Journal of Hydraulic Engineering

On August 29, 2005 Hurricane Katrina resulted in several breaches in the levees and flood walls protecting New Orleans. Of the 20 breaches, the 17th Street Canal breach caused much of the city flooding. In this paper, the results of studies on a 1:50 scale hydraulic model of this breach based on the Froude similitude relationships are presented. It is assumed in the model that the bed is fixed and the levee below the flood wall remains intact during breach closure. This was the case in the 17th Street Canal breach. Because of the many uncertainties in the values of various variables, a range of conditions were run on the model in an attempt to bracket the results for the flooding depths and the initial failed attempts to close the breach. Then, various possible methods for breach closure were investigated utilizing the procedures developed for cofferdam closure for river diversion, e.g., toe dumping, transverse dumping, single- and multibarrier embankments, etc. Closures of the breach and the closure of the canal at the Old Hammond Highway Bridge were investigated. Results from the case study show that some of these methods could have been utilized for closing the Katrina breaches. However, special care should be exercised when extending them for breach closure at other sites.

The hydraulic design of a tilting weir is presented, which allows for periodic exchange of potadromous fish between freshwater ecosystems. The application domain includes inland waters that need to be isolated hydraulically, preserving the existing ecological connection with the surrounding areas as much as possible. In the absence of a hydraulic gradient, the weir is opened in its neutral position and fish can bypass through passages sideways of the weir. When a hydraulic gradient develops in either of two directions, the weir rotates until a new balance of moments of force is reached, while fish can still bypass. Above a threshold value of the hydraulic gradient, the weir falls shut. When the hydraulic gradient returns to zero, the weir reopens, restoring the ecological connection. Four variants of the weir were investigated in the Hydraulics Laboratory at Wageningen University. The versatility of the design is demonstrated by showing that the hydraulic gradients required for closure and reopening can be manipulated largely independently. Patterns of flow velocity and turbulence kinetic energy were analyzed, which suggest that relevant fish species can bypass the weir unimpeded. The effectiveness of the weir will mainly depend on hydrostatic aspects, which determine when the weir is opened and closed, and on the absence of large hydraulic gradients in the migration season.

We present the results of a double-plume integral model for a positively buoyant multiphase plume of liquid carbon dioxide injected at mid-depths $(500–2000\mathrm{m})$ in the ocean. In addition to the relevant plume physics, the model accounts for dissolution of the carbon dioxide droplets, the effect on dissolution of clathrate hydrate films, and the increase in seawater density due to enrichment by dissolved carbon dioxide. Due to the creation of negative buoyancy in the entrained fluid through dissolution of carbon dioxide, the near-field mixing exhibits a complex set of energetic descending outer plume structures that do not converge to a steady state. The unsteady near field is shown to be inherent in the plume physics, resulting in enhanced distribution of the dissolved carbon dioxide over the plume height and formation of multiple, unsteady intrusion layers. Despite the complex plume near field, we demonstrate that the height of maximum plume rise has a steady solution. A general empirical design equation for the maximum height of plume rise is presented from the model results, dependent on the initial droplet diameter and buoyancy flux, the strength of the ambient stratification, and the linear mass transfer reduction factor due to hydrate formation. A sensitivity analysis of the design equation highlights the droplet diameter as the most important design variable and demonstrates that current uncertainty in the value of the mass transfer reduction factor results in uncertainty of the plume rise height within a factor of three.

The interaction of two rivers flowing into Coeur d’Alene Lake (United States) was investigated with a field experiment and three-dimensional numerical simulations. The focus was on the influence of basin morphology, wind speed, and wind direction on the fate and transport of the inflowing water. Data from the field campaign showed that intrusions from the two rivers propagated into the lake at different depths, with the trace element polluted Coeur d’Alene River flowing into the lake above the trace element poor and nutrient rich St. Joe River inflow. The inflows initially intruded horizontally into the lake at their level of neutral buoyancy and later mixed vertically. Model results revealed that, as the intrusions entered the main lake basin, a forced horizontal mode-two basin-scale internal wave interacted with the intrusions to frequently siphon them into the lake proper and where rapid vertical mixing followed. The results serve to show how detailed transport and mixing patterns in a lake can have important consequences for the plankton ecology in the lake.

