Ice Influences on Channel Stability: Insights from Missouri’s Fort Peck Reach

This paper presents insights from a comprehensive study of river ice influences on alluvial-channel bathymetry and stability. The study entailed unique wintertime fieldwork along the Fort Peck reach of the Missouri River. The insights show how ice formation, presence, and breakup can influence channel stability in several important ways, especially when channels must convey substantial water flow during winter. Ice may hasten the migration of channel bends, cause transient scour and sediment deposition during winter, and induce cyclic shifts of flow thalweg through sinuous-braided subreaches. The insights are of direct significance for engineering activities along the Missouri’s Fort Peck reach and other alluvial channels subject to frigid winters. They also are significant for understanding the frigid-winter habitat of aquatic life in such channels.     

Modeling of Hyperconcentrated Sediment-Laden Floods in Lower Yellow River

This paper presents a rapid forecast model for simulating hyperconcentrated sediment-laden floods in the Lower Yellow River. The model is a hybrid of a conventional one-dimensional mathematical model for unsteady sediment-laden flow and an artificial neural networks model for encapsulation of numerical results. The former provides detailed river flood routing information under typical scenarios, whereas the latter extracts modeling outputs from the former and establishes a station-specific model for efficient flood forecasting. Three typical floods that occurred in the Lower Yellow River in 1977, 1982, and 1996 are simulated. Not only the hybrid model predictions are found to be in close agreement with measured data, but also the computational speed is significantly enhanced. It is found that sediment transport is of significance with regard to the flooding behavior of hyperconcentrated flows. Therefore, the model presented herein is of particular use for rivers with high sediment concentration.     

Case Study: Channel Stability of the Missouri River, Eastern Montana

The construction of Fort Peck Dam in the 1930s on the Missouri River, eastern Montana, initiated a series of changes in hydrologic conditions and channel morphology downstream from the dam that impacted channel stability. Impacts included streambed degradation of up to 3.6 m and substantially altered magnitude, frequency, and temporal distribution of flows. To investigate the effects of the altered flow regime and bed degradation on bank stability, two independent bank-stability analyses (one for planar failures, the other for rotational failures) were performed on 17 outside meanders. Both included the effects of matric suction and positive pore-water pressures, confining pressures, and layering. Instability occurred from the loss of matric suction and the generation of positive pore-water pressures. In this semiarid region, such hydrologic conditions are most likely to occur from the maintenance of moderate and high flows (greater than 425–566 m3/s) for extended periods (5–10 days or more), thereby providing a mechanism for saturation of the streambank. For the postdam period, average annual frequencies of flows maintained above 566 m3/s for 5- and 10-day durations are 149 and 257% greater, respectively. The analyses indicated that 30% of the sites were susceptible to planar failures while 53% of the sites were susceptible to rotational failures under sustained moderate- and high-flow conditions, while under a worst-case rapid drawdown scenario, 80% of the banks were susceptible to failure. Despite the negative effects of the altered flow regime, analysis of maps and aerial photographs shows that closure of Fort Peck Dam has resulted in a fourfold reduction of the average rate of long-term channel migration between the dam and the North Dakota border.     

Wash Load and Bed-Material Load Transport in the Yellow River

It has been the conventional assumption that wash load is supply limited and is only indirectly related to the hydraulics of a river. Hydraulic engineers also assumed that bed-material load concentration is independent of wash load concentration. This paper provides a detailed analysis of the Yellow River sediment transport data to determine whether the above assumptions are true and whether wash load concentration can be computed from the original unit stream power formula and the modified unit stream power formula for sediment-laden flows. A systematic and thorough analysis of 1,160 sets of data collected from 9 gauging stations along the Middle and Lower Yellow River confirmed that the method suggested by the conjunctive use of the two formulas can be used to compute wash load, bed-material load, and total load in the Yellow River with accuracy.     

Hydraulic Evaluation of W-Weir for River Restoration

Various structural measures have been advocated for river restoration and habitat improvement schemes. The W-weir is one such structure that can be used in mobile bed alluvial rivers to diversify habitat and provide grade control. Laboratory studies have been carried out in a large-scale meandering channel with a mobile bed to investigate their effects on flow and sediment transport processes. A W-weir placed immediately downstream of a riffle section created a strongly three-dimensional flow pattern and high-turbulence zones. Two adjacent scour holes of different depths and substrate are formed under clearwater and live bed conditions. The continuity of sediment transport along the channel was not interrupted by the structure and the upstream afflux is minimal. Overbank flow significantly influenced the action of the weir and the scour hole was shifted closer to the structure. In a relatively tight bend followed by a short crossover reach, the weir may affect bed load transport pathways in the downstream bend. Finally, the study provides insights to guide their design for restoration projects.     

