Journal of Hydraulic Engineering

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.

The objective of this contribution is to analyze the formation of debris waves in natural channels. Numerical simulations are carried out with a 1D code, based on shallow-water equations and on the weighted averaged flux method. The numerical code represents the incised channel geometry with a power-law relation between local width and flow depth and accounts for all source terms in the momentum equation. The debris mixture is treated as a homogeneous fluid over a fixed bottom, whose rheological behavior alternatively follows Herschel-Bulkley, Bingham, or generalized viscoplastic models. The code is first validated by applying it to dam-break tests on mudflows down a laboratory chute and verifying its efficiency in the simulation of rapid transients. Then, following the analytical method developed by Trowbridge, the stability of a uniform flow for a generalized viscoplastic fluid is examined, showing that debris flows become unstable for Froude numbers well below 1. Applications of the code to real debris flow events in the Cortina d’Ampezzo area (Dolomites) are presented and compared with available measured hydrographs. A statistical analysis of debris waves shows that a good representation of wave statistics can be obtained with a proper calibration of rheological parameters. Finally, it is shown that a minimum duration of debris event and channel length are required for waves showing up, and an explanation, confirmed both by field data and numerical simulations, is provided.

Experimental data are presented for flow in compound channels with symmetrically narrowing floodplains. In such a geometry, the flow behavior presents similarities with the more complex flow in a meandering compound channel, yet without the curvature effects, because of mass transfers between the floodplains and the main channel, and secondary currents induced in the main channel. An estimation of the momentum transfer generated by the mass transfer is found significant compared to the frictional losses. It mainly depends on the geometrical parameters and is practically independent of the friction slope. Free-surface profile computations are performed with the exchange discharge model (EDM) to incorporate the effects of the momentum transfer in terms of an additional head loss. Agreement was found between measured and computed water surfaces, thus validating the EDM approach.

The interactions between vortical structures, free surface, and background shear in an open channel flow is experimentally investigated. Deterministic structures are generated by a vertical injection from the channel bottom and a number of disturbances characterized by different jet amplitudes and Reynolds number are considered. Three cases are selected as representatives of the whole phenomenology and their evolution is discussed in detail. The streamwise perturbation is visualized by fluorescein while the streamwise component of velocity is measured by a laser Doppler anemometer. The perturbation in the cross stream plane is analyzed quantitatively and the velocity field is reconstructed on the symmetry plane, to extract the traces of the individual vortical structures. Depending on the intensity of the external perturbation, the structures either evolve as in the absence of the interface (small intensity) or they approach the free surface to undergo a topological change due to their reconnection with the free surface. The process originating in the so-called blockage and viscous layers is analyzed in detail. As a major conclusion, the interaction with the free surface is found to deplete the velocity gradients generated by the liftup of the quasi-streamwise vortices. This effect is suggested to be a major cause of stabilization of the open channel flow, with respect to a closed channel at the same Reynolds number.

Distributions of bed shear stress across the width of a rotating circular flume with smooth and rough bed surfaces were obtained by measurement and model prediction. Results with flows over smooth beds showed that the flow in the central part may be considered to be two-dimensional and that effects of flow depth over the operating range of the flume are minor for flow depths not exceeding 0.14 m. For rough beds, the bed shear stress distributions were found to be skewed toward the inner wall. This can be corrected if a compensating roughness is added to the bottom of the ring. Such measures are also effective for flumes with smooth beds. Measured bed shear stress distributions agreed well with the predicted distributions for smooth beds and reasonably well for rough beds. The modified Preston tube, for measurement of bed shear stress in flows over rough beds, was found to give promising results. Further tests are required to completely define the uncertainty in bed shear stress measurements made with this instrument.

Dam-break flows in general geometries with complex bed topography are simulated numerically using a high-resolution Godunov-type cut cell method. The model is based on the shallow water equations with appropriate source terms. A vertical step in the bed is treated efficiently and accurately with the surface gradient method. For dam-break flows occurring in complicated geometries, the Cartesian cut cell method together with transmissive boundary conditions is incorporated. Verification of the method is carried out by predicting dam-break flows typical of practical situations, i.e., dam-break flows over a vertical step into bent channels and a dam-break flow over a bump in a bed with both transmissive and reflective boundary conditions at the channel end. The results are compared with experimental data showing good agreement. The method is simple, efficient, and conservative. It shows promise for handling a wide range of dam-break flows.

First- and second-order explicit finite volume (FV) Godunov-type schemes for water hammer problems are formulated, applied, and analyzed. The FV formulation ensures that both schemes conserve mass and momentum and produce physically realizable shock fronts. The exact solution of the Riemann problem provides the fluxes at the cell interfaces. It is through the exact Riemann solution that the physics of water hammer waves is incorporated into the proposed schemes. The implementation of boundary conditions, such as valves, pipe junctions, and reservoirs, within the Godunov approach is similar to that of the method of characteristics (MOC) approach. The schemes are applied to a system consisting of a reservoir, a pipe, and a valve and to a system consisting of a reservoir, two pipes in series, and a valve. The computations are carried out for various Courant numbers and the energy norm is used to evaluate the numerical accuracy of the schemes. Numerical tests and theoretical analysis show that the first-order Godunov scheme is identical to the MOC scheme with space-line interpolation. It is also found that, for a given level of accuracy and using the same computer, the second-order scheme requires much less memory storage and execution time than either the first-order scheme or the MOC scheme with space-line interpolation. Overall, the second-order Godunov scheme is simple to implement, accurate, efficient, conservative, and stable for Courant number less than or equal to one.

Prediction of water column concentrations of suspended sediment is often necessary for environmental impact assessment of point source industrial discharges. For example, in “flow lane” or “open water” disposal, suction dredges discharge large volumes of suspended sediment into shallow water disposal locations. A sediment density current mixing model is presented here as part of the D-CORMIX expert system for hydrodynamic simulation of mixing zone behavior. This density current model extends the CORMIX decision support system to simulate continuous negatively buoyant discharges with or without suspended sediment loads on a sloping bottom with loss of suspended particles by sedimentation. Sedimentation is modeled using Stokes settling for five particle size classes. Density current width and depth, trajectory, total solids, tracer concentration, dilution, and particle size concentration are predicted. In addition, location and widths of sediment deposits, accretion rates, including particle size fractions within the spoils deposit, are predicted. The model results are in good overall agreement with available field and laboratory data.

A high-resolution current profiler (HRCP), which belongs to a pulse-to-pulse coherent Doppler sonar, has been used to measure vertical profiles of turbulence parameters, such as the Reynolds stresses, eddy viscosity, production and dissipation rates, etc., and to test the parametrization of dissipation rate and eddy viscosity. The HRCP and automatic ascending/descending CTD are deployed during the autumn of 2001 for 24 h in a tidal estuary. Reliable velocities along the beams with HRCP are collected with 3 s intervals and a vertical resolution as fine as 0.03 m in the range 0.02–0.98 m above the bottom. Density profiles with the CTD are taken nominally every 30 sec. The turbulent velocity variables depend largely on the tidal phase; the variables during the ebb deviate from those in neutral equilibrium boundary layer. This deviation during the ebb presumably arises from the “inactive motion.” The stability function $S_M$ in the Mellor–Yamada (M–Y) model is smaller than 0.39 even when the stratification is negligible during the flood. The constant of proportionality $B_1$ in the dissipation model is larger than 16.6 used in M–Y model. There is room for improving some of the mixing parametrizations in estuarine tidal flows.