Showing papers in "Journal of Hydraulic Engineering in 1990"

Controlling Mechanism of Local Scouring

Abstract: Experimental study of clear water scouring around a circular cylinder shows that the scour mechanism is coupled to the three-dimensional separation of the upstream boundary layer and the periodic vortex shedding in the wake of the cylinder. The first scour appears in the wake of the cylinder. The main scouring agent in the upstream region is a system of horseshoe vortices. The vortices have a periodical character that causes a triple-scour profile to develop in the upstream region. During scouring, the number and periods of horseshoe vortex shedding undergo no appreciable change. Despite the clear water stage, the transport phenomenon is periodical. Transport of sediment takes place through turbulent scales of comparable size to macro-length scales. The size of horseshoe vortices are representative for the macroscale. Wake scouring is caused by the primary wake vortices and the accelerated side flow. The process is characterized by a strong periodical transport and the formation of ripples. The periodicity is controlled by the shedding frequency of the wake vortices. Collars attached to the cylinder cannot prevent the formation of the vortices.

Overland flow in wetlands: vegetation resistance.

Abstract: Emergent wetland vegetation frequently provides most of the resistance to flow of surface water. Stems are typically spaced many diameters apart. Therefore, fluid friction should be computed from drag on single objects, not channel or packed bed equations. Complication arises from vertical variation of vegetation density and nonoriented spatial variation of soil elevations. Further, flows are often in the transition region between laminar and turbulent; and the Manning equation is therefore not appropriate. A detailed approach requires knowledge of statistical distributions of wetland ground elevation, depth, and velocity. Ground slopes are typically in the range of 0.1–100 cm/km. However, the depth range is small, so the combined effects recorrelate to a simple power law equation for dense emergent vegetation with spatial uniformity on the scale of 10 m. Parameters in the model can be estimated with sufficient accuracy from vegetation and soil surveys, combined with relatively little hydrologic data. Dat...

Explicit Methods for 2‐D Transient Free Surface Flows

Abstract: MacCormack and Gabutti explicit finite‐difference schemes are introduced to integrate the equations describing two‐dimensional, unsteady gradually varied flows. Both schemes are second‐order accurate in space and time, allow sharp discontinuous initial conditions, and do not require isolation of the bores. Both sub‐ and supercritical flows may be present simultaneously in different parts of the channel or in a sequence in time. The inclusion of boundaries and stability conditions and the addition of artificial viscosity to smooth high‐frequency oscillations are discussed. To illustrate application of the schemes in hydraulic engineering, two typical problems are solved and the results of different schemes are compared.

Skimming Flow in Stepped Spillways

Abstract: Flow on stepped spillways can be divided into nappe and skimming flow regimes. This technical note presents an approximate method of predicting the characteristics of skimming flow on stepped spillways. Expressing the turbulent shear stress between the skimming stream and the recirculating fluid, trapped by the stream between the steps, in terms of a fluid friction coefficient and dynamic pressure (in terms of the mean velocity), a flow equation has been developed. Using the model studies of Sorensen on the new Monksville dam, this friction coefficient was found to be equal to about 0.18. An estimate has been made of the energy loss caused by the steps and it was found that this loss is significant. It was also found that the skimming flow sets in when the critical depth is greater than about 0.8 times the height of the steps.

Model for Water Distribution System Reliability

Abstract: The hydraulic reliability of a water distribution system can be defined as the probability that the system can provide the demanded flowrate at the required pressure head. Due to the random nature of future water demands, required pressure heads, and pipe roughness, the estimation of water distribution system reliability for the future is subject to uncertainty. A methodology is presented to estimate the nodal and system hydraulic reliabilities of water distribution systems that accounts for the uncertainties. The framework for the methodology is based upon a Monte Carlo simulation consisting of three major components: random number generation, hydraulic network simulation, and computation of reliability. For purposes of illustration, an example water distribution system consisting of 17 pipes and 12 demand nodes is used. This new methodology can be used in the analysis or expansion of existing systems, or the design of new systems.

