The effect of tilt on sediment trap efficiency

-Cylindrical traps with an aspect ratio of 5.2 were tilted from 0 ° to 70 ° upstream and downstream in the Hudson River with currents ranging between 2 and 62 cm s -~. Particle flux in the tilted traps increased with tilt up to about 45 ° where overcollection reached a maximum factor of three but decreased beyond 45 ° when traps were tilted downstream. There was no discernible velocity effect on the total flux collected with tilted traps. In a recirculating flume, dye used as a water tracer revealed the existence of boundary layers within traps. The exchange of particles across the boundaries appears to play an important role in controlling the collection rate of particles and the conditions during which traps overand under-collect particles. The tilt effect on traps is important, not only where high velocities can cause a mooring to lean, but also where highfrequency internal waves might pass either moored or freefloating traps since the vertical velocity component of an internal wave can cause an effective tilt o f the traps. I N T R O D U C T I O N SEVERAL studies have been made to determine the effect of trap geometry and current velocity on the mass of particles collected in sediment traps (PECK, 1972; GARDNER, 1977, 1980a, b; HARGRAVE and BURNS, 1979; BLOMQVIST and KOFOED, 1981) and to compare trap fluxes with other methods of measuring sedimentation rates (I~NNINGTON, 1974; GARDNER, 1977; SPENCER et aL, 1978; REYNOLDS, 1979; DYMOND et al., 1981; BRULAND et al., 1981; LORENZEN et al., 1981; GARDNER et al., 1983). Reported in this paper are the results of testing another parameter that affects the collection rate of traps, i.e., tilt. The empirical observations also have provided further insight into the dynamics of the way in which panicles are collected in traps. Although previous experiments have provided a sound rationale for using traps to measure the vertical flux of panicles over a range of environmental conditions, the burgeoning use of sediment traps (RBYNOLDS et aL, 1980; BLOESCH and BURNS, 1980; B LOMQVlST and HAKANSON, 1981) increases the necessity of understanding trap dynamics and determining any limitations to their use. The intent of using sediment traps is to collect a sample of the panicles settling through the water column. A question asked in designing and using traps is whether there are factors that cause the collection of particles at rates greater than, or less than, the rate at which they are settling across a horizontal plane. Undercollection could occur if water is flushed through the trap so quickly that panicles do not have time to settle out, or if panicles collected in the bottom of the trap were resuspended by turbulence, scour, or wave action and then carried out of the trap. Overcollection might be suspected since the mass of particles carried through a trap in moving water greatly exceeds the vertical flux of panicles. * Lamont/Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A.