Clear-Water Scour at Labyrinth Side Weir Intersection Along the Bend

Abstract

Weirs are among the oldest and simplest hydraulic structures, and have been used for centuries by hydraulic and environmental engineers for flow measurement, flow diversion, regulation of flow depth, flood passage, and other means. Although the definition of many different kinds of weirs is very simple and similar, the application and hydraulic behavior of each is quite different (Borghei et al., 1999). Weirs are normally placed perpendicular to the direction of flow of the main channel. The most significant parameters in determining the capacity of a weir are its height relative to the upstream depth, the crest shape, and the crest length. Here, capacity refers to the flow rate or discharge for a given depth of flow over the crest of the weir. Of these parameters, the crest length has the greatest influence on the discharge capacity. The crest length can be increased further and can still keep the downstream dimension small by folding the weir into several sections. One implementation of this idea is the duckbill spillway (Falvey, 2003). A Labyrinth weir is an overflow weir folded in plan view to provide a longer total effective length for a given overall weir width. A Labyrinth weir has advantages compared to the straight overflow weir and the standard ogee crest. The total length of the labyrinth weir is typically three to five times the weir width. Its capacity varies with head and is typically about twice that of a standard weir or overflow crest of the same width (Tullis et al., 1995). Labyrinth weirs have long been used as the spillway of a dam reservoir. Emiroglu et al. (2010) used the labyrinth weirs as a side weir structure. If the crest is placed parallel with the channel centerline, it is called a side weir. A side weir is an overflow weir framed in the side of a channel, which allows lateral overflow when the surface of the liquid in the channel rises above the weir crest. Side weirs, also known as a lateral weir, are control structures, widely used in canal systems and for storm water overflow from urban drainage systems. Like normal weirs, side weirs may be sharp, broad or rounded-crested. In addition, flow in the main channel along a side weir may be subcritical or supercritical. The flow over a side weir falls within the category of spatially varied flow. The existing studies of side weir flow deal mainly with the application of the energy principle. The energy principle assumes that the longitudinal component of velocity vector of spill flow at any section is equal to the average velocity of flow in the channel. Therefore the total energy per unit mass of water remaining in the channel is unaffected by the spill flow occurring and, apart from frictional losses, the total energy of the flow in the main channel remains constant. The concept of constant specific energy (De Marchi, 1934) is often adopted when