Boundary Shear Stress and Roughness Over Mobile Alluvial Beds

Abstract: The resistance to flow in the turbulent rough-flow range depends primarily upon the size, shape, and arrangement of the granular material making up the boundary. We have estimated the hydraulic roughness of mobile alluvial surfaces by inverting sediment-transport formulas to solve for the local boundary shear stress required to predict the observed sediment flux and size. Inserting this shear stress value and a near-bed velocity measurement into the law of the wall yields the roughness scale, \Iz\N\do defined as the height above the bed where velocity goes to zero. If the roughness is related to the coarse fraction of the bedload, such as \ID\N\d8\d4, then \Iz\N\do=0.1\ID\N\d8\d4. This roughness, obtained from mobile, naturally packed, and heterogeneous-in-size beds is three times greater than that predicted by the Nikuradse formula developed from nearly uniform and smoothly-packed surfaces. We detect no variation in roughness with transport stage, implying that the large static and slowly moving grains determine flow resistance and that momentum extraction by saltating grains is minor. Application of this simple roughness algorithm allows convenient and accurate calculation of the local boundary shear stress.

Mechanics of local scour around submarine pipelines

Abstract: The primary objective of this study is to improve understanding of the mechanism causing scour in unidirectional current. Experiments have shown how local scour develops around submarine pipelines in noncohesive sediments. The study shows that piping is the dominant cause of the initiation of scour. Piping and the stagnation eddy combine to undermine the pipeline, and mark the onset of scour. The critical hydraulic gradient associated with the initiation of scour is equal to the flotation gradient of the bed sediment. The pressure drop between the stagnation pressure upstream and wake pressure downstream of the pipe induces this hydraulic gradient. When a pipe is just embedded, the onset of scour does not occur if the ratio of the flow depth to pipe diameter exceeds 3.5. Similarly, the onset of scour does not occur for half‐buried pipes. The reduction in pressure gradient across the pipeline for these flow/pipe combinations accounts for the lack of scour. The onset of scour can be prevented by placing an ...

2‐D Depth‐Averaged Flow Computation near Groyne

Abstract: The depth-averaged velocity and bottom shear stress distributions in a rectangular channel near a groyne are computed by using a 2-D depth averaged model. The model uses a hybrid finite difference scheme and an iterative method to solve the governing equations of flow and turbulence transport. Due to streamline curvature effects in the region near the groyne tip, a correction factor is incorporated into the \Ik\N=ϵ\N turbulence model that significantly improves the agreement between the computed and experimental data of the velocities and of the streamline pattern compared to previous numerical methods. In this region the bottom shear stress is found to be largely influenced by the 3-D effects. A 3-D correction factor is introduced which considerably improves the computed bottom shear stresses. Sensitivity analysis is made on the \Ik\N=ϵ\N model coefficients and on the correction factors of the streamline curvature and the 3-D effects. The experimental errors in the velocity and bottom shear stress measurements are analyzed. The average errors between the computed and previous experimental results are presented with confidence intervals.

Transmission Losses in Arid Region

Abstract: Hydrographs of ephemeral streams in arid and semiarid regions usually decrease significantly in magnitude downstream, unless augmented by tributary flows. This process, termed transmission loss, is important not only in its obvious effect on flow reduction, but also as a source of ground water recharge to underlying alluvial aquifers. Most of the previous work in transmission loss was carried out on the semiarid regions of the United States. In this paper, three regression equations were developed for relating transmission loss to channel characteristics for an arid region in southwest Saudi Arabia. One of the equations seems to be reliable predictor of small losses associated with small upstream volumes. The other two equations were better predictors for large losses associated with large upstream volumes. Transmission loss data were also compared with some previously published. Although there was some scatter, the data is generally consistent with that previously published. This paper also adds to the d...

Closure of "Model for Flood Propagation on Initially Dry Land"

Abstract: A model is presented for the simulation of shallow water flow and, specifically, flood waves propagating on a dry bed. The governing equations are transformed to an equivalent system valid on a deforming coordinate system and are solved by a dissipative finite-element technique. A second-order difference scheme is employed for the integration in time. The implicit nonlinear equations resulting from the weak formulations are solved by the Newton-Raphson method, and the set of linear algebraic equations generated is solved by a frontal algorithm. A deforming grid generation scheme is introduced in the dissipative finite-element formulation to account for the effects of the propagating or receding wave fronts on dry land. The accuracy and stability of the model is examined by comparing the model results with observed data from an experimental field test. Results of trial runs for the simulation of overland flow are also presented.

Optimal Reliability‐Based Design of Pumping and Distribution Systems

Abstract: A reliability-based optimization model for water-distribution systems has been developed. The model is aimed at the following goals: (1) Design of the pipe network including the number, location, and size of pumps and tanks; (2) design of the pumping system using a reliability-based procedure considering both hydraulic failures of the entire network and mechanical failure of the pumping system; and (3) determination of the optimal operation of the pumps. The optimization problem is a large mixed-integer, nonlinear programming problem that is solved using a heuristic algorithm consisting of a master problem and a subproblem. The master problem is a pure 0–1 integer programming model, and the subproblem is a large nonlinear programming model solved in an optimal control framework. The conservation of flow and energy constraints are solved implicitly for each iteration of the nonlinear optimization procedure using a hydraulic simulation model, and the reliability constraints are also solved implicitly using a reliability model. The nonlinear programming problem is solved using a generalized reduced gradient code.

