Journal of Hydraulic Engineering

The purpose of this study is to analyze the combined effects of storm surge and inland rainfall on the floodplain of a coastal bayou in the Houston area by using dynamic hydraulic modeling. Most existing floodplains in the Houston area are defined using only rainfall as an input into steady-state hydraulic models and do not consider the impact of hurricane-induced storm surge on the floodplain. HEC-RAS, a one-dimensional flow model, was run for both steady- and unsteady-states to analyze the additional effect storm surge has on the coastal floodplain. Storm surge and rainfall data from Hurricane Ike were utilized to run an unsteady hydraulic model on Horsepen Bayou near Galveston Bay. The dynamic model generated a good match between the modeled hydrograph and measured data in the watershed. Additionally, a timing sensitivity analysis was completed by shifting the timing of the storm surge both earlier and later in time. The dynamic model revealed that the timing of both rainfall and storm surge play a significant role in the magnitude of inland flooding.

In this paper, we propose a system of numerical models for the compatibility assessment of reservoir sediment flushing and protection of downstream river environments. The model system is made up of two simulation models. The first model simulates soil erosion in watershed slopes and sediment transport in the tributary of the reservoir by means of a weighted essentially nonoscillatory (WENO) method, which is conservative and fourth-order accurate in space and time. The second model simulates velocity and suspended solid concentration fields in the reservoirs. This model is based on the three-dimensional (3D) numerical integration of motion and concentration equations, expressed in contravariant form on a generalized boundary-conforming curvilinear coordinate system by using a conservative and higher-order accurate numerical scheme. The proposed system of models is applied to the Pieve di Cadore (Veneto, Italy) reservoir and to its catchment area. By comparing suspended solid concentrations that are discharged through the bottom outlets during flushing operations with suspended solid concentrations in the main river during natural flooding, we perform an assessment of the compatibility between sediment flushing and the protection of the river ecosystem downstream.

A new theoretical equation is proposed to describe the streamwise variations of the transverse velocity along a curved channel with a constant curvature. Furthermore, based on this theoretical equation for the transverse velocity, a new equation for the transverse dispersion coefficient is developed to incorporate the effect of the secondary flow on the transverse dispersion in curved channels. The new equations for the transverse velocity and dispersion coefficient are verified with experimental data sets that were obtained from laboratory experiments conducted in two different channels. The results show that the proposed velocity equation properly describes the streamwise variations of the secondary flow developed in the curved channels. The reach-averaged values of the transverse dispersion coefficient calculated by the new equation are in relatively good agreement with the observed values from the laboratory channels. Sensitivity analysis reveals that both the secondary flow and the transverse dispersion coefficient are proportional to the roughness factor, and in inverse proportion to the aspect ratio of the channel.

We investigated sediment-water heat flux estimation in relation to applications (hydrodynamics simulation, evaporation studies, or global change effects assessment) in which sediment temperatures are not available because of technical complications (difficulties in installing sensors, for example) or because of methodology used (remote sensing, for example). We used field sediment temperature data measured every 10 min to 1 m depth at Doñana National Park marshland to obtain sediment thermal properties and to calculate diel sediment-water heat exchange through Beck’s sequential function specification method. We compare four models for the simulation of sediment-water heat flux by using surface temperatures. Two models need initial estimated inside sediment temperatures; the other two do not. Influence of estimated initial temperature profiles depends on the temperature distribution assumption and is significant for three days or fewer at the daily timescale. A model that does not use initial sediment temperatures provides accurate estimations with low computation time.

