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1.
The impact of wastewater flow on the channel bed morphology was evaluated in four ephemeral streams in Israel and the Palestinian Territories: Nahal Og, Nahal Kidron, Nahal Qeult and Nahal Hebron. Channel changes before, during and after the halting of wastewater flow were monitored. The wastewater flow causes a shift from a dry ephemeral channel with intermittent floods to a continuous flow pattern similar to that of humid areas. Within a few months, nutrient‐rich wastewater flow leads to rapid development of vegetation along channel and bars. The colonization of part of the active channel by vegetation increases flow resistance as well as bank and bed stability, and limits sediment availability from bars and other sediment stores along the channels. In some cases the established vegetation covers the entire channel width and halts the transport of bed material along the channel. During low and medium size flood events, bars remain stable and the vegetation intact. Extreme events destroy the vegetation and activate the bars. The wastewater flow results in the development of new small bars, which are usually destroyed by flood flows. Due to the vegetation establishment, the active channel width decreases by up to 700 per cent. The deposition of fine sediment and organic material changed the sediment texture within the stable bar surface and the whole bed surface texture in Nahal Hebron. The recovery of Nahal Og after the halting of the wastewater flow was relatively fast; within two flood seasons the channel almost returned to pre‐wastewater characteristics. The results of the study could be used to indicate what would happen if wastewater flows were introduced along natural desert streams. Also, the results could be used to predict the consequences of vegetation removal as a result of human intervention within the active channel of humid streams. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

2.
Channel erosion along the Carmel river,Monterey county,California   总被引：1，自引：0，他引：1  
Historic maps, photographs, and channel cross-sections show that the channel of the Carmel River underwent massive bank erosion, channel migration, and aggradation in a major flood in 1911, then narrowed and incised by 1939. The channel was stable until 1978 and 1980, when bank erosion affected some reaches but not others. The narrowing and incision were in response to a lack of major floods after 1914 and construction in 1921 of a dam that cut off sediment supply from the most actively eroding half of the basin. Localized erosion in 1978 and 1980 occurred during low magnitude events along reaches whose bank strength had been reduced by devegetation. These events illustrate that the stability of a fluvial system can be disrupted either by application of a large erosive force in a high magnitude event (the 1911 flood) or in a low magnitude event, by reducing the resistance to erosion (bank devegetation). The Carmel River is a potentially unstable system. Its discharge and slope characteristics place it near the threshold between meandering and braided. On the Lower Carmel, the presence of bank vegetation can make the difference between a narrow, stable meandering channel and a wide shifting channel with braided reaches.  相似文献   

3.
Percolation of flood waters into the bed and banks of ephemeral streams provides one of the key mechanisms responsible for transmission loss. However, there are very few published estimates of the rates at which water can enter stream‐bank sediments, and little is known about the variation in bank permeability with elevation above the bed and the resulting effects on transmission loss in floods of different magnitudes. This paper presents the results of 69 field determinations of bank infiltrability made on Fowlers Creek, an ephemeral dry‐land stream located in arid western New South Wales, Australia. Fowlers Creek carries high concentrations of suspended sediments, which are deposited as mud drapes on the bed, banks and floodplain. Results demonstrate that infiltration rates are lowest at the base of the banks, and tend to increase steadily with elevation on the bank, even above the apparent upper limit of mud drapes. In parallel, the texture of the bank sediments (assessed from samples of the uppermost 10 cm) becomes coarser with elevation above the bed. This pattern is inferred to relate to the delivery of silts and clays into pore spaces in the bank sediments by percolating flood waters. The patterns of infiltration rate and sediment texture mapped in the field are reasoned to be the product of many clogging episodes in past flood events having different peak stages. The increase in infiltration rate and mean particle size up the banks reflects lower frequencies of submergence and clogging of the upper banks by large floods, and more frequent inundation and clogging of the lower banks by sub‐bank‐full flows. The stage‐related changes in bank permeability provide a mechanism that can drive variations in transmission loss among floods having different peak stages and hydrograph shapes. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

4.
1 INTRODUCTION The plane shape of a river channel is very important for river improvement planning, because it must allow floodwater to flow off safely. Natural rivers wind from side to side, which creates meandering forms. From the history of river impro…  相似文献   

