Journal of Hydraulic Engineering

Three‐dimensional Lagrangian residual circulation computed from an intratidal hydrodynamic model is used to drive an intertidal water‐quality transport model. The sum of the Eulerian residual and Stokes' drift is used as a first‐order approximation for the Lagrangian residual current. The Stokes' drift approximates residual currents induced by the nonlinear interactions of the tidal currents. Computed residual velocities show close agreement with a two‐dimensional (vertical‐longitudinal) analytical result. The methodology is applied to Chesapeake Bay for the entire year of 1985. Salinity computed with the intertidal transport model (i.e., with Lagrangian residuals) shows good agreement with observed salinity data and that computed with the intratidal hydrodynamic model. The basic effect of the Stokes drift is to transport water mass (e.g., salinity) up estuary. The procedures provide a practical means of interfacing intratidal hydrodynamic model information to water‐quality models for making long‐term, intertidal transport computations for weakly nonlinear tidal systems.

A numerical model for the computation of turbulent flows in sedimentation tanks is presented. Based on a nonorthogonal grid formulation, it can handle fairly arbitrary two‐dimensional geometries. It solves the full two‐dimensional elliptic governing flow equations with the k‐ε turbulence model. Effects on momentum, as well as turbulent kinetic energy balances, due to sediment‐induced density currents are taken into account. Solids with a broad settling velocity distribution are treated by considering different settling velocity classes. A simple flocculation model assuming only turbulent shear‐induced flocculation is formulated, and the importance of this cause of flocculation is examined. Predicted velocity and concentration profiles and removal efficiencies are compared with observations in a tertiary settling basin in Sweden. Results suggest that the flow field is significantly changed by seemingly small density differences, that shear‐induced flocculation is relatively unimportant, and that the influent settling velocity distribution is critical for accurately predicting of removal efficiencies.

In the southwestern United States, flood‐frequency relations for streams that drain small arid basins are difficult to estimate, largely because of the extreme temporal and spatial variability of floods, many years of no flow, and short periods of systematic records of annual peaks. A new method is proposed that combines records for several streamflow‐gaging stations, as in the station‐year approach, and produces regional flood‐frequency relations using an iterative regression technique. This technique eliminates the need to extrapolate the flood‐frequency relation to the flood probability of interest. The resulting multiparameter regional flood‐frequency relation is based on all the available annual peak‐flow data. The method was applied to a group of records from 42 gaging stations in Nevada with many years of no flow and with many poorly defined flood‐frequency relations. One‐ and two‐parameter models were developed in which much of the variance in peak discharge is explained by drainage area. The log‐Pearson type III and Weibull probability distributions were used in the models. Part of the error is directly assessed using randomly selected subsamples of the annual peak discharges.

TO prevent bridge failure due to pier scour, engineers are being asked to determine the reliability of new and existing bridges. There is currently little guidance for decision making regarding the control of scour at existing piers and the design of piers for avoiding failure due to pier scour. The purpose of this paper is to present a method to assess the risk of bridge failure due to pier scour during the life of the bridge. The analysis involves simulating pier scour for a period of time and determining the probability that the bridge will fail at various points in time during that period. A linear performance function describes the probability of bridge failure due to scour. The distributions and coefficients of variation of the random variables in the performance function are then determined by simulation, and the probability of failure is computed directly. Trie risk‐based failure analysis will enable the engineer to base bridge pier designs and scour control on an estimated probability of failure, thus providing for the safer design of bridges.

It is postulated that movable sediment beds comprise of two types of grains, termed undisturbed and disturbed grains. The probability of entrainment by flow of the latter is higher than that of the former. The lag in bedload transport is due to the difference in the entrainment probabilities of the two types of grains. A model is developed to predict lag in bedload transport. The spatial‐ and temporal‐lag coefficients are functions of both the inner and outer variables. The 50% of the equilibrium bedload discharge is attained within about 26% of the total lag distance or lag time. The lag model was verified by conducting laboratory experiments and using experimental data of Bagnold. The proposed model agrees with the experimental data. Both the lag distance and lag time decrease with increasing equilibrium bedload discharge. The findings of the model can be used to improve the Einstein formula for the equilibrium bedload discharge.

Lower reaches of tributaries to the Missouri River were straightened in the late 19th and early 20th century. Channel degradation has progressed upstream as a result. Channel deepening and widening have caused problems at stream crossings and have resulted in gully encroachment into cultivated fields. A diffusion model and an hyperbolic model, each describing channel degradation, are solved using a Laplace transform approach. A closed‐form solution is obtained for the diffusion model, but numerical methods are necessary for evaluation of the inverse transform of the hyperbolic model. A closed‐form asymptotic solution is found for the hyperbolic case. This asymptotic solution is shown to contain the solution to the diffusion model as a special case. The asymptotic solution is compared with observed degradation of West Tarkio Creek, a small river in northwestern Missouri and southwestern Iowa, with good results.

The vertical distribution of suspended sediment in an open channel is studied theoretically by means of a new mixing length concept. The diffusion coefficient is represented as a function of the distance from the bed and dependent on the magnitude of sediment concentration. An equation to predict the sediment concentration is derived analytically, using the vertical mass-balance equation over the water depth. The theoretical concentration distributions are compared with previous experimental data. It is found that the present theory is applicable to sediment-laden flow with low and high concentrations. The ratio of the diffusion coefficient and the eddy viscosity, γ, is larger than unity and increases monotonically with the increase of the sediment load in flows with low sediment concentrations, and smaller than unity and decreases with the increase of the sediment load in flows with high sediment concentrations.