The dispersion capabilities of two types of marine wastewater diffusers, one with risers containing eight radial ports (rosette) and the other with the ports uniformly distributed, were directly compared in laboratory experiments for three scenarios: stationary and flowing unstratified, and flowing stratified. Tracer concentration fields were measured with a three-dimensional laser-induced fluorescence system. The dispersion capabilities, particularly dilution, of the two diffuser types were similar. The primary differences occurred with no current where the rosette diffuser plumes were bent inwards, causing a dynamic interaction. This resulted in a small reduction in near field dilution, an increase in the spreading layer thickness, and a decrease of the near field length. These differences were virtually eliminated by a flowing current when the eight individual plumes from a rosette first merged with themselves and then with those from neighboring risers to become laterally quite uniform at the end of the near field. The major near field characteristics were primarily determined by the buoyancy flux per unit length. Temporal concentration fluctuations were high close to the diffuser and then decreased and leveled off near the end of the near field where the discharge-induced turbulence collapses. The fluctuations do not go to zero, however, owing to remnants of previous fluctuations.

Mixing and dispersion processes affect the timing and concentration of contaminants transported within urban drainage systems. Hence, methods of characterizing the mixing effects of specific hydraulic structures are of interest to drainage network modelers. Previous research, focusing on surcharged manholes, used the first-order advection-dispersion equation (ADE) and aggregated dead zone (ADZ) models to characterize dispersion. However, although systematic variations in travel time as a function of discharge and surcharge depth were identified, the ADE and ADZ models did not provide particularly good fits to observed manhole mixing data, which meant that the derived parameter values were not independent of the upstream temporal concentration profile, and no rules for predicting parameter values based on manhole size and configuration were provided. An alternative, more robust, method is described by using the system’s cumulative residence time distribution (CRTD). This paper shows how a deconvolution approach derived from systems theory may be applied to identify, from laboratory data, the CRTDs associated with surcharged manholes. Archive laboratory data are reanalyzed to demonstrate that the solute transport characteristics of a surcharged manhole with straight-through inflow and outlet pipes over a range of flow rates and surcharge depths may be modeled using just two dimensionless CRTDs, one for prethreshold and the other for postthreshold surcharge depths. The model combines the derived manhole CRTDs with a standard (Gaussian) pipe dispersion model to provide temporal solute concentration profiles that are independent of both scale and the ratio of the pipe and manhole diameters.

The time-averaged characteristics of turbulent wall-wake flows downstream of a sphere placed on a rough wall are studied. The profiles of the defect of streamwise velocity, Reynolds shear stress, and turbulence intensities exhibit some degree of similarities when they are scaled by their respective peak defect values. For the velocity defect profiles, the vertical distances are scaled by the height of the location of the half-peak velocity defect. However, for the defect profiles of the Reynolds shear stress and the turbulence intensities, the vertical distances are scaled by the height of the location of the half-peak Reynolds shear stress defect. The magnitudes of the peak defect of all the quantities diminish with the distance downstream of the sphere characterizing the recovery of their undisturbed profiles. Additionally, the theoretical similarity solution for the velocity defect profiles is obtained. The third-order correlations imply that in the inner layer of wall wakes, a streamwise acceleration is prevalent and associated with a downward flux, suggesting sweeps. In contrast, in the outer layer, a streamwise deceleration exists and is associated with an upward flux, suggesting ejections. The profiles of the energy budget show that the turbulent and pressure energy diffusions oppose each other. The turbulent production has a positive peak, and the pressure energy diffusion has a negative peak, indicating a large gain in turbulence production in the wall-wake flows. The quadrant analysis confirms that in wall-wake flows, sweeps are the governing mechanism resulting from an inrush of fluid streaks. The bursting events have shorter duration, but they are more frequent than those in upstream.

Comprehensive measurements of bed-load sediment transport through a section of the Danube River, located approximately 70 km downstream from Bratislava, Slovakia, are used to assess the accuracy of bed-load formulas implemented in 1D modeling. Depending on water discharge and water level, significant variations in the distribution of bed load across the section were observed. It appeared that, whatever the water discharge, the bed shear stress $\tau$ is always close to the estimated critical bed shear stress for the initiation of sediment transport ${\tau }_{cr}$. The discussion focuses on the methods used in 1D models for estimating bed-load transport. Though usually done, the evaluation of bed-load transport using the mean cross-sectional bed shear stress yields unsatisfactory results. It is necessary to use an additional model to distribute the bed shear stress across the section and calculate bed load locally. Bed-load predictors also need to be accurate for $\tau$ close to ${\tau }_{cr}$. From that point of view, bed-load formulas based on an exponential decrease of bed-load transport close to ${\tau }_{cr}$ appear to be more appropriate than models based on excess bed shear stress. A discussion on the bed-load formula capability to reproduce grain sorting is also provided.