5.
Alternate bars have the property that they migrate downstream whenever floods occur. However, in meander channels whose bend angles are larger than a critical value, the migration of bars can be suppressed, and the positions of bank erosion and flood attack also will be steady. In this study, the bed morphology in flume channels with bends of various lengths and angles is investigated at various flow discharges, and the relation of bed morphology to surface flow is investigated in detail using fluid measuring software. An effort is made to obtain guidelines for the plane shape design of meander channels. Based on the experimental results of bed topography and measurement of surface flow direction and velocity distribution, from the viewpoint of bank erosion and the concentration and dispersion of flood flow the most suitable plane shape for meandering channels is suggested through which the migration of alternate bars is suppressed.  相似文献   

6.
Many models of river meander migration rely upon a simple formalism, whereby the eroding bank is cut back at a rate that is dictated by the flow, and the depositing bank then migrates passively in response, so as to maintain a constant bankfull channel width. Here a new model is presented, in which separate relations are developed for the migration of the eroding bank and the depositing bank. It is assumed that the eroding bank consists of a layer of fine‐grained sediment that is cohesive and/or densely riddled with roots, underlain by a purely noncohesive layer of sand and/or gravel. Following erosion of the noncohesive layer, the cohesive layer fails in the form of slump blocks, which armor the noncohesive layer and thereby moderate the erosion rate. If the slump block material breaks down or is fluvially entrained, the protection it provides for the noncohesive layer diminishes and bank erosion is renewed. Renewed bank erosion, however, rejuvenates slump block armoring. At the depositing bank, it is assumed that all the sediment delivered to the edge of vegetation due to the transverse component of sediment transport is captured by encroaching vegetation, which is not removed by successive floods. Separate equations describing the migration of the eroding and depositing banks are tied to a standard morphodynamic formulation for the evolution of the flow and bed in the central region of the channel. In this model, the river evolves toward maintenance of roughly constant bankfull width as it migrates only to the extent that the eroding bank and depositing bank ‘talk’ to each other via the medium of the morphodynamics of the channel center region. The model allows for both (a) migration for which erosion widens the channel, forcing deposition at the opposite bank, and (b) migration for which deposition narrows the channel forcing erosion at the opposite bank. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

7.
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8.
Channel bars and banks strongly affect the morphology of both braided and meandering rivers. Accordingly, bar formation and bank erosion processes have been greatly explored. There is, however, a lack of investigations addressing the interactions between bed and bank morphodynamics, especially over short timescales. One major implication of this gap is that the processes leading to the repeated accretion of mid‐channel bars and associated widenings remain unsolved. In a restored section of the Drau River, a gravel‐bed river in Austria, mid‐channel bars have developed in a widening channel. During mean flow conditions, the bars divert the flow towards the banks. One channel section exhibited both an actively retreating bank and an expanding mid‐channel bar, and was selected to investigate the morphodynamic processes involved in bar accretion and channel widening at the intra‐event timescale. We repeatedly surveyed riverbed and riverbank topography, monitored riverbank hydrology and mounted a time‐lapse camera for continuous observation of riverbank erosion processes during four flow events. The mid‐channel bar was shown to accrete when it was submerged during flood events, which at the subsequent flow diversion during lower discharges narrowed the branch along the bank and increased the water surface elevation upstream from the riffle, which constituted the inlet into the branch. These changes of bed topography accelerated the flow along the bank and triggered bank failures up to 20 days after the flood events. Four analysed flow events exhibited a total bar expansion from initially 126 m2 to 295 m2, while bank retreat was 6 m at the apex of the branch. The results revealed the forcing role of bar accretion in channel widening and highlighted the importance of intra‐event scale bed morphodynamics for bank erosion, which were summarized in a conceptual model of the observed bar–bank interactions. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

9.
Hydrological process in arid zones differs substantially from that in better documented humid environments. The ponding point for infiltration is reached within 10 mins of first rain and overland flow forms the major component of basin runoff. Drainage densities are high, approaching 100 km.km?2, maximising the opportunity for both water and eroded soil to reach the channel network. The typical flood bore is not as abrupt as the mythology of desert streams would suggest. Nevertheless, the time of rise of the flood hydrograph is usually between 4 and 16 mins, giving credance to the notion of ‘flash flood’. Measured flows remain subcritical in the main, though Froude number exceeds unity for short periods around peak discharge. Flow is exceedingly turbulent, with Reynolds number > 105 even for much of the recession limb. As a result, suspended sediment concentrations by size grade are shown to be hydraulically controlled. However, the high degree of turbulence and wide availability of sediment from hillslope and channel sources also means that the mean size of the suspended load varies systematically with flow parameters. In this respect, ephemeral streams differ from perennial counterparts in humid environments where no clear-cut relationships exist. There is greater prospect of deriving a physically deterministic model of suspended sediment transport in desert streams. Implications for soil erosion and reservoir siltation are discussed, and sediment is traced from its source to its various sinks within the drainage basin.  相似文献   

