Spur Dike Research Articles

Local Scour at Permeable Spur Dikes

Local Scour at Permeable Spur Dikes

Model-based evaluations of spur dikes for fish habitat improvement: A case study of endemic species Varicorhinus barbatulus (Cyprinidae) and Hemimyzon formosanum (Homalopteridae) in Lanyang River, Taiwan

Abstract Two endemic species of fish in Taiwan, Varicorhinus barbatulus (Cyprinidae) and Hemimyzon formosanum (Homalopteridae), were considered to evaluate habitat areas. Numerical models, NETSTARS and TABS-2, were employed to simulate the hydraulic characteristics, i.e. water depth and flow velocity, in the presence as well as absence of constructed spur dikes. The results were used to calculate the weighted usable area (WUA) and percentage usable area (PUA); both of which are employed to evaluate the fish habitat area. The habitat area WUA of V. barbatulus and H. formosanum increases 404.8–442.4% and 38.4–435.8%, respectively after installing spur dikes. The maximum PUA for V. barbatulus was obtained when the dimensionless spur dike distance ( D / L ) fell between 3 and 3.4. However, the PUA for H. formosanum increased with D/L. This study also concluded that the fish habitat area was inversely proportional to the Froude number. The optimums of spur dikes numbers are 5–7 and 3–4 for V. barbatulus and H. formosanum , respectively. Regressive equations of PUA acquired from 240 cases indeed provided engineers with paradigms for fish habitat improvement. These two regressive equations are suitable for V. barbatulus and H. formosanum habitat evaluation and limited with the Froude number between 0.17 and 0.25.

An investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel: 2. Conditions corresponding to the final stages of the erosion and deposition process

Large eddy simulation (LES) is used to investigate the flow around a vertical spur dike in a straight channel with equilibrium scour bathymetry and the scour mechanisms in the later stages of the erosion deposition process. The equilibrium bathymetry is obtained from an experiment conducted at the same relatively low channel Reynolds number (Re = 18,000). Flow visualizations are used to complement the information obtained from the numerical simulation. The present investigation demonstrates that large‐scale bimodal oscillations of the primary necklace vortex are the main reason for the amplification of the turbulence inside the scour hole. The nature of these oscillations is similar to the one observed in the horseshoe vortex (HV) system forming in the flow past bluff bodies (e.g., spur dikes) mounted on flat surfaces. The presence of the scour hole stabilizes the HV system compared to the case when the bed is flat. It is observed that random ejection of vorticity patches from the legs of the necklace structures takes place at random times. The axis of the vorticity patch remains approximately parallel to the bed. These patches of high vorticity move predominantly against the mean slope of the scour hole and can cause sediment entrainment as they are convected over the bed before they dissipate. This mechanism explains, at least partially, the lateral growth of the scour hole in the later stages of the scouring process. As a result of merging phenomena, some of the eddies convected in the detached shear layer (DSL) can also induce large local value of the bed shear stress. The variation in the mean shape of the DSL as the bed is approached is another phenomenon observed in the current study. Close to the bed, at some time instances the DSL tends to curve toward the back of the spur dike, whereas at higher elevations the DSL regains the shape typically encountered in the flow over a surface‐mounted wall of infinite width. In the near‐bed region, the DSL is observed to oscillate between these two extreme positions. This explains how the scour hole grows in the region situated just behind the spur dike and how the entrained sediment is then transported in the deposition area behind the spur dike. The role of the two streamwise‐oriented vortices, present in between the elongated submerged deposition hill and the channel sidewall, in the sediment transport processes inside the wake region is discussed. The predicted mean bed shear stress distribution around the spur dike is consistent with the equilibrium conditions present in the experiment.

An investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel: 1. Conditions corresponding to the initiation of the erosion and deposition process

The present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start (flat bed) of the scouring process. Large eddy simulation (LES) is performed at a relatively low channel Reynolds number (Re = 18,000), in the range where most flume studies with clear water scour conditions are conducted. Similar to these studies, the incoming flow is fully turbulent and contains realistic turbulence fluctuations. Visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process. It is found that the structure of the horseshoe vortex (HV) system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex. However, its intensity is the largest at vertical sections situated around the tip of the spur dike. It is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes. In one of them (zero‐flow mode), the necklace vortex is closer to the spur dike and more compact, and the near‐bed jet flow beneath it is weak. In the other one (back‐flow mode), a strong near‐bed jet flow convects the primary necklace vortex away from the spur dike, and its core is more elongated and less compact. This explains the large amplification (by about 1 order of magnitude compared to the surrounding turbulent flow) of the turbulent kinetic energy and pressure fluctuations inside the HV system in the region situated around the tip of the spur dike and the double‐peak distribution of the turbulent kinetic energy. Past the spur dike, in the legs of the necklace vortex, the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike. It is found that the legs of the horseshoe vortices can interact, at times, with the vortex tubes shed in the detached shear layer (DSL) and with the tip of the spur dike. These events typically result in a significant change in the coherence of the HV system. The largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike, but high bed shear stress values are also observed beneath the upstream part of the DSL. The bed shear stress fluctuations around the local mean values can be very high, especially in the region situated beneath the upstream part of the DSL. At random times, some of the vortices shed in the DSL merge or interact with eddies from the recirculation region. This leads to an increase in their strength and to a large increase of the bed shear stress along their path.

