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
Köken, Mete (American Geophysical Union (AGU), 2008-08-05)
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 conta...
The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation
Constantinescu, George; Köken, Mete; Zeng, Jie (American Geophysical Union (AGU), 2011-05-12)
[1] Results of a detached eddy simulation (DES) are used to better understand the effects of the mean flow three-dimensionality and secondary currents on turbulence and boundary shear stresses and the mechanisms through which the momentum and Reynolds stresses are redistributed in a strongly curved 193 degrees bend with fixed deformed bed corresponding to the later stages of the erosion and sedimentation process. The ratio between the radius of curvature of the curved reach and the channel width is close to...
A double-porosity model for a fractured aquifer with non-Darcian flow in fractures
Altinors, Altay; Onder, Halil (Informa UK Limited, 2008-08-01)
Non-Darcian flow in a finite fractured confined aquifer is Studied. A stream bounds the aquifer at one side and all impervious Stratum at the other. The aquifer consists of fractures capable of transmitting water rapidly, and Porous blocks which mainly store water. Unsteady flow in the aquifer due to a Sudden rise in the stream level is analysed by the double-porosity conceptual model. Governing equations for the flow in fractures and blocks are developed using the continuity equation. The fluid velocity in...
Ünlü, Kahraman; NIELSEN, DR (American Geophysical Union (AGU), 1989-12-01)
Unsaturated hydraulic conductivity K values as a function of soil‐water pressure head h were measured in the soil at 75 cm depth at 70 different sites separated from one another by a distance of l m along a horizontal transect. K field was viewed as a random function of spatial location x. Field data were analyzed (1) to examine the isotropy and stationarity of K, (2) to check the ergodicity of K in the mean and covariance functions, and (3) to characterize the distribution properties of K by estimating the...
A process-based diagnostic approach to model evaluation: Application to the NWS distributed hydrologic model
Yılmaz, Koray Kamil; Wagener, Thorsten (American Geophysical Union (AGU), 2008-09-11)
Distributed hydrological models have the potential to provide improved streamflow forecasts along the entire channel network, while also simulating the spatial dynamics of evapotranspiration, soil moisture content, water quality, soil erosion, and land use change impacts. However, they are perceived as being difficult to parameterize and evaluate, thus translating into significant predictive uncertainty in the model results. Although a priori parameter estimates derived from observable watershed characteris...
Citation Formats
M. Köken, “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,” WATER RESOURCES RESEARCH, pp. 0–0, 2008, Accessed: 00, 2020. [Online]. Available: