Aeration of Roberts Splitters through an Internal Gallery of a Dam

Calitz, Gerard (2016-12)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Roberts splitters have effectively been used for more than 75 years to dissipate the flood discharge energy of more than 30 dam spillways in South Africa and abroad. Roberts’ (1943) standard, unaerated splitter design procedure is, however, limited to a spillway head (𝐻) of 3.0 m, equivalent to a unit discharge (𝑞) of 12 m2/s. In order to avoid cavitation at higher design spillway heads, the flow is aerated by local air vents positioned on the splitters. On current dams, these air vents are individually fed by intake pipes placed in the step directly below the splitters. However, problems emerge when these pipes need to drain water entering the air vents. To avoid drowning the intake pipes, aeration through an internal gallery that is open to the atmosphere is proposed, with the end goal of increasing the unit discharge limit of Roberts splitters. A 1:20 scale hydraulic model of an ogee spillway equipped with Roberts splitters was constructed. Two aerated models, with different sized air vents, were compared to an unaerated control model in order to determine the effect that the proposed aeration system has on the cavitation risk of the splitters at prototype unit discharges (𝑞) of up to 50 m2/s. At the maximum tested spillway head of 7.6 m (𝑞 = 50 m2/s), the minimum pressures and air concentration around the splitters of both aerated models increased considerably. This indicated that the proposed aeration system completely alleviates the high cavitation risk of unaerated splitters. It was further observed that the unaerated splitters were prone to drowning at high spillway heads, leading to unfavourable hydraulic conditions that should be avoided at all cost. To summarise, the addition of aeration through an internal aeration gallery can increase the unit discharge capacity of Roberts splitters to at least 50 m2/s (up by 43%, from the unaerated limit of 35 m2/s, as tested during this study) while the proposed aeration gallery provides a solution to the problem of draining the inflow water of the air vents, without jeopardising the effective aeration of the splitters.

AFRIKAANSE OPSOMMING: Roberts stroombrekers word al vir langer as 75 jaar op meer as 30 damoorlope in Suid-Afrika en oorsee gebruik om die energie van vloede te dissipeer. Die oorspronklike ontwerpriglyne vir onbelugte stroombrekers is beperk tot oorloophoogtes ( 𝐻 ) van hoogstens 3.0 m, gelykstaande aan ‘n eenheidsdeurstroming (𝑞) van 12 m2/s. By hoër oorloophoogtes word kavitasie verhoed deur die vloei te belug deur middel van luggate in die stroombrekers. Tipies word lug aan die luggate voorsien deur individuele inlaatpype in die trap direk onder die stroombrekers. Dit veroorsaak egter probleme omdat die inlaatpype water moet dreineer wat by die luggate ingeloop het. Die belugting van Roberts stroombrekers deur middel van ‘n interne gallery wat oop is na buite, word voorgestel om te verhoed dat die inlaatpype versuip sal word, met die einddoel om Roberts stroombrekers se eenheidsdeurtromingskapasiteit van te verhoog Vir hierdie studie is ‘n 1:20 skaalmodel van ‘n ogee-oorloop met Roberts stroombrekers gebou. Twee belugte modelle met verskillende luggatgroottes is met ‘n onbelugte kontrole model vergelyk om die uitwerking te bepaal wat die voorgestelde belugtingsisteem gehad het op die kavitasierisiko van die stroombrekers. Dit is gedoen vir eenhiedsdeurstromings (𝑞) tot en met 50 m2/s. By die maksimum toetsoorloophoogte van 7.6 m (𝑞 = 50 m2/s), het die minimum drukke en luginhoud rondom die stroombrekers van beide belugte modelle aansienlik verhoog. Dit het veroorsaak dat die hoë kavitasierisiko van onbelugte stroombrekers heeltemal verhoed word deur die voorgestelde belugtingsisteem. Daar is verder waargeneem dat onbelugte stroombrekers geneig is om versuip te word by hoë oorloophoogtes. Dit veroorsaak ongunstige hidrouliese toestande wat te alle tye verhoed moet word. Ter opsomming is dit duidelik dat die eenheidsdeurstromingskapasiteit van Roberts stroombrekers tot ten minste 50 m2/s verhoog kan word deur belugting te verskaf (verhoog met 43%, vanaf 35 m2/s vir onbelugte stroombrekers). Verder het die voorgestelde belugtingsgallery ‘n oplossing gebied aan die probleem om invloeiwater vanaf die luggate te dreineer, sonder om doeltreffende belugting van die stroombrekers in gedrang te bring.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100062
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