edgers, and especially of suction dredgers
in sand (see DREDGE), together with the increasing draught of vessels,
has resulted in a considerable increase being made in the available
depth of rivers and channels leading to docks, and has necessitated the
making of due allowance for the possibility of a reasonable improvement
in determining the depth to be given to a new dock. On the other hand,
there is a limit to the deepening of an approach channel, depending upon
its length, the local conditions as regards silting, and the resources
and prospects of trade of the port, for every addition to the depth
generally involves a corresponding increase in the cost of maintenance.

[Illustration: FIG. 6.--Dunkirk Docks and Jetty Channel.]

At tidal ports the available depth for vessels should be reckoned from
high water of the lowest neap tides, as the standard which is certain to
be reached at high tide; and the period during which docks can be
entered at each tide depends upon the nature of the approach channel,
the extent of the tidal range and the manner in which the entrance to
the docks is effected. Thus where the tidal range is very large, as in
the Severn estuary, the approach channels to some of the South Wales
ports are nearly dry at low water of spring tides, and it would be
impossible to make these ports accessible near low tide; whereas at high
water, even of neap tides, vessels of large draught can enter their
docks. At Liverpool, with a rise of 31 ft. at equinoctial spring tides,
owing to the deep channel between Liverpool and Birkenhead and into the
outer estuary of the Mersey in Liverpool Bay, maintained by the
powerful tidal scour resulting from the filling and emptying of the
large inner estuary, access to the river by the largest vessels has been
rendered possible, at any state of the tide, by dredging a channel
through the Mersey bar; but the docks cannot be entered till the water
has risen above half-tide level, and the gates are closed directly after
high water. A large floating landing-stage, however, about half a mile
in length, in front of the centre of the docks, connected with the shore
by several hinged bridges and rising and falling with the tide, enables
Atlantic liners to come alongside and take on board or disembark their
passengers at any time.

[Illustration: FIG. 7.--Tilbury Docks.]

Comparatively small tidal rivers offer the best opportunity of a
considerable improvement in the approach ch