EFFECT OF EXTERNAL SHIPPING TRAFFIC ON THE TRANSPORT OF POLLUTED SEDIMENTS INTO THE INNER-CITY CANALS OF DELFT (THE NETHERLANDS)

The effect of external shipping traffic on the transport of polluted s ediments into the inner city canals of Delft was investigated in a fie ld study, measuring flow velocities and suspended solids contents in t he water prior to, during and directly after passage of commercial ves sels on the main navigation canal bordering the inner city canal syste m. 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 D elft inner city canals. Mass budget calculations as well as sludge thi ckness measurements revealed that of the ca. 520 tons of sludge materi al 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, t he 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. (C) 1998 IAWQ. Published by Elsevier Science Ltd.