Hydraulic model study of the blowback behaviour of the bottom outlet of the Berg River Dam, South Africa

Abstract

The Berg River Dam is equipped with the first multi-level draw-off environmental flood release outlet in South Africa and can release flows up to about 200 m3/s. The outlet is controlled by a radial gate at the outlet end, and is protected by a vertical emergency gate near the inlet end. Commissioning tests of the emergency gate in 2008 found that large volumes of air were expelled, instead of the expected air entrainment into the air vent, designed to reduce expected negative pressures in the conduit during emergency gate closure. This paper describes the testing of a 1:14 physical model representing the outlet works of the Berg River Dam to determine the reasons for the unexpected release of air from the outlet work's air vent, as observed in the field during the commissioning tests of the emergency gate in the outlet conduit. Simulations of continuous gate closure on the as-built physical model of the Berg River Dam outlet showed predominant inflow of air into the air vent during emergency gate closure, with intermittent short duration high-speed air releases during the stages of emergency gate openings between 37% and 25% open. The problem was determined to be one of intermittent air blowback from the outlet conduit via the air vent during the latter stage, rather than continuous air release for all stages of the gate opening operation. The cause of the blowback was found to be the constriction of flow due to a reduction in the conduit cross-section at the radial gate chamber located at the downstream end of the outlet conduit.

Mitigation Strategies to Reduce Detrimental Effects of Air Bubbles in Shaft Spillways: A Literature Review

“…A significant quantity of air bubbles is ejected from the exit and subsequently burst. Bosman et al [28] introduced a remedial strategy for the lower ventilation system of the Berg River Dam in South Africa. According to their investigation, the expulsion of air bubbles is caused by the small size of the radial gate.…”

“…Air bubbles inside the pipe migrate in the opposite direction of the incoming flow and forcefully leave the intake under certain hydraulic conditions, especially as the input discharge decreases. The lower ventilation system of the Berg River Dam in South Africa was evaluated by Bosman et al [28] for reverse flow. The water flow is regulated by a bulkhead emergency gate at the entering point and a radial gate at the exit point.…”

“…The blowback action of the bottom overflow of the Berg River Dam in South Africa was studied using numerical and physical model studies by Bosman et al [28]. Behind the emergency gate, in the conduit, is an air vent.…”

Hybrid Artificial Intelligence Methods for Predicting Air Demand in Dam Bottom Outlet

“…A wide study was carried out on the Berg river dam model to investigate the air demand in the bottom conduit. The outcome shows that the air velocity was independent of the time of valve closure [14]. Among all hydraulic parameters, flow rate effect on air demand was investigated in the spillway of highest arched dam in the world to observe that increasing flow rate can lead to rising air demand [15].…”