Journal of Hydraulic Engineering

This study aims to clarify the mechanism of riverine levee breach and propose a new numerical model for that phenomenon. Large-scale experiments of overtopping breach were performed using an experimental flume located on the floodway of an actual river channel. By taking advantage of the scale of the flume, the levee breach process was monitored with state-of-the-art observation devices under highly precise hydraulic conditions. Four test cases were performed with variations of inflow rate, levee material, and levee shape, and the levee breach was monitored quantitatively using acceleration sensors installed in the levee body. From the results of the experiments, the breach process is categorized into four stages, focusing on the breach progress and hydraulic characteristics. It was determined that the correlation between the breached volume and the hydraulic quantities of velocity, water level, and Shields number can be expressed by an equation similar to that for bed load transport. Finally, a two-dimensional numerical model is proposed by integrating the experimental results into geomechanics, and a good reproduction result is obtained.

One-dimensional (1D) hydraulic models for phreatic aquifers are commonly based on Dupuit’s approximation for near-horizontal flows. In the past, improved 1D models were developed by inclusion of streamline curvature in the governing equations. However, the models developed for steady flow are quite different from those developed for unsteady curvilinear flow. In this paper, a single mathematical model is proposed for application to both steady and unsteady curvilinear phreatic flows. Unsteady flow equations for phreatic aquifers are presented by adopting the shallow flow approximation. The resulting model converges to that originating from perturbation methods, thereby providing a generalized result. Limitations of the second-order shallow flow theory and Dupuit’s approximation are investigated for steady flow through a dam and unsteady flow in the bank storage problems. The results obtained from the present model are compared with two-dimensional data, indicating that Dupuit’s theory is valid under a wider range of conditions than previously found.

The overarching goal of this study is to perform a comprehensive bank stability analysis that is phenomenologically sound by considering both mass failure and fluvial erosion. The nature of this study is twofold. First, field and experimental analyses are conducted to generate data for channel cross-section properties, soil index properties, and mechanical and erosional strengths at two sites in a representative, midsize, midwestern stream in southeastern Iowa that is subjected to frequent flash floods and characterized by active fluvial erosion and cantilever failure. Second, the channel surveys and data obtained from the field and laboratory analyses are used as input parameters for an established one-dimensional, channel evolution model, namely, the conservational channel evolution and pollutant transport system (CONCEPTS, version 2.0, Langendoen and Alonso 2008), to estimate the factor of safety for mass failure ($F{S}_m$) and fluvial erosion ($F{S}_f$) and simulate the bank retreat as a result of either fluvial erosion or mass failure or the interaction between the two modes of erosion. In CONCEPTS, a bank profile can be divided into several layers, allowing the user to account for heterogeneity in soil properties. The results show that estimation of $F{S}_m$ must be complemented with the estimation of $F{S}_f$ for not underestimating mass failure. Otherwise, based on mass failure criteria alone, the stability analysis fails to consider the potential for the interconnection between bank toe undercutting and planar failure and may lead to the underestimation of mass failure over time. Second, bank soil heterogeneity plays an important role in bank stability analysis. The variability of mechanical and erosional strengths shown in this study, along the downslope of the banks, highlight the need to acquire both mechanical and erosional strengths for the three layers along a bank profile (crest, midbank, and toe) to improve the commonly adopted protocols that typically assume homogeneous, well-compacted soils along a bank profile. The predicted bank profile obtained from the model compares favorably with cross-sectional measurements obtained for a period of two hydrological cycles.

We develop, validate, and demonstrate the potential of Virtual StreamLab (VSL3D), a novel three-dimensional hydromorphodynamics computational model capable of simulating turbulent flow and sediment transport in natural waterways with embedded and arbitrarily complex hydraulic structures under live-bed conditions. The numerical model is based on the curvilinear immersed boundary (CURVIB) approach and can solve the unsteady Reynolds-averaged Navier-Stokes (URANS) equations closed with the $k-\omega$ turbulence model in arbitrarily complex waterways with mobile sediment beds. Bed material transport is simulated by solving the nonequilibrium Exner equation for the bed surface elevation coupled with a transport equation for suspended load. Field-scale measurements obtained from experiments carried out in the St. Anthony Falls Laboratory Outdoor StreamLab are employed to validate the predictive capabilities of the numerical model. The VSL3D is used to develop a virtual testing environment of unprecedented resolution and physical realism for designing and optimizing the arrangements and installation of stream restoration rock structures. To our knowledge, the present work is the first systematic attempt to employ unsteady, three-dimensional, hydro-morphodynamics numerical modeling coupled with high-performance computing to develop an engineering framework for designing hydraulic structures for stream restoration applications.

The mean flow in a nature-like fishpass can be highly modified by the Froude number. It is important to understand this evolution to correctly design the structure. The studied configuration is an emergent staggered arrangement of obstacles. The hydraulic resistance of a fishpass is experimentally investigated that depends on several geometric parameters: block shape, ramp slope, block density, and bed roughness. An analytical model based on the balance momentum allows one to quantify the influence of each hydraulic parameter. The bed roughness has a weak influence, whereas the block shape and the Froude number are significant. The variation of the drag coefficient was analyzed to improve the stage-discharge relationship. To this end, a correlation with the block diameter and water level is proposed. The maximal velocity reached in the fishpass can also be estimated. These results have to be compared with the fish swimming ability to assess the fishpass passability.

