Longshore transport in the South Basin of Lake Winnipeg

Abstract

In the past six years, the shoreline of the Southern Basin of Lake Winnipeg has been receding at an alarming rate, causing property loss to local dwellers and cottage owners. It is the objective of this study to evaluate the mechanism causing this sudden recession of shoreline. A quick beach survey was conducted during the summer of 1970. It records the beach profiles and sand sizes of beaches in the Southern Basin. A resurvey of the lines previously surveyed by Veldman at Winnipeg Beach in 1968 showed that the quantity of sand loss from the area was not significant. The wave spectrum approach was used to compute wave energy. The combination of Liley's atmospheric pressure spectrum, Miles-Phillips' wave generation mechanism, and a wave refraction model yields a wave generation model which can be suitably applied to the Southern Basin. Wave energy distribution along the shoreline of Southern Basin was then computed for the year 1968 for wind data recorded at Gimli Meteorological Station. The underlying cause of the sudden recession of shoreline is not the 10 percent increase of wave energy due to the increase in lake depth by about 5 feet, rather the fact that the waves can now attack areas of the bluff which were not reached by the waves before lake stage increase. The tendency of the beach to return to the equilibrium beach profile, and to a lesser extent, the capacity of the littoral current to transport sediment, efficiently disperse the material eroded from the bluff and allow the waves to continue eroding inland. The process continues until an equilibrium beach profile is again established. The extent of erosion is inversely proportional to the slope of the equilibrium beach profile. Using the values of beach slope measured in the survey, estimates were made on the limit of shoreline recession. It is felt that shoreline recession can be predicted in qualitative terms if these estimates, in conjunction with the estimates of beach erosion calculated from the rates of longshore transport, were used with caution, provided that all the physical characteristics of the beaches are taken into consideration.