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Channel Banks (channel + bank)
Selected AbstractsSediment transport in a highly regulated fluvial system during two consecutive floods (lower Ebro River, NE Iberian Peninsula)EARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2005Damia Vericat Abstract The transfer of sediment through a highly regulated large fluvial system (lower Ebro River) was analysed during two consecutive floods by means of sediment sampling. Suspended sediment and bedload transport were measured upstream and downstream of large reservoirs. The dams substantially altered flood timing, particularly the peaks, which were advanced downstream from the dams for flood control purposes. The suspended sediment yield upstream from the dams was 1 700 000 tonnes, which represented nearly 99 per cent of the total solid yield. The mean concentrations were close to 0·5 g l,1. The sediment yield downstream from the dams was an order of magnitude lower (173 000 tonnes), showing a mean concentration of 0·05 g l,1. The dams captured up to 95 per cent of the fine sediment carried in suspension in the river channel, preventing it from reaching the lowermost reaches of the river and the delta plain. Total bedload transport upstream from the dams was estimated to be about 25 000 tonnes, only 1·5 per cent of the total load. The median bedload rate was 100 gms,1. Below the dams, the river carried 178 000 tonnes, around 51 per cent of the total load, at a mean rate of 250 g ms,1. The results of sediment transport upstream and downstream from the large dams illustrate the magnitude of the sediment deficit in the lower Ebro River. The river mobilized a total of 350 000 tonnes in the downstream reaches, which were not replaced by sediment from upstream. Therefore, sediment was necessarily entrained from the riverbed and channel banks, causing a mean incision of 33 mm over the 27 km long study reach, altogether a significant step towards the long-term degradation of the lower Ebro River. Copyright © 2005 John Wiley & Sons, Ltd. [source] Processes and forms of an unstable alluvial system with resistant, cohesive streambeds ,EARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2002Andrew Simon Abstract As a response to channelization projects undertaken near the turn of the 20th century and in the late 1960s, upstream reaches and tributaries of the Yalobusha River, Mississippi, USA, have been rejuvenated by upstream-migrating knickpoints. Sediment and woody vegetation delivered to the channels by mass failure of streambanks has been transported downstream to form a large sediment/debris plug where the downstream end of the channelized reach joins an unmodified sinuous reach. Classification within a model of channel evolution and analysis of thalweg elevations and channel slopes indicates that downstream reaches have equilibrated but that upstream reaches are actively degrading. The beds of degrading reaches are characterized by firm, cohesive clays of two formations of Palaeocene age. The erodibility of these clay beds was determined with a jet-test device and related to critical shear stresses and erosion rates. Repeated surveys indicated that knickpoint migration rates in these clays varied from 0·7 to 12 m a,1, and that these rates and migration processes are highly dependent upon the bed substrate. Resistant clay beds of the Porters Creek Clay formation have restricted advancement of knickpoints in certain reaches and have caused a shift in channel adjustment processes towards bank failures and channel widening. Channel bank material accounts for at least 85 per cent of the material derived from the channel boundaries of the Yalobusha River system. Strategies to reduce downstream flooding problems while preventing upstream erosion and land loss are being contemplated by action agencies. One such proposal involves removal of the sediment/debris plug. Bank stability analyses that account for pore-water and confining pressures have been conducted for a range of hydrologic conditions to aid in predicting future channel response. If the sediment/debris plug is removed to improve downstream drainage, care should be taken to provide sufficient time for drainage of groundwater from the channel banks so as not to induce accelerated bank failures. Published in 2002 John Wiley & Sons, Ltd. [source] Interpreting sediment delivery processes using suspended sediment-discharge hysteresis patterns from nested upland catchments, south-eastern AustraliaHYDROLOGICAL PROCESSES, Issue 17 2009Hugh G. Smith Abstract In this study, suspended sediment concentration (SSC) and discharge (Q) hysteresis patterns recorded at the outlets of two nested upland catchments in south-eastern Australia were examined. Detailed monitoring of sediment flux was undertaken in a 1·64 km2 sub-catchment located within a 53·5 km2 catchment for which sediment yield was measured and the extent of incised channels mapped. The analysis of