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The rate and extent of riverbed degradation resulting from sediment interruption are determined by means of computerbased numerical experiments. Momentum and continuity equations for water and sediment flows in a wide prismatic channel are solved numerically using a recently verified model for the bedarmoring process. The bed profiles for different times are found to be similar and are coalesced to a single normalized curve. The changes in bed levels and mean sediment size, and the length of degradation, determined from the numerical experiments, are correlated by multiple regression analysis to the independent dimensionless variables. The depth of degradation depends on the Froude number, the geometric standard deviation, and the normalized median size of the bed material. The proposed relationship for bed degradation is verified with the field data of Missouri River. The computation procedure is illustrated by an example.

The effect of swirl on air transport in a vertical drop shaft is described. Laboratory experiments on a vortexflow drop shaft showed that most of the air is entrained in the annular hydraulic jump that is formed when the annular jet flow changed to the pipefull flow. A significant portion of the air bubbles converged due to an inward pressure gradient toward the center of the drop shaft, coalesced to form large bubbles, and then rose toward the surface. A large number of air bubbles still were carried downstream for flow conditions where the jump occurred at the location of low circulation. The air transport rate down the drop shaft was virtually zero if the jump formed in the strong circulation flowregion. The theoretical analysis indicates that the amount of air transported downstream depends upon the swirl number. The air concentration decreases with the increase in the swirl number.

A new type of spiralflow dropshaft, composed of a vertical circular shaft with tangential inlet and outlet conduits and one or more continuous helicoidal ramps attached to the shaft wall, is described. Experimental results indicate that these helicoidalramp dropshafts are superior to other types of dropshafts both in their energydissipation and airhandling characteristics. Additionally, the needs for energydissipation and deaeration chambers at the bottoms of these structures are obviated. The friction factors of flows along the channels formed by the helicoidal ramps are somewhat higher than those of equivalent straightchannel flows. Prototypeinstallation experience indicates that helicoidalramp dropshafts are significantly cheaper than conventional ones in installations requiring low air discharges from and intensive energy dissipation in the dropshafts.

Analytical solutions for freesurface swirling flows in a vertical dropshaft are presented. The swirling flows are governed by the circulation number and the Froude number. Two types of flow problems are analyzed. The transition flow problem, where the shaft diameter varies, is solved using specific energy curves. In the gradually variedflow problem, the shaft diameter is kept constant. Some results of practical interest are presented.

The necessary condition for, and the process of, evolution of a coarse layer over finer sediment underneath are presented. Flow regions for pavement and armor formation are identified. A new definition for armor and pavement is proposed. The relevation of the two flow regions contributes towards an improved understanding of the distinction between armor and pavement. With increasing shear velocity, the surface layer coarsens in the armor region and becomes smoother in the pavement region. The eroded material (EM) in the armor region becomes coarser with increasing shear velocity but remains finer than the parent-bed material (PBM). In the pavement evolution region, the EM is finer than PBM; and in the equilibrium pavement region, the composition of EM is identical to that of the PBM. The available experimental data are in agreement with the proposed hypothesis of evolution of the coarse surface layer.

Excessive production of frazil ice in rivers may lead to the occurrence of ice jams and attendant flooding during winter. An oftendiscussed means of minimizing frazilice production, and thereby for mitigating frazil jams, is regulation of river flow in order to facilitate rapid icecover formation. A numerical model of icecover formation by frazilice accumulation over stageregulated pools is exercised to assess the durations of flowdischarge regulation that would be required for ice covers to form over river pools of diverse length, regulated downstream depth, and channel slope. The numerical model indicates that for a given navigation pool, there exists an optimum river discharge that corresponds to a minimum time required for an ice cover to form. Optimal discharge is a function of pool length, channel slope, and downstream flow depth, but is invariant with air temperature.

Watersurface profiles in a channel that includes either a contraction in channel width, or a rise in channel bottom are analyzed using the specificenergy concept. It is discovered that the watersurface profiles in this channel for particular flow conditions are not unique; two stable solutions and one unstable solution are found. The flow conditions leading to nonunique solutions are delineated.

The rate and extent of bed aggradation resulting from sediment overloading are determined by means of computerbased numerical experiments. A nondimensional momentum equation, along with continuity equations for water and sediment of unsteady flow for a wide, prismatic channel, are solved numerically using recently developed relations for sediment discharge and friction factor. Numerical experiments are executed for a range of values of nondimensional input data. Principal dependent variables, such as changes in bed levels from equilibrium levels and the length of aggradation, determined from the computer experiments are correlated by multiple linear regression analysis to the independent nondimensional variables. Numerical results show that the sediment diffusion coefficient is a function of the rate of sediment overloading. The agreement between the numerical results and the available experimental data is satisfactory.

An analysis to predict the hydraulic performance of a tangential vortexinlet which is relatively simple in shape and compact in size is presented. The theory agrees with the experimental data. A relation to determine the dropshaft diameter for a design discharge is given.

A nonlinear parabolic model for nonequilibrium processes in alluvial rivers is presented. Analytical expressions for the characteristic parameters of relevant aggradation and degradation processes are derived. The validity and limitations of the model are assessed by comparing the analytical results with the available experimental data.