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not entirely conclusive, but it shows that there is a strong presumption in favour of its stability.
Notwithstanding this weakness in the data, I consider that, taking all the evidence cumulatively, the conclusion that no appreciable deterioration in the harbour or its approaches has taken place is a sound one.
In dealing, then, with the barrier question on the lines before indicated, it would appear that conservancy consideration need not be taken into account, and that all that is necessary is to provide that the barrier shall not embarrass navigation.
But early in my study of these matters a possibly alternative scheme presented itself to me. Half-way between the Front Reach and the Back Reach is the Elliot Reach. There is reason to believe that before the placing of the one stone barrier on it, it had a continuous deep channel free from shallows. If then a sufficiency of water still existed in it, the removal of this one barrier would enable any vessel that A survey of the nine miles could cross the second bar to get up to Cantou Harbour.
of this reach was therefore undertaken. The result, however, was disappointing. It showed that for half a mile above, and more than half a mile below, the barrier no channel existed, and that elsewhere over long stretches the channel was comparatively shallow-14 feet-and tortuous. I judged that the difficulties of opening this channel were such as not to justify a departure from the more obvious course of improving the navigation of the other reaches.
I mention this matter partly with the object of bringing forward the evidence which the condition of this reach adduces, as to the reason for the immunity of the approaches to Canton generally from detrimental effects consequent on the existence of the barriers.
The Elliot Barrier, unlike the other barriers, has practically no channel through it. It is continuous right across the river, and reaches within 4 or 5 feet of the level of of low-water spring tides. The natural result of this has obviously been a very considerable general silting up of the whole reach. This being so, a much less volume of water passes through it now than before. It is probable that in its former condition the greater part of the ebb water flowing down the South Reach entered the Elliot Reach, while that from the Fatshan Branch flowed down from the Blenheim Reach, the so-called Taimei Spit, with 104 feet on it at low-water spring tide, forming a natural guide. But with the partial closing of the Elliot Eranch a part of the water which used to flow down the Elliot Reach must have been forced over the Taimei Spit into the Blenheim Passage. In this connection the important thing to realize is that, under existing conditions, the excess of ebb over flood in the Blenheim Passage is now greater than it was before the Elliot Passage was closed. Now it is this excess that enables the silt to be carried down to the sea, and I take it that it is this increased excess that has at least counteracted such deteriorating tendencies as the barriers in Blenheim Reach caused.
And also as regards such deteriorating tendencies there is the following considera- tion. With the exception of Taishek none of the barriers, in effect, raise the river bed, and at Taishek the bed is raised only to within 11 feet of the level of low-water spring tide. When we realize that at the second bar the river bed is only 14 feet below low-water spring tide, and that the tidal range there is a foot more than at Taishek, it will be seen that the depth, 11 feet at Taishek, is not a serious matter.
Again, as regards lessening the cross-section of the river at Taimei-a natural obstruction-the cross-section is some 11,000 square feet. At Taishek, some 2 miles lower down, I estimate the cross-section to be about 10,000 square feet. The reduction of the cross-section by 10 per cent. is the deteriorating feature of this barrier. At the Iron Barrier the cross-section is not less than 30,000 square feet, i.c., it is about three times as great as at a natural obstruction 12 miles further up, and it is, therefore, improbable that the restricted cross-section here even tends to deterioration. These aspects of the barriers in the Back Reach, namely, the comparative unimportance of the decrease in depth and cross-section, together with the effect of the increased excess of ebb over flood tide, enable one to understand how it is that the barriers in the Back Reach have not had that deteriorating effect that was expected of them.
In the Front Reach the cross-section of the river at the Salt Flats and Sulphur Point have not been ascertained, but it seems likely that in this reach also the diminution of the cross-section of the river by the barriers is of comparative unimportance.
The foregoing explanation being concluded, I will proceed with a description of the work now undertaken. Plans Nos. 6, 7, 8, and 9 are surveys of the several barriers.
5
At the Iron Barrier.
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(a.) A widening of the channel through it to about 570 feet. (6.) The securing of a depth of not less than 16 feet through the centre width of this passage.
The width 570 feet has been taken as that corresponding to a certain section of the piles which give, promise of being more easily removed than the remainder, i.e., stones are not heaped up round them to the same extent as elsewhere.
Looking to possible requirements in the future, it seems proper to do now all that these requirements may possibly call for-as regards the screw piles-because as years go on their removal will become more and more difficult. For this reason the channel here is made wider than elsewhere, and every effort will be made to entirely remove these piles so as to admit in the future of any depth being obtained.
At the Bridge Barrier.
The entire removal of this barrier, and where stone exists its removal to a depth of 16 feet.
At the Taishek Barrier,
The removal of stone to a depth of 12 feet in a channel 400 feet wide where the bed of the river admits of it.
At the Cambridge Barrier.
The removal of piles, stones, and sunken junks, so as to secure a channel of 400 feet wide with a depth of not less than 16 feet.
At the Whampoa Barrier.
The removal of piles, stones, and sunken junks, so as to secure a channel of 400 feet wide with a depth of not less than 9 feet; and, perhaps, also the removal of the south- east elbow of the barrier, so as to straighten the course of vessels passing through. The amount of work to be done in this latter respect to depend on the difficulties encountered.
(Signed) W. FERD. TYLER,
Coast Inspector's Office, Canton, November 2, 1904.
APPENDIX.
Coast Inspector.
WITH the object of ascertaining whether there has been any significant alteration in the level of high and low water and in the tidal range at Canton, the mean of the daily recorded heights have been ascertained for the years 1882, 1888, 1893, and 1903. As a check on these the mean of the daily ranges has also been ascertained.
The value of these tidal data and of the deductions obtained from them will, of course, depend on the stability of the tide-gauge, and this stability cannot be taken for granted, for on the one hand it appears to have been originally intended to show the depth of water on the Salt Flats, and on the other it is known to have recently been 6 inches in error. Before, therefore, the value of deductions from these tidal records can be estimated, it is necessary to get some idea of the accuracy of the records themselves.
At Canton a datum mark, 10, is cut on the steps immediately to the eastward of the Custome examination shed; its date is unknown. When 10 on the gange was on the same level as this mark the gauge was supposed to show the depth of water on the Salt Flats. There is every reason to suppose that the gauge has been set by this mark for the past twenty years and
more.
On the 1st November, 1900, the gauge is reported to have been lifted 6 feet, and its zero then adopted as a standard low-water level. When, however, I examined the gauge in June last there was a
difference of 5 ft. 6 in. instead of 6 feet between the datum and the gauge.
Was this discrepancy due to the setting in November 1900, or was it due to a subsidence? To ascertain the answer to this, a coinparison of the records of high and low waters at Canton and Whampoa were made with the object of seeing the degree to which the zeros of these places conformed, and whether any change had taken place in this conformity.
It should be understood that the difference in the level of these gauges has never been ascer- tained by lovelling.
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