Investigation of air concentration and pressures of a stepped spillway equipped with a crest pier

Thesis (MEng)--Stellenbosch University, 2015.ENGLISH ABSTRACT: The evolution of roller compacting concrete has led to stepped spillways becoming increasingly popular over recent decades, mainly credited to the fact that the stepped profile of the downstream dam wall can be incorporated into a spillway chute. However, the discharge over stepped chutes in current use has been limited, due to the risk of local cavitation damage to the concrete of stepped chute structures. A general accepted practice to combat cavitation is to aerate the flow. Would it be possible, when adding a pier to the spillway crest, to introduce air into the flow upstream of the inception point in order to reduce the cavitation potential on the chute, and subsequently allow discharges greater than the current recommended values to be safely passed? A physical hydraulic model was constructed at a scale of 1:15 to investigate the air concentration along the pseudo-bottom and minimum pressures at the upper vertical step face for areas on the spillway chute where cavitation could be imminent for large discharges. The tests were conducted using a conventional stepped spillway with no pier as the control, to which the test results of two different pier configurations fixed on the spillway crest were compared. The recorded results showed an increase in air concentration and minimum pressures downstream of the pier for both tested crest pier designs. The Type 1 pier is recommended over the Type 2 pier due to the increased ability of the former pier to aerate the flow that consequently alleviates minimum pressures found on the spillway chute. In summary, the literature recommends a maximum discharge of 18 m²/s, but the experimental study has shown through a cavitation evaluation of the air concentration and minimum pressures that, for a no-pier stepped spillway with a chute angled at 51.3° and a prototype step height of 1.5 m, a maximum discharge of up to 25 m²/s can be allowed. For a spillway equipped with a Type 1 pier, Method A suggested that a unit discharge of at least 30 m²/s could be safely passed.AFRIKAANSE OPSOMMING: Die ontwikkeling van rollergekompakteerde beton het daartoe gelei dat damwal-konstruksie met die kenmerkende trap-profiel ʼn gewilde konstruksie-opsie oor die afgelope paar dekades geword het. Dit is grotendeels te danke aan die feit dat die trap-profiel aan die stroomafkant van die dam terselfdertyd as ʼn oorloopgeut gebruik kan word. Nietemin word die eenheidsdeurstroming oor trap-oorlope beperk weens die kavitasierisiko vir lokale skade aan die beton van damoorloopstrukture. Om kavitasieskade te voorkom is dit algemeen aanvaarde praktyk om die watervloei te belug. Sou dit moontlik wees om ʼn pyler aan die oorloopkruin te heg wat die vloei stroomop van die beluggingspunt belug om kavitasiepotensiaal op die oorloop te verminder en sodoende die toelaatbare eenheidsdeurstroming te verhoog? ʼn Fisiese hidrouliese model op ʼn skaal van 1:15 is gebou om die lugkonsentrasie op die pseudobodem, asook minimum drukke op die boonste vertikale trap te ondersoek vir gebiede op die oorloop waar kavitasie gewisse gevaar inhou vir groot deurstromings. Die toetse is gedoen met behulp van ʼn konvensionele trapoorloop sonder ʼn pyler wat as kontrolegeval gedien het, en is vergelyk met die toetsresultate van twee oorlope, elk met ʼn unieke pylerontwerp. Die waargenome toetsresultate toon 'n toename in die lugkonsentrasie en minimum drukke stroomaf van die pyler vir albei pylerontwerpe. Die Tipe 1 pyler word bo die Tipe 2 pyler aanbeveel weens die verhoogde vermoë van eersgenoemde pyler om die vloei te belug, wat tot verhoogde minimum drukke op die oorloop lei. Ter opsomming beveel die literatuur ʼn maksimum eenheiddeurstroming van 18 m²/s aan, maar die eksperimentele studie het deur middel van die kavitasie-evaluering van lugkonsentrasiedata en minimum drukke bewys dat maksimum deurstromings van tot 25 m²/s toegelaat kan word vir ʼn trapoorloop sonder ʼn pyler, maar met ʼn oorloopgeut teen ʼn hoek van 51.3° en ʼn prototipe traphoogte van 1.5 m. Vir ʼn trapoorloop toegerus met ʼn Tipe 1-pyler het Metode A bewys dat ʼn eenheidsdeurstroming van ten minste 30 m²/s veilig toelaatbaar is