NOROINNAM

This article appears in Volume V23, Page 384 of the Encyclopedia Britannica.

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the fresh-water discharge, generally forms the navigable- channel, which is scoured out during floods. Narrowing the river between the bends to bring the two channels together would unduly restrict the tidal flow; and in a river like the Hugh dependent on the tidal influx for the maintenance of its depth for two-thirds of the year, and with channels changing with the wet and dry seasons, so that deepening by dredging in the turbid river could not be permanent, training works below low water to bring the ebb-tide current into the flood-tide channel, which latter must not be obstructed at all, offer, aided by dredging, the best prospects of improvement.
The average rate of enlargement adopted for the trained channel of the Nervion, in proportion to its length, is I in 75 between Bilbao and its mouth, and I in 71 for the Weser from Bremen to Bremer-haven; and these ratios correspond very nearly tothe enlargement of the regulated channel of the

Clyde

from Glasgow to Dumbarton of i in 83, and of the Tyne from Newcastle to its mouth of r in 75. Accordingly, a rate of enlargement comprised between I in 70 and I in 80 for the regulated or trained channel of the lower portion of a tidal river with a fairly level. bed may be expected to give satisfactory results.
Works at the Outlet of Tidal Rivers.Tidal rivers flowing straight into the sea, without expanding into an estuary, are subject to the obstruction of a bar formed by the heaping-up action of the waves and

drift

DRIFT (from "drive ")

along the

coast

COAST (from Lat. costa, a rib, side)

, especially when the fresh-water discharge is small; and the scour of the. currents is generally concentrated and extended across the beach by parallel jetties for lowering the bar, as at the outlets of the Maas (figs. 11 and 12) and of the Nervion (figs. 19 and 20). In the latter case, however, the trained outlet was still liable to be obstructed by

drift

DRIFT (from "drive ")

during north-westerly storms in the Bay of Biscay; and, except in the case of large rivers, the jetties have to be placed too close together, if the scour is to be adequate, to form an easily accessible entrance on an exposed

coast

COAST (from Lat. costa, a rib, side)

. Accordingly, a harbour has been formed in the small bay into which the Nervion flows by two converging breakwaters, which provides a sheltered approach to the river and protects the outlet from drift (fig. 19), and a similar provision has been made at Sunderland for the mouth of the

Wear

WEAR

; whilst the

Tynemouth

TYNEMOUTH

piers formed part of the

original

ORIGINAL

design

DESIGN (Fr. desiin, drawing; Lat. designare, to mark out)

for the improvement of the Tyne, under shelter of which the bar has been removed. by dredging (fig. 17).
Training Works through Sandy Estuaries.Many tidal rivers flow through bays, estuaries or arms of the sea before reaching the open sea, as, for instance, the Mersey through Liverpool Bay, the Tees through its enclosed bay, the Liffey through Dublin Bay, the Thames, the Ribble, the Dee, the Shannon, the Seine, the Scheldt, the Weser and the Elbe through their respective estuaries, the Yorkshire Ouse and Trent through the Humber estuary, the

Garonne

GARONNE (Lat. Garumna)

and Dordogne through the Gironde estuary, and the

Clyde

, the Tay, the Severn and the St Lawrence through friths or arms of the sea. These estuaries vary greatly in their tidal range, the distance inland of the ports to which they give access, and the facilities they offer for navigation. Some possess a very ample depth in their outer portion, though they generally become shallow towards their upper end; but dredging often suffices to remedy their deficiencies and to extend their deep-water channel. Thus the St Lawrence, which possesses an ample depth from the Atlantic up to Quebec, has been rendered accessible for sea-going vessels up to Montreal by a moderate amount of dredging; whilst dredging has been resorted to in parts of the Thames and Humber estuaries, and on the Elbe a little below Hamburg, to pro-vide for the increasing draught of vessels; and the Mersey bar in Liverpool Bay, about 11 m. seawards of the actual mouth of the river, has been lowered by suction dredging from a depth of about 9 ft. down to about 27 ft. below low water of equinoctial

spring

SPRING (from " to spring," " to leap or jump up," " burst out," O. Eng., springau, a common Teut. word, cf. Ger. springer, possibly allied to Gr. QnrFpxecOac, to move rapidly)

tides, to admit Atlantic liners at any state of the tide.
Some estuaries, however, are so encumbered by sand banks that their rivers can only form shallow, shifting channels through them to the sea; and these channels require to be guided or fixed by longitudinal training walls, consisting of mounds of rubble stone, chalk, slag or fascines, in order to form sufficiently deep stable channels to be available for navigation. The difficulty in such works is to fix the wandering channel adequately, and to deepen it
\MIl1 NOI1N11iOro11Nrt
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sufficiently by the scour produced between the training walls, without placing these walls so close together and raising them so high as to. check the tidal influx and produce accretion behind them, thereby materially reducing the volume of tidal water entering and flowing out of the estuary at each tide. The high training works in the Dee estuary, carried out in the 18th century with the object of land reclamation, unduly narrowed the channel, and led it towards one side of the estuary; and though they effectually fixed the navigation channel, they produced very little increase in its depth, but caused a very large amount of sand to accumulate in the estuary beyond, owing to the

great

GREAT

reduction in tidal volume by the reclamations, and diminished considerably the channel through the lower estuary in width and depth without checking its wanderings.' The training of the channel of the Ribble through its estuary below Preston, for improving its depth and rendering it stable, was begun in 1839, and has been gradually extended at intervals; but the works have not yet been carried out to deep water, and a, shifting, shallow channel still exists through the sand banks, between the end of the training walls and the open sea. The high training walls adopted along the upper part of the channel enabled the upper end of the estuary on both sides to be
tide (figs. 25 and 26). The channel, however, was made too narrow between Aizier and Berville and was subsequently enlarged, and large tracts of land were reclaimed in the upper estuary. The reduction in tidal capacity by the reclamations, together with the fixing and undue restriction in width of the channel, occasioned very large accretions at the back of the lower portions of the training walls and at the sides of the estuary beyond them, and an extension of the sand banks seawards. Moreover, the channel has always remained shallow and unstable beyond the ends of the training walls down to deep water near the mouth of the estuary.'
Conclusions about Training ,. Works in Estuaries.Experience has proved that training works through sandy estuaries, by stopping the wanderings of the navigable channel, produce an increase in its depth, and, consequently, in the tidal scour for maintaining it. This scour, however, being concentrated in the trained channel, is withdrawn from the sides of the estuary, which in its natural condition is stirred up periodically by the wandering channel; and, therefore, accretion takes place in the parts of the estuary; from which the tidal scour and fresh-water discharge have been permanently diverted, especially where an abundance of sand from outside, put in suspension by the action of the prevalent
cAUOEBEC.

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