Statistical characteristics of velocity fluctuations at intermediate scales in a coastal plain estuary

Motion at intermediate scales has been observed in a coastal plain estuary with four propeller current meters, each consisting of two propellers mounted one above the other at right angles. Three runs were made with the current meters arranged in a longitudinal, a vertical, and a lateral array, respectively. Data were recorded every 10 sec for 50 hours during each run. During the longitudinal run, spectra of kinetic-energy density of the components of horizontal velocity were similar over distances of 500 meters. The spectra could be satisfactorily represented in two frequency bands by straight lines (on log-log plots) with a −5/3 slope. Between the two bands with −5/3 slopes there was a twofold increase in the energy-density level in both velocity components. The natural period, about 7.5 min, for cross-channel seiching occurred in the same interval. During the vertical run, spectra in a region of large vertical shear in the average current had a frequency band with a slope of −1. These spectra had a roughly −5/3 slope at higher frequencies. During the lateral run, the spectra were observed to have comparable energy density in both velocity components. Significant time changes in the spectra were observed during a 2-week field experiment.