WaterSubject

As water flows over a spillway, oxygen can dissolve into water. This process has environmental importance along rivers. For example, if the amount of dissolved oxygen (DO) is too small in a river, then aquatic life such as fish die, and the river may emit an odour. Thus, there is a range of DO concentrations that define an acceptable level of water quality. Recent researches have focused on developing measurement and predictive techniques for oxygen transfer at hydraulic structures to maintain and enhance water quality.

Hydraulic structures have an impact on the amount of DO in a river system, even though the water is in contact with the structure for only a short time. The same quantity of oxygen transfer that would normally occur over several kilometres in a river can occur at a single hydraulic structure, because the flow over a structure is typically highly turbulent, resulting in increased interfacial renewal. Aeration performance of hydraulic structures has been studied experimentally by a number of investigators.

Stepped spillways have regained popularity over the last one and a half decades with the evolution of the roller compacted concrete (RCC) dam construction technique. A stepped chute can be economically integrated on the downstream face of an RCC gravity dam. Another common application is the use of stepped overlays on the downstream face of embankment dams as emergency spillways to safely pass the probable maximum flood over the dam. Advantages of stepped spillways include ease of construction, reduction of cavitation risk and reduction of the stilling basin dimensions at the downstream toe of the dam because of continuous energy dissipation along the chute.

Stepped flows can be classified into nappe flow, transition flow and skimming flow. In nappe flow, the steps act as a series of overfalls with the water plunging from one step to another. In skimming flow, the water flows as a coherent stream over the pseudo-bottom formed by the step corners. Generally speaking, nappe flow is found with low discharges and wide steps. For a range of intermediate discharges, a transition flow regime takes place. The dominant feature is stagnation on the horizontal step face associated with significant splashing and a chaotic appearance. For narrow steps or larger discharges, such as the design discharge, the water skims over the step corners and recirculating zones develop in triangular niches formed by the step faces and the pseudo-bottom, as shown.

Stepped spillway flows are characterized by a large amount of self-entrained air. The macro roughness of the steps leads to a sharp increase in the thickness of the turbulent boundary layer. As the boundary layer reaches the free surface, air is entrained at the inception point of air entrainment. The entrainment inception point is located further upstream than on conventional smooth spillways because of increased roughness. For a spillway designer, the location of the inception point is important because the unaerated spillway zone is potentially prone to cavitation damage owing to large subpressures.

Mainly focused flow regimes and energy dissipation on stepped spillways. Presented some aeration data for stepped chutes with nappe and skimming flows.  Local void fractions, bubble count rates, bubble size distributions and gas–liquid interface areas were measured simultaneously in the air–water flow region using resistivity probes. However, further work was required to compare estimated aeration efficiencies with detailed interfacial area data based upon dissolved gas measurements. Recently, Baylar & Emiroglu studied the aeration efficiency of flat stepped-channel chutes (14.48°??? 22.55°) and Emiroglu & Baylar studied the aeration efficiency of stepped-channel chutes with end sills. This paper describes an experimental investigation of aeration efficiency of steep stepped-channel chutes (30°???50°), in particular the effects of chute angle and step height.