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M. de Rijke proposes to remove by dredging, carried out in two belts having depths of 9 and 13 feet respectively at low-water ordinary spring tide, the total quantity of material to be removed being estimated at 1,700,000 cubic yards.
Cutting Back of Pootung Point.
82. At Pootung Point it is also proposed to eut back the sharp angle formed in the right bank to the extent shown on drawing No. 1, the dredging being carried out in two belts having depths of 15 and 18 feet, respectively, at low-water ordinary spring tide. The total quantity of this dredging is estimated at 931,000 cubic yards.
Cutting Back of Bank at Native City.
83. M. de Rijke's complete scheme contemplates the cutting back of the bank at the Native City, to the extent likewise shown on drawing No. 1. The dredging, in this case also, would be carried out in two belts, having depths of 12 and 15 feet respectively at low-water ordinary spring tide. The total quantity of materials to be removed at this place is estimated at 2,204,000 cubic yards.
Further Works.
84. The total of the dredging works proposed but not commenced is thus about 7,500,000 cubic yards, and upon their relative desirability, as also upon the other works suggested, we shall offer our remarks in subsequent parts of this report. Various smaller works are also proposed by M. de Rijke, thus carrying the improve- ments to the upper harbour limit.
IX. THE EFFECTS PRODUCED BY THE WORKS ALREADY EXECUTED.
$5. We now come to a consideration of the effects which have been produced in In the river, by the construction of the works so far as they have been carried out. approaching this question it will be of interest, if, in the first instance, we give some figures showing the results actually achieved, and afterwards refer to the opinions expressed by those directly interested in the navigation of the river.
Effect on Outer Bar.
86. In April 1902 the harbour-master stated in a report that there was at that time, on the Outer Bar, a depth of only 13 feet below Customs datum (15 feet at low- water ordinary spring tide). In February 1907, before the commencement of the works, the depth, according to the same authority was 14 ft. 6 in, below Customs datum (16 ft. 6 in, low-water ordinary spring tide).
87. From M. de Rijke's report for the quarter ending the 31st December, 1909, it appears that there was then a depth of 18 feet at low-water ordinary spring tide over a width of 1,050 feet from the jetty, and of 22 feet at low-water ordinary spring tide over a width of 600 feet. Thus the Outer Bar practically no longer exists, notwithstanding that there has been no dredging over this portion of the river.
Effect on Inner Bar.
88. With respect to the Inner Bar, we have, in paragraph 36, given the small depths of water which were, at various dates, available over that obstruction. By the recent closing of the Ship Channel and the opening out of the Juuk Channel (now called "Astra" Channel, on account of His Majesty's ship "Astræa" having been the first vessel to pass through it on the 5th May, 1909), all questions regarding the old Inner Bar have been disposed of. The depth of water adopted for the " Astræa Channel, although less than that which existed for a limited width in the old Ship Channel, is throughout its length considerably greater than was formerly available over the Inner Bar, at the outlet of the Ship Channel. Inasmuch as the minimum depth over the Inner Bar limited the draught available for navigation between Woosung and Shanghai, it is evident as will be further explained in succeeding paragraphs, that a very valuable improvement has been effected by the disappearance of the Inner Bar as one result of the works which have been already carried out,
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89. In the achievement of this satisfactory result it should be noted that through- out the whole of the constructive operations the minimum or fouling depth previously available between Shanghai and the Yang-tsze, by way of the Ship Channel, was never diminished nor was the navigation of the river interfered with.
Depth over Inner Bar during Progress of Works.
90. In support of this last-named statement particulars supplied by the harbour- master show that the "official" minimum depth below Customs datum in the Ship Channel in 1906, the year before the works were commenced, was 10 ft. 6 in. in August; in 1907 the least depth was 11 feet in September; in 1908 it was 10 ft. 6 in. in the same month; and in 1909 it was not less than 11 ft. 6 in. up to the 1st July, at which date the Astræa Channel was opened, for general navigation by day, with a minimum depth of 13 feet below Customs datum. On the 15th September following lights were exhibited and the channel opened for general traffic both by day and by night, the old Ship Channel being finally abandoned,
Depth in Astrea Channel.
91. With regard to the present depth in the Astræa Channel, the harbour-master, in a memorandum dated the 14th February, 1910, states that "a fairway at least 600 feet wide at its narrowest part, with a depth of not less than 13 feet at lowest low- water spring tides (ie., Customs datum) has been maintained since the opening of the "it is confidently channel." He further states, "within the next two weeks
It considered that we will be able to signal 15 feet at lowest low-water spring tides. must be borne in mind that this office's low-water datum is 2 feet below that of the Conservancy Board." (See paragraph 13 above.)
92. We may add that the depth signalled indicates the draught of the vessel which may pass through the channel, so that, allowing, say, 1 foot of water under the keel, and 2 feet, the difference between the Customs datum and the Conservancy datum, the depth of 15 feet, when signalled by the harbour-master, would correspond with 18 feet at low-water ordinary spring tides aimed at by M. de Rijke. It further appears from M. de Rijke's report for the quarter ending the 31st December, 1909, that, with the exception of one small patch having a depth of 17 ft. 6 in., 18 feet at low-water ordinary spring tides, was available over an average width of, say, 950 fect. Messrs. Wilson and Matthews inform us that while at Shanghai they satisfied themselves by actual soundings of the accuracy of the above figures.
Effect of Scour in the Astrea Channel.
93. With regard to the scouring action of the currents in Astræa Channel, it may be noted for by comparing soundings taken in September 1907 with those of June 1908 M. de Rijke found that the material actually removed was 2,643,000 cubic yards (2,464,000 cubic yards in barges) had been removed by dredging, leaving a balance removed by scour of 1,056,000 cubic yards. The same calculation was repeated in September 1909, when it was found that a total quantity of 9,976,000 cubic yards has been removed, of which 4,716,000 cubic yards (7,078,000 cubic yards in barges) had been dredged, leaving a balance of 5,260,000 cubic yards as having been secured. These results appear to us to be very satisfactory as affecting the prospects of the future maintenance of the channel as now developed by the works which have been carried out.
94. It is also interesting in this connection to note that last autumn, prior to the letting of the latest dredging contract referred to in paragraph 74, after all dredging operations in the Astra Channel had been suspended for two months the official depth was not reduced, although it is right to add that the pilots of two of the steam-ship companies state that they observed some shoaling.
95. In auswer to enquiries by Messrs. Wilson and Matthews as to whether any increase in the range of tide at Shanghai had been observed since the opening of the Astræa Channel, or as to the time of high water having become relatively earlier in conse- quence of improved tidal development, M. de Rijke informed them that, although no definite observations had been made, he was of opinion there had been certain changes of In this connection the harbour-master also stated that he the character referred to. considered there had been a marked improvement in the channel above Gough Island
was apparent. and that an "increased flow of tide"
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