Backwater Prediction due to the Blockage Caused by a Single, Submerged Spur Dike in an Open Channel

A method is proposed for predicting the backwater effect due to a single, submerged spur dike located within an open channel flow. A theoretical analysis based on the momentum principle relates the backwater effect to the drag force exerted by the spur dike on the flow. Experimental data obtained in laboratory flumes having subcritical flow conditions throughout the flow field have been used in developing predictive relationships for the spur dike drag coefficient, which is found to be strongly correlated to the blockage created by the spur dike within the flow cross section. The predictive relationships provide a means of obtaining a first-level estimate of the backwater effect due to a single, submerged spur dike in an open channel flow.

Measured and Simulated Flow near a Submerged Spur Dike

To improve knowledge of the flow and scour processes associated with spur dikes, three-dimensional flow velocities were measured at 2,592 points using an acoustic Doppler velocimeter over a fixed flat bed with a trapezoidal shaped submerged spur dike in a laboratory flume. General velocity distribution and detailed near field flow structures were revealed by the measurements and numerical simulations performed using a free surface turbulent flow model with a k–e closure scheme. The three-dimensional flow separation characterized in this study was found to yield forces on the bed that were significantly different from nonsubmerged vertical obstructions that have been measured in other studies. Values of bed shear stress derived from both measured and simulated values were similar but indicated that local scour would be initiated in one rather than in the two locations of initial local scour measured in previous experiments with a similar flow.

Scour and flow field around a spur dike in a 90° bend

Spur dike is an important element in river training that creates rapid variations in flow field, sediment transport and bed topography. The mechanism of flow and sediment transport in a channel bend is very complex, especially when a spur dike is constructed in a bend. Most of previous investigations on flow behavior and scour around spur dike were carried out in straight channels. In this paper results of experiments on flow field and scour around a spur dike in a 90 degree channel bend are presented. Sand with uniform grain size was used as the bed material. Experiments were conducted for different locations and different lengths of spur dikes at the bend with different values of discharge. The three dimensional flow fields around a spur dike were investigated. The maximum depth of scour was correlated to the Froude numbers, lengths and the locations of spur dike in the bend.

Generalized Approach for Clear-Water Scour at Bridge Foundation Elements

Extensive studies on the temporal evolution of scour were recently conducted at Versuchsanstalt fur Wasserbau, Hydrologie und Glaziologie VAW, ETH Zurich, Switzerland, along with a computational model for the scour depth estimation. The VAW scour data are revisited in the present study to present a further refinement. Additional experiments conducted on the scour entrainment at piers, at rectangular and sloping abutments, as well as at singular and multiple spur dikes are reported. A general criterion is proposed to determine the densimetric particle Froude number for scour entrainment. A new relationship for the temporal scour evolution at bridge foundation elements is then developed based on the similitude of Froude by relating the scour depth to the difference between the actual and the entrainment densimetric particle Froude numbers. The new relationship is validated by the complete VAW scour data set, and verified by the available literature data. The combination of sediment characteristics, approach...

Application of two- and three-dimensional computational fluid dynamics models to complex ecological stream flows

Complex flow patterns generated by irregular channel topography, such as boulders, submerged large woody debris, riprap and spur dikes, provide unique habitat for many aquatic organisms. Numerical modeling of the flow structures surrounding these obstructions is challenging, yet it represents an important tool for aquatic habitat assessment. In this study, the ability of two- (2-D) and three-dimensional (3-D) computational fluid dynamics models to reproduce these localized complex flow features is examined. The 3-D model is validated with laboratory data obtained from the literature for the case of a flow around a hemisphere under emergent and submerged conditions. The performance of the 2-D and 3-D models is then evaluated by comparing the numerical results with field measurements of flow around several boulders located at a reach of the Smith River, a regulated mountainous stream, obtained at base and peak flows. Close agreement between measured values and the velocity profiles predicted by the two models is obtained outside the wakes behind the hemisphere and boulders. However, the results suggest that in the vicinity of these obstructions the 3-D model is better suited for reproducing the circulation flow behavior at both low and high discharges. Application of the 2-D and 3-D models to meso-scale stream flows of ecological significance is furthermore demonstrated by using a recently developed spatial hydraulic metric to quantify flow complexity surrounding a number of brown trout spawning sites. It is concluded that the 3-D model can provide a much more accurate description of the heterogeneous velocity patterns favored by many aquatic species over a broad range of flows, especially under deep flow conditions when the various obstructions are submerged. Issues pertaining to selection of appropriate models for a variety of flow regimes and potential implication of the 3-D model on the development of better habitat suitability criteria are discussed. The research suggests ways of improving the modeling practices for ecosystem management studies.

