Study of a self-scouring outfall in Lake Ontario by physical and computer modeling

Coastal outfalls that discharge storm water and/or sewerage into bodies of water are part of a collection of critical municipal infrastructure that must be kept functioning properly at all times so as to avoid expensive frequent maintenance and environmental problems. The Green Road costal outfall pipe, located in an embayment on the shores of Lake Ontario in the City of Hamilton, is subject to sediment plugging by waves that transport sediments from an eroding bluff to the east into the study outfall pipe, thereby reducing its hydraulic discharge capacity. To alleviate the problem of outfall blockage, a 1:15 scale undistorted physical (hydraulic) model ws designed and built at the National Water Research Institute (NWRI) to study the performance of a self-scouring outfall (SSO), a structure that utilizes combination of wave run-up slopes, converging walls and steep outlet channels to promote self-cleansing with respect to sediment to prevent direct sediment intrusion into the outfall pipe. A commercial computational fluid dynamics (CFD) model FLUENT, was used to study the internal hydrodynamics of the complex outfall structure. Results from the physical model determined that the performance of the SSO is a function of wave direction, water level, wave height and period, as well as sediment characteristics. Model results indicate that a SSO built on the shores of Lake Ontario would be able to scour a greater amount of sediment in conjunction with a high water level and wave height of 75.07 m and 1.95 m, respectively. A modified SSO design reduced sedimentation on the wave run-up slopes by more than 25% and was 100% effective in preventing direct sediment intrusion. The structure performed exceptionally better than a traditional outfall in terms of sediment handling. Therefore, it is expected a municipality can save a great deal of money on cleanouts by installing a self-scouring outfall to prevent outfall plugging.