chien-Yuan Abstract The study contains descriptions of landslides and debris flows in a small upland catchment in Taiwan between 1986 and 2004. This catchment, the Chushui Creek, is situated near the epicentre of the 1999 M7·6 Chi-Chi earthquake. This is an area with high background erosion rate. The Chi-Chi earthquake caused a sharp increase in the rate of mass wasting in the epicentral area, and the data presented in this study illustrates the geomorphic change associated with the earthquake. The measurements of the geometry of the trunk stream of the Chushui catchment also show its change due to the impact of a strong typhoon in 1996. Two channel reaches that were affected by separate debris flows during this event were identified. Each reach has three sections: scour, transfer and deposition. Cross profiles of these sections show a systematic change from V-shaped bedrock channel in the scour areas to flat-floored channel in the transfer and deposition areas. Debris flows also occurred on other occasions in this channel, and their frequency has increased since the 1999 earthquake. In addition, this study contains precise geographic and statistical descriptions of the landslides triggered by three typhoons and an earthquake that affected the catchment since 1996. The total landslide area is measured for different time intervals, and a marked increase in landslide incidence is found after the 1999 earthquake. Frequency,area plots of the 126 landslides in the catchment indicate a power-law scaling with an exponent of about -1·5 of these quantities, as has been observed in other studies. We propose that a link exists between landslide frequency,area distributions and initialization of debris flow in the catchment. It may be classified as a self-organized criticality process with a critical frequency,area distribution for the landslide population. In this case, the landslide frequency,area distribution is useful in quantifying the severity of the trigger and the contribution of landslides to debris flow. Copyright © 2006 John Wiley & Sons, Ltd. [source] Understanding the temporal dynamics of the wandering Renous River, New Brunswick, CanadaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2005Leif M. Burge Abstract Wandering rivers are composed of individual anabranches surrounding semi-permanent islands, linked by single channel reaches. Wandering rivers are important because they provide habitat complexity for aquatic organisms, including salmonids. An anabranch cycle model was developed from previous literature and field observations to illustrate how anabranches within the wandering pattern change from single to multiple channels and vice versa over a number of decades. The model was used to investigate the temporal dynamics of a wandering river through historical case studies and channel characteristics from field data. The wandering Renous River, New Brunswick, was mapped from aerial photographs (1945, 1965, 1983 and 1999) to determine river pattern statistics and for historical analysis of case studies. Five case studies consisting of a stable single channel, newly formed anabranches, anabranches gaining stability following creation, stable anabranches, and an abandoning anabranch were investigated in detail. Long profiles, hydraulic geometry, channel energy, grain size and sediment mobility variables were calculated for each channel. Within the Renous study area, the frequency of channel formation and abandonment were similar over the 54 years of analysis, indicating that the wandering pattern is being maintained. Eight anabranches were formed through avulsions, five were formed through the emergence of islands from channel bars and 11 anabranches were abandoned. The stable anabranch pair displayed similar hydraulic geometry and channel energy characteristics, while unstable anabranch pairs did not. The anabranch pair that gained stability displayed more similar channel energy characteristics than the anabranch pair that was losing stability (abandoning). It appears that anabranch pairs with similar energy characteristics are more stable than anabranches where these characteristics are out of balance. This is consistent with the hypothesis that anabranch pairs of similar length will be more stable than those with dissimilar lengths. Copyright © 2005 John Wiley & Sons, Ltd. [source] | ||||||||||