DEEPENING THE NAVIGATION APPROACH TO THE PORT OF LONDONDERRY

The deepening of the navigation channel through Lough Foyle by approxi mately 1.10 m, over a total length of 15 km, is described. Deepening o f the channel and the creation of a turning area required the removal of almost 1 x 10(6) m(3) of mixed materials, comprising silts, sands, gravels and medium to stiff clays. The work was executed by a combinat ion of two trailer dredgers, one grab dredger and a powerful plough. V ery considerable time and effort was expended in obtaining a licence t o dispose of the materials arising from dredging. The work was complic ated by the cross-border situation, the presence of extensive shell fi sheries and the River Foyle being host to migratory salmon. The presen ce of shell fisheries, some located close alongside the channel to be dredged, dictated the need for careful control of the dredging operati on and monitoring of conditions before, during and after dredging. The dredging specification imposed limits on various aspects of the opera tion of dredgers and on the effect on water quality. The paper describ es in general terms the site and the soils to be dredged, the plant em ployed, and the results achieved in terms of completion times, quantit ies removed and environmental effects. Of particular interest are the results of turbidity and suspended solids measurements during dredging and the use of acoustic Doppler methods to record in real time the pa ttern of dispersion of sediments suspended during dredging.