RIVER SEICHES BETWEEN COUPLED BASINS UNDER WIND FORCING

Abstract

River seiches between coupled basins under wind forcing are investigated by numerical model and field data. Especially, this paper is focusing on revealing nonlinear resonance features of river seiches both in ideal case and field case. For an ideal couple-basin system (two equal rectangular basins connected by a short river) under periodic wind forcing, the response curve (amplification as a function of external forcing frequency) shows some interesting new resonance modes apart from the traditional seiches modes based on linear wave theory (linear wave modes). Frequency analysis for the cases when external forcing frequencies among those resonance modes clearly reveals that those interesting modes in the response curve are the outcome of energy transfer due to nonlinear interactions so we name them nonlinear interaction modes. It also shows that each linear mode has a nonlinear interaction mode. To further investigate the nonlinear features of river seiches between coupled basins, Monona-Waubesa study sites are investigated. Frequency analysis for free wave slosh test at Monona-Waubesa shows all the linear wave modes including two Helmholtz modes in the system and many sloshing modes coming from each basin. The spatial pattern of water level fluctuations, wave amplification shows distinct features at different linear wave modes and similar features between each linear wave mode and its corresponding nonlinear interaction mode.