Maintenance dredging is carried out approximately five times a year, to ensure that Harwich Harbour’s deepwater channel remains at a minimum 14.5 metres below chart datum.
Each round of dredging, by the Van Oord and Boskalis joint venture, involves removal of about 500,000 cubic metres of silt from the harbour.
The contractors’ trailer suction hopper dredger generally has a full load after working for 30 minutes in the harbour. This material is then taken out to the licensed Inner Gabbard disposal site – it is not dumped on the sea bed, but released in 30 metres of water in what is known as a ‘dispersive site’.
Because of the distance to the current disposal site, which is a 40-mile round trip from the harbour, the Authority is seeking permission to move its licensed disposal site five miles closer to shore to reduce cost and CO2 output. The new site is still outside the Shipwash Bank area.
After lengthy studies and consultations, Harwich Haven Authority was permitted by the Marine Management Organisation (MMO) to dispose twice to the new site during summer 2016, while monitoring was carried out. Earlier studies indicated that using this site would not have a major impact. The results of this latest monitoring exercise are now being compiled and analysed by maritime and coast experts HR Wallingford.