This work focuses on the sediment transport that is dominated by collisional exchange of momentum between particles and can be identified as the transition regime between bed-load and suspended-load transport. Idealizing the particles as inelastic frictional spheres and accounting for the role of the interstitial fluid on particle collisions, three regions characterize such flows: a diffuse collisional layer neighboring the free surface, in which a simple trapezium rule is employed to solve a boundary-value problem based on the kinetic theory; a dense, algebraic layer, in which there is an algebraic balance between production and dissipation of particle fluctuation energy, the concentration is approximately constant, and correlated motion between the particles exists; and a macroviscous layer, close to the erodible bed, in which the collisions are inelastic and the fluid viscous force dominates the momentum exchange. Using boundary conditions of no-slip and yielding at the erodible bed and vanishing of the particle stresses and energy flux at the top of the sediments, an analytical description of the flow field is obtained. After a sensitivity analysis of the approximate theoretical solution to the model parameters—only one of them has a phenomenological origin and cannot be directly measured—comparisons with experiments performed on sheet flows of water and plastic cylinders, sand, or gravel assess the validity of the theory.

Chute aerators are applied to high-velocity spillways to entrain air into the flow so that cavitation damage is avoided. Air entrainment occurs locally at the aerator, whereas further downstream the flow is deaerated. This process is relevant because it defines the influence range of an aerator. A preliminary study investigated the effect of the aerator geometry and of the approach flow conditions on the streamwise bottom and average air concentration characteristics. Two aspects were excluded, namely, the effect of (1) steep deflectors, which operate more efficiently regarding air entrainment yet with simultaneously poor flow features; and (2) cavity subpressure effect on the streamwise air concentration field. A cavity subpressure reduces, in particular, the streamwise bottom air concentrations or it provokes aerator choking so that the cavitation protection is not ensured. Physical model tests indicate that optimum aerator performance results at deflector angles around 10°, i.e., a slope of $1:5$ relative to the chute bottom with acceptable shock wave formation, spray generation, and jet height.

Three issues concerning the iterative solution of the nonlinear equations governing the flows and heads in a water distribution system network are considered. Zero flows cause a computation failure (division by zero) when the Global Gradient Algorithm of Todini and Pilati is used to solve for the steady state of a system in which the head loss is modeled by the Hazen-Williams formula. The proposed regularization technique overcomes this failure as a solution to this first issue. The second issue relates to zero flows in the Darcy-Weisbach formulation. This work explains for the first time why zero flows do not lead to a division by zero where the head loss is modeled by the Darcy-Weisbach formula. In this paper, the authors show how to handle the computation appropriately in the case of laminar flow (the only instance in which zero flows may occur). However, as is shown, a significant loss of accuracy can result if the Jacobian matrix, necessary for the solution process, becomes poorly conditioned, and so it is recommended that the regularization technique be used for the Darcy-Weisbach case also. Only a modest extra computational cost is incurred when the technique is applied. The third issue relates to a new convergence stopping criterion for the iterative process based on the infinity-norm of the vector of nodal head differences between one iteration and the next. This test is recommended because it has a more natural physical interpretation than the relative discharge stopping criterion that is currently used in standard software packages such as EPANET. In addition, it is recommended to check the infinity norms of the residuals once iteration has been stopped. The residuals test ensures that inaccurate solutions are not accepted.