Reliability‐Constrained Pipe Network Model

Abstract: A new methodology for incorporating reliability considerations directly into leastcost optimization design models for looped water distribution networks is presented. The essence of the methodology...

Explicit calculation of pipe-network parameters

Abstract: An explicit algorithm is presented for directly determining a variety of design, operating, and calibration parameters for pipe networks. The problem solution is based on the reformulation of the steady‐state network equilibrium equations in terms of specified pipe‐system parameters. Since these equations are nonlinear, the incremental Newton‐Raphson method is used as the basic solution procedure. A continuous variable space is assumed for the various parameters, which are selected to exactly satisfy stated pressure and flow requirements at critical nodes and pipes throughout the pipeline systems for a range of operating conditions. This approach offers a basis for determining the optimum values for the various design, operating, and calibration parameters in the sense that the parameters can be calculated to exactly meet the specified pressure and flow constraints. All types of pipe distribution systems can be considered. The solution space is secured through a well‐arranged interaction between network t...

Tests of Selected Sediment‐Transport Formulas

Abstract: Eleven existing, sediment‐transport formulas were tested against the field data measured at two United States Geological Survey (USGS) gauging stations along the Sacramento River in California, whose bed‐material sizes are classified as those ranging from fine sand to coarse gravel. They include the Ackers‐White Einstein‐Brown, Engelund‐Fredsoe, Engelund‐Hansen, Inglis‐Lacey, Karim, Meyer‐Peter and Mueller, Rijin, Schoklitsch, Toffaleti, and Yang formulas. General flow and sediment input variables required by each transport function, as well as dependent variables, are summarized and detailed, and measured hydraulic and sediment quantities including bed‐material size distributions are presented. The computed sediment discharges are tabulated for comparison, and are directly compared against the measured suspended‐sediment discharges. The computed values, however, are found to deviate significantly from the measured values except for a very few cases. The test results clearly demonstrate how difficult a ta...

Watershed delineation with triangle-based terrain models

Abstract: An algorithm is presented for tracing the path of steepest descent from a given starting point on a terrain model defined by a triangulated irregular network. This algorithm is then extended to solve several problems. The flow patterns for a site are generated by tracing flow paths from a large number of starting points. The approximate stream network or channel network is found by tracing the channels upstream from pits or exit points. Once the stream network is found, the source areas or contributing areas for each of the sections of the stream are delineated. The source areas are then used to delineate the watersheds of selected nodes in the channel network. The format of a possible data structure for the channel network is presented along with pseudocode examples for traversing the channel network and the source areas. Execution times on a desktop computer are presented along with suggestions for optimal use of the algorithms.

Surging in urban storm drainage systems

Abstract: The ocurrence of surge and related problems is conceptually described in this paper, and then illustrated with both a mathematical model and field records. The paper also proposes and analyzes alternatives. It is noted that an urban storm drainage system could pressurize if its conveyance capacity is not sufficient or if it is also desinged for in-line storage. Surges caused by pressurization have been explained as a major source of transient problems such a sharp-peaked overflow, manhole blow-off, geysering, and structural damages. Severe shocks are produced whenever the surge front collides with an upstream end or a flow-restricting structure. The application is described of a mixed transient flow model to the Chicago Tunnel and Reservoir Plan (TARP), Illinois. It was used to study possible means of mitigating the surge problems. The most critical factor was found to be the surge intensity at the end of the pressurization process. Four possible operational and structral methods for in-line storage systems were investigated.

Three-Dimensional Computation of Flow and Bed Deformation

Abstract: A three-dimensional (3D) flow model is developed to improve the defects of a two-dimensional (2D) model proposed by Shimizu and Itakura. Calculated results are compared with experiments as well as with results calculated by the 2D model. The flow field is predicted more correctly by the 3D model than the 2D model. A simplification of the full 3D model is made, in which the depth-wise profile of the velocity is assumed to be logarithmic. This approach is favorably tested in the calculation of flow in a meandering bend. The simplified 3D model is applied to the computation of bed deformation by bed load as well as suspended load transport in meandering channels. Good agreements are found in comparison with the experimental results. A definite difference between the 3D and 2D models is found in the predicted bed deformation with suspended load. Evaluations of the flow and bed deformation by the proposed model are clearly demonstrated through application examples, and the validity of the new calculation model is verified.

