In many parts of the world fluid-mnud suspensions exist on the bottom of navigational channels which create difficulties in defining the navigational depth. When lead lines were used, the depth recorded was to d fairly solid bottom and any overlying mud layer was not usually detected. When echo sounders were introduced, the water-mud interface was not always clearly defined. The interface shown on the records may depend on the instrument used and the frequency employed. The lack of clear definition of water-mud interface can cause unnecessary depth restriction and possibly excessive dredging.
During the 25th PIANC Congress in Edinburgh, in May 1981, a Working Group was formed under the auspices of the Permanent Technical Committee II to prepare a short report which was to include :
A definition of the term nautical depth.
Methods of measuring the characteristics of bottom layers.
A description of the effect of muddy layers on the manoeuvrability of ships.
The Terms of Reference were reworded in the following way: To give a dear definition of navigable depth in soft bed areas, to describe factors affecting this depth and to recommend survey techniques and chart presentation. The work of the group took about one and a half years, during which time two plenary meetings were held.