Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (''ground truth'') flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (u 2 , w 2) and covariance (uw) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempirical models. Closer to the boundary, the measured Reynolds stresses deviate from those predicted by the model, probably due to the size of the ADV sample volume. Turbulence spectra computed using the ADV records agree with theoretical spectra after corrections are applied for the spatial averaging due to the size of the sample volume and a noise floor. The noise variance in ADV velocity records consists of two terms. One is related to the electronic circuitry of the sensor and its ability to resolve phase differences, whereas the second is flow related. The latter noise component dominates at rapid flows. The error in flow measurements due to the former noise term depends on sensor velocity range setting and ranges from 0.95 to 3.0 mm s 1. Noise due to shear within the sample volume and to Doppler broadening is primarily a function of the turbulence dissipation parameter. Noise variances calculated using spectral analysis and the results of the ground truthing technique are compared with theoretical estimates of noise.

15 16The coupled ocean-atmosphere-wave-sediment transport modeling system (COAWST) 17 enables simulations that integrate oceanic, atmospheric, wave and morphological processes in 18 the coastal ocean. Within the modeling system, the three-dimensional ocean circulation module Kumar et al. (2012) Ocean Modeling (doi: 10.1016/j.ocemod.2012

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Abstract. Simultaneous in situ observations of floc size, waves, and currents in a continental shelf bottom boundary layer do not support generally accepted functional relationships between turbulence and floc size in the sea. In September and October 1996 and January 1997, two tripods were deployed in 70 m of water on the continental shelf south of Woods Hole, Massachusetts. On one a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 •m, and on the other, three horizontally displaced acoustic current meters measured flow velocity 0.35 m above the bottom. The tripods were separated by ---150 m. Typically, maximal floc diameter stayed relatively constant, around 1 mm, and it showed a dependence on turbulence parameters that was significantly weaker than that predicted by any model that assumes that turbulence-induced stresses limit floc size. Occasionally, when waves and currents generated intense near-bed turbulence, flocs were destroyed. These precipitous decreases in maximal floc size also were not predicted by conventional models. The correlation in time between episodes of floc destruction and elevated combined wavecurrent stresses provides the first quantitative support for the hypothesis that floc size throughout bottom boundary layers can be controlled by breakup in the intensely sheared near-bed region. These observations demand a reassessment of the forces limiting floc size in the sea, and they indicate the potential for significant simplifying assumptions in models of floc dynamics. Repackaging of particles into flocs affects numerous industrial processes that employ gravitational settling to remove solids from suspension. Disciplines such as wastewater treatment and mineral processing therefore share a long history of research into the forces that limit floc size [Hunt, 1986]