Long-term morphological river response to hydrological changes

Hydrologic changes have a great impact on the long-term river morphology. The most common anthropic cause is the construction of dams, which often reduces both the discharge regime and sediment transport, producing a narrowing and degradation of the river bed. In this study we propose a simple, lumped morphodynamic model that describes fluvial cross-section dynamics consequent to changes in discharge and sediment transport induced by external factors. The model provides the temporal dynamics of the river width and bed quote. These dynamics result not to be trivial and can exhibit non monotonic behavior, with aggradations and degradations, and narrowing and widening. The model has been tested on real rivers using data obtained from field studies. The agreement between the outcomes and the data measured in the field works is always satisfactory.     

Numerical simulation of low-flow channel evolution due to sediment augmentation

For the conservation and restoration of river environment,a sediment replenishment technique, which conveys a part of the sediments excavated and/or dredged from reservoirs to the river below dams is developed and has been implemented tentatively in several dams.Sediments placed as replenishment can be flushed out and transported downstream by floodwater or dam releases.The flushed sediments are expected to contribute to the control of degradation and the variation of the low-flow channel.However,this technique is in the development stage because there are many unknown factors.Therefore,systematic investigations are necessary for practical management of the technique.In this study,the effects of the location of replenishment sediment on sediment flushing and on control of degradation were investigated by means of flume experiments.A two-dimensional numerical model was also developed to further investigate the effects of sediment augmentation on river restoration.The numerical model treats bank erosion and sediment transport over fixed beds. The simulation results were verified against the experimental results.The flushing process of replenishment sediment was investigated first,and then its effect as a countermeasure for river bed degradation was analyzed.Results are summarized as follows:(1) Augmentation at upper riffles is effective for flushing of replenishment sediment and variation of low-flow channel.(2) Amelioration of degradation can be found in the cases of two types of placement.The amelioration effect of upper riffle placement was larger than that of lower riffle placement(3) Aggradation rate of the bed near the replenishment site in the fixed bed is large in the lower riffle placement compared with the upper riffle placement.(4) The numerical model was found to be generally successful as a predictive tool.     

NUMERICAL SIMULATION OF SEDIMENT TRANSPORT IN ALLUVIAL RIVER WITH FLOODPLAINS

1 INTRODUCTION The transport of sediment in rivers with active floodplains is a two-dimensional process because the main channel and the floodplain can have very different transport capacities. Therefore, two-dimensional (2D) models are often used to simulate the streamwise and transverse variations of sediment erosion and deposition. Many 2D numerical models have been presented to simulate sediment transport in floodplains (James, 1985; Pizzuto, 1987; Howard, 1992; Nicholas and Walli…     

A new mixing equation for bed material composition in bed form dominant conditions

Vertical mixing beneath the bed surface and its effect on sediment composition has long been underestimated. This paper proposes a mixing equation to illustrate the temporal and spatial variations of bed material composition for bed form dominant rivers. A continuous mass conservation equation for elevation-specific nonuniform bed material composition is initially presented. A vertical mixing equation is finally derived as a diffusion-type partial differential equation with a source term charact...     

Geomorphic monitoring and response to two dam removals: rivers Urumea and Leitzaran (Basque Country,Spain)

Dam removal has been demonstrated to be one of the most frequent and effective fluvial restoration actions but at most dam removals, especially of small dams, there has been little geomorphological monitoring. The results of the geomorphological monitoring implemented in two dams in the rivers Urumea and Leitzaran (northern Spain) are presented. The one from the River Urumea, originally 3.5 m high and impounding 500 m of river course, was removed instantaneously whereas that in the River Leitzaran, 12.5 m high, and impounding 1500 m of river course, is in its second phase of a four‐stage removal process. Changes in channel morphology, sediment size and mobility and river bed morphologies were assessed. The monitoring included several different techniques: topographical measurements of the channel, terrestrial laser scanner measurements of river bed and bars, sediment grain size and transport; all of them repeated in four (May, August, November 2011 and May 2012) and five (July and September 2013, April and August 2014 and June 2015) fieldwork campaigns in the River Urumea and River Leitzaran, respectively. Geomorphic responses of both dam removals are presented, and compared. Morphological channel adjustments occurred mainly shortly after dam removals, but with differences among the one removed instantaneously, that was immediate, whereas that conducted by stages took longer. Degradational processes were observed upstream of both dams (up to 1.2 m and 4 m in the River Urumea and River Leitzaran, respectively), but also aggradational processes (pool filling), upstream of Inturia Dam (2.85 m at least). Less evident aggradational processes were observed downstream of the dams (up to 0.37 m and 0.50 m in the River Urumea and River Leitzaran, respectively). Flood events, especially a 100 year flood registered during the monitoring period of Mendaraz Dam removal, reactivated geomorphological processes as incision and bank erosion, whereas longitudinal profile recovery, grain‐size sorting and upstream erosion took longer. Copyright © 2016 John Wiley & Sons, Ltd.     

