Journal of Hydraulic Engineering

Engineering methods to deal with pier-scour problems include placing coarse riprap stones around the bridge pier. This study aims to investigate the stability of a riprap layer around a cylindrical bridge pier. Experiments conducted in a laboratory flume identified three different modes of riprap failure. They can act singly or combine to render a complete failure of the riprap layer. These modes of failure are riprap shear failure, winnowing failure, and edge failure. The study proposes a semiempirical method to size stones for riprap protection. The procedure for this method was illustrated with field data from three different bridge sites. The experimental data show that a thick riprap layer can prevent winnowing in the absence of a filter layer. The thick riprap layer can also sustain a partial breakup of the layer with the capability of rearmoring the scour hole, preventing a total disintegration of the riprap layer. Finally, the study proposes two empirical relationships, which describe the effect of riprap thickness and cover on the stability of the riprap layer.

This paper introduces the concept of a submerged hydraulic jump being used for energy dissipation. A baffle wall is used to produce a stable deflected surface jet, thereby deflecting the high-velocity supercritical stream away from the bed to the surface. Based on a series of experiments, a diagram was developed that predicts the conditions under which such a surface jet would be produced. A second series of experiments were performed to study the characteristics of the deflected jet, as it travels upward first as a curved turbulent jet to eventually become a turbulent surface jet. The decay of the maximum velocity in the deflected as well as the surface jet was studied and compared with that of a plane turbulent wall jet that is a model for deeply submerged jumps.

The pore pressure increase in a deep sandy seabed due to the discharges from a buried source pipe was studied analytically. Darcy's law was assumed. The injection flow from the discharge ports of the source pipe was simulated as line discharges. An analytical solution of the pore-pressure increase was obtained using the technique of conformal mappings and the method of images. The pore-pressure distribution was analyzed for a burial depth 5–20 times the pipe diameter. The results indicated that the effect of the pipe size was confined to a distance from the center of the pipe of approximately half of the burial depth. Based on the analytical solution, the flow rate for the incipient fluidization of the cover sands was determined using the fluidization criterion adopted in 1990 by Lennon, Chang, and Weisman.

The geometry of lower regime bed forms in several large sand-bed rivers is investigated during average and flood conditions. The van Rijn method is revised because it generally underpredicts the dune height of most large rivers around the world. During floods in large sand-bed rivers, upper-regime plane bed is not necessarily obtained when T = 25. Both parameters describing dune height and dune steepness do not decrease as the transport-stage parameter T increases in the range 10 <T< 25. The analysis of bed-form data during large floods on the Meuse River and the Rhine River branches indicates that both the dune height and length generally increase with discharge while dune steepness remains relatively constant. A reasonable approximation of the wavelength is λ≅ 6.5 h , where h is the flow depth. The dune height Δ varies as a function of the depth h and median grain size d₅₀ . Estimates can be obtained by Δ≅ 2.5 h ^0.7d^0.3₅₀ .

Singular value decomposition is used to calibrate the Manning's roughness coefficients in a one-dimensional unsteady flow model of the Upper Niagara River. The method is used to solve for the parameters after formulating the calibration problem as a generalized linear inverse problem. Singular value decomposition is useful in solving underdetermined, overdetermined or even-determined problems, and can provide information to compute matrices describing parameter resolution, covariance, and correlation. This information is useful in identifying the important parameter groups in the model. Calibration is repeated with different numbers of parameter groups to determine the variation of the output error and uncertainty of the parameters with the parameter dimension. For purposes of comparison, the model with a selected group of parameters is calibrated using the Gauss-Newton method and minimax methods. The study shows the relationship of the parameters to the geometric layout of the river and the gauging stations.

Spatial trends in the statistical parameters (mean, standard deviation, and skewness coefficient) of a Pearson Type III distribution of the logarithms of annual flood peaks for small rural basins (less than 90 km ² ) are delineated using a climate factor, C _T ( T = 2-, 25-, and 100-yr recurrence intervals) which quantifies the effects of long-term climatic data (rainfall and pan evaporation) on observed T -yr floods. The statistical parameters are computed from gridded estimates of the geographically varying climate factors and regression models relating at-site T -yr floods as a function of drainage area, climate factor, hydrograph shape, and infiltration. Spatial portrayals, covering a large part of the United States east of the 105th meridian, are presented for averaged statistical parameters and their variability obtained using data from a sample of 200 small basins scattered over 12 states. Maps showing trends in average parameter values demonstrate the geographically varying influence of climate on the magnitude of Pearson Type III statistical parameters. The spatial trends in variability of the parameter values characterize the sensitivity of statistical parameters to the interaction of basin-runoff characteristics (hydrology) and climate.