Consequences of dredging, especially harmful effects produced by dredging and by dumping of dredged material

The turbid water along the western bank of the Swedish river Göta älv is due to dredging operations further upstream in the part of Trollhätte canal called Karls grav.

Compartment model for the elongated mouth area of the river Göta älv at Gothenburg. Suspended-sediment concentration, suspended-sediment load, deposition of sediment, and transportation of dredged material. Mean values for February 1975. The sampling stations Ä (Älvsborgsbron) and S (just upstream Sannegårdshamnen) are situated about 5 respectively 3 km downstream of G (the bridge Götaälvbron). Sampling station L (Lärjeholm) is situated about 6 km upstream of G. The water depth along the channel was about 5 m upstream of G and about 10 m downstream of G. Modified after Axelsson 1980 (UNGI Rapport Nr 52).

This schematic type of compartment model has been used to calculate transportation and deposition of sediment as well as the transportation of pollutants in the estuary of the river Göta älv in connection with dredging operations. The increase in suspended-sediment load due to dredging amounted in february 1975 to 15 - 16 % of the dry weight of the dredged material.

Nomogram for estimating the increase in silt content as a function of dredging intensity and water discharge where the increase in silt discharge amounts to 10 and 20 per cent respectively of the dry weight of the dredged material. The nomogram is based on the dry weight 0.9 tonnes/m3 wet substance.

The increase in silt concentration due to dredging in the river Göta älv has been estimated by this nomogram, and documented by recording turbidimeters, and by analyses of water samples taken upstream and downstream the dredging areas.

Radiographic comparison between the uppermost part of sediment cores 510, 514, and 513 from the coastal bay Hallsfjärden. The arrows mark the position of the sediment surface on 18 October 1978 and 15 May 1979, i.e. just before and after a period of dredging and dumping operations. From Axelsson 2002 (Geo-Marine Letters 21/4).

Parts of a marker layer formed during the winter of 1978 - 1979, when dredged material was transported to and dumped in the deepest part of Hallsfjärden, a bay of the Swedish Baltic coast, are shown by the radiographs above. A considerable part of the dredged material was suspended during the dumping and later redeposited from a turbid underflow which spread out from the disposal area and covered the deeper parts of the bay. The thickness of the redeposited dredged material decreased with decreasing water depth and with distance from the disposal area. Coring sites 510, 514, and 513 were situated at depths of 20, 23, and 22 m, and about 150 m west of, 800 north of, and 400 m south of the disposal area respectively.

The dumped and redeposited material consisted mainly of old clay deposits. Because of the low organic content, the density of this layer was relatively high. Also, contents of heavy metals and other pollutants were low in this marker layer, and even very low compared to the levels of pollutants in the deposits just below.

The increase of pollutants such as heavy metals and chlorinated hydrocarbons in the water due to dredging may sometimes be rather low, when dredging is going on in the erosive or mobile types of river beds, lake-, or sea bottoms with only thin covers of more or less temporarily deposited, modern sediments.

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