2D NUMERICAL SIMULATION OF FLOOD AND FLUVIAL PROCESS IN THE MEANDERING AND ISLAND-BRAIDED MIDDLE YANGTZE RIVER

The characteristics of water flow and sediment transport in a typical meandering and island-braided reach of the middle Yangtze River is investigated using a two-dimensional （2D） mathematical model. The major problems studied in the paper include the carrying capacity for suspended load, the incipient velocity and transport formula of non-uniform sediment, the thickness of the mixed layer on the riverbed, and the partitioning of bed load and suspended load. The model parameters are calibrated using extensive field data. Water surface profiles, distribution of flow velocities, riverbed deformation are verified with site measurements. The model is applied to a meandering and island-braided section of the Wakouzi-Majiazui reach in the middle Yangtze River, which is about 200 km downstream from the Three Gorges Dam, to study the training scheme of the navigation channels. The model predicts the processes of sediment deposition and fiver bed erosion, changes of flow stage and navigation conditions for the first 20 years of impoundment of the Three Gorges Project.     

River adjustment to changes in sediment load: The effects of tin mining on the Ringarooma River,Tasmania, 1875–1984

The mining of alluvial tin in the Ringarooma basin began in 1875, reached a peak in 1900–20, and had virtually ceased by 1982. During that time 40 million m³ of mining waste were supplied to the main river, quickly replacing the natural bed material and requiring major adjustments to the channel. Based on estimates of sediment supply from more than 50 widely scattered mines and the frequency of flows capable of transporting the introduced load, the river's transport history is reconstructed using a mass-conservation model. Because of the lengthy time period (110 years) and river distance (75 km) involved, the model cannot predict detailed change but it does reproduce the main pattern of sediment movement in which successive phases of aggradation and degradation progress downstream. Peak storage is predicted in that part of the river where braiding and anastomosis are best developed. Aggradation was most rapid in the upper reaches close to major supply points, becoming slower and later with distance downstream. Channel width increased by up to 300 per cent where the valley floor was broad and braiding became relatively common. Bridges had frequently to be replaced. While bed levels were still rising in lower reaches, degradation began in upper ones, notably after 1950, and by 1984 had progressed downriver over 30 km. Rates of incision reached 0·5 m yr^?1, especially in the early 1970s when record high flows occurred. As a result of degradation the bed material became gravelly through either reexposure of the original bed or lag concentration of coarser fractions. Also a narrower unbraided channel has developed. The river is beginning to heal itself and upper reaches now have reasonably stable beds but at least another 50 years will be required for the river to cleanse its channel of mining debris.     

Theory of delayed response in river morphodynamics: Applicability and limitations

The delayed response of fluvial rivers to external disturbances has been described by many researchers.To simulate such behavior,the rate law model(or the delayed response model) was developed by previous researchers,and has been applied to a series of river morphological problems.However,to date,the applicability of the rate law model has not been fully understood.In the current paper,a physicallybased analysis of the rate law model is presented to assess the responses of bed elevation and the ...     