10.
Alluvial channel bed incision and bank widening have been reported in both the south‐western and south‐eastern US throughout the past century. Distinct regional differences in climate and landscape properties likely influence the rate of erosion. This study discusses regional differences in hydraulic driving forces and substrate resistance and tests the hypothesis that regional differences exist in average rates of channel incision, bank erosion, and knickpoint retreat. Specifically, we hypothesize that erosion rates are higher in south‐western US streams and reason that this is because of greater flood magnitudes and limited substrate resistance. A review of the literature documenting incision, bank erosion, and knickpoint retreat, however, indicates that intra‐regional differences are larger than inter‐regional differences and that average rates in the south‐western US are either statistically similar or less than the rates in parts of the south‐eastern US. This could either be a result of strong intra‐regional hydroclimatic and substrate variability or because average erosion rate may not be an appropriate metric for inter‐regional comparisons because of the variability between case studies associated with the field methods to measure erosion, duration of study period, and time since disturbance to the channel. Nevertheless, these findings provide a basis for future evaluations of the relative importance of different controls on driving and resisting forces in these and other landscapes characterized by rapid channel incision and arroyo formation. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

11.
An updated linear computer model for meandering rivers with incision has been developed. The model simulates the bed topography, flow field, and bank erosion rate in an incised meandering channel. In a scenario where the upstream sediment load decreases （e.g., after dam closure or soil conservation）, alluvial river experiences cross section deepening and slope flattening. The channel migration rate might be affected in two ways： decreased channel slope and steeped bank height. The proposed numerical model combines the traditional one-dimensional （1D） sediment transport model in simulating the channel erosion and the linear model for channel meandering. A non-equilibrium sediment transport model is used to update the channel bed elevation and gradations. A linear meandering model was used to calculate the channel alignment and bank erosion/accretion, which in turn was used by the 1D sediment transport model. In the 1D sediment transport model, the channel bed elevation and gradations are represented in each channel cross section. In the meandering model, the bed elevation and gradations are stored in two dimensional （2D） cells to represent the channel and terrain properties （elevation and gradation）. A new method is proposed to exchange information regarding bed elevations and bed material fractions between 1D river geometry and 2D channel and terrain. The ability of the model is demonstrated using the simulation of the laboratory channel migration of Friedkin in which channel incision occurs at the upstream end.  相似文献   

12.
Variability of suspended sediment concentration (SSC) versus discharge relationships in streams is often high and illustrates variable particle origins or availability. Particle availability depends on both new sediment supply and deposited sediment stock. The aim of this study is to improve SSC–discharge relationship interpretation, in order to determine the origins of particles and to understand the temporal dynamics of particles for two small streams in agricultural catchments from northwestern France. SSC and discharge were continuously recorded at the outlets and data were examined at different time‐scales: yearly, monthly, with distinction between flood periods and non‐flooding periods, and individual flood events. Floods are classified in relation to SSC–discharge hysteresis, and this typology is completed by the analysis of SSC–discharge ranges during rising and falling flow. We show that particles are mainly coming from channel, banks, either by hydraulic erosion or by cattle trampling. Particle availability presents a seasonal dynamics with a maximum at the beginning of autumn when discharge is low, decreasing progressively during autumn to become a minimum in winter when discharge is the highest, and increasing again in spring. Bank degradation by cattle is the determining factor in the suspended sediment dynamics. Cattle bank‐trampling produces sediment, mostly from spring to autumn, that supplies the deposited sediment stock even outside floods. This hydrologically independent process hides SSC–discharge correlation classically linked to hydraulic erosion and transport. Differences in SSC–discharge relationships and suspended sediment budgets between streams are related to differences in transport capacity and bank degradation by cattle trampling and channelization. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

13.
Severe bank erosion at lowland rivers in Bangladesh devours vast tract of lands and renders thousands of people homeless at high flood; also,rapid deposition changes bed topographies and seriously redu...  相似文献   