SSC,Q hysteresis patterns was supplemented by these additional datasets to contribute to the explanation of observed patterns. Clockwise SSC,Q hysteresis loops (with the suspended sediment peak leading the discharge peak) were recorded most frequently at both sites. This was attributed to initial rapid delivery of sediment from channel banks, the dominant sediment source in the sub-catchment and probably also for the catchment, in conjunction with remobilization of in-channel fine sediment deposits. Sediment exhaustion effects were considered to enhance clockwise hysteresis, with reduced SSC on the falling limb of event hydrographs. Pronounced exhaustion effects were observed on some multi-rise events, with subsequent flow peaks associated with much reduced sediment peaks. To compare SSC,Q hysteresis patterns between the two catchments, a dimensionless similarity function (SF) was derived to differentiate paired-event hysteresis patterns according to the extent of pattern similarity. This analysis, coupled with the other datasets, provided insight into the function of erosion and sediment delivery processes across the spatial scales examined and indicated the dependency of between-scale suspended sediment transfer on defined flow event scenarios. Quantitative measures of event SSC,Q hysteresis pattern similarity may provide a mechanism for linking the timing and magnitude of process response across spatial scales. This may offer useful insights into the between-scale linkage of dominant processes and the extent of downstream suspended sediment delivery. Copyright © 2009 John Wiley & Sons, Ltd. [source] MODELING THE LONG TERM IMPACTS OF USING RIGID STRUCTURES IN STREAM CHANNEL RESTORATION1JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 6 2006Sue L. Niezgoda Abstract: Natural channel designs often incorporate rigid instream structures to protect channel banks, provide grade control, promote flow deflection, or otherwise improve channel stability. The long term impact of rigid structures on natural stream processes is relatively unknown. The objective of this study was to use long term alluvial channel modeling to evaluate the effect of rigid structures on channel processes and assess current and future stream channel stability. The study was conducted on Oliver Run, a small stream in Pennsylvania relocated due to highway construction. Field data were collected for one year along the 107 m reach to characterize the stream and provide model input, calibration, and verification data. FLUVIAL-12 was used to evaluate the long term impacts of rigid structures on natural channel adjustment, overall channel stability, and changing form and processes. Based on a consideration of model limitations and results, it was concluded that the presence of rigid structures reduced channel width-to-depth ratios, minimized bed elevation changes due to long term aggradation and degradation, limited lateral channel migration, and increased the mean bed material particle size throughout the reach. Results also showed how alluvial channel modeling can be used to improve the stream restoration design effort. [source] Wheat field erosion rates and channel bottom sediment sources in an intensively cropped northeastern Oregon drainage basin,LAND DEGRADATION AND DEVELOPMENT, Issue 1 2004G. N. Nagle Abstract Sediment tracers were used to quantify erosion from cultivated fields and identify major source areas of channel bottom sediment within the Wildhorse Creek drainage, an intensively cropped tributary of the Umatilla River in northeastern Oregon, USA. Available data indicated that Wildhorse Creek was one of the largest sediment yielding tributaries of the Umatilla River. Carbon, nitrogen and the nuclear bomb-derived radionuclide 137Cs were used as tracers to fingerprint sediment sources. Sediment was collected from the stream bottom and active floodplain and compared to samples from cultivated fields and channel banks. Samples were characterized on the basis of tracer concentrations and a simple mixing model was used to estimate the relative portion of bottom sediment derived from cultivated surface and channel banks. The results indicate that the amount of bottom sediment derived from cultivated surface sources was less than 26,per,cent for the 1998 winter season, although this estimate has a high margin of error. Cesium-137 was also used to estimate surface erosion from three cultivated fields in the watershed. Annual estimates of erosion since 1963 from the three sampled fields were from 3 to 7,5,t,ha,1 yr,1. For the 1998 season, it appears that most channel-bottom sediment was of subsurface origin with much of it likely coming from channel and gully banks indicating that significant reductions in sediment in Wildhorse Creek might be accomplished by the stabilization of eroding riparian areas and swales on the lower slopes of agricultural fields. Published in 2004 by John Wiley & Sons, Ltd. [source] | |||||||||||||