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

Koen, Jaco (2017-12)

Thesis (MScEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Stepped spillways have been used for approximately 3500 years and, with the recent technical advances in Roller Compacted Concrete construction, these spillways have received a renewed interest over the past few decades. However, because of the possibility of cavitation damage to the spillway chute at higher discharges, the maximum discharge that these spillways can safely handle has been limited. A pre-emptive measure to combat cavitation damage is to introduce flow aeration at the pseudo-bottom. In order to aerate the flow, various crest pier aeration structures were investigated to ultimately increase the maximum safe unit discharge 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, USBR stepped spillway with transitional crest steps and a constant step height of 1.5 m. The spillway performance of each aeration structure was determined by measuring the air concentration at the pseudo-bottom and the minimum pressure at the step riser. Experiments on the Type A spillway were carried out at a prototype unit discharge of 30 m²/s for the investigation of different pier configurations near the spillway crest. The crest pier configurations comprised two pier nose shapes, two pier lengths and the addition of a flare to the pier. The pier configuration results were compared with the performance of an unaerated stepped spillway. The maximum safe unit discharge capacity of an unaerated stepped spillway had previously been 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 of cavitation 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 step height 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 the Dachaoshan Dam (China), which has a design unit discharge of 165 m²/s. The FGP designs consisted of an X-Shape and a Y-Shape FGP, together with a slit-type flip bucket. The performance of these aerators was compared to an unaerated stepped spillway for prototype unit discharges of 50 m²/s to 200 m²/s. The most notable improvement was the increase in the maximum 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 spillways increased the maximum safe unit discharge capacity to 30 m²/s. In the case of a high head/velocity stepped spillway, while the X-Shape FGP improved the maximum safe discharge capacity to 50 m²/s.

AFRIKAANSE OPSOMMING: Getrapte oorlope is vir meer as 3500 jaar al in gebruik, en met die onlangse tegniese vooruitgang in roller-gekompakteerde beton konstruksie het hierdie oorlope die afgelope paar dekades ‘n hernude belangstelling aangewakker. As gevolg van die moontlike kavitasie-skade aan die oorloop oppervlakte met hoë deurstromings, is die maksimum deurstroming wat hierdie oorlope veilig kan hanteer, beperk. ‘n Voorkomingsmaatreël om kavitasie-skade te verhoed, is om die vloei naby die pseudo-bodem kunsmatig te belug. Ten einde die vloei kunsmatig te belug, is verskeie kruin belugting strukture in hierdie tesis ondersoek met die doel om die maksimum veilige eenheidsdeurstroming van getrapte oorlope te verhoog. Verskillende belugtingstrukture is op twee tipes getrapte oorlope (Tipe-A en Tipe-B) met behulp van twee fisiese, hidrouliese modelle ondersoek. Tipe-A oorloop was ‘n 1:15 skaal, USBR-getrapte oorloop met oorgangs trappe op die kruin en ‘n konstante trap hoogte van 1.5 m. Die gedrag van elke belugtingstruktuur is bepaal deur die lug konsentrasie by die pseudo-bodem en die minimum drukke by die vertikale trap te meet. Eksperimente is uitgevoer op die Tipe-A oorloop met ‘n prototipe eenheidsdeurstroming van 30 m²/s vir die ondersoek van verskillende pyler konfigurasies naby die oorloop kruin. Hierdie pyler konfigurasies bestaan uit twee pyler neus vorms, twee pyler lengtes en die byvoeging van ‘n vlerkie aan die pyler. Die resultate van die verskillende pyler konfigurasies was vergelyk met ‘n onbelugte getrapte oorloop. Calitz (2015) het voorheen die maksimum veilige eenheidsdeurstroming van ‘n onbelugte getrapte oorloop bepaal as 25 m²/s. Die implementering van ‘n kort, bul neus pyler, het die maksimum veilige eenheidsdeurstroming vermeerder tot 30 m²/s deur die risiko van kavitasie-skade in die omgewing van die natuurlike aanvangs belugtingspunt uit te skakel. Tipe-B oorloop was ‘n WES getrapte oorloop bestaande uit ‘n gladde ogee kruin en ‘n konstante trap hoogte van 1 m. Die model was gebruik om die Chinees-ontwikkelde “Flaring Gate Pier” (FGP) ontwerp te evalueer. ‘n Skaal van 1:50 was vir die oorloop gebruik. Die ontwerp van die oorloop is gebaseer op die Dachaoshan Dam wat geleë is in China, met ‘n ontwerp eenheidsdeurstroming van 165 m²/s. Die verskillende FGP ontwerpe bestaan uit ‘n X-Vorm en ‘n Y-Vorm FGP tesame met ‘n spleetvormige “flip bucket”. Die gedrag van hierdie belugters is vergelyk met ‘n onbelugte getrapte oorloop vir prototipe eenheidsdeurstromings van 50 m²/s tot en met 200 m²/s. Die mees noemenswaardigste verbetering was die toename van die maksimum veilige eenheidsdeurstromings kapasiteit tot 50 m²/s vir die X-Vorm FGP. In opsomming, die toevoeging van ‘n kort, bul neus kruin pyler het op lae hoogte/snelheid getrapte oorlope die maksimum veilige eenheidsdeurstromings kapasiteit vermeerder tot 30 m²/s. Vir die geval van ‘n hoë hoogte/snelheid getrapte oorloop, het die X-Vorm FGP die veilige eenheidsdeurstromings kapasiteit verbeter tot 50 m²/s.

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