黄河兰州段COD_(Cr)降解系数的实验研究

在环境容量的确定过程中,降解系数为关键影响因素,其确定的合理性,直接影响到容量的可靠性。该研究课题通过实验模拟法对黄河兰州段CODC r降解系数进行了研究,考虑到实验条件与实际河流的差异,对降解系数做了修正,并利用实际监测端面监测数据对修正后降解系数进行验证,得出黄河兰州段COD降解系数介于0.185~0.240 d-1。     

Velocity and Turbulence Measurements for Two Overbank Flow Events in River Severn

Field measurements of velocity and turbulence have been carried out in a study reach of the River Severn at Lower Farm near Shrewsbury during overbank flow. Acoustic Doppler velocity meters have been used for the field measurements of velocity and turbulence, particularly in the interface region between river channel and floodplain. The values of local shear velocity and roughness length for the reach under study were calculated using measured velocity data. The distributions of turbulent intensities, and the Reynolds stresses are also presented. The variation of horizontal shear stress in the vertical direction deviates from linear for the main channel/floodplain interaction region due to the existence of a lateral shear and momentum transfer from the floodplain towards the main channel. Comparisons are made between the field data and previous experimental data from the Flood Channel Facility.     

Assessment of the Effectiveness of a Constructed Compound Channel River Restoration Project on an Incised Stream

Compound channels are often constructed in restoration projects on rivers and streams that have been channelized or are deeply incised. This design allows for flow over a wider cross-sectional area during high flows and is expected to reduce both flow velocities and bed-shear stresses in the channel during high flows. Using a compound channel restoration project on Tassajara Creek as a case study, the effectiveness of a constructed compound channel in reducing channel velocities and bed-shear stresses during high flow events was tested in two ways. First, since this is an a posteriori analysis, postproject surveys and assessments of the project are used to demonstrate the geomorphic and ecological benefits of the constructed compound channel for reducing further channel incision, improving channel stability, and enhancing native riparian vegetation, while still providing conveyance capacity for design flood flows. Second, the effectiveness of a constructed compound channel in reducing channel velocities and bed-shear stresses during high flow events is evaluated using both the one-dimensional (1D) model, HEC-RAS, and the three-dimensional (3D) numerical model, UnTRIM. This analysis demonstrates that the 1D analysis does not accurately portray the benefits of the compound channel, and is therefore not a suitable tool for evaluating the effectiveness of compound channel designs. These results demonstrate the advantages of using a 3D model and make a strong case for the implementation of more detailed hydrodynamic modeling in evaluating the suitability of restoration alternatives to improve the planning and design of river restoration projects.     

黄河水源污染对画匠营子水厂供水的影响

黄河是包头市的主要水源.本文分析了由于黄河过境流量减少及河水污染对画匠营子自来水总厂造成的影响,并对此提出了改进建议.     

Three-Dimensional Modeling of Sediment Transport in a Narrow 90° Channel Bend

A three-dimensional numerical model was used for calculating the velocity and bed level changes over time in a 90° bended channel. The numerical model solved the Reynolds-averaged Navier-Stokes equations in three dimensions to compute the water flow and used the finite-volume method as the discretization scheme. The k-ε model predicted the turbulence, and the SIMPLE method computed the pressure. The suspended sediment transport was calculated by solving the convection diffusion equation and the bed load transport quantity was determined with an empirical formula. The model was enhanced with relations for the movement of sediment particles on steep side slopes in river bends. Located on a transversally sloping bed, a sediment particle has a lower critical shear stress than on a flat bed. Also, the direction of its movement deviates from the direction of the shear stress near the bed. These phenomenona are considered to play an important role in the morphodynamic process in sharp channel bends. The calculated velocities as well as the bed changes over time were compared with data from a physical model study and good agreement was found.     

Three-Dimensional Numerical Modeling of Mixing at River Confluences

Lateral mixing of a pollutant is considered as a slow process that is usually complete within 100–300 river widths. Recent studies on flow dynamics at river confluences revealed that lateral mixing can be markedly enhanced when the tributary channel is shallower than the main channel. This study uses a three-dimensional model to examine mixing processes immediately downstream of confluences as well as further downstream in the mainstream. Simulations are presented for a concordant and discordant laboratory junction and a field confluence for a low and a high flow condition. The decrease in standard deviation at a cross section of a tracer over a distance of 5 channel widths is 30% for discordant beds but only 10% for concordant beds in the laboratory simulation. At the natural site, bed discordance is more important at the low flow than at the high flow with corresponding decreases in the standard deviation of 31 and 18% over 3.5 channel widths. Mixing is completed after a distance of 25 and 37 channel widths for the low and high flow conditions, respectively. Further downstream, mixing is mainly affected by planform curvature of the channel.     

