A vertical drop is an abrupt change in elevation to offset the difference between the channel slope and the ground slope. The free falling jet impacts the channel floor and produces a recirculating pool. An air pocket is trapped in the area between the pool surface and the free falling jet. Due to the mixing process of the free falling jet, negative pressures develop in the air pocket, fill the pocket with air and increase the falling jet vibration. Usually, a ventilation shaft is provided to aerate the air pocket. In this investigation, contraction transitions are used to eliminate the negative pressures in the vertical drops. By using contraction transition at the brink of the drop, the falling jet width decreases and the air pocket vanishes. The main objective of this study is to examine the effect of contraction transition on characteristics of hydraulic jump downstream of drop and the energy dissipation. The experimental work was conducted in two channels with three different drops and six different transitions. The relative critical depth d c /h (where d c is the critical depth and h is the vertical drop height) varied between 0.07 and 0.459. Empirical equations were presented to estimate brink depth profile, average bring depth, relative pool length and relative energy loss. Energy loss at the drop was calculated and compared with the result of the previous studies. It was observed that contraction transitions increase the energy loss about 10% to 15%. By using experimental observation, assumptions from previous researchers and assuming that the present hydraulic jump characteristics are similar to those of the justify; MARGIN: 0in 0in 0pt; unicode-bidi: embed; DIRECTION: ltr" Keywords : vertical drop, energy loss, hydraulic jump, contraction transition.