River system discontinuities due to lateral inputs: generic styles and controls

In alluvial river systems, lateral inputs of water and/or sediment at junctions or undercut hillsides can disrupt what would otherwise be smooth downstream trends in mainstream bed elevation, channel gradient, and bed grain size. Generic styles of mainstream response to lateral inputs are investigated using a one‐dimensional sediment routing model with multiple grain size fractions. Numerical experiments isolate the effects of three para‐meters: ratio of tributary to mainstream water flux (QR), ratio of tributary to mainstream bedload flux (FR), and ratio of tributary to mainstream bedload diameter (DR). The findings are not unduly sensitive to the choice of initial conditions or to approximations made in the model. The primary distinction is between junctions that aggrade, causing local profile convexity with interrupted downstream fining, and junctions that degrade. The immediate effects of aggradation extend further upstream than downstream, whereas degradation is much more subdued and has no upstream impact. Aggradation is typical of coarse inputs (DR > 2), and degradation of fine inputs (DR < 1), but very high ratios of QR to FR also promote degradation. Both aggrading and degrading junctions can lead to a change in mainstream bed grain size well below the junction, with higher ratios of QR to FR producing a coarser distal bed. The effect of a tributary reflects the interplay between additional bed load and additional discharge to transport it. Copyright © 2006 John Wiley & Sons, Ltd.     

Assessment of the flow regime alterations in the middle reach of the Yangtze River associated with dam construction: potential ecological implications

Hydrological regimes strongly influence the biotic diversity of river ecosystems by structuring physical habitat within river channels and on floodplains. Modification of hydrological regimes by dam construction can have important consequences for river ecosystems. This study examines the impacts of the construction of two dams, the Gezhouba Dam and the Three Gorges Dam, on the hydrological regime of the Yangtze River in China. Analysis of hydrological change before and after dam construction is investigated by evaluating changes in the medians and ranges of variability of 33 hydrological parameters. Results show that the hydrological impact of the Gezhouba Dam is relatively small, affecting mainly the medians and variability of low flows, the rate of rise, and the number of hydrological reversals. The closure of the Three Gorges Dam has substantially altered the downstream flow regime, affecting the seasonal distribution of flows, the variability of flows, the magnitude of minimum flows, low‐flow pulses, the rate of rise, and hydrological reversals. These changes in flow regime have greatly influenced the aquatic biodiversity and fish community structure within the Yangtze River. In particular, populations of migratory fish have been negatively impacted. The results help to identify the magnitudes of hydrological alteration associated with the construction of dams on this important large river and also provide useful information to guide strategies aimed at restoration of the river's ecosystems. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatial prediction of river channel topography by kriging

Topographic information is fundamental to geomorphic inquiry, and spatial prediction of bed elevation from irregular survey data is an important component of many reach‐scale studies. Kriging is a geostatistical technique for obtaining these predictions along with measures of their reliability, and this paper outlines a specialized framework intended for application to river channels. Our modular approach includes an algorithm for transforming the coordinates of data and prediction locations to a channel‐centered coordinate system, several different methods of representing the trend component of topographic variation and search strategies that incorporate geomorphic information to determine which survey data are used to make a prediction at a specific location. For example, a relationship between curvature and the lateral position of maximum depth can be used to include cross‐sectional asymmetry in a two‐dimensional trend surface model, and topographic breaklines can be used to restrict which data are retained in a local neighborhood around each prediction location. Using survey data from a restored gravel‐bed river, we demonstrate how transformation to the channel‐centered coordinate system facilitates interpretation of the variogram, a statistical model of reach‐scale spatial structure used in kriging, and how the choice of a trend model affects the variogram of the residuals from that trend. Similarly, we show how decomposing kriging predictions into their trend and residual components can yield useful information on channel morphology. Cross‐validation analyses involving different data configurations and kriging variants indicate that kriging is quite robust and that survey density is the primary control on the accuracy of bed elevation predictions. The root mean‐square error of these predictions is directly proportional to the spacing between surveyed cross‐sections, even in a reconfigured channel with a relatively simple morphology; sophisticated methods of spatial prediction are no substitute for field data. Copyright © 2007 John Wiley & Sons, Ltd.     

