1887

oa Civil Engineer in South Africa - Air-regulation of a siphon spillway

Volume 26, Issue 10
  • ISSN : 0009-7845

 

Abstract

The 20 km long raw water canal between Vaal Dam and Zuikerbosch discharges into a large terminal reservoir which acts as a forebay for the Rand Water Board treatment plant and high-lift pumping station. A spillway was required for the reservoir to avoid overtopping of the embankments following a trip-out of the pumping station due to power failure. As an alternative to the conventional black-water siphon with its large discharges and hunting propensities, an air-regulated, smooth-discharge, white-water siphon spillway was chosen. The paper is concerned with its design and model-testing.


Planning and model-testing revealed a number of design problems. Care was needed in the design of the de-aeration mechanism in the siphon barrel and the downstream stilling basin to ensure that the siphon primed properly. On the upstream side the elevation and shape of the lip of the siphon had to be such that the air flow into the siphon was uniform to avoid hunting.The uniform inflow of air was upset by the draw-down of the water surface associated with the increase in velocity head at the siphon entrance, leading to instability in this region. Extensive modeling was needed to remedy the problem.
Full-scale tests of the siphon spillway at Zuikerbosch showed that although the de-aeration mechanism and stilling basin operated satisfactorily, the problem of upstream surging had not been fully resolved.

Die 20 km lang rouwater kanaal tussen Vaaldam en Zuikerbosch vloei uit in 'n groot eindreservoir wat as voorbak dien vir die Randwaterraad se suiweringswerke en hoë-druk pompstasles. 'n Oorloop is vir die reservoir benodig om oorstroming van die kanaalwalle te verhoed indien die pompe afskakel as gevolg van 'n kragonderbreking. 'n Lug-gereguleerde wit-water heweloorloop is gekies as 'n alternatief tot die konvensionele swart-water hewel met sy neiging tot groot uitvloei en swewing. Hierdie referaat behandel die ontwerp en modeltoetsing van eersgenoemde metode.


Die beplanning en modeltoetsing het 'n aantal ontwerpprobleme aan die lig gebring. Aandag moes geskenk word aan die ontwerp van die ontlugtingsmeganisme in die hewelbuis en die stroomaf stilbak om te verseker dat die hewel voldoende voorgevoer word. Die hoogte en vorm van die hewellip aan die stroomopkant moes sodanig wees dat die lugvloei eenvormig in die hewel is om swewing te verhoed. Die eenvormige invloei van lug is versteur deur die aftrek van die watervlak wat gepaard gaan met die toename in snelheidsdrukhoogte by die hewelingang wat lei tot onstabiliteit in hierdie gebied. Uitgebrilde modeltoetse was nodig om die probleem te oorbrug.
Volskaalse toets van die heweloorloop by Zuikerbosch het getoon dat alhoewel die ontlugtingsmeganisme en stilbak bevredigend funksioneer, is die probleem met stroomop stuwing nog nie bevredigend opgelos nie.

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/content/civeng1/26/10/EJC24931
1984-10-01
2017-11-23

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