The two principal plans of water-front improvements adapted to shipping are, in general, the basin plan and the pier plan. If we look to see in which ports each has been put into operation, we will find that in those ports where there is a large tidal difference the basin plan is adopted, while in those ports where the tidal difference is small, say, below eight feet, the pier plan has been adopted. The plan of water-front improvement depends in reality upon the tidal differences in a port. The most typical example in the world of the basin plan is to be found in Liverpool, where there is a rise and fall of the tide varying from eighteen to twenty-five feet, the system, in brief, consisting of a number of basins in which the water and ships are allowed to enter when the tide is full. As it begins to ebb, locks, similar to canal locks but much stronger, are closed and the depth of water obtained in the basin at high tide is thus preserved for the flotation of the ships which have entered, until a new high tide makes it possible to open the locks again. Ships are limited to a very small part of the day in which to enter and leave basins. This system requires the most solid and expensive form of masonry construction, and is the most costly plan of improvement which can be adopted, but, of course, presents a handsome appearance, and is lasting and provides fireproof structure. The other plan consists in building a marginal quay, wharf or street with a bulkhead (either crib or wall), from which, generally at right angles, piers, built on cribs or spiles, project toward the channel. This system is the one adopted in New York, where the rise and fall of the tide is between four and a half and eight feet, generally five. The pier system is constructed at much lower cost than the basin system, and has a number of advantages, among others that of elasticity to ships lying against piers, and is economical of wharfage room. Its chief disadvantage is the generally inflammable character of the sub-structure, as spiles are still generally of wood, though ferro-concrete piling, which is absolutely fireproof and which has been in use in Europe for some time, is beginning to be considered for water-front work in this country.

The most expensive feature of the "new plan" system used around Manhattan to-day is building the bulkhead wall, of which there are over twenty different types, the cost of which may be generally estimated from $300 to $450 per running foot. The crib bulkhead, which characterized the "old plan," may cost as low as $30 a running foot, and is still used in many places. The tendency of the weight on the top of a marginal street is to push the bulkhead wall out or to overturn it; hence, various arrangements of platforms have been devised to suit local conditions, and to change the direction of this overturning force, known to engineers as a thrust. It is to these various attempts to change the direction of the thrust, and the different character of the bottom at various places, that the different types of wall are due. Dredging is the first operation in the building of the bulkhead wall. This is known as crib dredging, to distinguish it from dredging alongside piers and bulkheads already constructed, to maintain a certain depth of water, which is known as mud dredging. As to the cost of the construction of piers, this varies very much with the price of materials, such as spiles, and particularly with the length of the spiles required to sustain the pier.