An individual-component approximation to quadratic stress for flow-decomposition analysis

An individual-component approximation (ICA) to the one-dimensional quadrati c stress is presented for analysis of hydrodynamic systems that contain mul tiple-frequency fluid motion such as tidal current and nearshore currents. Criteria governing development of the approximation are accuracy, symmetry, and preservation of contributions (magnitude and direction) of individual and coupled velocity components. Preservation of directionality is required to isolate functioning of individual terms for flow-decomposition analysis . The ICA is compared with the small-amplitude approximation analytically a nd through numerical examination for two velocity components. The two appro ximations differ at second order within the range of validity of the small- amplitude approximation. The ICA is exact in the limits of components with large magnitudes and components of equal magnitudes. Based on calculations comparing the exact numerical solution for velocity components with a wide range of relative magnitudes, relative periods, and phase differences, the ICA is accurate except in restricted regions of small-magnitude quadratic v elocity. The ICA is generalized to multiple components without restriction on magnitude, period, or phase, and example calculations are given for flow s comprised of three components and six components. The method is applied t o velocity tidal constituents for Galveston Bay, Texas, and Tampa Bay, Flor ida. (C) 1999 Elsevier Science Ltd. All rights reserved.