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 reduction in tidal volume
by the reclamations, and diminished considerably the channel
through the lower estuary in width and depth without checking its
wanderings.[1] 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
Figs. 25 and 26.—Training Works in Sandy Estuary: River Seine.
reclaimed for a length of 4 m.; whilst the half-tide training walls below, placed unduly close together, have led to considerable accretion at the sides of the estuary and some extension of the sand banks seawards. Moreover, by fixing the channel near the northern shore they have enabled the landowners to carry out large reclamations on the southern foreshore. These works, however, besides fixing the navigable channel, have increased, its depth, especially in the upper part, and augmented the tidal scour.along it by lowering the low-water line; and the trained channel is further deepened by dredging. The training works in the Weser estuary have been confined to constructing a single low training wall at the upper end, which forms a trumpet-shaped outlet for the river below Bremerhaven, and to guiding the navigable channel by occasional low dikes at the side and closing minor channels, so as to concentrate the tidal scour and fresh water discharge in it, whilst additional depth is obtained by dredging (fig. 21). A remarkable improvement has been effected in the navigable condition of the upper portion of the Seine estuary by training works, begun in 1848; for in place of a shallow, intricate channel through shifting sand banks, whose dangers were at times intensified by a bore, a stable deep channel has been provided down to about half-way between Berville and St Sauveur, rendering access easy to the river above at high 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 accretion sat 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.[2]
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°wh ere an abundance of sand from outside, put in suspension by the action of the prevalent winds blowing into the estuary, is brought in by the flood-tide, as in the cases of the estuaries of the Dee, the Ribble and the Seine. This accretion, reduces the tidal capacity of the estuary, and, producing diminution in the tidal volume passing through the outlet, promotes the extension of the sand banks seawards, as indicated by the difference in the outer portions of the longitudinal section os different dates of the Weser and Seine estuaries (figs. 22 and 26), To prevent as far as possible the reduction in tidal capacity, the training walls should not be raised more above low-water level than absolutely necessary to fix the channel; and the rate of enlargement of their width apart should not be less than 1 in 80 at the upper end, and should increase considerably towards the mouth of the estuary so as to form a trumpet-shaped outlet. The loss of scour in the channel resulting from this enlargement must be compensated for by dredging to attain the requisite depth. Training works partially carried out through an estuary have the advantage of reducing the length of shallow channel to be traversed between deep water and the entrance to the deepened river; but as these works produce no influence on the channel for any distance beyond their termination, a shallow, shifting channel is always found between their the end of the trained channel and deep water. Accordingly, when training works are started at the head of a sandy estuary, provision should always be made in their design for their eventual
