Case Study: Dillon Dam Trashrack Damage

In 1986 the trashracks on the intake to the outlet works at Dillon Dam in Colorado were damaged due to filling the intake tunnel too rapidly. As the air from the tunnel was discharged through the intake structure, high velocities displaced two horizontal trashracks and removed one vertical trashrack from its guides. This damage was discovered through an underwater inspection in 1996. The addition of an air vent and the establishment of a filling procedure provided a solution to the problem. The quantity of air in the intake tunnel was reduced through the installation of a 102 mm (4 in.) diam air valve and 63.5 mm $\left(2\frac{1}{2} \mathrm{in}\text{.}\right)$ vent hole in the crown of the intake tunnel at the control tower. Since the vent could not remove all the air in the tunnel, the tunnel was filled by opening the emergency gate 7.9 mm $(\frac{5}{16}$ in.) and letting the conduit fill over a 12 h period.