Flow resistance and associated backwater effect due to spur dikes in open channels
A spur dike is a hydraulic structure built on the bank of a river at some angle to the main flow direction. A series of spur dikes in a row may also be placed on one side or both sides of a river to form a spur dike field. Spur dikes are used for two main purposes, namely river training and bank protection. For river training, spur dikes may be used to provide a desirable path for navigation purposes or to direct the flow to a desirable point such as a water intake. For bank protection, spur dikes may be used to deflect flow away from a riverbank and thus protect it from erosion. It has also been observed that spur dikes provide a desirable environment for aquatic habitat. Despite the fact that spur dikes are useful hydraulic structures, they have been found to increase the flow resistance in rivers and hence increase the flow stage. The present study focuses on the quantification of the flow resistance and associated flow stage increase due to a single spur dike and also that of a spur dike field. Increased flow stage is referred to herein as a backwater effect. In the first stage of the study, the flow resistance due to a single spur dike, expressed as a drag force exerted on the flow in an open channel, was studied and quantified. The work was carried out in a rigid bed flume, with the model spur dike being simulated using various sizes of a two-dimensional (2-D) rectangular plate. Several discharge conditions were studied. The drag force exerted by the spur dike for both submerged and unsubmerged flow conditions was determined directly from measurements made using a specially designed apparatus and also by application of the momentum equation to a control volume that included the spur dike. It was found that the unit drag force (i.e., drag force per unit area of dike) of an unsubmerged spur dike increases more rapidly with an increase in the discharge when compared with that of a submerged spur dike. The results also showed that an increase in the blockage of the open channel cross-section due to the spur dike is the main parameter responsible for an increase in the spur dike drag coefficient, hence the associated flow resistance and backwater effect. Based on these findings, relationships were developed for estimating the backwater effect due to a single spur dike in an open channel. In the second stage of the study, the flow resistance due to a spur dike field expressed as a drag force exerted on the flow was quantified and subsequently related to the backwater effect. The work was carried out in a rigid bed flume, with the model spur dikes simulated using 2-D, rectangular plates placed along one side of the flume. For various discharges, the drag force of each individual spur dike in the spur dike field was measured directly using a specially-designed apparatus. For these tests, both submerged and unsubmerged conditions were evaluated along with various numbers of spur dikes and various relative spacings between the spur dikes throughout the field. It was concluded that the configuration of a spur dike field in terms of the number of spur dikes and relative spacing between the spur dikes has a substantial impact on the drag force and hence the flow resistance and backwater effect of a spur dike field. The most upstream spur dike had the highest drag force amongst the spur dikes in the field, and it acted as a shield to decrease the drag force exerted by the downstream spur dikes. From the experiments on the submerged spur dikes, it was observed that the jet flow over the spur dikes has an important effect on the flow structure and hence the flow resistance. In the third stage of the study, the flow field within the vicinity of a single submerged spur dike was modeled using the three-dimensional (3-D) computational fluid dynamic (CFD) software FLUENT. Application of the software required solution of the 3-D Reynolds-averaged Navier-Stokes equations wherein the Reynolds stresses were resolved using the RNG ê-å turbulence model. One discharge condition was evaluated in a smooth, rectangular channel for two conditions, including uniform flow conditions without the spur dike in place and one with the spur dike in place. The CFD model was evaluated based on some experimental data acquired from the physical model. It was found that the CFD model could satisfactorily predict the flow resistance and water surface profile adjacent to the spur dike, including the resulting backwater effect. Furthermore, the CFD model gave a good prediction of the velocity field except for the area behind the spur dike where the effects of diving jet flow over the spur dike was not properly modeled.
DegreeDoctor of Philosophy (Ph.D.)
DepartmentCivil and Geological Engineering
ProgramCivil and Geological Engineering
CommitteePutz, Gordon; Mazurek, Kerry; Sumner, David; Bugg, Jim; Rennie, Colin
Copyright DateFebruary 2010
Computational Fluid Dynamics