Spillway discharges by modification of weir shapes and overflow surroundings

Almost all spillways of mid-size dams in Korea are overflow weir. To investigate the flood control capacity of overflow spillway, 3D numerical simulations were analyzed the discharge of a dam spillway for different weir shapes. Hydraulic model experiments, downsized as scale factor of 40 to 1 by Froude similitude were also carried out to verify the numerical results. Overflow phases and discharges of the linear labyrinth weir and the curved labyrinth weir were compared with those of the linear ogee weir. It was found that the discharge of the labyrinth weir was 71 % greater than that of the linear ogee weir at low reservoir elevation because the labyrinth weir had a longer effective length. However, as the elevation of the water surface rose, the discharge slope of the labyrinth weir became milder by submergence and nappe interference in the side channel. Structural modification of the overflow surroundings such as changing the approaching channel elevation, lowering the bed elevation of the side channel, and moving the weir structure toward the reservoir were also considered to examine the discharge capacity of the spillway. Lowering the bed elevation of the side channel was found to be the most effective option.