Modeling Hurricane-Driven Currents on the Continental Shelf and Slope: Comparisons With Data
Authors: Gordon R.B. 
Published: October 2008

This paper describes the use of a 3D, layered, numerical current model to simulate hurricane driven currents on the continental shelf and slope. A novel feature of the model is its ability to represent the intersection of the thermocline with the sea bed. This allows for a realistic representation of the current response on the shelf and upper slope during intense hurricane forcing. Hindcast currents are compared with an extensive set of current measurements recorded during the passage of two hurricanes (Frederic and Belle). Water depths at the measurement sites (30-470 m) encompass both the shelf and slope. Model error statistics are presented and the effect of these errors on calculated platform loads is discussed.

INTRODUCTION

Calculation of loads due to winds, waves and currents is an important aspect in offshore structure design. Measurements of these environmental factors are rarely of sufficient duration to determine design criteria for installations that will remain in place for decades. To circumvent this, it is necessary to simulate these factors for past storm events using the available meteorological data base.

Techniques for hindcasting storm winds and waves are well developed. Considerable attention has been given to establishing accuracy, and the techniques have been routinely used as a basis for establishing offshore design criteria for over a decade. However, the development and verification of storm current hindcasting techniques have lagged somewhat behind.

Historically, work on hindcasting techniques for storm driven currents has focused on two regimes the inner continental shelf and the deep ocean. On the inner shelf, the density throughout the water column is uniform. Consequently, density stratification plays no role in the dynamics of the storm response. During hurricane season in the Gulf of Mexico, this well-mixed zone typically extends from the coast out to water depths on the order of 30 m. In this regime, the response is dominated by the effects of wind and bottom stresses, Coriolis acceleration and topography. Fairly reliable current model predictions can be made using relatively simple, single layer models [1].

In the deep ocean (typically, water depths greater than about 1,000 meters), topography is of little or no importance, but stratification is important. The response is dominated by the effects of the local wind stress, Coriolis acceleration, inertia and entrainment. Reliable predictions can be made using multilayer models that do not include topography [2].

Little attention, however, has been paid to the regimes between the inner shelf and deep ocean (water depths from roughly 50 to 1,000 meters) where the complicating effects of both stratification and topography can be important. Recently, Cooper and Thompson [3] (subsequently referred to as CT) have developed a multilayer current model that has shown good success in reproducing the observed storm response in this regime. While their model is quite general, it is limited in its representation of shelf topography. This restricts the use of the model to water depths greater than about 200 m.

This paper presents a multilayer model that builds on the work of CT.