EFFECT OF SPUR SPACING ON FLOW RESISTANCE IN AN OPENCHANNEL

摘要

Spurs or groynes are often used in river training or bank protection schemes. They work bydeflecting the main current away from the bank along which they are located. In many instances, spursare constructed as low-height structures, which are specifically intended to be overtopped at higher flows.In some cases, however, such as in the protection of a bank against erosion, it may be necessary to usehigher spurs so that the bank is protected over a wider range of flow conditions than would be the casewith a low-height spur. In either instance, but particularly so for the higher spurs, the spurs or spur fieldalong a bank will result in increased water levels through and upstream of the reach containing the spurs.In essence, the increased water levels, or backwater effect, are due to the increased hydraulic resistanceor drag imposed by the spurs on the flow. It is expected that the bulk drag force and the resultingbackwater effect is influenced by a number of variables, including the length of the spur, the height of thespur and the spacing between spurs, as well as by the relative depth and Froude number of the flow.The present study, which is part of a larger study, was specifically directed at evaluating the effect of spurspacing and associated hydraulic characteristics of the flow on the bulk drag coefficient of an array ofspurs. The study was undertaken using a physical hydraulic model in which an array of five equallyspacedspurs was arranged within a smooth boundary channel. Three values of each of three studyparameters were evaluated, including the relative spur spacing (i.e., spur length to spacing ratio), relativeflow depth (i.e., flow depth to spur height ratio), and Froude number of the flow (subcritical flow only). Thedrag or resistance offered by the spur array was computed using the momentum equation, which wasapplied between a flow cross section upstream of the spur array and a cross downstream from the spurarray. All terms in the equation were measured or otherwise independently evaluated except for theresistance force, which is the bulk drag force due to the array of spurs. Frictional effects were accountedfor separately by a series of tests undertaken with no spurs in the flow.The preliminary results show that the overall bulk drag coefficient for the array of spurs decreases with anincrease in the Froude number up to a Froude number of 0.4. For Froude numbers greater than 0.4, thebulk drag coefficient is constant, with a value of 5.5. For the three relative spur spacings, it was found thatthe bulk drag coefficient increased with an increase in the spacing. The effects of relative flow depth onthe bulk drag coefficient were mixed. For constant Froude number conditions, there was no apparenteffect. However, for constant Reynolds number conditions, the bulk drag coefficient increased with anincrease in the relative flow depth. More work is needed to completely explain the observations noted.