annel to a port; for they can
be converted into artificially deep channels by dredging, and their
necessary maintenance is somewhat aided by the increased influx and
efflux of tidal water due to the lowering of the low-water line by the
outflow of the ebb tide being facilitated by the deepening. Thus
systematic, continuous dredging in the Tyne and the Clyde has raised the
Tyne ports and Glasgow into first-class ports. In large tidal rivers and
estuaries, docks should be placed alongside a concave bank which the
deep navigable channel hugs, as effected at Hull and Antwerp, or close
to a permanently deep channel in an estuary, such as chosen for Garston
and the entrance to the Manchester ship canal at Eastham in the inner
Mersey estuary, and for Grimsby and the authorized Illingham dock in the
Humber estuary; for a channel carried across an estuary to deep water
requires constant dredging to maintain its depth. Occasionally,
extensive draining works and dredging have to be executed to form an
adequately deep channel through a shifting estuary and shallow river to
a port, as for instance on the Weser to Bremerhaven and Bremen, on the
Seine to Honfleur and Rouen, on the Tees to Middlesborough and Stockton,
on the Ribble to Preston, on the Maas to Rotterdam and on the Nervion to
Bilbao (see RIVER ENGINEERING). Southampton possesses the very rare
combination of advantages of a well-sheltered and fairly deep estuary, a
rise of only 12 ft. at spring tides, and a position at the head of
Southampton Water at the confluence of two rivers (fig. 4), so that,
with a moderate amount of dredging and the construction of quays along
the lower ends of the river with a depth of 35 ft. in front of them at
low water, it is possible for vessels of the largest draught to come
alongside or leave the quays at any state of the tide. This circumstance
has enabled Southampton to attract some of the Atlantic steamers
formerly running to Liverpool.

Ports on tideless seas have to be placed where deep water approaches the
shore, and where there is an absence of littoral drift. The basins of
such ports are always accessible for vessels of the draught they provide
for; but they require most efficient protection, and, unlike tidal
ports, they are not able on exceptional occasions to admit a vessel of
larger draught than the basins have been formed to accommodate.
Occasionally, an old port whose approach channel has become inadequate
for modern vessels, o