A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 239 nearly 7 miles. One training-wall was situated on the left bank about 4| miles long, opposite Bremerbaven, and lying between that port and Imsum, and the second wall was on the right bank, 1 mile long, between Imsum and Wremen. These walls had the effect of reducing the excessive width of the river, and the increased velocity of the water produced a scour which removed completely 7 million cubic yards of sand in two years, and changed the position of anotlier 4| million cubic yards—all in a length of 12 miles. The movement of material, however, simply led to the formation of banks elsewhere, and a suction dredger of 500 I.H.-P. was ultimately obtained to keep open the channel to Bremerhaven for deep-draughted vessels, which it has done in a perfectly satisfactory manner. A second dredger of greater capacity was added, in 1898, as a substitute in case of damage to the earlier boat, possibly entailing lengthy repairs, and also in order to be in a position to deal with simultaneous shoaling of distant parts of the estuary. The Robben Plate divides the outlet of the Weser into two streams, and there is a further subdivision at the West Ever Sand. In consequence of these obstructions, an outer bar had formed at the north end of the right-hand channel. To remedy this defect, in 1896 the branch stream between the East and West Ever Sands, which has a breadth of about two-thirds of a mile, was dammed by depositing a layer of weighted fascines, 3| feet in height, and in the following year a second layer was added. Simultaneously, protection works were carried out along the frontage of the Wurster Watt. The object in view during these operations was to attain a low-water depth throughout of 26 feet, so that there might be no restriction to the passage of deep- draughted vessels. This object was in due course attained. It was also at one time proposed to block the channel lying to the left of the Robben Plate by means of a training wall and dam, as indicated in fig. 214. The works, however, have not been carried out, as the channel has manifested a natural tendency to silt up, and it is considered that any artificial assistance in this direction is superfluous. Writing in 1906 to the author, Herr de Thierry remarks:— “The works begun in 1896 and 1897 between the East and West Ever Sands, and for the protection of the Wurster Watt, have been carried out and maiutained. The two dredgers, “Columbus” and “Franzius,” have been nearly constantly at work deepening and regulating the channel. The dredging of a deep channel caused difficulties between Imsum and Wremen, owing to a bank of boulders bedded in very stiff clay. The dredgers have worked chiefly at this place, and lower down, where the velocity of flow is sensibly decreased, on account of the branching off of the stream between the East and West Ever Sands. A low-water depth of 28 feet throughout was, however, notwithstanding these difficulties, achieved two years ago, and has since been easily maintained.” Training-jetties at Tampico Harbour, Mexico.1—The improve- ment works at the mouth of the river Panuco, for the port of Tampico in Mexico, 1 Corthell on Tampico Harbour Works, Afin. Proc. Inst. C.E., vol. cxxv.