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Complex Surveys for New Gibraltar Outfall
01 May 2003
Gibraltar is at the meeting point of the Atlantic Ocean and the Mediterranean Sea, a geography which produces a highly complex pattern of currents and tides. When the Government of Gibraltar planned a new main outfall a comprehensive integrated survey was necessary to determine the preferred route of the pipe, the ability of the receiving waters to disperse the effluent and the likely effects of effluent discharge on flora and fauna.
The Halcrow Group, the designers appointed by the Government of Gibraltar, chose EMU Ltd UK to conduct the surveys as it had all the relevant disciplines required in-house.
There were three main segments to the survey; geophysical, oceanographic and benthic.
The geophysical survey was to determine the optimum route for the outfall and involved several considerations including the suitability of the geology and the presence of obstacles. Over 200 man made objects were identified on the seabed, including cars. Geophysical data was obtained by sidescan sonar, sub-bottom profiler, echo sounder and magnetometer.
Because of the complex regime, a detailed series of oceanographic studies was made to determine rates and direction of dispersion of effluent under various met-ocean conditions. This involved dye tracing, drogue tracking, current measurements from vessel mounted acoustic Doppler instruments and spore tracing.
Vido surveys and grab sampling with subsequent laboratory analysis provided information on the types of biota and thus the sensitivity of the seabed environment to the pipeline installation process and to effluent discharge.
MJ Information No: 18201
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