Journal of Hydraulic Engineering

This paper presents the results of a physical and numerical model study on the entrainment by a high-concentration, cohesive sediment suspension. This turbulent, near-bed suspension is referred to as a concentrated benthic suspension (CBS). In the current configuration, the CBS entrains low-concentrated overlying water. The results of the physical and numerical experiments are in line with the results of similar experiments, published in literature, in which, in contrast, the upper, turbulent layer entrains water and sediment from the lower layer. The entrainment rate for these experiments is almost inversely proportional to the bulk Richardson-number $E\propto {\mathrm{Ri}}_{*}^{-n}$, with $n\approx 1$, indicating a profound effect of the side wall of the experimental facility. We anticipate that the conditions studied in this paper are characteristic for the behavior of many high-concentration systems, such as the mouth of the Amazon River, and in the Loire Estuary and Ems River. A sequence of entrainment during high flow velocities, and settling/consolidation during low flow velocities, driven by tidally induced horizontal pressure gradients, keeps the sediment fluid and mobile; this process can be referred to as tidal pumping. The experimental results could be reproduced to a fair degree by a 1DV point model. This, however, does not necessarily imply that state-of-the-art numerical engineering models are readily applicable for simulations of the hydro-sedimentological conditions in complicated systems, such as the Amazon mouth and Loire Estuary. In these systems, one expects a gradual transition from a fully turbulent upper part of the CBS layer, to laminar conditions deeper down within this layer, with increasing values of viscosity and yield strength; such transition is not accounted for in state-of-the-art turbulence models. Yet, the current work provides a framework for a better understanding of these systems.

Standard sumps (manholes) are common features of urban storm water collection systems, and there are anecdotes suggesting that standard sumps can improve storm water quality. However, no data on the effectiveness of sumps as treatment devices for suspended sediment removal and the associated required maintenance schedule of the sumps could be found. Such data could justify giving pollution prevention credit for the use of standard sumps to transportation departments of cities, counties, and state agencies. To assess the effectiveness of standard sumps as storm water treatment devices, a laboratory study was conducted. Three goals were achieved in this study: (1) sediment capture and sediment washout were measured in four configurations of a straight flow through standard sump; (2) performance functions for the efficiency of suspended sediment removal in a standard sump were developed; and (3) performance functions for sediment washout from a standard sump were developed. To determine whether they remove suspended sediment from storm water runoff, two standard sumps of different sizes were tested in a laboratory setting. Removal efficiency under low flow conditions and washout rates under high flow conditions were measured. The sumps did remove suspended sediment at low flows, but at high flows the washout rate was substantial. The data collected were used to develop two performance functions: one for suspended sediment removal (deposition) in a sump at low flow, and one for sediment washout from the sump at high flow. Four sump configurations were tested under a wide range of flow characteristics. The principal independent variables such as sump dimensions, sediment settling velocities, and hydraulic parameters were grouped into dimensionless numbers that were related to performance of all designs tested. These performance functions can be used to select appropriate designs and analyze the performance of existing standard sumps. Overall, the data collected show that standard sumps can be used as pretreatment devices for storm water if properly selected and maintained.

A depth-averaged two-dimensional model has been developed in this study to simulate the unsteady flow and noncohesive sediment transport due to embankment break and overtopping breaching. The model adopts the generalized shallow-water equations that consider the effects of sediment transport and bed change on the flow, thus leading to coupled calculations of these processes. It computes the non-equilibrium total-load sediment transport and considers the noncohesive embankment slope avalanching. The model solves the governing equations using an explicit finite-volume method on a rectangular grid, with the Harten, Lax and van Leer (HLL) approximate Riemann solver to handle the mixed-regime flows generated by embankment break/breaching and the monotonic upstream scheme for conservation laws (MUSCL) piecewise reconstruction method to reach second-order accuracy in space. It uses a varying time step length that satisfies both the Courant-Friedrichs-Lewy condition and the limitation that the bed change is less than about ten percent of the local flow depth at each time step. Validations using laboratory and field experiments showed that the developed model generally predicts well the embankment-break wave propagation over movable beds, the induced sediment transport and bed changes, and the temporal evolution of noncohesive embankment breach.

