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River floods are usually featured by a downstream flattening discharge peak whereas a downstream increasing discharge peak is observed at a rate exceeding the tributary discharge during highly silt-laden floods (hyperconcentrated floods) in China's Yellow River. It entails a great challenge in the downstream flood defence and the underlying mechanisms need to be unravelled. Previous study on this issue only focuses on one possible mechanism, while the present work aims to reveal the relative importance of bed roughness change and bed erosion in the hyperconcentrated flood. Using a newly developed fully coupled morphodynamic model, we have conducted a numerical study for the 2004 hyperconcentrated flood in the Xiaolangdi-Jiahetan reach of the Lower Yellow River. In order to focus on the physical mechanism and to reduce uncertainty from low-resolution topography data, the numerical modeling was carried out in a schematized 1-D channel of constant width. The basic understanding that bed roughness decreases with concentration at moderate concentrations (e.g. several 10 s to 100 s g L−1) was incorporated by a simple power-law relation between Manning roughness coefficient and sediment concentration. The feedback between the bed deformation and the turbid flow, however, was fully accounted for, in the constituting equations as well as in the numerical solutions. The model successfully reproduced the downstream flood peak increase for the 2004 flood when considering the hyperconcentration-induced bed roughness reduction. As the hyperconcentration lags shortly behind the flood peak, later parts of the flood wave may experience less friction and overtake the wave front, leading to the discharge increase. In comparison, bed erosion is much less important to the discharge increase, at least for hyperconcentrated flood of moderate sediment concentration.

A sediment transport equation based on universal stream power is presented for the prediction of bed-material concentrations in large sand-bed rivers. The universal stream power, which is derived from the energy concept, has the advantage of eliminating the energy slope as a parameter. The energy slope, which is in the order of 10-5 for large rivers, is a major source of uncertainty in measurements. The analysis shows that relationships derived from flume experiments with shallow flows cannot be universally applied to large rivers with deep flows. Also the use of dimensionless homogeneous parameters in an equation is not sufficient to ensure its applicability to flow conditions where flow depths are several orders of magnitude larger. The comparisons between computed and measured sediment concentrations indicate that the commonly used Engelund and Hansen, Ackers and White, and Yang equations which were developed using mainly flume experiments are not applicable for large rivers with flow depths and Reynol...

A study on the design of highway cross culverts in an area of potential debris flow along the Snowmass Canyon segment of SH-82 in Colorado is presented. Considering the characteristics of the drainage network and the highway cross culverts, FHWA’s BRI-STARS model is enhanced to simulate surface profiles for debris flows. Appropriate sizing of highway cross culverts is provided on the basis of the results of the model simulations for various approaches utilizing both clear-water and debris flow techniques. It is shown that the traditional clear-water modeling results in considerably undersized culverts for debris flows. The modified BRI-STARS model was verified using field data and was shown to be an effective tool in the design of cross culverts to pass debris flows across highways.

Total bed-material load computations for nonuniform sediment mixtures were evaluated for the commonly used Engelund and Hansen, Ackers and White, and Yang transport formulas. The results of sediment transport computations indicate considerable scatter is in bed-material load transport relations for nonuniform sediment mixtures. The analysis demonstrates that a single fixed size, such as D35 or D50, cannot reflect the presence of various size fractions on the transport of nonuniform sediment mixtures. A size gradation compensation factor incorporates the effect that the size distribution has on the transport of sediment mixtures. The size gradation compensation factor, Kg, which is a function of the geometric standard deviation, σg, of the bed material and the flow conditions, in conjunction with the D50 of the bed material yields an equivalent representative diameter, De. By including De in the Engelund and Hansen, Ackers and White, and Yang formulas, the agreement between computed and measured bed-materi...

Shear stress distribution around vertical wall abutments is studied experimentally, and maximum shear stresses that occur at the upstream corner of vertical wall abutments are determined. Shear stress distributions are presented for Froude numbers ranging from 0.30 to 0.90 and for protrusion ratios (ratio of protrusion length perpendicular to direction of flow to total channel width) of 0.1, 0.2, and 0.3. Shear stress at the nose region of an abutment can be expressed as the sum of stresses due to contraction and the abutment structure alone. An expression for the nose shear amplification factor, Λnose, defined as the ratio of the bottom shear stress at the nose region to the approach flow shear stress, is derived. This amplification is due to the contraction and to the presence of a protrusion. In the computation of total shear stresses due to vertical wall abutments, expressions for the shear stress amplification at short contracted reaches, Λcont, and amplification due to abutment structure alone, Λnos...