Experimental and numerical investigations on secondary flows and sedimentations behind a spur dike

For a physical model of the approach navigation channel of Three Gorges Project(TGP), flow patterns around a non-submerged spur dike placed as a mountain in a long rectangular glass flume were experimentally investigated with and without “sucking-spouting” water respectively. Based on rigid lid assumption and Dynamic Smagorinsky Model, all these vortex flows around the spur dike were numerically simulated and analyzed, which probably affect the whole flow field and then probably lead to sedimentations for silt-laden two-phase flows. Meanwhile, silt-laden flows were also investigated experimentally. Both the secondary flow region and the silt sedimentations downstream of the spur dike decrease greatly with “sucking-spouting” water compared to those ones without “sucking-spouting” water. The Finite Volume Method (FVM) was used to discretize the governing equations together with a staggered grid system, where the second order difference is employed for the diffusion terms and the source terms while the upwind difference QUICK is used for the convection terms. The computational results are all in fairly good agreement with the experimental data.

Backwater effect due to a single spur dike

Spur dikes are river engineering structures that project from the bank of a stream at some angle to the main flow direction. They are principally used for river training and protection of the riverbank from erosion. A spur dike might be considered a form of macroscale boundary roughness, which produces a backwater effect upstream from the spur dike location. Despite this impact, spur dike design often proceeds without regard to the effect that the spur dike might have on the stream system. The work presented herein is on the backwater effect due to a single, vertical-walled spur dike. It is based on a momentum analysis in which the resistance offered by the spur dike is represented by a drag equation, for which the key parameter is the spur dike drag coefficient. Experimental data acquired for various configurations of a single spur dike within fixed-bed flumes have been used to calibrate and validate the proposed backwater model. The results show that the spur dike drag coefficient, hence the computed backwater effect, depends on the channel contraction caused by the spur dike, the degree of spur dike submergence, the aspect ratio of the spur dike, and the Froude number of the flow.Key words: spur dike, backwater effect, physical model, momentum principle, drag force, drag coefficient, river engineering.

NUMERICAL SIMULATION ON LOCAL SCOURING AROUND A SPUR DIKE USING EQUILIBRIUM AND NON-EQUILIBRIUM SEDIMENT TRANSPORT MODELS

Around a spur dike, local scouring is occurred and it is necessary to estimate the maximum scour depth and the bed elevations around the dike. In this paper local scouring around the dike is simulated using the equilibrium and non-equilibrium sediment transport models. To evaluate the model performance, both calculated results are compared with experiments conducted by Michiue & Hinokidani (1992) and the scouring processes and the flow near bed, especially in the initial stage, are examined. It was shown that the time scale of maximum scour depth and scouring processes are different between the equilibrium and non-equilibrium sediment transport models.

Flow and sediment simulation around spur dike with free surface using 3-D turbulent model

Treatments of irregular boundary, bed deformation and free surface are main problems in 3-D flow and sediment model. Based on control equations of turbulent flow considering irregular boundary from liquid-solid two phase flow theory, the k-ɛ model and the Wall Function are adopted to develop a new model. In the model, apart from k-ɛ model and the Wall Function, the VOF method and the porosity concept are adopted to simulate water surface, irregular boundary and bed deformation. Using the new model, flow field, changing of water level, turbulent kinetic energy and its dissipation rate, and local scour-deposition around spur dike are simulated. Computed result is compared with experimental data. The model developed can successfully simulate the separating structure of flow and circumfluence around the spur dike, secondary flow in the circumfluence area, scour hole profile and bed contours and agree well with the experimental data.

Two-dimensional depth-averaged model simulation of suspended sediment concentration distribution in a groyne field

Summary River-training structures, such as spur dikes, are effective engineered methods used to protect banks and improve aquatic habitat. This paper reports the development and application of a two-dimensional depth-averaged hydrodynamic model to simulate suspended sediment concentration distribution in a groyne field. The governing equations of flow hydrodynamic model are depth-averaged two-dimensional Reynold’s averaged momentum equations and continuity equation in which the density of sediment laden-flow varies with the concentration of suspended sediment. The depth-averaged two-dimensional convection and diffusion equation was solved to obtain the depth-averaged suspended sediment concentration. The source term is the difference between suspended sediment entrainment and deposition from bed surface. One laboratory experiment was chosen to verify the simulated flow field around a groyne, and the other to verify the suspended sediment concentration distribution in a meandering channel. Then, the model utility was demonstrated in a field case study focusing on the confluence of the Kankakee and Iroquois Rivers in Illinois, United States, to simulate the distribution of suspended sediment concentration around spur dikes. Results demonstrated that the depth-averaged, two-dimensional model can approximately simulate the flow hydrodynamic field and concentration of suspended sediment. Spur dikes can be used to effectively relocate suspended sediment in alluvial channels.

