5.9.4.2. Open Flume Outlet An open flume outlet is a weir with a sufficiently constricted throat to ensure supercritical flow and long enough to ensure that the controlling section remains within the throat at all discharges up to the maximum. A gradual expansion is provided downstream of the throat. The entire structure is built in brick masonry but the controlling section is generally provided with cast iron or steel bed and check plates. This arrangement ensures the formation of hydraulic jump and, hence, the outlet discharge remains independent of the water level in the watercourse. Figure 5.6 shows the type of open flume outlet commonly used in Punjab. The discharge through the outlet is proportional to H3/2. The efficiency of the outlet varies between 80 and 90 per cent.
The throat width of the outlet should not be less than 60 mm as a narrower throat may easily get blocked by the floating material. For the range of outlet discharges normally used, the outlet is either deep and narrow, or shallow and wide. While a narrow outlet gets easily blocked, a shallow outlet is not able to draw its fair share of sediment.
