Drag coefficient at hurricane wind speeds as deduced from the numerical simulation of dynamical water level changes in Lake Okeechobee

Abstract

A time dependent, two-dimensional storm surge algorithm has been used to estimate the drag coefficient C[subscript D], over the wind speed range: 20 [less than or equal to] W?éü?éÇ [less than or equal to] 40 m/s, where W?éü?éÇ is the wind speed 10 m above MWL. The algorithm represents a vertically integrated physical model which includes non-linear boundary conditions representing flooding and recession. Wind and water level data for the investigation was gathered by the U.S. Army Corps of Engineers in the Lake Okeechobee, Florida region. The lake is a roughly circular feature 60 km in diameter with a maximum normal depth of 4 m. The effect of extensively grassed, shallow areas of the lake on wind-driven circulation was simulated by a multi-layer canopy flow model and included as a subroutine in the numerical analogue. The surge model was calibrated for empirical constants with two seiches and quasi-static wind-induced condition within the lake. Two wind stress relationships, a quadratic one and the Keulegan-van Dorn expression, were used to model the air-sea interaction. Multiple surge calculations were performed with a range of C[subscript D] and statistical analyses made of the difference between the observed and computed water levels for the hurricane of August 1949. The results indicate that the Keulegan van Dorn wind stress model yields superior results over the wind speed range. This conclusion was verified by a simulation of the surge associated with the hurricane of October 1950. The analysis yields the stress relation C[subscript D] = 0. 00228 + (1.0 - 7.0/W?éü?éÇ)?? 0.00263, which appropriately relates the drag coefficient to wind speed in the range: 20 [less than or equal to] W?éü?éÇ [less than or equal to] 40 m/s.