Vertical profiles and horizontal scales of bubble clouds in the surf zone as measured with a distributed array of upward-looking sonars

Abstract

A collaborative, multi-institute experiment was recently performed in the vicinity of the Scripps Pier, whose goal was the study of bubble-field generation, transport, and distribution, as influenced by surf zone conditions. The collaboration produced an ensemble of instrumentation at the site for measuring bubbles, ambient noise, temperature and salinity, currents, and surface waves. This paper emphasizes data gathered by APL-UW using a set of four upward-looking transducers (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles from four locations. The primary locus of measurement activity was defined by NRL-SSC’s triangular frame (Delta Frame) and the four transducer locations for the APL-UW system bracketed this region. The transport of bubbles via rip currents emerged as a key feature surf zone bubble environment. Images of volumetric backscattering strength versus time and depth reveal episodic events (by way of increased scattering level) which last O(100) s and can occur in intervals of O(10) min. Time lags for increased scattering at the four locations were consistent with a seaward flow of rip currents equal to ∼0.25 m/s. Other features of the data that complement the single-depth measurements made from the Delta Frame will be discussed. [Work supported by ONR.]