Two experimental sediment traps: operation and solids characteristics

Abstract

Specific sediment traps for solids transported as bed load during storm events could be an interesting alternative to usual grit chambers. Two field experiments have been carried out in Bordeaux (south-west of France) in 1993 and 1994 to characterise the solids caught in the traps and to assess the sediment trap efficiency. These two sites have been chosen because it was easy to transform the existing usual grit chambers with a set of boxes used as sediment traps. The mass of trapped solids is well correlated with the rainfall height, for rainfalls of less than 50 mm. For greater rainfalls, the experimental sediment traps were too small to catch all solids. The efficiency of the sediment traps located at the grit chamber inlet side is assessed at about 70-80% in mass. Only 20-30% of solids are trapped at the outlet side. The efficiency decreases rapidly for the traps that are not in alignment with the median axis of the inlet pipe. The grain size distribution of the trapped solids is very characteristic: the d10, d50 and d90 values decrease when the traps are farther away from the median axis of the inlet pipe. This distribution of the trapped solids is due to the enlargement of the grit chamber which leads to a decrease of the flow velocity. The organic fraction increases when the d50 decreases: the finer the particles, the more organic the fractions.