The 14-m-tall Marmot Dam was removed during the summer of 2007, and the cofferdam protecting the working area was breached during a storm on October 19, 2007, allowing approximately $\mathrm{750,000}{\mathrm{m}}^3$ of reservoir deposit to be eroded freely and released downstream to the Sandy River. Prior to the Marmot Dam removal, sediment transport models were developed to predict the transport dynamics of both gravel and sand, providing key pieces of information for stakeholders and regulatory agencies to select the most appropriate dam removal alternative. A monitoring program was implemented following dam removal that was designed to examine model predictions and assess when potential fish passage issues related to dam removal were no longer of concern. Comparisons of model predictions with field observations indicate that the model successfully predicted the erosion of the impoundment deposit, the deposition of sediment in a short reach downstream of the dam, and the lack of deposition in the majority of the Sandy River. The model overpredicted sediment deposition in a reach 7 to 12 km downstream of the dam. Further examinations indicated that the overpredicted sediment deposition may be attributed to underestimating the gravel particle abrasion coefficient during initial modeling. Comparisons also indicated that the model significantly underpredicted the suspended sediment concentration during the first 10 h immediately following cofferdam breaching but correctly predicted the minimal increase in suspended sediment concentration thereafter.

A new surface flow routing algorithm based on numerical solutions of shallow water equations and the kinematic wave approximation (SWE-KWA) is proposed in this paper. The shallow water equations are discretized by the first-order Godunov-type finite-volume method. The stability analysis showed that the friction source term increased exponentially as flow depth became very small. This breaks the balance between the friction and the slope source terms. An approximate solution to the kinematic wave equation is introduced to restore this balance. This kinematic wave approximation makes it possible to apply the shallow water equations to both overland and channel flows. Test results show that this algorithm is accurate, robust, and stable for both very shallow overland and concentrated channel flows. The minimum allowable flow depth used in the tests is ${10}^{-10}\mathrm{m}$, two orders of magnitude smaller than the common rainfall excess rate (${10}^{-5}$ to ${10}^{-8}\mathrm{m}/\mathrm{s}$). Because this algorithm applies to both the overland and the channel flow, the developed model can directly use the raw digital elevation data as topographic data without a predefined channel network. This feature makes the new algorithm extremely valuable for solving flow routing problems in ungauged watersheds.

In the last decades the design of stepped spillways regained some interest because of their suitability with new construction methods including gabions. The hydraulic performances of gabion stepped weirs were investigated experimentally in terms of the flow patterns, air-water flow properties, and energy dissipation. A laboratory study was conducted in a 26.6° slope (1V:2H) and 0.10-m step height facility, with both smooth impervious and gabion steps. The visual observations highlighted the seepage flow through the gabions, inducing a modification of the cavity flow especially in the skimming flow regime. In skimming flows, higher velocities were measured at the downstream end of the gabion stepped chute, associated with smaller energy dissipation rates and lower friction factors, compared to the smooth impervious stepped chute data.

In this study, transport of mixed-size sediment particles under steady flow is described using the continuous-time Markov process. The continuous behavior of particle movement among the immobile bed material, bedload, and suspended load layer is modeled using a three-state continuous-time Markov process. Therefore, the probability of particles staying in each layer can be obtained by the proposed model, which can then be used to quantify the number of particles, and the bedload and suspended load transport rate, respectively. In addition, since the finer particles are more likely sheltered by coarse particles on the bed, mixed-size particles are considered in this paper to account for such an effect. The influence of dimensionless effective shear stress on the mean transient rates, representing the average rate of particle transitions among the immobile bed material, bedload layer, and suspended load layer, is quantified and discussed. The proposed model is verified against the experimental data with both bedload and suspended load particles. The modeling results of both bedload and suspended load transport rate show a reasonable agreement with laboratory measurements.

Transient flow events can potentially damage infrastructure and threaten public safety in the vicinity of violent flow eruptions that occur where tunnel drop shafts interconnect with the surface-combined sewer system. Transient events have been observed in tunnels and at drop shafts in the tunnel and reservoir plan (TARP) system in Chicago on a number of occasions since operations first began in 1985. For the most part, the phenomenon has been controlled effectively by operating the system in a conservative mode through controlling the rate of filling. Nevertheless, more recently, not only water transients but also geysering events—the intermittent discharge of a combination of water and air from a hydraulic system—have occurred at the terminus of branch tunnels where controlled filling is not effective. The Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), the operating agency for the system, has been prompted to develop engineering solutions to minimize transient flow occurrence and negative effects at the affected drop shaft locations. To this end, numerical modeling was conducted to investigate the cause and potential solutions. Such modeling included one-dimensional (1D) hydraulic transient modeling of a large domain in the system, followed by more detailed three-dimensional (3D) two-phase flow simulations within a smaller domain. The 1D approach revealed the interaction between localized events of interest and other transient flow features that originate far from the drop shaft under consideration. Computational fluid dynamics (CFD) modeling results show the complex interaction of trapped air and water in the tunnels. Even though more field observations are needed to test the accuracy of model predictions, careful interpretation of the results made it possible to pinpoint the problem and propose a solution. The findings also provided insight into the importance of considering both inertial instabilities as well as air–water interactions in geysering mechanics.

Former studies on combining flows resulted in an efficient layout of sewer junctions operated under supercritical approach flow conditions. Straight extensions allowed a reduction in the shock wave heights generated by the merging flows, so that the global discharge capacity was significantly increased. Herein, an extensive experimental campaign is presented on a physical model with the aforementioned layout, although with generalized geometrical conditions now including various conduit diameters. The effects of the main parameters governing the energy losses for combining flows were ascertained to enhance the information available from the literature. The results and their analysis provide a basis for the prediction of energy losses at junction manholes with different upstream and lateral conduit diameters and various flow conditions.