Journal of Hydraulic Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

The National Oceanic and Atmospheric Administration (NOAA) is working toward improving water forecasts in the river-estuary transition zone. One operationally viable method is to extend one-dimensional (1D) hydraulic models downstream well into the tidal estuarine environment. Recent advances in NOAA’s National Weather Service (NWS) operational software infrastructure have made this method easier to implement. This paper evaluates the strengths and limitations of an unsteady implementation for the tidal Potomac River. The model was calibrated and validated to propagate tidal signals upstream for a wide range of freshwater events. Focusing on water-level gauges near Washington, DC, root-mean-squared error for tide simulation at the Washington Waterfront gauge was 0.05 m (0.16 ft) with a phase error of approximately 2 h. For historic flood events, simulated peak water-level error varied from $-0.40\mathrm{m}$ ($-1.34\mathrm{ft}$) to 0.40 m (1.34 ft) with a mean absolute error of 0.25 m (0.82 ft) at the Wisconsin Avenue gauge. While the NWS implemented model shows considerable potential to improve upon an existing empirical forecast technique for freshwater, tidal, and freshwater-tidal events, it did not adequately model wind-driven events because it does not include an explicit wind forcing term. Therefore, to further understand the influence of wind on Potomac water levels, both a 1D and a two-dimensional (2D) model were developed for Hurricane Isabel. With storm-specific calibration, wind-driven simulations from the models improved peak water-level simulation for Hurricane Isabel over the NWS model. The NWS model without wind forcing missed the Hurricane Isabel peak at the Washington Waterfront by 0.66 m, while the 1D and 2D models came within 0.08 and 0.01 m, respectively. Little improvement was seen over the NWS model in prediction of peak time. While the use of dynamically coupled 2D or three-dimensional hydraulics models for operational river forecasting may be the wave of the future, the value of loosely coupled, 1D river hydraulic models to efficiently provide water-level forecasts in should not be discounted.

The jet erosion test (JET) is an experimental device increasingly used to quantify the resistance of soils to erosion. This resistance is characterized by two geotechnical parameters: the critical shear stress and the erosion coefficient. A previously published JET interpretation model provides an estimation of these erosion parameters. But the existing model is simplified and semiempirical and several assumed hypotheses can be discussed. The aim of this study is to determine the relevance of the JET interpretation model. Therefore, a numerical model was developed that is able to predict the erosion of a cohesive soil by a turbulent flow. The numerical model was first validated on a benchmark: erosion of an erodible pipe by a laminar flow. The numerical results were satisfactorily compared with the theoretical solution. Then, three JETs were modeled numerically with values of erosion parameters obtained experimentally. A parametric study was also conducted to validate the accuracy of the numerical results and a good agreement was observed. The erosion parameters found experimentally permit the numerical prediction of the evolution of the erosion pattern within good accuracy. This result contributes to the validation of the JET’s semiempirical model. The numerical model also gives a complete description of the flow, including vortices which can be observed in the cavity created by erosion. The entire erosion pattern evolution was given by the numerical results. This numerical model gives information that is not available otherwise.

The model of junction of four orthogonal channels is used to investigate mass transport within viscous eddies developing through channel networks. These viscous eddies may develop in low flow rate branches of channel networks and may control solute transport. Thus, to contribute to the knowledge of the influence of viscous eddies on solute transport in channel networks, the analysis of the streamline patterns is conducted for various flow rates and the threshold of the flow rate for the appearance of eddies is determined. Following this, the calculation of the mass transport is carried out in function of time when a permanent source of solute is applied at the inlet of the system. This calculation is carried out for different values of the Péclet number in order to highlight diffusive and convective transport mechanisms. The results show that the effect of eddies for transporting the solute may be significant.

Laboratory experiments were conducted to investigate the influence of suction on sediment transport in closed-conduit flows in this study. The results show that the bed load transport rate essentially remains unchanged for small values of suction rates before abruptly increasing beyond a certain threshold. Theoretical analyses of the forces acting on a spherical particle also are conducted and a conceptual model set up to analyze suction effects on particle mobility by considering the near-bed velocities. The model hypothesizes that (1) the bed particle experiences an additional downward vertical drag force induced by suction; and (2) increasing suction will lead to larger horizontal and vertical near-bed velocities, which enhances both the driving force and effective weight of the particle. To qualitatively examine how suction affects the near-bed flow behavior, physical modeling capability software was used to simulate the physical system and the results confirm that the near-bed velocities increase with suction.

