SEDIMENT FLUXES, RE-SUSPENSION AND ACCUMULATION RATES AT 2 WIND-EXPOSED COASTAL SITES AND IN A SHELTERED BAY

Sediment fluxes measured by sediment traps placed at different heights above the sea-bed at two near-coastal sites and in a sheltered bay ar e analysed in relation to wave and current conditions. Traps were depl oyed for 13, 15 (coastal sites) and 6 months and recovered with a mean period of 16.6, 14.0 and 30 days, respectively. The traps with an asp ect ratio of 5 were placed at 0.3, 0.5, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0 a nd 10.0 m above the sea-bed at one location, whereas an aspect ratio o f 2.3 was used at the second coastal location with similar trap interv als. The trap in the sheltered bay was placed at 1.0 m above the sea-b ed. Results showed that sediment fluxes varied between 505.0 and 38.9 g m (-2) day(-1) at 0.3 m above the sea-bed at a coastal site, whereas only small flux variations occurred in the sheltered bay. Accumulatio n rates were measured by Pb-210 at the three sites and was 2.4 g m (-2 ) day (-1) at the same site where fluxes varied between 505.0 and 38.9 g m(-2) day(-1) at 0.3 m above the sea-bed. This shows that re-suspen sion here is high and frequent. A similar pattern was found at the oth er coastal site, whereas accumulation rate and sediment flux were very similar in the sheltered bay. Analyses showed that re-suspension was induced both by surface wave activity related to periods of high wind speeds, and by current shear stress in more calm periods. Re-suspensio n is quantified as a time series by applying the introduced depth-inte grated re-suspension flux (DIRF). High DIRF occurred during periods of high surface wave activity, and one re-suspension event accounted for about 25% of the total re-suspension flux at the sire, although the e vent had a very low frequency of occurrence, less than 5% of the time. About 50% of the total re-suspension flux occurred in the interval be tween 0.0 and 20.0 g m(-2) day(-1). Comparison of different time scale s for measuring sediment fluxes and re-suspension using sediment traps , shows that a time scale of 24-12 h gives an adequate resolution. (C) 1997 Academic Press Limited.