Sediment Transport in River Models with Overbank Flows

Some laboratory sediment-transport experiments are described in which a compound channel with a mobile-bed composed of uniform sand with a d50 of 0.88?mm was subjected to overbank flows. The main river channel was monitored to determine the effect of floodplain roughness on conveyance capacity, bed-form geometry, resistance, bed-load transport, and dune migration rate. The floodplain roughness was varied to simulate a wide range of conditions, commensurate with conditions that can occur in a natural river. For a given discharge, the main river channel bed was found to adjust itself to a quasi-equilibrium condition governed by the lateral momentum transfer between the floodplain and main channel flows and the local alluvial resistance relationship appropriate for the proportion of total flow in the main river channel. The sediment transport rate was found to reflect all these influences. The data are summarized in equation form for comparison with other experimental studies and for checking numerical river simulation models.     

A new closure methodology for 1D fully coupled models of mobile-bed alluvial hydraulics: application to silt transport in the Lower Yellow River

A new closure approach involving a common parameter has been incorporated into a 1D fully coupled model of mobile-bed alluvial hydraulics. The objective is to simplify the methodology of 1D river routing models and to improve their accuracy. The common parameter, called control factor m,introduces the concept of Rossiter modes in alluvial hydraulics and represents the interactions between the flow, the sediment transport and the bed morphology. The feasibility of the new closure approach has been established by reproducing numerically the 2002 silt flushing experiment conducted on the Lower Yellow River (LYR) downstream the Xiaolangdi reservoir. From the comparison between the experimental data and the numerical results, a time evolution of the control factor m reproducing the characteristics of the flow has been extracted. This time evolution agrees with analysis conducted previously on other datasets and with data measured during the flush. The results obtained with this time evolution for the hydraulics, the sediment transport and bed adaptation are encouraging but still need improvements and further feeding from complementary experimental data.     

Effect of Bed Armoring on Bed Topography of Channel Bends

The two-dimensional numerical model previously developed by the writers for modeling the bed variations in a channel bend with uniform sediment is upgraded to incorporate the nonuniformity of sediment particles as well as bed armoring. In this model, the two-dimensional, depth-averaged, unsteady flow equations along with the bed-load mass conservation equation are solved in a body-fitted coordinate system by using the Beam and Warming alternating-direction implicit (ADI) scheme. A one-dimensional bed surface armoring approach is extended herein for application to a two-dimensional domain. The model is applied to a 180° bend with a constant radius under unsteady flow conditions. Numerical simulations are carried out to study the effect of bed armoring on the bed deformations in channel bends. Results show that bed armoring reduces scour in channel bends.     

Sediment and Metals Modeling in Shallow River

It is a challenge to apply coupled hydrodynamic, sediment process, and contaminant fate and transport models to the studies of surface water systems. So far, there are few published modeling studies on sediment and metal transport in rivers that simulate storm events on an hourly basis and use comprehensive data sets for model input and model calibration. The United States Environmental Protection Agency (USEPA) in 1997 emphasized the need for credible modeling tools that can be used to quantitatively evaluate the impacts of point sources, nonpoint sources, and internal transport processes in 1D/2D/3D environments. A 1D and time-dependent hydrodynamic, sediment, and toxic model, within the framework of the 3D Environmental Fluid Dynamics Code (EFDC), has been developed and applied to Blackstone River, Mass. The Blackstone River Initiative (USEPA) in 1996, a multiyear and multimillion-dollar project, provided the most comprehensive surveys on water quality, sediment, and heavy metals in the river, and served as the primary data set for this study. The model simulates three storm events successfully. The river flow rates are well calculated both in amplitude and in phase. The sediment transport and resuspension processes are depicted satisfactorily. The concentrations of sediment and five metals (cadmium, chromium, copper, nickel, and lead) during the three storm events are also simulated very well. Numerical analyses are conducted to clarify the impacts of contaminant sources and sediment resuspension processes on the river. While point sources are important to sediment contamination in the river, other sources, including nonpoint sources from watershed and bed resuspension, were found to contribute significantly to the sediment and metals in the river. Point sources alone cannot account for the total metals in the river. The model presented in this paper can be a useful tool for studying sediment and metals transport in shallow rivers and for water resource management.     

