Ports and harbours vary in size, use and importance; for example, from relatively small local fishing ports to the really large international ports of major importance to international commerce and the doorways for connection with other parts of the world. Hydrographic work in ports and harbours varies too, not only in relation to the size of the port but also in hydrographic aspects. A port situated on a natural river which may bring silting problems demonstrates different aspects of hydrography than does a deepwater port on a deep and hard bottom.
This all implies variation in creativity from simple methods involving, perhaps, two men on the quay as mobile leading lines, to the highly sophisticated capabilities and organisational aspects shown by some of the big ports. The latter may include high refresher rates for (multi) beam surveys, silt density measurements for determining navigable depth to reduce dredging costs, magnetometer-surveys for heavy metals, (near) real-time processing and forwarding of updates to pilots and their PPUs (Personal Pilot Units) and to the dredging contractor (see the article on New Orleans in our last OI Americas-issue) and intensive use of GIS-applications. But, in quite a lot of cases, less cutting-edge technology may also fulfil the needs: see the ‘As it Is’ column from the Seychelles in this issue.
In harbour and port hydrography, both private sector and public sector hydrographers work on various tasks and, also due to the local aspect, a variety of definitions for depths are used. To name a few: design depth, bottom depth, contract depth, dredged depth, intervention depth, maintenance depth, operational depth, target depth. These terms have to be translated by the nautical cartographers into their (international) definitions and hopefully there are no wrong interpretations.
From a port authority point of view the various terms are understandable, but does the mariner really know in depth what lies behind the words he finds on his chart when he reads: ‘7,2 metres’, ‘maintained depth 7,2 m’ or ‘dredged to 7,2 m (1994)’. On top of this, and each with good reason, terms like nautical depth, navigable depth etc. are used by harbour authorities. And just to add to the mariner’s confusion, he has to deal with various vertical datums.
Do we, as hydrographers, realise how much trust the mariner places in the data with which we supply him? He navigates his ship with a minimum keel clearance and he cannot see the bottom ahead, only what depth is underneath his ship. Would he navigate his ship along a quay, with the same speed, at a similar minimal distance as the keel clearance, could he see only the quay abreast of him? I doubt it.
I still recall the experience, as guest aboard a VLCC Ð and not one of very great size - of hearing the keel clearance alarm while approaching Rotterdam harbour. I had personally surveyed this sand-dune affected channel before. We entered safely - the alarm was set at 3 metres - but it certainly brought home in a practical way the responsibility of surveying.
The same formidable trust is placed in the skills of the hydrographer when, for example, a harbour is extended (see the articles on the port of Monaco in this issue).