Effect of external shipping traffic on the transport of polluted sediments into the inner city canals of Delft (The Netherlands)

Abstract

The effect of external shipping traffic on the transport of polluted sediments into the inner city canals of Delft was investigated in a field study, measuring flow velocities and suspended solids contents in the water prior to, during and directly after passage of commercial vessels on the main navigation canal bordering the inner city canal system. Turbulent conditions caused strongly increased flow rates, viz. up to 1 m/sec. These flows were dominantly directed inwards, thus causing a net accumulation of pollutant-loaded sediment material inside the Delft inner city canals. Mass budget calculations as well as sludge thickness measurements revealed that of the ca. 520 tons of sludge material imported annually, about 40% can be attributed to external shipping traffic. The above observations were further supported in experiments with sediment traps located at stations within, as well as outside, the influence of the external shipping traffic. Here, also an apparent import of heavy metals (Pb, Zn, Cu), attached to the sludge material, could be observed.