Sea Technology

FEB 2014

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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56 st / February 2014 www.sea-technology.com for in-situ maintenance and data downloading. It provides an additional slot for the possible deployment of further oceanographic instrumentation. Current velocities along the water column are recorded every 600 seconds within 50-centimeter cells, with a pre- cision of 1.1 centimeters per second and 3.4 centimeters per second on the vertical and horizontal directions, re- spectively. Waves are measured hourly by collecting 2,400 samples at a 2-hertz sampling rate. The expected duration of a standard alkaline battery in these conditions is approxi- mately 100 days, but the bottleneck for system maintenance is caused by severe biofouling occurring in the shallow working environment, requiring at least monthly upkeep inspections. Video Monitoring Station A video monitoring system has been designed to record, without gaps, the response of the whole beach stretch to the different meteomarine events characterized in detail by the acoustic hydrodynamic measurements described above. The identifcation of key topographic information is based on the analysis of time exposure (timex) images obtained by digitally averaging image intensity over a fxed time interval of image acquisition, eliminating random momentary sea conditions and removing variability in run-up height. This image processing technique leads to an increase in pixel color intensity, enabling identifcation of peculiar beach features, such as sand bar morphology, shoreline position, intertidal beach profle and slope, and morphological for- mations in the shoreface. The video monitoring station consists of two refex digital cameras (10.1 megapixel, optical zoom), each hosted in a single waterproof case, and a management module (com- puter control unit of 1.6 megahertz, HD 160 gigabytes; network interface controller Ethernet, modem UMTS/GSM/ GPRS) allowing online monitoring and confguration. The system was installed in July 2013 on the roof of a hotel fac- ing the test area, which was chosen after a detailed survey of the coastal tract, aiming to maximize the optical perfor- mance while minimizing the beach landscape alteration. The control software package is composed of three mod- depth (approximately the limit of the active beach) and moored in an aluminum framework stabilized by concrete blocks. Besides the traditional computation of velocity via analysis of the echo of a known emitted signal (1-megahertz frequency for this device), the confguration of the four acoustic transducers installed in this system together with an embedded pressure gauge allows the reconstruction of the spectral parameters from an estimate of free water level oscillations (acoustic surface tracking, or AST) and orbital velocity statistics. In addition, given prior calibration, the intensity of the returning echo registered by the transducer can provide an estimate of suspended sediment concen- tration along the water column. An integrated orientation sensor records the rotation components of the instrument around three dimensions, permitting a retrospective control on the geotechnical stability of the system and on the fulfll- ment of the tolerance requirements for an appropriate op- eration of the wave measurements. The case hosting the instrument and its battery canister has a permeable bottom that allows sediment outfow and can be easily disengaged from its framework and retrieved Refex digital cameras installation. Feb2014.indd 56 2/11/14 1:11 PM

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