Artificial aeration on stepped spillways with piers and flares to mitigate cavitation damage

[摘要] ENGLISH ABSTRACT: Stepped spillways have been used for approximately 3500 years and, with the recent technicaladvances in Roller Compacted Concrete construction, these spillways have received arenewed interest over the past few decades. However, because of the possibility of cavitationdamage to the spillway chute at higher discharges, the maximum discharge that thesespillways can safely handle has been limited. A pre-emptive measure to combat cavitationdamage is to introduce flow aeration at the pseudo-bottom. In order to aerate the flow, variouscrest pier aeration structures were investigated to ultimately increase the maximum safe unitdischarge capacity of stepped spillways.Different aeration structures were investigated, on two types of spillways (Type A and Type B),with the aid of two physical hydraulic models. The Type A spillway was a 1:15 scale, USBRstepped spillway with transitional crest steps and a constant step height of 1.5 m. The spillwayperformance of each aeration structure was determined by measuring the air concentration atthe pseudo-bottom and the minimum pressure at the step riser. Experiments on the Type Aspillway were carried out at a prototype unit discharge of 30 m²/s for the investigation ofdifferent pier configurations near the spillway crest. The crest pier configurations comprisedtwo pier nose shapes, two pier lengths and the addition of a flare to the pier. The pierconfiguration results were compared with the performance of an unaerated stepped spillway.The maximum safe unit discharge capacity of an unaerated stepped spillway had previouslybeen determined by Calitz (2015) to be 25 m²/s. The implementation of the bullnose, short pier,increased the maximum safe unit discharge capacity to 30 m²/s, by eliminating the risk ofcavitation damage in the vicinity of the natural aeration inception point.The Type B spillway was a WES stepped spillway, with a smooth ogee crest and constant stepheight of 1 m, which was used to evaluate the Chinese developed Flaring Gate Pier (FGP)design. The model scale for this spillway was 1:50. The design of the model was based on theDachaoshan Dam (China), which has a design unit discharge of 165 m²/s. The FGP designsconsisted of an X-Shape and a Y-Shape FGP, together with a slit-type flip bucket. Theperformance of these aerators was compared to an unaerated stepped spillway for prototypeunit discharges of 50 m²/s to 200 m²/s. The most notable improvement was the increase in themaximum safe unit discharge capacity to 50 m²/s in the case of the X-Shape FGP.In summary, the addition of a short, bullnose crest pier on low head/velocity stepped spillwaysincreased the maximum safe unit discharge capacity to 30 m²/s. In the case of a highhead/velocity stepped spillway, while the X-Shape FGP improved the maximum safe dischargecapacity to 50 m²/s.

[发布机构] Stellenbosch University