Journal of Hydraulic Engineering

An experimental study was conducted to determine the influence of length, width, and protrusion of noncontinuous riprap protections on shear failure conditions. The incipient motion of particles as a failure criterion and the reference transport method as the threshold of motion were used. In each test, riprap transport rates were measured at different time intervals using a sediment trap placed immediately downstream from the test reach so that time dependence could be well described. Results reveal that incipient motion conditions of transverse (cross-sectional) protections are strongly influenced by both the protrusion and length of bed protection, which indicates that stability significantly increases as protection length increases and decreases as protrusion increases. In the case of longitudinal protections, almost the same failure conditions were found as in the case of continuous protection. Furthermore, these conditions are unrelated to the width of the protection. A coefficient to correct design formulas obtained by other authors is proposed to take into account the effect of the geometry of transverse protections on their stability.

This study addresses the design of riprap mattresses as a scour countermeasure near vertical-wall bridge abutments under clear water flow conditions. It specifically deals with the diameter of riprap, $D_{r50}$ , the lateral extent of mattresses, $w$ , and their thickness, $t$ . Experiments were performed in a rectangular, sand-bed open channel using different abutment lengths, three riprap stone sizes, and two different sands. The minimum size of stable stones as well as the mattress dimensions depend on the ratio between the abutment length and the flow depth. New equations for the evaluation of $D_{r50}$ and $w$ are suggested. The geometric properties of the scour holes which develop at the edge of riprap mattresses are similar to those reported in the literature for spill-through abutments. Although it is not possible to fully arrest scour by winnowing, the corresponding scour depth is negligible when the mattress layer thickness is at least $6D_{r50}$ .

The temporal development of clear-water local scour depth at bed sills in uniform gravel beds is considered. Experiments are presented on the development of scour holes under unsteady hydraulic conditions, with the triangular-shaped hydrographs tested being of different durations and different rates of flow variation. Based on the experimental results and a theoretical framework, a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter. A “flash flood” is here defined as an event for which the scour doesn’t attain its potential magnitude, i.e., the equilibrium value for the peak hydrograph flow rate. This flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream. A quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter. Results show that the ratio between the final scour depth and the potential scour depth at a bed sill for a given hydrograph can be estimated as a function of the identified temporal parameter.

Incision and the ensuing widening of alluvial stream channels represent important forms of channel adjustment. Two accompanying papers have presented a robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors. This work reports on applications of the model to two incised streams in northern Mississippi, James Creek, and the Yalobusha River, to assess: (1) its capability to simulate the temporal progression of incised streams through the different stages of channel evolution; and (2) model performance when available input data regarding channel geometry and physical properties of channel boundary materials are limited (in the case of James Creek). Model results show that temporal changes in channel geometry are satisfactorily simulated. The mean absolute deviation (MAD) between observed and simulated changes in thalweg elevations is $0.16\mathrm{m}$ for the Yalobusha River and $0.57\mathrm{m}$ for James Creek, which is approximately 8.1 and 23% of the average degradation of the respective streams. The MAD between observed and simulated changes in channel top width is 5.7% of the channel top width along the Yalobusha River and 31% of the channel top width along James Creek. The larger discrepancies for James Creek are mainly due to unknown initial channel geometry along its upper part. The model applications also emphasize the importance of accurate characterization of channel boundary materials and geometry.

This paper presents results from an extensive experimental study of sheet flow on rough impervious surfaces that are used to represent highway pavement. Experiments were performed on three surfaces under no-rainfall and simulated rainfall conditions, and with slopes of 1, 2, and 3%. Measurements include flow depth and unit discharge. Turbulent boundary layer theory for a rough surface is used to describe the depth-discharge relationship, resulting in a model with a single parameter directly related to the surface roughness. Comparisons are made with Manning’s equation, and the variability of the Manning coefficient is assessed. Hydraulic effects of rainfall are generally found to be small compared to other factors.

Recent developments in flood modeling have led to the concept of coupled (sewer/surface) hydraulic models. In this paper two coupled models are examined; a one-dimensional (1D) sewer model coupled with a 1D surface network model (1D/1D) and a 1D sewer model coupled with a two-dimensional (2D) surface flow model (1D/2D). Flow over the terrain is better modeled by 2D models, whereas in confined channels 1D models provide a good approximation with less computational effort. This paper presents a comparison of the simulation results of 1D/1D model and a 1D/2D model. The methodology adopted for setting up the models is outlined and explained in detail as well as the 1D/1D modeling techniques used for reproducing the 1D/2D model results. The surface flow comparison clarifies the limitations of the 1D/1D model and indicates that the definition of the surface pathways, the linking elements sewer/surface, and inclusion of virtual manholes are key factors for setting up a more accurate 1D/1D model.

Discontinuous gravity currents in rectangular channels are modeled numerically by solving the 3D unsteady Reynolds-averaged Navier-Stokes (URANS) equations closed with a buoyancy corrected low-Reynolds number (LRN) $k\text{-}\epsilon$ model using second-order accurate finite-volume numerics. It is shown that, on moderately fine computational meshes (with $\sim {10}^6$ grid nodes) and with careful modeling of the near-wall flow, the URANS model can capture the essential large-scale 3D dynamics of gravity current flows, which previously had been resolved only by DNS and/or large-eddy simulation (LES) on very fine computational meshes. These 3D dynamics include the onset of the well-known lobe-and-cleft instability at the current head, the onset of large-scale Kelvin-Helmholtz billows at the head of the gravity current, and the breakdown of the interfacial billows in the rear part of the current head due to intense three-dimensional mixing. The computed results underscore the importance of careful modeling of the near wall flow in URANS simulations. The standard $k\text{-}\epsilon$ model with wall functions fails to capture the aforementioned complex 3D dynamics, which are only resolved by the LRN $k\text{-}\epsilon$ model on grids that resolve the near-wall region. Furthermore, numerical experiments show that including in the simulation the lateral no-slip end walls of the channel has a profound effect on the accuracy of the computed solutions. End-wall effects enhance the three-dimensionality of the flow, result in increased mixing of the dense and the ambient fluids behind the head of gravity currents, and yield results in good agreement with measurements. On the other hand, when end-wall effects are omitted, by imposing periodicity in the spanwise direction, three-dimensional mixing is suppressed and the breakdown of interfacial billows is significantly underestimated. Grid sensitivity studies are also carried out using three successively refined meshes and show that the URANS LRN model yields grid-converged solutions at affordable computational resources. As URANS modeling requires only a fraction of the computational cost of DNS or LES with near-wall resolution, the present results underscore the potential of unsteady statistical turbulence models for predicting and elucidating the physics of gravity current flows in complex geometries and at Reynolds numbers of engineering relevance.

Block ramps are a short length of a steep channel armored with rocks, which produce high dissipation of energy. This technical note presents the results of experiments performed on a ramp having a staggered arrangement of hemispherical boulders with base material on its bed. Compared to row and random arrangements of boulders, a staggered arrangement produces more dissipation of energy. Relative energy loss in the ramp depends not only on the discharge but also on boulder concentration and their sizes. A relationship is proposed that satisfactorily determines the dissipation of energy on the ramp with base material and boulders placed with a staggered arrangement.

This technical note, applying dimensional analysis and incomplete self-similarity, proposes a new functional relationship for the sequent depth ratio for hydraulic jumps over both smooth and rough horizontal beds. For the smooth bed condition, experimental measurements in the literature were used to calibrate the new relationship. For the rough bed condition the data of a previous investigation were used with new measurements carried out in a rectangular horizontal flume having a gravel bed. Finally, a generalized solution of the sequent depth ratio is proposed.