Hydraulic transient field tests have been conducted in a water distribution network. Existing transient models are applied to model the measured responses, but poor matches are obtained apart from the estimation of the initial rise of pressure. Possible reasons for these discrepancies include the effects of demands, entrained air, unsteady friction, friction losses associated with small lateral pipes, and mechanical damping caused by the interaction of pipes and joints with surrounding soils (including the effects of vibration and different degrees of restraint). These effects are systematically investigated by inclusion of the previously mentioned phenomena in conceptual transient models and calibration to the measured field responses. A mechanical damping-based conceptual transient model is shown to be the only model that can be accurately calibrated to the measured field responses.

A higher-order elastic model of the flow in long pressurized pipelines is expected to be utilized for stability analysis of the governor-turbine-hydraulic system in hydropower stations. Because traditional elastic models are limited in lower order application because of their difficult decoupling in addition to the rigid model, a new linear elastic model of the flow in pressurized pipelines is derived on the basis of the equations of hydraulic vibration, in which each oscillatory flow with a different order has been obtained with ordinary differential equations in decoupling form. For water conveyance systems with branching pipes or parallel pipes in hydropower stations, the state equations to describe hydraulic characteristics of the governor-turbine-hydraulic system are established with the application of this new elastic model for diversion pipeline flow or tail tunnel flow. The influence of the elastic models with different order on a system’s stability are revealed in detail by two cases that illustrate that an elastic model with proper order should be used for the flow in pressurized pipelines of hydropower stations, according to their length, to improve the accuracy of stability analysis.

The Federal Highway Administration recommends that stream stability analyses at bridges over water begin with a Level 1 assessment. Following this assessment of the physical condition of the stream in the vicinity of the bridge, the user must determine whether or not the relative risk is low. If it is low, then no action is needed. If the risk is greater than low, then a Level 2 analysis is recommended; however, no method is given for determining the risk. In this paper, the relative risk of unsatisfactory conditions at bridge foundations is assessed as a simple function of vulnerability and criticality. Vulnerability is based on a stream stability assessment and data from the National Bridge Inventory (NBI). Criticality is determined indirectly as a function of the bridge serviceability and cost by using data extracted from the NBI. Relative risk is then qualitatively determined by combining vulnerability and criticality. Three examples in central Pennsylvania are provided in which the relative level of risk is used to determine the need for a Level 2 analysis.

Bubbles entrained by spilled water at hydroelectric projects increase the concentration of total dissolved gas (TDG), which may lead to gas bubble disease in fish. In this paper, the TDG dynamics downstream of Wells Dam are investigated using a two-phase flow model that accounts for the effect of the bubbles on the flow field. The TDG is calculated with a transport equation in which the source is the bubble/liquid mass transfer, a function of the gas volume fraction and bubble size. The model uses anisotropic turbulence modeling and includes attenuation of normal fluctuation at the free surface to capture the flow field and TDG mixing. The model is validated using velocity and TDG field data. Simulations under two plant operational configurations are performed to gain a better understanding of the effect of spill operations on the production, transport, and mixing of TDG. Model results indicate that concentrated spill releases create surface jets that result in the lowest TDG concentration downstream. On the other hand, spreading the spill release, with moderate flow through each gate, produces the highest TDG values downstream as a result of more air available for dissolution and smaller degasification at the free surface.

Sediment concentration distribution and the basic probability of sediment movement near the channel bed are two of the most important and fundamental issues in the study of sediments. Based on statistical analysis and considering the transport mechanisms, the rules of sediment concentration distribution near a channel bed are studied. Analytical expressions for the near-bed sediment concentration distribution and mean sediment concentration are derived, and the expression for the mean sediment concentration near the bed is verified by measured data, which were obtained from previous experiments. With the help of statistical theory, the expressions of basic probabilities, i.e., rolling, saltating, and suspending probabilities, for sediment movement near the bed are also derived. The expression for starting probability is verified by the measured data. The verification shows that the results from the proposed expression agree well with the measured data. This research has both theoretical and practical significance for further investigation concerning rules of bed load and suspended sediment transport.