An Eulerian two-phase flow model was presented and employed to investigate wave-induced tunnel scour beneath marine pipelines. The model is based on the Euler-Euler coupled governing equations for the fluid and sediment phases, i.e., time-averaged continuity and momentum equations were solved for both phases with a modified $k-\epsilon$ turbulence closure for the fluid phase. Fluid-particle, particle-particle, and fluid-structure interactions were implemented in the simulation. The model accounts for the interphase momentum exchange by considering the drag, lift, and added mass forces. The flow model was validated against an oscillatory flow around an isolated cylinder and a cylinder close to a rigid wall. The two-phase model was also validated against an oscillatory sheet-flow motion above a plane bed. Then, the two-phase model was used to simulate the wave-induced tunnel scour beneath the pipeline laid on a plane erodible bed. Comparison between the numerical results and experimental measurements indicates that the model simulates the bed profile successfully during the tunnel scour stage. Investigations revealed that the tremendous sediment transport takes place during the tunnel scour stage under high turbulence intensity. A phase-lag was observed between the flow velocity in the scour hole and the free stream velocity.

A smoothed particle hydrodynamics (SPH) numerical model for shallow water equations (SWEs) is presented for simulating flood inundation owing to rapidly varying flow, such as dam breaks, tsunamis, and levee breaches. Important theoretical and numerical developments have recently been made, and the model in this paper incorporates these developments and implements open boundary conditions, resulting in a general, accurate computational tool suitable for practical application. The method is attractive for flood simulation over large domains in which the extent of inundation is unknown because computation is carried out only in wet areas and is dynamically adaptive. The open boundary algorithm is very general, on the basis of a simplified version of the characteristics method, handling both supercritical and subcritical inflow and outflow. This is tested against reference solutions for flows over a hump involving shocks. The model is then applied to two very different flood inundations resulting from the Okushiri tsunami in Japan and from a hypothetical dyke breach at Thamesmead in the United Kingdom. The SPH-SWE model compares well with established commercial and state-of-the-art finite-volume codes.

The longitudinal dispersion coefficient is a key parameter in determining the distribution of pollution concentration, especially in temporally time-varying source cases after full cross-sectional mixing has occurred. Several studies have been performed to present simple formulas to predict it. However, they may not always result in an accurate prediction because of the complexity of the phenomenon. In this study, a M5′ model tree was used to develop a new model for predicting the longitudinal dispersion coefficient. The main advantages of the model trees are that (1) they provide transparent formulas and offer more insight into the obtained formulas and (2) they are more convenient to develop and employ compared with other soft computing methods. To develop the model tree, extensive field data sets consisting of hydraulic and geometrical characteristics of different rivers were used. By using error measures, the performance of the model was also compared with the performance of other existing equations. Overall, the results showed that the developed model outperforms the existing formulas and can serve as a valuable tool for predicting of the longitudinal dispersion coefficient.

The discharge capacity and hydraulic efficiency of a labyrinth weir can be increased with an Arced cycle configuration. An Arced geometric layout for labyrinth weirs is presented, including nomenclature for arc-specific geometric variables. Discharge coefficients as a function of $H_T/P$ for half-round trapezoidal Arced labyrinth weirs with 6 and 12° sidewall angles are also presented. The hydraulic performance of the tested Arced labyrinth weir geometries is compared to Projecting and in-channel labyrinth configurations and to an arced weir (horseshoe weir) with a half-round crest, including the effects of approaching flow conditions and local submergence. Differences between geometrically similar and geometrically comparable Arced labyrinth weirs are identified and discussed.

Orifice spillways are in vogue for the dams in the hilly regions where the spillway has to serve the dual function of flood disposal and flushing of sediment through the reservoir. Deep-seated orifice spillways are subjected to cavitation damage as the cavitation index drops below the critical cavitation index of 0.2 because of negative pressures on the profile and high flow velocities. Aerators are provided for mitigating cavitation damage. Design guidelines for aerator of orifice spillways are scanty and not reported much in the literature so far; thus, there still remains gray area in the field of spillway aerator design. The present study investigates the performance of an offset aerator with and without a ramp for deep-seated orifice spillway on a physical and numerical model. Performance of the aerator for varying discharges, heads, and gate openings is studied for varying cavity subpressures. Results with respect to jet length, cavity subpressure and air entrainment coefficients are presented in the form of nondimensional plots. From this study, the nondimensional jet length is in the range of 2 to 35 and increases as the cavity subpressure, approaches atmospheric pressure. It is also found that the air entrainment increases with increase in Froude number, ramp height and cavity pressure. From the present study results, equations for jet length and air entrainment coefficient are developed and are presented for the orifice spillway aerator.