The water resources development has changed the water and sediment situation in the waterway and ruined the balance between the lake system and the eco-environment. It is important to realize the new environment after the change of the water and sediment situation, and to explore the effective environmental improvement measures for the sustainable development and the lake health. Healthy lake is a lake whose social functions and natural functions can be balanced or compromised in corresponding periods. The indexes reflecting a healthy lake should be favorable lake bed, acceptable water quality and favorable lake ecosystem, and the social functions and natural functions of a lake perform in a balanced way. The lake health indexes should reflect the lake's natural function status which includes the lake bed, water quality, lake ecosystem and the relative runoff. The variety and quantity would be different for different lakes because of different natural features and social background. The indexes should be decided according to the requirement of maintaining lake's normal natural functions and the extent whether the social functions and the natural functions could perform in the balanced way, and also the indexes should be adjusted according to the change of the natural features and social background. The key to describe the lake health situation is to find out the relatively balanced lake situation based on several critical quantified indexes such as the runoff discharge into the lake, water quality etc., which are influenced by the delayed response of the lake to the changing upstream water and sediment conditions. This paper proposed a method to identify the relatively balanced lake condition and lake health indexes.

Bankfull area is highly related to the incoming discharge and sediment load, as well as the boundary condition of river channel. Based on the computation of erosion and sedimentation, a method for bankfull area prediction was proposed in this paper. The bankfull area at Gaocun station on the Lower Yellow River measured from 1960 to 2002 was used to test the proposed method. Results show that the computed bankfull areas are in good agreement with the measured values. The proposed method has a clear physical meanings and revealed the inherent relation between bankfull area and the incoming discharge, sediment load and channel boundary conditions.

Due to the continuous intensification of human activities, the global water demand has dramatically increased. As a result, great changes have taken place in water runoff and sediment load in rivers all over the world. This paper selected fourteen major rivers of the world as its study objects on the basis of the significance of the rivers and the degree of difficulty in getting water and sediment data. Evidence from long-term water runoff and sediment load records indicates that water runoff of rivers located in humid and semi-humid areas has changed little while it has decreased significantly in rivers located in arid and semi-arid areas. Sediment load of rivers located in humid and semi-humid areas has changed little, while it has decreased dramatically in rivers located in arid and semi-arid areas. Though both water runoff and sediment load of rivers located in arid and semi-arid areas have decreased, the decrease of sediment load is much more significant than water runoff.

[1] Reservoir sedimentation management is of critical importance for the sustainable development of surface water resources. Sanmenxia Dam, located on the middle reach of the Yellow River, in China, is notorious for its severe sedimentation problems. Because of the alarming rate of loss of reservoir storage capacity and the unacceptable negative impact induced by the rapid upstream extension of sediment deposited in the river's backwater region, the dam has been reconstructed to provide high sediment releasing capacity, and the dam operation has been changed in order to achieve a balance between sediment inflow and outflow. As a result, the dam is still providing the basin with flood control, irrigation, and hydropower generation, even though some benefits are lower than the original design. Complex sedimentation processes in response to the dam reconstruction and changes of dam operation are discussed in this paper. The engineering experiences and management practices of Sanmenxia Dam are valuable assets to the sustainable use of reservoirs built on sediment-laden rivers.

Two kinds of bank erosion mechanisms were analyzed, including fluvial and non-fluvial controlled mechanisms, and mechanical methods of simulating the erosion processes of cohesive, non-cohesive and composite riverbanks were improved. Then a two-dimensional numerical model of the channel deformation was developed, consisting of a 2D flow and sediment transport submodel and bank-erosion submodels of different soil riverbanks. In the model, a new technique for up dating the bank geometry during the bed evolution was presented, which combines closely two kinds of submodels. The proposed model is capable of not only predicting the processes of flood routing and longitudinal channel deformation in natural rivers, but also simulating the processes of lateral channel deformation, especially the processes of lateral erosion and failure of cohesive, non-cohesive and composite river banks.