Flood and dambreak hazards are not limited to moderate terrain, yet most shallow-water models assume that flow occurs over gentle slopes. Shallow-water flow over rugged or steep terrain often generates significant nonhydrostatic pressures, violating the assumption of hydrostatic pressure made in most shallow-water codes. In this paper, we adapt a previously published nonhydrostatic granular flow model to simulate shallow-water flow, and we solve conservation equations using a finite volume approach and an Harten, Lax, Van Leer, and Einfeldt approximate Riemann solver that is modified for a sloping bed and transient wetting and drying conditions. To simulate bed friction, we use the law of the wall. We test the model by comparison with an analytical solution and with results of experiments in flumes that have steep (31°) or shallow (0.3°) slopes. The law of the wall provides an accurate prediction of the effect of bed roughness on mean flow velocity over two orders of magnitude of bed roughness. Our nonhydrostatic, law-of-the-wall flow simulation accurately reproduces flume measurements of front propagation speed, flow depth, and bed-shear stress for conditions of large bed roughness.

Flood risk studies require hydraulic modeling in order to estimate flow depths and other hydraulic variables in the floodplain for a wide range of input conditions. Currently there is a need to improve the computational efficiency of fully two-dimensional numerical models for large-scale flood simulation. This paper describes an adaptive quadtree grid-based shallow water equation solver and demonstrates its capability for flood inundation modeling. Due to the grid dynamically adapting to dominant flow features such as steep water surface gradients and wet-dry fronts, the approach is both efficient and accurate. The quadtree model is applied to a realistic scenario of flood inundation over an urban area of $36{\mathrm{km}}^2$ , resulting from the flood defenses breaching at Thamesmead on the River Thames, United Kingdom. The results of the simulation are in close agreement with alternative predictions obtained using the commercially available software TUFLOW.

Because of complicated channel geometry, horizontal coordinates are important in accurate and efficient computations of river flow. We propose a simplified horizontal coordinate system for river-flow simulation, referred to here as a horizontal sigma coordinate system that uses boundary-fitted grids along river channels like a generalized curvilinear coordinate system, without losing the simplicity of Cartesian or orthogonal curvilinear coordinate systems. The proposed approach is based on a sigma coordinate system. The depth-averaged continuity and momentum equations for horizontal two-dimensional fluid motion in the Cartesian and orthogonal curvilinear coordinate systems are transformed to those in the horizontal sigma coordinate system. The form of the transformed continuity and momentum equations in the proposed system is a fairly minor modification of the form of the equations for the Cartesian and orthogonal curvilinear coordinate systems. Computational results for a straight open-channel flow with a narrow pass indicate that the horizontal sigma coordinate system is as numerically accurate as the generalized curvilinear coordinate system and that the CPU times required to calculate the horizontal sigma and Cartesian coordinate systems are comparable. We also computed flood flow in an actual river using the numerical models based on the horizontal sigma, orthogonal curvilinear, and generalized curvilinear coordinate systems. The results show that the horizontal sigma coordinate system may dramatically reduce the numerical error generated in the orthogonal curvilinear coordinate system. These facts demonstrate that the horizontal sigma coordinate system is a promising tool for numerical approximation of flow in the horizontal direction that is computationally efficient and numerically accurate.

This study proposes relationships between mean streamflow depth and cohesionless particle bed characteristics at the beginning of the sediment transport at high slopes in low submergence conditions. The inception of sediment transport is theoretically approached by the incipient motion condition. A previous theoretical analysis of the equilibrium of a gravel particle based on the exposure of the particle itself to the streamflow is generalized by introducing the relative position of the exposed part of the particle with respect to the bottom and by considering the case of a partially submerged particle. Using a low-submergence flow velocity profile in the particle balance of forces at the limit of equilibrium leads to a relationship between the mean flow depth and the sediment size. Another velocity profile of low-submergence flow is added to that used in the previous study, and two analytical expressions of flow depths corresponding to the beginning of sediment transport are obtained for both fully submerged particles and partially submerged particles. The theoretical flow depths given by the analytical expressions are then compared with those from previous experimental tests at the beginning of sediment transport at high slopes with fully submerged particles. The comparison shows a satisfactory agreement between theoretical and experimental values of flow depth corresponding to the degree of exposure and the relative position of the exposed part of the surface layer bed particles.

The temporal development of plunge pool scour was investigated using a novel experimental approach. Longitudinal profiles along the scour hole were recorded with an optical method to allow its definition at any time, from the initiation of scour to nearly the end-scour condition. The characteristics of the scour hole geometry were investigated, namely the maximum scour hole depth, the maximum ridge height, and their locations relative to the scour hole origin. It is demonstrated that the evolution is logarithmic, similar to that found for bridge pier and abutment scour. A distinction is further made between the developing and the developed scour hole phases. The main issue of the present research was to define the developed scour hole characteristics because the developing scour phase is influenced by turbulence features that may be difficult to assess. This work therefore allows for an appreciation of the temporal evolution of a scour process of engineering interest.