Large Eddy Simulations of Three-Dimensional Flows Around a Spur Dike

Large eddy simulations were used to model the three-dimensional flows around a non-submerged spur dike. Based on the rigid lid assumption, all these vortex flows around the spur dike, which probably affected the whole flow field, were numerically simulated and analyzed. The largest circulating flow region caused by the spur dike behind it was weak, which would naturally lead to sedimentation for silt-laden twophase flows. Based on the Smagorinsky model, the finite volume method was used to discretize the NavierStokes equations, and the SIMPLEC algorithm was used to solve them. Meanwhile, these flows were investigated experimentally in a 0.5-m wide flume with a dike placed as a barrier. The computational results are in a fairly good agreement with the experimental data.

EFEECTS OF ORIENTATION ANGLE ON RIVER-BED FORMATION USING SUBMERGED SPUR-DIKE GROUPS

Continuous spur dikes were often used to create diverse river-bed forms. We investigated the effects of the orientation angle of spur dikes on the flow structure and bed formation. Three submerged spur dikes were included in a group structure and they were set on both sidewalls alternately. The velocity distributions were measured and calculated by using a simple 3-D numerical model. Calculated results show a fairly good agreement with the experimental data. The overflow on spur dikes was deflected to the direction normal to the spur-dike line. In upstream-oriented spur-dike groups, large scour holes were observed in front-edge regions of spur dikes and deposition regions were formed beside the sidewalls. In downstream-oriented spur-dike groups, front regions of spur dikes and sidewall regions were largely scoured. These bed forms were created by characteristic 3-D flow structures around submerged spur dikes.

Influence of the Structure Permeability on Scouring and Flow around a Spur Dike during a Surge Pass

Influence of the Structure Permeability on Scouring and Flow around a Spur Dike during a Surge Pass

Case Study: River Training and Its Effects on Fluvial Processes in the Lower Yellow River, China

More than 50 years' river training practices in the Lower Yellow River provide valuable experience in river management for flood control in rivers having rapid flow changes, silting beds, and active channel migrations and are of importance in understanding the fluvial processes in regulated rivers with high sediment loads. Planned channel alignments for river training in the Lower Yellow River usually consist of a series of consecutive moderate bends representing the natural tendency of flows. Flow guide works, namely spur dikes, were constructed on the concave banks of the planned bends to protect the channel against scouring and migration by deflecting the current away from bends and further guiding the main flow from one bend to the next one. As a result, well-planned flow guide works can play a crucial role in limiting channel shifting and migration and in establishing a relatively stable channel. Enough flow guide works, on both sides together reaching about 80% of the channel length, may change the transitional and braided channel patterns to a confined meandering pattern.

Further Results to Time-Dependent Local Scour at Bridge Elements

This research intends to clarify the limitations of a local scour equation recently proposed, based on extended laboratory data collected at VAW, Zurich, Switzerland. The present project is concerned with four items: (1) clarification of the minimum laboratory dimensions required to apply Froude similitude; (2) effect of sloping abutments on scour advance; (3) extension of scour formula to spur dikes; and (4) effect of unsteady flow on scour development. These items were investigated mainly from an experimental point of view based on some 150 laboratory experiments and accounted for by a hydraulic approach. It was found that the basic scour equation mentioned may be applied provided additional limitations are specified. These are discussed in the light of the densimetric particle Froude number, the threshold Froude number, and other important parameters that influence the progress of local scour. The results of this study may be applied to practice, provided the limitations of the computational approach are respected.

EFFECT OF ASPECT RATIO ON BED TOPOGRAPHY AND HYDRAULIC CHARACTERISTICS AROUND SPUR DIKES

the spur dikes has gathered great public attentions from a point of view not only in river engineering but also in water environment, because the spur dikes offers a comfortable environmento aquatic lives and plants as well as human beings in eco-systems. At present, some artificiality side-cavity has been constructed in the rivers for the purpose of the maintenance of the river environment. However, there is little information of form effects of the spur dikes on the bed shape and hydraulic characteristics. In this study, the bed topography and velocity in and around spur dikes in an equilibrium condition were measured with changing the aspect ratio (=streamwise distance of each spur dike/spanwise length of spur dike). It was found that the maximum depth of the scour hole increases with an increase of the aspect ratio and also that crest is located near the junction between the main-channel and side-cavity. The latter result is corresponding to the phenomenon of the Kiso River.