Fast flows and avalanches of rock and debris are among the most dangerous of all landslide processes. Understanding and predicting postfailure motion (runout) of this kind of flowlike landslides is thus key for risk assessment, justifying the development of numerical models able to simulate their dynamics. In this paper a numerical method for the resolution of the depth-averaged debris flow model is presented. This set of nonlinear differential equations is formed by a variation of the shallow water equations, including strong bed slope, and a rheology resistance term. This paper focus on the numerical discretization of the resistance term, exploring three different approximations: pointwise, implicit, and unified. Well balance between numerical flux and source terms is only achieved using the unified discretization. In order to avoid nonphysical values of the water depth and discharge, a limitation of the unified resistance term is also needed. This correction is made following three conditions that identify the physical boundaries of the resistance term in the debris flow. This technique does not affect the computational efficiency of the method, keeping the original time step. Furthermore, proposed analytical test cases show that the three resistance limitations do not significantly perturb the numerical solution. The properties of the resulting numerical scheme are studied using a set of numerical experiments that include steady and transient flows. The results show the convenience of the unified discretization and the need of the three-condition limitation in order to avoid unphysical solutions.

The erosion rate of cohesive soils is typically computed using an excess shear stress model based on the applied fluvial shear stress. However, no mechanistic approaches are available for incorporating additional forces, such as groundwater seepage into the excess shear stress model parameters. Seepage forces are known to be significant contributors to streambank erosion and failure. The objective of this research was to incorporate seepage forces into a mechanistic fundamental detachment rate model to improve the predictions of the erosion rate of cohesive soils. The new detachment model, which is referred to as the modified Wilson model, was based on two modified dimensional soil parameters ($b_0$ and $b_1$) that included seepage forces due to localized groundwater flow. The proposed model provided a general framework for studying the impact of soil properties, fluid characteristics, and seepage forces on cohesive soil erodibility. Equations were presented for deriving the material parameters from both flume experiments and jet erosion tests (JETs). In order to investigate the influence of seepage on erodibility, the erodibility of two cohesive soils (silty sand and clayey sand) was measured in flume tests and with a new miniature version of the JET device (mini JET). The soils were packed in three equal lifts in a standard mold (for JETs) and in a soil box (for flume tests) at a uniform bulk density (1.5 and $1.6\text{Mg}/{\mathrm{m}}^3$) near the soil’s optimum water content. A seepage column was utilized to induce a constant hydraulic gradient on the soils tested in the flume and with the mini JET. The modified Wilson model parameters, $b_0$ and $b_1$, were derived from the erosion rate data both with and without the influence of seepage from the flume and JETs. Seepage forces had a significant but nonuniform influence on the derived $b_0$ and $b_1$ as functions of the hydraulic gradient and soil density. The more fundamental detachment model can be used in place of the excess shear stress model with parameters that can be derived using JETs, transforming the modeling of cohesive soils for hillslopes, embankments, gullies, streambeds, and streambanks.

This paper presents a new semianalytical approach for the solution of suspended sediment transport in channels under uniform conditions using generalized integral transform technique (GITT). Comparing with previous analytical work, this new approach is not limited by the sediment diffusivity distribution and boundary conditions. Instead, it solves the suspended sediment transport equation with arbitrary distribution of eddy diffusivity. The solutions also consider realistic boundary conditions at the free surface and the bed. The new solutions converge fast and compare well with previous analytical solutions as well as numerical solutions from a high-resolution spectral code. With this new analytical approach, a library of solutions for suspended sediment concentration can be developed. It gives deeper insight into the physical process of the problem and provides benchmarks for the validation of numerical models.

Failure of the Utatsu concrete girder highway bridge in Minamisanriku, Miyagi Prefecture during the 2011 Great East Japan Tsunami was puzzling because the bridge decks were not pushed off their piers but rather were flipped off the landward side of the bridge piers after being deeply submerged by the surging tsunami. To determine what caused this to happen, two simulations were conducted. The first was a large-scale Delft shallow-water simulation (beginning with published tsunami source free surface deviation) to determine the behavior of the tsunami (time series of flow depth and speed) at the bridge site. The second was a small-scale two-dimensional (2D) (profile view) software volume-of-fluid (VOF) simulation of flow over the bridge deck, with boundary conditions taken from the Delft model. The VOF model then allowed calculation of lift force, drag force, and overturning moment on the bridge deck. Results show that factors contributing to failure included the presence of a seawall near the bridge, inclination (superelevation) of the deck upward toward the ocean, sediment entrained in the water, and air trapped between girders.

