The Weser from Bremen down to Bremerhaven, a distance of over 4o m., has been regulated by constructing longitudinal training walls, cross dikes and groynes, while in the estuary between Bremerhaven and the Hoheweg light, a distance of about 18 m., there are occasional low training-walls and dikes and minor channels have been closed. These operations have secured a minimum low-water depth in the channel up to Bremer haven of about 25 ft. and to Bremen of about 17 feet.
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 chan nel through shifting sand banks, whose dangers were at times intensified by a bore, a stable channel has been provided down to St. Sauveur, rendering access easy as far up the river as Rouen for vessels drawing up to 23 ft. at high-water neaps and up to 27 ft. at high water of spring tides. The channel itself, however, was originally made too narrow between Aizier and Berville and was subsequently enlarged, and large tracts of land were reclaimed in the upper estuary.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 due to the reclamations, together with the fixing and undue restriction in width of the channel, occasioned 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.
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 This scour, however, being concentrated in the trained channel, is withdrawn from the sides of the estuary, which in its natural condition is stirred up periodically by the wandering channel; and, therefore, accretion takes place in the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 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 a diminution in the tidal volume passing through the outlet, promotes the extension of the sand banks seawards. 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 r in 8o at the upper end, and increase towards the mouth of the estuary. Training works carried partially out through an
estuary have the advantage of reducing the length of the shallow channel to be traversed between deep water and the entrance to the deepened river; but as the influence of these works on the channel terminates close to their seaward end, a shallow, shifting channel is always found between 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 prolongation to deep water at the mouth of the estuary, to ensure the formation of a stable, continu ous, navigable channel. Experiments with a model, moulded to the configuration of the estuary under consideration and repro ducing in miniature the tidal ebb and flow and fresh-water dis charge over a bed of very fine sand, in which various lines of training walls can be successively inserted, are capable in some cases of furnishing valuable indications of the respective effects and comparative merits of the different schemes proposed for works which have often evoked very conflicting opinions.
BIBLIOGRAPHY.--See list at end of article CANALS AND CANALIZED RIVERS. The works by Vernon-Harcourt, D. Stevenson and Thomas & Watt (vol. 1, Free Rivers, vol. 2, Canalised Rivers) mentioned therein are important. See also W. H. Hunter, Rivers & Estuaries (1913) ; J. L. Van Ornum, The Regulation of Rivers (1914 an excellent and comprehensive work dealing mainly with non-tidal rivers) ; C. McD. Townsend, Hydraulic Principles of River and Harbour Construction (1922 contains bibliography) ; Encyclopedie de Genie Civil (2 vols. by Vidal deal with rivers, 1921-22) ; and L. Fargue, La Forme du Lit des Rivieres (1908). See "308 Reports" of Chief of Engineers, U.S. Army, for the waterways of the United States. The Proceedings Inst. Civ. Eng. ; Transactions American Soc. C. E. and the International Navigation Congress publications contain valuable papers. The Bibliographies issued by the I.N.C. bureau (Brussels) are exhaustive. (L. F. V-H.; N. G. G.; X.) RIVER BRETHREN: see BRETHREN IN CHRIST.