Modeling Flow and Mixing in Sedimentation Tanks

Abstract: Numerical results from a computer model for the prediction of the neutrally buoyant flow within sewage treatment plant settling basins are compared with results of several new experiments. The flow field calculations performed using the k‐e turbulence model agree well with experiments for simple geometries. The quality of the computations, however, deteriorates with increasing flow complexity. It is argued that the effects of flow curvature, which are not included in the standard k‐e model, are mainly responsible for the differences between computation and experiment. The results of the calculation of dye transport within the basin after pulse injection show that the use of low‐order numerical models results in predictions in which the solution is badly polluted by numerical errors. The quality of the prediction of the dye concentration field is strongly related, to the quality of the flow field prediction. The passing time of the peak output concentration is well predicted if the size of the recirculatio...

Characteristics of Self‐Formed Straight Channels

Abstract: Laboratory experiments were conducted to study various aspects of the mechanics of self‐formed, stable, straight alluvial channels in the presence of bed‐load transport. The assumptions used for the formulation of the turbulent diffusion model of Parker (1978) were examined first. Velocity measurements indicated that the logarithmic law for rough walls is a valid approximation to the velocity profile, along normals to the boundary for the whole channel depth. The equivalent sand‐grain roughness, ks, is about 2d90. The experiments show that the turbulent‐diffusion model is more realistic for flow in a straight channel than the threshold‐channel model. Regime relations based on the straight‐channel model, and a resistance equation, predict accurately the center depth and the top width of a self‐formed straight channel. The shape of the bank is found to follow closely an empirically fitted exponential function that requires knowledge of only the center‐channel depth. Data obtained from other studies support ...

Energy Relations in Transient Closed‐Conduit Flow

Abstract: When the rate of flow in a closed conduit is changed, large-scale conversions of mechanical energy often occur, particularly if the pipeline is carrying water or some other slightly compressible liquid. Mathematical expressions describing these transient energy transformations are motivated from first principles and derived by mathematical manipulation of the governing continuity and momentum equations. The resulting expression accounts for the kinetic energy of the fluid, the internal energy associated with fluid compressibility and pipeline elasticity effects, the energy dissipated by friction, and the work done at the ends of the conduit. The energy approach provides an integrated view of transient conditions in the pipeline and is thus a simple, efficient, and logically consistent way of comparing the transient response of different systems and solution techniques. In particular, compressibility effects are shown to be negligible when the ratio of the change in internal energy to the change in kinetic energy is much less than one. This rule helps to distinguish the “rigid water column” model of unsteady flow from the more complex water-hammer theory.

Development of Bed Features

Abstract: The concept of unification, the opposite of branching theories used in biological sciences, is introduced as a new avenue to investigate the behavior of bed features in alluvial channels under unidirectional flow. The approach starts with the theoretical concepts initiated by Exner, according to which the features propagate at speeds inversely proportional to their heights. This leads to coalescence and rearrangement of the general pattern of bed features. Numerical experiments show that an initial large number of arbitrary bed disturbances rapidly reduces to a small number. If one feature reaches the maximum height for given flow conditions, smaller features pass through it. An initial uniform distribution of heights of disturbances slowly changes into a broad distribution of heights with some at the maximum height, similar to what is observed in nature.

Menu of Coupled Velocity and Sediment‐Discharge Relations for Rivers

Abstract: Nonlinear multiple‐regression analysis is used to derive relations among the velocity, sediment discharge, bed‐form geometry, and friction factor of alluvial rivers. A data base comprising 339 river flows and 608 flume flows is used in the analysis. A hierarchy of formulations involving progressively more interdependencies among velocity, depth, sediment discharge, and bed‐form geometry is developed. Procedures for computing velocity‐ and sediment‐rating curves are presented. A simplified procedure for preparing depth‐discharge relations for streams with strongly, discontinuous rating curves is also presented. Rating curves for four rivers, and computed depths and velocities for two others, are presented. The accuracies of the different methods are evaluated in terms of the mean normalized errors of predicted friction factors, depths, velocities, and sediment discharges. It is concluded that these formulations yield velocity‐ and sediment‐rating curves that are as reliable as is permitted by the accuracy ...

Review of Geostatistics in Geohydrology. I: Basic Concepts

Abstract: Geostatistics offers a variety of tools that can be used in groundwater estimation problems, including interpolation, integration, and differentiation. This paper introduces the basic concepts of g...

