Greater Vernon Diversion and Disposal Facility Spillway Design

Water Street was retained by the Regional District of North Okanagan to design a spillway for the Greater Vernon Diversion and Disposal Facility leachate pond dam. A spillway was identified as a recommended upgrade to provide a controlled release for excess inflows and prevent overtopping of the dam.

During the initial stages of design, an appropriate return period for the spillway upgrade was established. By analyzing the consequences of failure, the 1:1000-year Annual Exceedance Probability (AEP) flow event was selected. The 1:1000 AEP design flow was then determined through detailed hydrologic modelling of the 20 ha catchment area. The climate adjusted rainfall volume and distribution was determined through analysis of regional rainfall intensity-duration-frequency data. For comparison, a simplified analysis of the Probable Maximum Flood event was completed to understand the upper limit for extreme flow events.

After establishing the design flow, a detailed hydraulic design for the spillway was completed. Through analysis of the feasible design alternatives, an articulated concrete block liner was selected for the chute. Downstream of the chute, a transition to 250 kg riprap was designed for the reduced slope section. An energy dissipation apron was designed at the downstream extent of the spillway to reduce flow energy and target a specific flow velocity and depth at the downstream extent of armouring. The scope also included detailed design of a granular filter layer and geotextile beneath the armouring to allow for seepage (prevent buildup of excessive hydrostatic pressures) and prevent undermining.