Observations of the lateral structure of wind-driven flow in a coastal plain estuary

Abstract

Wind-stress is one of the main drivers of estuarine circulation. The lateral structure of wind-induced flows in estuaries has been described mainly with analytical and numerical models. The aim of this study is to investigate, with observations, the lateral structure of wind-induced flow in a coastal plain estuary, the James River, Virginia. Such structure was explored with the first Empirical Orthogonal Function (EOF1) mode derived from month-long moored ADCPs deployed at two cross-sections in the estuary. Mode 1 explained 53% of the variance and was attributed to wind forcing. Its lateral structure (eigenvectors) consisted of upwind flow centered in the channel at middle depth, downwind flow over the shoals, and a vertically sheared bidirectional flow over the channel slope. Wavelet coherence analysis showed that along-estuary wind was highly coherent with mode 1 (coherence squared > 0.9), especially at the period band between 4 and 6 days. The along-estuary wind stress and the temporal variation of mode 1 showed an anti-phase coherence, with the wind stress leading mode 1 persistently. In addition, an analytical solution showed a lateral structure similar to the eigenvectors obtained with EOF1. The conclusion is that the lateral structure described by EOF1 was induced by wind-stress and was consistent with analytical and numerical model results. These results are among the handful available that confirm the theoretical structure of wind-driven flows over laterally varying bathymetry.