14.
This paper uses numerical simulation of flood inundation based on a coupled one‐dimensional–two‐dimensional treatment to explore the impacts upon flood extent of both long‐term climate changes, predicted to the 2050s and 2080s, and short‐term river channel changes in response to sediment delivery, for a temperate upland gravel‐bed river. Results show that 16 months of measured in‐channel sedimentation in an upland gravel‐bed river cause about half of the increase in inundation extent that was simulated to arise from climate change. Consideration of the joint impacts of climate change and sedimentation emphasized the non‐linear nature of system response, and the possibly severe and synergistic effects that come from combined direct effects of climate change and sediment delivery. Such effects are likely to be exacerbated further as a result of the impacts of climate change upon coarse sediment delivery. In generic terms, these processes are commonly overlooked in flood risk mapping exercises and are likely to be important in any river system where there are high rates of sediment delivery and long‐term transfer of sediment to floodplain storage (i.e. alluviation involving active channel aggradation and migration). Similarly, attempts to reduce channel migration through river bank stabilization are likely to exacerbate this process as without bank erosion, channel capacity cannot be maintained. Finally, many flood risk mapping studies rely upon calibration based upon combining contemporary bed surveys with historical flood outlines, and this will lead to underestimation of the magnitude and frequency of floodplain inundation in an aggrading system for a flood of a given magnitude. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

15.
Recent research into flood modelling has primarily concentrated on the simulation of inundation flow without considering the influences of channel morphology. River channels are often represented by a simplified geometry that is implicitly assumed to remain unchanged during flood simulations. However, field evidence demonstrates that significant morphological changes can occur during floods to mobilize the boundary sediments. Despite this, the effect of channel morphology on model results has been largely unexplored. To address this issue, the impact of channel cross‐section geometry and channel long‐profile variability on flood dynamics is examined using an ensemble of a 1D–2D hydraulic model (LISFLOOD‐FP) of the ~1 : 2000 year recurrence interval floods in Cockermouth, UK, within an uncertainty framework. A series of simulated scenarios of channel erosional changes were constructed on the basis of a simple velocity‐based model of critical entrainment. A Monte‐Carlo simulation framework was used to quantify the effects of this channel morphology together with variations in the channel and floodplain roughness coefficients, grain size characteristics and critical shear stress on measures of flood inundation. The results showed that the bed elevation modifications generated by the simplistic equations reflected an approximation of the observed patterns of spatial erosion that enveloped observed erosion depths. The effect of uncertainty on channel long‐profile variability only affected the local flood dynamics and did not significantly affect the friction sensitivity and flood inundation mapping. The results imply that hydraulic models generally do not need to account for within event morphodynamic changes of the type and magnitude of event modelled, as these have a negligible impact that is smaller than other uncertainties, e.g. boundary conditions. Instead, morphodynamic change needs to happen over a series of events to become large enough to change the hydrodynamics of floods in supply limited gravel‐bed rivers such as the one used in this research. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

16.
A study on flood water infiltration and ground water recharge of a shallow alluvial aquifer was conducted in the hyperarid section of the Kuiseb River, Namibia. The study site was selected to represent a typical desert ephemeral river. An instrumental setup allowed, for the first time, continuous monitoring of infiltration during a flood event through the channel bed and the entire vadose zone. The monitoring system included flexible time domain reflectometry probes that were designed to measure the temporal variation in vadose zone water content and instruments to concurrently measure the levels of flood and ground water. A sequence of five individual floods was monitored during the rainy season in early summer 2006. These newly generated data served to elucidate the dynamics of flood water infiltration. Each flood initiated an infiltration event which was expressed in wetting of the vadose zone followed by a measurable rise in the water table. The data enabled a direct calculation of the infiltration fluxes by various independent methods. The floods varied in their stages, peaks, and initial water contents. However, all floods produced very similar flux rates, suggesting that the recharge rates are less affected by the flood stages but rather controlled by flow duration and available aquifer storage under it. Large floods flood the stream channel terraces and promote the larger transmission losses. These, however, make only a negligible contribution to the recharge of the ground water. It is the flood duration within the active streambed, which may increase with flood magnitude that is important to the recharge process.  相似文献   

17.
This study focused on a spatial and temporal analysis of the active channel and associated floodplain lakes using aerial photographs spanning five decades (1942, 1962, 1985, 1999) over a 140 km long reach of the Sacramento. Planimetric changes were analysed longitudinally and temporally to highlight the spatial structures and their evolution through time. The results underline complex changes and space–time pattern in bank erosion, channel length and active channel width. The bank erosion and also channel lengthening were higher between 1962 and 1985 than in the two periods studied before and after. Active channel width significantly decreased from 1942 to 1999; partly progressively from upstream to downstream with local widening whatever the studied periods. Similarly the floodplain lakes observed before 1942–1962 were significantly different in size and geometry from those which appeared during the most recent period. The creation of lakes is less frequent after the 1940s, with a secondary peak of occurrence during the 1962–1985 period. The interpretation of these changes is complex because of various human pressures acting over different time scales (bank protection, flow diversion, sediment starvation, land‐use changes) and various natural influences (flood sequences through out the period, geological setting). The findings are discussed by comparison with previous work, and highlight the important effect of dam impact on peak flow and sediment starvation modifying longitudinally hydraulic conditions within the channel, but also the increase in riprap protection which induced change in bank erosion, channel planimetry and floodplain lake characters (geometry, frequency of renewal). Variation in flood intensities is also observed as having positive effects on the bank erosion pattern. Secondarily, land‐use changes also controlled bank erosion intensity. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