Downstream effects of impounding a natural lake: the Snake River downstream from Jackson Lake Dam,Wyoming, USA

A sediment mass balance constructed for a 16‐km reach of the Snake River downstream from Jackson Lake Dam (JLD) indicates that river regulation has reduced the magnitude of sediment mass balance deficit that would naturally exist in the absence of the dam. The sediment budget was constructed from calibrated bed load transport relations, which were used to model sediment flux into and through the study reach. Calibration of the transport relations was based on bed load transport data collected over a wide range of flows on the Snake River and its two major tributaries within the study area in 2006 and 2007. Comparison of actual flows with unregulated flows for the period since 1957 shows that operations of JLD have reduced annual peak flows and increased late summer flows. Painted tracer stones placed at five locations during the 2005 spring flood demonstrate that despite the reduction in flood magnitudes, common floods are capable of mobilizing the bed material. The sediment mass balance demonstrates that more sediment exits the study reach than is being supplied by tributaries. However, the volume of sediment exported using estimated unregulated hydrology indicates that the magnitude of the deficit would be greater in the absence of JLD. Calculations suggest that the Snake River was not in equilibrium before construction of JLD, but was naturally in sediment deficit. The conclusion that impoundment lessened a natural sediment deficit condition rather than causing sediment surplus could not have been predicted in the absence of sediment transport data, and highlights the value of transport data and calculation of sediment mass balance in informing dam operations. Copyright © 2011 John Wiley & Sons, Ltd.     

A study of river channel pattern information recorded by grain size parameters of fluvial sediment

River channel pattern may be regarded as the outcome of streamflow, sediment load, and channel boundary conditions, as can the grain size distribution of bed material. It may therefore be expected that connections should exist between river channel pattern characteristics and the corresponding river bed material grain size parameters. Using data from some Chinese rivers, an attempt has been made to express these connections quantitatively by using statistical methods. The work demonstrates that the river's bed load can be related to the percentage of the traction subpopulation of the bed material shown by the probabilistic plot of grain size cumulative-frequency curve. The study has also revealed some correlations between the bed material grain size parameters of rivers and their channel geometry such as channel width-depth ratio and channel sinuosity. For instance, the higher the ratio of the traction to suspension subpopulation in bed material, the more sinuous, more shallow, and wider the river channel would be. Furthermore, a discrimination function has been given to distinguish between meandering and wandering braided rivers. If the existence of these relationships can be supported by data from more rivers in other regions, then by using them we can postdict palaeoriver channel geometry and its channel pattern character from fluvial sediment grain size parameters of the palaeoriver. This would open a new way to reconstruct the physicogeographical environment in which palaeorivers developed.     

Two-dimensional modeling of channel evolution under the influence of large-scale river regulation works

In the middle and lower reaches of alluvial rivers, various kinds of river regulation projects affecting natural channel evolution often are distributed due to the requirements of flood control, navigation,and channel stability. However, the influence of large-scale river regulation works on fluvial processes is not fully known. Therefore, a two-dimensional(2D) morphodynamic model has been improved to address this problem. The new detailed procedure is presented in this paper:(i) First, each nod...     

Modelling of meander migration in an incised channel

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.     

Morphological responses in a meandering and island-braided reach of the Middle Yangtze River to the Three Gorges Reservoir impoundment

Lateral migration is an important form of morphological changes on the Middle Yangtze River (MYR), particularly for the lower Jingjiang reach. The Three Gorges Reservoir (TGR) has substantially reduced sediment supply to the downstream river channels since its impoundment in June 2003. The scientific understanding of how decrease of sediment influences the processes of bank erosion and channel adjustments is complex and limited. In the present paper, the morphological responses in a typical meandering and island-braided river segment of the MYR to the filling of the TGR were investigated by a 3-D morphodynamic model. The potential of the 3-D model has been demonstrated by the observed data. The morphological evolutions in the Shishou bend during the first 12 years of the TGR impound-ment were predicted. The effects of the TGR operation on the planform evolutions in the study reach were analyzed based on the simulated results. Sediment load is decreased by 75%due to the early filling of the TGR. The magnitude of bed degradation with less sediment load due to the TGR operation is increased compared with the pre-dam situation. Qualitatively, the overall planform evolution trends in the Shishou bend after the TGR operation are similar to that without the TGR operation. The magnitude of lateral migration has been increased in some part of the channel bend, where the morphological response of the TGR operation exhibits more lateral migration rather than vertical degradation. Scouring at the bank toe enhances bank failure. Decrease of sediment load and weak bank anti-scour ability as well as the significant helical flow can be responsible for intensified bank erosion in the channel bend.     