The dynamic behavior of bed-load sediment transport under unsteady flow conditions is experimentally and numerically investigated. A series of experiments are conducted in a rectangular flume (18 m in length, 0.80 m in width) with various triangular and trapezoidal shaped hydrographs. The flume bed of 8 cm in height consists of scraped uniform small gravel of $D_{50}=4.8\mathrm{mm}$. Analysis of the experimental results showed that bed-load transport rates followed the temporal variation of the triangular and trapezoidal hydrographs with a time lag on the average of 11 and 30 s, respectively. The experimental data were also qualitatively investigated employing the unsteady-flow parameter and total flow work index. The analysis results revealed that total yield increased exponentially with the total flow work. An original expression which is based on the net acceleration concept was proposed for the unsteadiness parameter. Analysis of the results then revealed that the total yield increased exponentially with the increase in the value of the proposed unsteadiness parameter. Further analysis of the experimental results revealed that total flow work has an inverse exponential variation relation with the lag time. A one-dimensional numerical model that employs the governing equations for the conservation of mass for water and sediment and the momentum was also developed to simulate the experimental results. The momentum equation was approximated by the diffusion wave approach, and the kinematic wave theory approach was employed to relate the bed sediment flux to the sediment concentration. The model successfully simulated measured sedimentographs. It predicted sediment yield, on the average, with errors of 7% and 15% of peak loads for the triangular and trapezoidal hydrograph experiments, respectively.

Sediment grains transported as supply-limited bed load on a rigid surface move either discretely or collectively as bed forms, with significantly different effective grain speeds and active storage volumes. The adopted mode has implications for sediment sorting and heavy mineral placer formation, dispersal of grain-associated pollutants, and accumulation and flushing of sediment deposits in unlined canals and sewers. The threshold condition between the two modes has been established for a smooth surface from flume experiments with different sediment types, flow conditions, and sediment supply rates. This is expressed in terms of a relationship between the sediment movability number, a dimensionless bed load parameter, and a grain shear Reynolds number.

A three-dimensional computational fluid dynamics model is applied to predict local scour around an abutment in a rectangular laboratory flume. When modeling local scour, steep bed slopes up to the angle of repose occur. To predict the depth and the shape of the local scour correctly, the reduction of the critical shear stress due to the sloping bed must be taken into account. The focus of this study is to investigate different formulas for the threshold of noncohesive sediment motion on sloping beds. Some formulas only take the transversal angle (perpendicular to the flow direction) into account, but others also consider the longitudinal angle (streamwise direction). The numerical model solves the transient Reynolds-averaged Navier-Stokes equations in all three dimensions to compute the water flow. Sediment continuity in combination with an empirical formula is used to capture the bed load transport and the resulting bed changes. When the sloping bed exceeds the angle of repose, the bed slope is corrected with a sand-slide algorithm. The results from the numerical simulations are compared with data from physical experiments. The reduction of the bed shear stress on the sloping bed improves the results of the numerical simulation distinctly. The best results are obtained with the formulas that use both the transversal and the longitudinal angle for the reduction of the critical bed shear stress.

A dispersion model for a wide range of depthwise vertical shear is derived by using perturbation analysis and power ($m$) law velocity profile. For $m=1$, the velocity profile provides linear shear, whereas $m>1$ provides nonlinear shear, and for $m>20$, the velocity profile resembles the flow through emergent vegetation. The power law represented parametrically simulates well the complex shear profiles involved in emergent and submerged vegetated flows. The proposed model shows reasonable agreement with past data on vegetated flows for a wide range of nonlinear shear velocities.

An experimental study of the subcritical dividing flow in an equal-width, four-branch junction with two inflows and two outflows is presented. A brief description of the flow characteristics is given. From the analysis of test data, a linear relationship among five nondimensional parameters, including inflow ratio, outflow ratio, Froude number of inflow in one direction, outflow depth ratio, and outflow aspect ratio, is proposed and proven to successfully predict the flow distribution in the junction. It can be used to solve the dividing flow problem in a street junction, and it can be included as a part of a numerical model of a street network involving subcritical flows.