Predicting the Morphodynamic Response of Silt-Laden Rivers to Water and Sediment Release from Reservoirs: Lower Yellow River, China

The high sediment load of the Yellow River results in rapid infilling of its reservoirs when sediment is not regularly flushed. Simultaneously, the downstream reaches of the Yellow River experience extremely high siltation rates, which are reduced when sediment is retained in its reservoirs. To minimize siltation in the reservoirs and the downstream river bed, water and sediment are released from the reservoir in a controlled way through flushing experiments. In this paper, we analyze the effect of such a flushing event on the downstream river bed through data analysis and numerical modeling. Sedimentation may be minimized by relating the amount of sediment released from the reservoir to the sediment available for release through operational monitoring and by releasing relatively clear water after turbid water. Despite this flushing of sediment, the reservoir will eventually fill up, and more sediment released again into the lower Yellow River. The change in discharge magnitude and frequency brought about by the reservoir will then probably result in increased siltation rates in the lower Yellow River compared to the predam situation.     

New Method for Meander-Loop Cutoffs

When a reach of a meandering river becomes very sinuous, and thereby causes significant problems for navigation and flood discharge, cutoff works in the narrow-neck portion of the river bend may be carried out. It is common practice to excavate a small slightly bending cutoff channel, referred to as pilot channel, which is gradually scoured and enlarged toward the concave bank by the river flow. In the design of the Wujiadu cutoff in the Caoe River (already constructed in 1998), the authors proposed a new method for constructing meander-loop cutoffs. Using this technique, the new cutoff channel on the concave bank side and the flood protection embankments are formed during construction, allowing the nonprotected convex bank and the bed of the new cutoff channel to be scoured and be enlarged by flood. Thus, the amount of excavation of the cutoff works can be significantly reduced and the construction period can be largely shortened. In this way, the lines of the new cutoff channel may be unified with the overall river alignment. This paper presents the design philosophy and major points of the method for meander-loop cutoffs.     

Case Study: Modeling the Lateral Mobility of the Rio Grande below Cochiti Dam, New Mexico

The Cochiti reach of the Rio Grande served as a case study to test the hypothesis that the lateral mobility of an alluvial river decreases as the river approaches equilibrium. The lateral mobility of the river was measured using a geographic information system from digitized aerial photographs of the nonvegetated active channel between 1918 and 2001. Reach-averaged lateral mobility was quantified in terms of width change, lateral migration, and total lateral movement. By 2001, the width of the Cochiti Reach was close to the expected equilibrium width indicating that the river had adjusted to the incoming water and sediment load. An exponential equation based on deviation from equilibrium width described 95–96% of the variance in channel width, 78–90% of variance in migration rates, and 92% of the variance in total lateral movement between 1918 and 1992. For validation of the model, the 2001 width and migration rates were predicted with errors as low as 19 and 8%, respectively. The exponential width model was also applied to four other rivers that exhibited narrowing trends following dam construction and explained 82–89% of the variance in width change on those rivers.     

3D Numerical Modeling of Flow and Sediment Transport in Laboratory Channel Bends

The development of a fully three-dimensional finite volume morphodynamic model, for simulating fluid and sediment transport in curved open channels with rigid walls, is described. For flow field simulation, the Reynolds-averaged Navier–Stokes equations are solved numerically, without reliance on the assumption of hydrostatic pressure distribution, in a curvilinear nonorthogonal coordinate system. Turbulence closure is provided by either a low-Reynolds number k?ω turbulence model or the standard k?ε turbulence model, both of which apply a Boussinesq eddy viscosity. The sediment concentration distribution is obtained using the convection-diffusion equation and the sediment continuity equation is applied to calculate channel bed evolution, based on consideration of both bed load and suspended sediment load. The governing equations are solved in a collocated grid system. Experimental data obtained from a laboratory study of flow in an S-shaped channel are utilized to check the accuracy of the model’s hydrodynamic computations. Also, data from a different laboratory study, of equilibrium bed morphology associated with flow through 90° and 135° channel bends, are used to validate the model’s simulated bed evolution. The numerically-modeled fluid and sediment transportation show generally good agreement with the measured data. The calculated results with both turbulence models show that the low-Reynolds k?ω model better predicts flow and sediment transport through channel bends than the standard k?ε model.     

Numerical Study of Ice Jam Dynamics in Upper Niagara River

A two-dimensional numerical model for the dynamics of surface ice transport and ice jam in rivers was developed and applied to the upper Niagara River. The model treats river ice dynamics as a two-layer system consisting of a layer of surface ice coupled with the underlying water flow. A Lagrangian discrete-parcel method was used to model the ice dynamics, and a finite-element method to model the hydrodynamics. These two components are coupled through the interaction at the interface between the ice layer and the flowing water. The model was validated using recorded data and observations of ice runs and ice jams. Subsequently, the model was used to evaluate possible measures for mitigating ice jams that adversely affect hydropower operations on the upper Niagara River.