Plate sediment traps for slowly moving waters

Abstract

In rivers and shallow lakes, the traditional cylindrical sediment trap often overestimates particle flux towards the bottom. Factors up to 5 were found in different environments. In order to overcome the lack of reliable sedimentation rates for organic and inorganic matter in moving waters, a round plate trap has been developed and tested. Its basic principle of operation is the dependence of particle settling on water flow. Experiments in the River Spree showed that trapping on a plate decreased nonlinearly with increasing approach velocity (bottom shear stress), whereas trapping in a cylinder demonstrated no systematic variation with velocity. The loss of collected material during surfacing is the major technical problem of such a plate sediment trap. A solution was found by lowering the plate hydraulically into the trap body after collection. Tests in the flume at approach velocities up to 15 cm s −1 showed that all peripheral compartments do not disturb the settling process on the plate, that a “run-up margin” of 10 cm width and an undercut bevel, with a slope only on the lower side of the plate are sufficient to avoid interference from the streamed edge, and that inclinations of the plate up to 12° do not play an important role for particle collection. In the flume, trapping rates are found on the plates, which are very similar to those found) at the bottom. Field tests in the River Spree has been shown that a trap with a diameter of 35 cm, together with suspension equipment and a hydraulic closure system works well. Trapping rates were found which were systematically lower than those for cylindrical traps.