Seasonal and annual maximum streamflow forecasting using climate information: application to the Three Gorges Dam in the Yangtze River basin,China / Prévision d'écoulements saisonnier et maximum annuel à l'aide d'informations climatiques: application au Barrage des Trois Gorges dans le bassin du Fleuve Yangtze,Chine

Abstract

This paper explores the potential for seasonal prediction of hydrological variables that are potentially useful for reservoir operation of the Three Gorges Dam, China. The seasonal flow of the primary inflow season and the peak annual flow are investigated at Yichang hydrological station, a proxy for inflows to the Three Gorges Dam. Building on literature and diagnostic results, a prediction model is constructed using sea-surface temperatures and upland snow cover available one season ahead of the prediction period. A hierarchical Bayesian approach is used to estimate uncertainty in the parameters of the prediction model and to propagate these uncertainties to the predictand. The results show skill for both the seasonal flow and the peak annual flow. The peak annual flow model is then used to estimate a design flood (50-year flood or 2% exceedence probability) on a year-to-year basis. The results demonstrate the inter-annual variability in flood risk. The predictability of both the seasonal total inflow and the peak annual flow (or a design flood volume) offers potential for adaptive management of the Three Gorges Dam reservoir through modification of the operating policy in accordance with the year-to-year changes in these variables.     

Key morphological changes and their linkages with stream power and land-use changes in the Upper Tapi River basin,India

Knowledge on spatio-temporal variations in planform, hydraulic geometry, and bed-level variations of alluvial streams is required for planning and development of hydraulic structures and bank protection works. In the current study, a Geographic Information System (GIS) has been used to analyze topographical maps, multi-temporal remotely sensed imagery, and hydrologic and hydraulic data to extract the morphological parameters of the Upper Tapi River, India. The river has been found to have consistent migration towards the northern direction, with erosion/deposition on right/left banks. The river has not experienced any major meander except in the lower reaches of the Upper Tapi Gorge and minor braiding conditions at the location where the river emerges from mountainous topography to the plain region. The analyzed river cross sections were found to be depth dominated, and contain large flows within the channel banks. The cross-sections exhibited moderate channel bed adjustments in 1994, 2006, and 2007 wherein excessive sediment flux and stream power were capable of causing morphological changes in the river. High intensity rainfall in the subcatchment resulted in high sediment flux into the river during 1994, which was reported to cause significant aggradation at the downgauging station. The analysis of sediment flux into the river in conjunction with decadal land use land cover, revealed that sediment yield from the catchment was reduced during 2000–2010 due to an increase in water bodies in the form of minor hydraulic structures. The entry of comparatively less sediment laden water into the river, resulted in moderate bed degradation especially in 2006 and 2007 as observed at the downstream station. The methodology applied in the current study is generic in nature and can be applied to other rivers to identify their morphological issues.     

DEPTH-INTEGRATED MODELING FOR AGGRADING/DEGRADING MOBILE CHANNELS I： MODEL DEVELOPMENT

1 INTRODUCTION The study of sediment transport in alluvial river is one of the most important fields in hydraulic engineering. Sediment transport has direct influence on the evolution of riverbeds, estuaries and coastlines, and, in turn, affects decision-making of flood control, operating rule of reservoir, design of hydraulic structure and many other aspects. Models with different orders of dimensions were presented in the literature, and most of them had common basis that they were formu…     

控制—反调节水库协同生态调度的优化策略:以三峡—葛洲坝梯级水库为例

全球主要河流已成为受梯级水库控制的人工调节系统.河流鱼类作为淡水生态系统的重要组成部分,在人类对河流水能资源开发利用的进程中,面临着种群退化、多样性丧失的巨大胁迫.水库生态调度是在鱼类关键生命期人为营造满足鱼类需求的水文水动力条件,减缓水库不利生态影响的一种生态环保措施.然而,在生态调度的实践过程中,受水库不同运行方式...