MOORING LINE MOTIONS AND SEDIMENT TRAP HYDROMECHANICS - IN-SITU INTERCOMPARISON OF 3 COMMON DEPLOYMENT DESIGNS

Trap array characteristics were monitored concurrent with particle col lections for surface-tethered and bottom-moored cones and cylinders (M ultiPITs) at the North Atlantic OFPJGOFS site in the Sargasso Sea. At depths ranging from 145 to 3200 m, velocities of approaching fluid and those inside the traps were recorded at 5 Hz in bursts of 3-10 min ev ery half hour during particle collections. A thermistor, a high resolu tion pressure gauge and two inclinometers concurrently monitored trap movements. Burst-averaged slip velocities experienced by both shallow and deep tethered traps reached 37 cm s-1, while a bottom-moored trap recorded 10-day averaged speeds of 4 cm s-1. Independent of deployment technique, for both cones and cylinders, flow cells inside the traps led to an intense flushing of fluid and particles. None of the surface -tethered traps tilted more than 8 degrees from vertical, even under s trong flow accelerations. Tether-line motions, induced by the surface waves, generated high flow acceleration peaks of trap arrays at all de pths, even for bungie-cord decoupled MultiPIT arrays. The flow cells i nside traps were thus agitated with the result of intense turbulence p revailing close to the collection cup in the apex of tethered cones. M oored cone arrays recorded less dynamic environments. Trap fluxes by t ethered cones were up to a factor of 8 smaller than by tethered MultiP ITs at the same depth and time, cones collected more material with hig her approaching fluid flows (untested so far for cylinders), and for t he same conical geometry tethered traps collected less material than b ottom-moored traps. The in situ deployments revealed substantial flow- and geometry-related differences in collection behavior among the dif ferent trap arrays, all of which deviated from steady-state flume simu lation results. The diameter of the retention cup at the trap apex rat her than the trap mouth diameter may be a controlling design parameter of particle collection rates for conical traps. Efforts to link trap and in situ fluxes require that hydrodynamics of individual trap array s at depth are monitored, including line motions. Drift velocities rar ely coincided with trap-experienced approach velocities. Trap simulati on studies utilizing steady-state flume flows may be accurate only und er very specialized conditions. Our data provide a hydrodynamic ration ale for earlier recommendations by others of cylinders with adequate l ength-width ratio.