Seiches, Circulation, and Storm Surges of an Ice-Free Lake Winnipeg

Abstract

Predictions of numerical hydrodynamical models of seiches, steady circulation, and water level setup and time-dependent motions known as storm surges are compared to limited amounts of water level data during the ice-free season on Lake Winnipeg. Two-dimensional gravitational seiches are computed by minimizing an expression for the total energy. The fundamental inviscid period is 39 h which is considered to be a maximum for any inland water body. Four higher modes of oscillation have been detected in spectra of water level fluctuations at three stations on Lake Winnipeg. Frictional effects are shown to have appreciable influences on the seiches. Predicted water level setup has an asymmetrical distribution with the largest excursions occurring in the southern basin. A number of observations of water level setup are in accordance with model predictions at Winnipeg Beach if a drag coefficient of 1.0 × 10−3 is assumed. Setup is relatively insensitive to vertical eddy diffusivities in the range 10–100 cm2/s. A modeling strategy utilizing the previously obtained solutions for the free oscillations and for the steady state setup results in a simple prediction scheme for arbitrary forcing in time involving the convolution of the time history of the wind stress components with the response of the lake. Predicted storm surges are in reasonable agreement with two storm surges measured at two locations in the southern basin, considering the imperfect knowledge of the forcing obtained from winds at a single station in the northern basin.