18.
The effects of large floods on river morphology are variable and poorly understood. In this study, we apply multi‐temporal datasets collected with small unmanned aircraft systems (UASs) to analyze three‐dimensional morphodynamic changes associated with an extreme flood event that occurred from 19 to 23 June 2013 on the Elbow River, Alberta. We documented reach‐scale spatial patterns of erosion and deposition using high‐resolution (4–5 cm/pixel) orthoimagery and digital elevation models (DEMs) produced from photogrammetry. Significant bank erosion and channel widening occurred, with an average elevation change of ?0.24 m. The channel pattern was reorganized and overall elevation variation increased as the channel adjusted to full mobilization of most of the bed surface sediments. To test the extent to which geomorphic changes can be predicted from initial conditions, we compared shear stresses from a two‐dimensional hydrodynamic model of peak discharge to critical shear stresses for bed surface sediment sizes. We found no relation between modeled normalized shear stresses and patterns of scour and fill, confirming the complex nature of sediment mobilization and flux in high‐magnitude events. However, comparing modeled peak flows through the pre‐ and post‐flood topography showed that the flood resulted in an adjustment that contributes to overall stability, with lower percentages of bed area below thresholds for full mobility in the post‐flood geomorphic configuration. Overall, this work highlights the potential of UAS‐based remote sensing for measuring three‐dimensional changes in fluvial settings and provides a detailed analysis of potential relationships between flood forces and geomorphic change. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

19.
This study assessed the effect of the largest flood since dam regulation on geomorphic and large wood (LW) trends using LW distributions at three time periods on the 150 km long Garrison Reach of the Missouri River. In 2011, a flood exceeded 4390 m3/s for a two‐week period (705% above mean flow; 500 year flood). LW was measured using high resolution satellite imagery in summer 2010 and 2012. Ancillary data including forest character, vegetation cover, lateral bank retreat, and channel capacity. Lateral bank erosion removed approximately 7400 standing trees during the flood. Other mechanisms, that could account for the other two‐thirds of the measured in‐channel LW, include overland flow through floodplains and islands. LW transport was commonly near or over 100 km as indicated by longitudinal forest and bank loss and post‐flood LW distribution. LW concentrations shift at several locations along the river, both pre‐ and post‐flood, and correspond to geomorphic river regions created by the interaction of the Garrison Dam upstream and the Oahe Dam downstream. Areas near the upstream dam experienced proportionally higher rates of bank erosion and forest loss but in‐channel LW decreased, likely due to scouring. A large amount of LW moved during this flood, the chief anchoring mechanism was not bridges or narrow channel reaches but the channel complexity of the river delta created by the downstream reservoir. Areas near the downstream dam experienced bank accretion and large amounts of LW deposition. This study confirms the results of similar work in the Reach: despite a historic flood longitudinal LW and channel trends remain the same. Dam regulation has created a geomorphic and LW pattern that is largely uninterrupted by an unprecedented dam regulation era flood. River managers may require other tools than infrequent high intensity floods to restore geomorphic and LW patterns. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

20.
The peak of river floods usually decreases in the downstream direction unless it is compensated by freshwater inflow from tributaries. In the Yellow River (China) the opposite is regularly observed, where the peak discharge of river floods increases in the downstream direction (at a rate far exceeding the contribution from tributaries). This flood peak discharge increase is probably related to rapid morphological changes, to a modified bed friction, or to a combination of both. Yet the relative role of these processes is still poorly understood. This paper aims to analyze the relative contribution of bed erosion and friction change to the peak discharge increase, based on available data and a recently developed numerical model. Using this high-resolution, fully coupled morphodynamic model of non-capacity sediment transport, two hyperconcentrated floods characterized by downstream peak discharge increase are numerically reproduced and analyzed in detail. The results reveal that although erosion effects may contribute to the downstream discharge increase (especially in case of extreme erosion), for most cases the increase must be mainly due to a reduction in bed friction during peak discharge conditions. Additionally, based on the concept of channel storage reduction, the effects of decreasing bed friction and (very strong) bed erosion can be integrated in explaining the peak discharge increase.  相似文献   

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