Sediment Fluxes, Re-suspension and Accumulation Rates at Two Wind-exposed Coastal Sites and in a Sheltered Bay

Abstract

Sediment fluxes measured by sediment traps placed at different heights above the sea-bed at two near-coastal sites and in a sheltered bay are analysed in relation to wave and current conditions. Traps were deployed 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 aspect ratio of 5 were placed at 0·3, 0·5, 0·8, 1·0, 2·0, 4·0, 6·0, 8·0 and 10·0m above the sea-bed at one location, whereas an aspect ratio of 2·3 was used at the second coastal location with similar trap intervals. The trap in the sheltered bay was placed at 1·0m above the sea-bed. Results showed that sediment fluxes varied between 505·0 and 38·9gm−2day−1at 0·3m above the sea-bed at a coastal site, whereas only small flux variations occurred in the sheltered bay. Accumulation rates were measured by210Pb at the three sites and was 2·4gm−2day−1at the same site where fluxes varied between 505·0 and 38·9gm−2day−1at 0·3m above the sea-bed. This shows that re-suspension here is high and frequent. A similar pattern was found at the other 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-suspension is quantified as a time series by applying the introduced depth-integrated 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 site, although the event 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 between 0·0 and 20·0gm−2day−1. Comparison of different time scales for measuring sediment fluxes and re-suspension using sediment traps, shows that a time scale of 24–12h gives an adequate resolution.