Journal of Hydraulic Engineering

Destratification techniques are often used to improve water quality in reservoirs. The majority of investigations have concentrated on the use of bubble aerators; however, an interesting alternative is the use of mechanical mixing systems consisting of a large propeller immersed in the water column. A laboratory study of such a system was used to examine the efficiency of energy conversion from mechanical energy to buoyancy flux as a function of impeller diameter, the speed of rotation, and the degree and type of stratification. The results were shown to depend on a generalized Froude number with the peak efficiency of energy conversion as high as 12%. A direct scaling mechanism enabling the results found in the laboratory model to be applied to a prototype situation was achieved via the application of derived scaling parameters. A simple design methodology for prototype versions of mechanical mixing systems based on this study is suggested.

A quasi‐three‐dimensional numerical model composed of two coupled modules has been developed for computing circulations in shallow waters. The depth‐averaged flow is computed in one of the modules, and the vertical distribution of the flow velocities is determined in the other. A simple algebraic formulation is assumed for the vertical eddy viscosity. Based on measurements of velocity profiles in a wind‐water flume, it was found that the vertical eddy viscosity should be scaled using the larger value of the bed‐ and surface‐friction velocities. For the cases considered, computations using the $k‐\epsilon$ model supported the results. Secondary currents caused by curvature effects and Coriolis force in open channel flow can be simulated by using the model. The model was also applied to compute circulations in a model harbor basin where measurements of velocities were available. The advective terms in the equations of motion are shown to be important to model the three‐dimensional features of these flows.

The accuracy of studies concerning the characteristics of gravel‐bed waterways is often dependent upon the techniques used to sample and quantify the material found on the channel boundary. Due to the vertical stratification present in the riverbed, the use of sampling techniques that remove only the particles within a thin surface layer is often necessary. Attributes of clay and grid sampling are considered in the present work. Criteria for determining the minimum sample size for a desirable level of accuracy are also presented. Many of the current surface sampling techniques truncate either the upper or lower size range of particles. Truncation of part of a size distribution not only limits the available information, but can also bias the rest of the distribution. The present work rectifies this problem by proposing the use of a hybrid technique that can sample the entire size range of either dry or submerged bed material. Results of field tests that utilize this new method are described in detail.

The critical shear stress of individual fractions ${\tau }_{\mathrm{ci}}$ in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture. For strongly bimodal sediments, ${\tau }_{\mathrm{ci}}$ increases with grain size, an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes ${\tau }_{\mathrm{ci}}$ to deviate from size independence in the direction of unisize (Shields) values. A quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of ${\tau }_{\mathrm{ci}}$ is observed and to predict the variation of ${\tau }_{\mathrm{ci}}$ with grain size in bimodal sediments. Because properties of sediment grain‐size distributions other than bimodality appear to have little influence on ${\tau }_{\mathrm{ci}}$ the trends presented here for 14 unimodal and bimodal sediments may be quite general.

Tuning a three‐dimensional (3‐D) coastal circulation model for vertical eddy viscosities can be a laborious task. An efficient alternative is to use adjoint inverse estimation methods, and one such method is developed here. A cost functional that consists of two terms is formulated: The first describes the lack of fit between model results and available data, and the second measures the variance of the eddy viscosity with depth. This latter term has been introduced with a view to eliminate the instabilities commonly associated with inverse methods. Weights are used to vary the relative influence of each term. The “optimized” eddy viscosity estimate is obtained by a variational minimization of the functional. The procedure is tested using velocity data at the center of a rectangular sea domain. Wind and tidal forcing are considered along with known eddy viscosity profiles. The inverse model retrieves extremely good estimates of the eddy viscosities in all cases, even when data are sparse. The estimates are all smooth, and the method and the solutions exhibit no sign of instability.