Transport of suspended sediment between the two reservoirs of a pumped-storage plant in Switzerland was monitored over an 8-month period. A turbidity probe was installed on the upstream end of the pressure shaft, in which real-time monitoring was ensured by automatic data acquisition and remote control. The suspended sediment concentration in the reservoirs varies considerably over the course of a year, with low particle loads in the winter, when the reservoirs are covered by ice and the catchment area is entirely covered by snow. In late spring, when snowmelt starts, the suspended sediment concentration increases and remains high until summer. The short-term evolution of the suspended sediment concentration was found to be correlated to the pumped-storage sequences, especially when the reservoir levels are low. Higher concentrations during pumping mode were observed for low levels in the lower reservoir, and low levels in the upper reservoir corresponded to higher sediment concentrations in turbine mode. Concentration ratios between pumping and turbine mode between 0.93 and 1.16 were observed. Periods with higher particle load were observed more frequently during pumping mode. Nevertheless, the suspended sediment balance due to pumped-storage operations is equilibrated, which indicates a high degree of correlation between the transported sediment volume and hydropower operation. Connecting the applied monitoring system in an existing plant control and management system would allow the plant operator to detect increased suspended sediment loads in the pressure shaft and to adapt the operation mode to actively manage reservoir sedimentation or to estimate and predict the long-term sediment balance.

The propagation dynamics of the turbid density currents of the Imha Reservoir is investigated using the FLOW-3D computational fluid dynamics code. The renormalization group (RNG) $\kappa \text{-}\epsilon$ turbulence scheme in a Reynolds-averaged Navier–Stokes (RANS) framework was applied for the field investigation. A new particle dynamics algorithm was developed and coupled with the FLOW-3D model to simulate the settling of sediment particles, and the model was tested with field measurements during Typhoon Ewiniar. The coupled model correctly predicted the spatial and temporal evolution of intrusive density currents with plunging flows at a depth of 20 m and interflows propagating downstream at about $0.2\mathrm{m}/\mathrm{s}$. The simulation results with sediment settling demonstrated that the ratio of transported to initial sediment concentration ($C_0/C_i$) varies as a function of particle size $d_s$ and sediment concentration. Density currents can be classified into three regimes: (1) a suspended regime ($d_s<10\mathrm{\mu m}$), where interflows will transport most of the sediment inflows from the reservoir inlet to Imha Dam; (2) a transition regime ($10\mathrm{\mu m}<d_s<40\mathrm{\mu m}$), where deposition rates alter the flow dynamics; and (3) a deposition regime ($d_s>40\mathrm{\mu m}$), where sediment particles rapidly settle out without forming a density current. For the suspended regime, river inflows will define three different types of density currents in the reservoir: (1) interflows when $C_i<\mathrm{2,000}\mathrm{mg}/\mathrm{L}$; (2) multiple intrusions when $\mathrm{2,000}<C_i<\mathrm{3,000}\mathrm{mg}/\mathrm{L}$; and (3) plunging underflows when $C_i>\mathrm{3,000}\mathrm{mg}/\mathrm{L}$.

Articulated concrete block (ACB) revetment systems are widely used for channel lining and embankment protection. Current approaches for prediction of ACB system stability use a moment stability analysis with a ratio of the boundary shear stress to critical shear stress to account for all hydrodynamic forces, which results in the exclusion of the flow velocity. Two embankment-overtopping laboratory data sets for a given ACB system were initially evaluated using the current industry-accepted design method. A new stability analysis method, termed the shear and velocity stability assessment method, was developed for ACB systems with embankment-overtopping flow, which uses a moment stability analysis in which hydrodynamic forces are computed using both boundary shear stress and flow velocity. A database was developed that included overtopping tests for three ACB systems with varying embankment slopes and lengths. The current design methodology accurately predicted stability for 67% of the tests and the instability for 40% of the installations. The shear and velocity assessment method accurately predicted stability for 91% of the tests and instability for 80% of the installations.

A method is proposed to improve the accuracy of the approximation commonly used with the friction term in the water hammer equations. This method partially corrects the truncation error of the friction integral by incorporating the truncation error associated with a linear flow profile into the solution algorithm. The effectiveness and utility of this accuracy enhancement are demonstrated by examples.

This research intends to provide new insights on sediment transport at bridges, exploring the features of migrating dune-like bed forms generated immediately behind piers or abutments by local scour processes. An extended VAW (Laboratory of Hydraulics, Hydrology, and Glaciology, Swiss Federal Institute of Technology Zurich) data set collected from 1999 to 2002 was analyzed. In total, 63 temporal runs involving 480 observations for cylindrical piers, and 54 runs with 384 observations for rectangular-shaped abutments were considered. It was found that the main dimensionless parameters governing the dune processes are the relative pier diameter (or abutment width), densimetric Froude number, wash-out parameter, and relative time. The dune height is mainly controlled by the relative width of the scour element, and its relative length depends on the wash-out parameter. The morphologic dune evolution and its downstream propagation are described using straightforward equations as a function of the key parameters. The results apply to practice and are suitable to assess the performance of computational modeling.