The Gupo Bridge crosses the Nakdong River near the city of Busan, South Korea. During Typhoon Maemi in 2003, the old Gupo Bridge collapsed due to excessive pier scour. More recently, the highway construction on the left-bank floodplain required right-bank channel widening to restore the channel flood-carrying capacity. This $7\mathrm{m}$ deep floodplain excavation is expected to cause significant local scour around the $8–10\mathrm{m}$ wide and $3\mathrm{m}$ thick spread footings of Piers 11 and 12 of the Subway Bridge and Piers 15 and 16 of the Gupo Bridge. Three design options are examined for retrofitting floodplain bridge piers with massive spread footings. A solution with sheet piles and riprap was recommended in 2006 as the most appropriate design, but Plan III with a conical riprap structure around the footings was ultimately constructed in 2007 for economic reasons. Laboratory experiments also highlight the need to place gravel and synthetic filters under the designed riprap.

Highway drainage channels often approach slopes of 0.5 (50%). Single layer riprap-lined channels at this slope have been constructed for highway drainage and appear to perform satisfactorily, but no quantitative information is available to guide their hydraulic design. A $1∕6$ scale laboratory hydraulic model, using a single layer of sized crushed limestone, was constructed to determine the relationship between depth of flow and discharge. The flow range investigated took place at depths near or below the top of the riprap, since riprap instability occurred at greater depths. Standing waves and localized hydraulic jumps dominated the surface of the flow, which held streamwise-averaged Froude numbers in a narrow range near the critical condition for those flow depths for which the channel is designed. This resulted in a simple relationship between unit discharge and depth of flow that applies to the model and prototype channels.

A new approach for predicting local scour downstream of grade-control structures based on neural networks is presented. An explicit neural networks formulation (ENNF) is developed using a transfer function (sigmoid) and optimal weights obtained from a training process. A genetic algorithm was used to optimize the neural network architecture and the optimal weights for input and output parameters were obtained using the Levenberg–Marquardt back-propagation algorithm. Experimental data available in the literature, including large-scale results were used for training and validation of the proposed model. The predictive performance of the ENNF was found superior to other regression-based equations and the robustness of ENNF was evaluated using field data.

The critical flow principle is a useful approach for the hydraulic analysis of round-crested weirs due to their single head-discharge relationships. The hydraulics of circular-crested weirs is examined using simplified models incorporating streamline curvature effects, comparing their predictions with experimental data. A generalized one-dimensional model based on the critical flow in curvilinear motion has been developed. The discharge coefficient increases with the specific energy normalized with the radius of curvature, $E∕R$ , when streamline curvature effects are included. The relative flow depth at the crest decreases as $E∕R$ increases. The flow at the weir crest is only critical for a normalized specific energy value of $E∕R\approx 0.5–0.6$ . For larger heads, the flow at the weir crest has been found to be supercritical.

Since predictions of scalar dispersion in small estuaries can rarely be predicted accurately, new field measurements were conducted continuously at relatively high frequency for up to $50\mathrm{h}$ (per investigation) in a small subtropical estuary with semidiurnal tides. The bulk flow parameters varied in time with periods comparable to tidal cycles and other large-scale processes. The turbulence properties depended upon the instantaneous local flow properties. They were little affected by the flow history, but their structure and temporal variability were influenced by a variety of parameters including the tidal conditions and bathymetry. A striking feature of the data sets was the large fluctuations in all turbulence characteristics during the tidal cycle, and basic differences between neap and spring tide turbulence.

Results of a numerical simulation investigating the complicated flushing process of an isolated trapped volume of salt water from a bar-blocked estuary are presented. A multiphase model, a part of the commercial code FLUENT 6.2, is applied. The governing equations together with initial and boundary conditions and the numerical scheme are described. The time-dependent salt-wedge position, vertical-density distribution, and proportion of total input kinetic energy converted into potential energy are examined for various incoming flow densimetric Froude number and estuary bed slope. The vertical position and thickness of the interfacial mixed layer between freshwater and salt water as well as the local gradient Richardson number are determined from simulated density profiles and velocity fields. The good agreement between the simulated and measured results indicates that the numerical model can be successfully applied to investigate the complex flushing process involving stratified flow.