Problems with Logarithmic Transformations in Regression

Abstract: The power model is widely used in engineering as the structure for empirical models. The coefficients are fitted using a logarithmic transformation of the data. The logarithmic transformation leads to a biased model, which is not usually corrected for. Even when the traditional approach to eliminating the bias is used, only the intercept coefficient is changed; the other coefficients are not corrected, so they remain biased estimators. A numerical method for fitting the coefficients of the power model is discussed; the method enables the coefficients to be fit so they provide unbiased estimates and a minimum-error variance in the \Iy\N-space, rather than the log \Iy\N-space. The numerical method is easily modified to fit the coefficients using an objective function based on the relative errors. Examples using actual engineering data are provided.

Diffusion and Dispersion in Coastal Waters

Abstract: The paper presents the results of oceanographic drogue and current meter studies performed in the coastal waters of southern California. The purpose of the study was to evaluate the transport and dispersing properties of the oceanographic currents at this location and relate them to extant theories of diffusion and dispersion in coastal waters. The measurements provide useful results concerning Lagrangian and Eulerian estimates of turbulent diffusivity. In particular, they show the importance of regarding diffusivity as an ensemble average property of the turbulent flow field and that great care is necessary in applying the concept to single realisation studies. The studies show that under some tide conditions significant negative diffusivities are possible. Furthermore, it is found that at this location coastal motions tend to be very coherent up to scales of 4-5 km, at which point they become rapidly dispersive. The results provide information of value to engineers concerned with the design of ocean disposal systems.

Effects of momentum transfer in compound channels

Abstract: Different methods of considering the shear stresses between a main channel and its flood plains are discussed The methods considered include the simplistic method of considering the entire hydraulic cross section as one, dividing the cross section into independent sections, assuming limited shear stresses between main channel and the flood plain, trying to find an interface with zero shear stress, or allowing for the shear stresses from a momentum balance Based on evaluation of alternative steady‐state models, it is concluded that the area method is the most promising for computation of discharge and that the Prinos‐Townsend equation gives accurate results for apparent shear stresses at the main channel‐flood plain interface The results of the theoretical study are used in an unsteady‐flow model, prepared based on hydrodynamic principles The model facilitates the investigation of the significance of the momentum‐transfer phenomenon between the main channel and its floodplains The analysis demonstrate

Dividing Flow in Open Channels

Abstract: Junctions are of considerable importance in the study of openchannel flows. For dividing flow at rightangled junctions of rectangular open channels, an estimate of the discharge ratio is obtained i...

Air Entrainment by Spillway Aerators

Abstract: Aerators on spillways may be regarded as an effective protection against cavitational erosion. The air entrainment of aerators is governed by a number of independent parameters, that include the sl...

Effect of Drought on Urban Water Supplies. I: Drought Analysis

Abstract: This paper summarizes studies conducted for the City of Fort Collins, Colorado, to determine the effects of prolonged droughts on the city's water supplies. As a result of the analysis, various water‐supply strategies were developed to be used by the city for handling the problems associated with extreme droughts. This study utilized stochastic modeling coupled with water‐resource‐system modeling to evaluate drought effects on the water supplies of the city. A stochastic model was developed for the five river basins that account for the city's water supplies. The stochastic model was verified with 400 years of tree‐ring data obtained for the area. Periods of records were then generated to determine potential droughts that could occur. Representative l‐in‐20‐, l‐in‐50‐, l‐in‐100‐, and 1‐in‐500‐year droughts were selected for the analysis.

Bed Form Resistances in Open Channel Flows

Abstract: Many previous studies suggest the division of total resistance into skin resistance and form resistance without direct evidence. In this study, the skin resistance, form resistance, and total resistance are independently measured for a series of rigid bed forms in the laboratory. The data shows that the sums of skin resistance and form resistance are close to the total resistance for all experimental runs conducted. Furthermore, data collected in this and other studies indicate that form resistance can be predicted from known bed form shapes. The Darcy-Weisbach form factor and the pressure drag coefficients are related only to the relative height of the dunes and independent of the flow velocities. The variations of form resistance for alluvial bed forms are similar to the expansion losses in pipelines. Skin resistance on the bed form has no significant effect on form resistance of the same bed form. The measured local skin stress increases almost linearly from a zero value at the flow reattachment point to a maximum value at the dune crest. A relationship is derived to predict the skin resistance as a function of flow Reynolds number, relative roughness, and relative height of dunes based on data collected in this and other studies. The skin and form drag for nonuniform bed forms are also measured and analyzed.