網頁圖片
PDF
ePub 版

of 1913 and 1914 and was a successful experiment until a better method was found in 1914 and the depot was finally closed in 1915. The losses in transport when the Wegyi boom was in vogue were often as high as 5 per cent, and undoubtedly a large number of logs which escaped from the boom were irretrievably lost in the silt deposits of the lower reaches of the Shewele stream. Mr. Leete took the systematic training of the rivers in hand in 1914 in the Prome, Zigon and Tharrawaddy Divisions, and since then it has been incontrovertibly proved that given a timber that will float no form of transport can hope to compete for cheapness with water transport.

All the Yoma torrents, due to their reception basins being highly susceptible to the excavating power of the rainfall, carry enormous quantities of silt and rubbish of all sorts during floods, and when their flow is checked by the backing-up action of the Irrawaddy the silt is rapidly dropped. In the past no adequate control was maintained, with the result that there have been continual changes in the channels in their lower reaches and the torrents have traversed the plains in all directions, but these divagations have at any rate served to raise the level of the plains considerably. Various reasons were given to account for the deterioration of the streams, and both Civil Officers and Railway Surveyors assailed the Forest Department as the cause of the evil, and there can be no doubt that the lack of adequate floating control and the failure to remove obstructions as soon as possible after formation were the principal causes. In 1914, after consultation with the late Mr. Samuelson, Leete decided that the most effective way to improve matters would be to double embank the channels at the places where the well-defined channels began to deteriorate. High embankments were therefore made at critical places on four of the streams before the 1915 rains. These measures were most successful so far as the embankments extended, but no good channel was obtained below the end of the embankments and logs were spread out fanwise over a large area. The problem of how to carry on the further improvement in the channels subsequent to the rains of 1915 now arose. Engineers strongly recommended that the high embankments should be extended, but on the grounds of economy an experiment was made in 1916 substituting low embankments for high embankments. Since 1916 it has been proved that the high embankments are not a necessity,

[graphic]

WILD ELEPHANT TIED UP FORE AND AFT AFTER CAPTURE FOR TRAINING. BURMA H. R. Blanford, photo.

GORGE AT THETUGEBIN LOOKING

UP STREAM AFTER A JAM OF 3000 TEAK LOGS HAD BEEN BROKEN UP AND LAID
OUT ALONG THE STREAM. BURMA

[graphic]

A. Rodger, photo.

but that in addition to involving considerable capital outlay, many disadvantages follow in their train, especially to cultivation. Low embankments were made in the Taungnyo stream for a distance of 1 mile in 1916, but, as the moment an embankment was overtopped by a flood, scouring immediately took place from the outer side of the embankment towards the stream the embankments were breached in many places during the first jungle flood. Therefore the experiment proved a failure. Experience, however, gained in dealing with the numerous breaches which destroyed the low embankments proved the value of bamboo-stake fences in controlling the flow of silt-laden water. This induced Leete, in his recommendation for the preparations for the 1917 rains, to suggest systematic application of the method which has been known to the cultivators in these riverain plains for many years. Instead, therefore, of continuing the training of these Yoma torrents with embankments, bamboostake fences alone have been used and no embankments whatever have been made since 1916. The continuation of all channels into the laha has been brought about and controlled by means of simple bamboo fences. For purposes of description of the manner in which the " No Embankment " method is applied an average state of affairs in any stream at the end of a rainy season may be considered. The condition of the stream is that a channel has been forced to a certain point and kept in place by herring-boned strings of logs. Below this point the channel for some distance is clearly defined in the silt deposits, the late rises having scoured a passage through the fan of silt thrown out in front of what may be considered the stream mouth. Side channels from this passage have been blocked with logs. Below this the channel is merely a depression in the stiff laha clay. The line along which it is desired to form the new channel is then pegged out and usually follows more or less a natural depression. All jungle growth to a width of 150 feet each side of the line is cut down flush with the ground and burned or cleared away. One hundred feet on each side of the line simple bamboo fences are made. These consist of bamboos, 5 to 6 feet long and pointed, driven into the ground about 9 inches apart and their tops dressed to an even height of about 3 feet above ground level. These stakes are lashed to an horizontal rail with coir rope about 6 inches from their tops, to hold them in position. Where this fence crosses side channels the bamboos naturally stand higher

out of the ground and must be strutted to enable them to withstand the additional pressure at these points. Any bad bends are eliminated by cuts. The low fences are continued just beyond the point which it is anticipated will be the limit of the silt deposit of the ensuing rains. The fences catch up the many kinds of small rubbish brought down on every jungle rise and form a barrier checking the flow of water, the finer particles being carried beyond it. In this way each rise serves to heighten the banks now forming, as well as to raise the level of the surrounding country. It will thus be seen that the channels being formed are not made by scouring a bed, but, on the contrary, the stream is induced to deposit its silt evenly along its course and to raise banks for itself. Each successive rise further heightens the banks and lessens the overflow, thereby leaving more water available to flow farther downstream to enlarge the section there. Care must be taken to keep the fences in repair. With the continuance of this work the channels are being carried year by year farther downstream. Along the shelf of the deposit on either bank of the stream cultivation of paddy has now been rendered possible where ten years and less ago annually inundated scrub jungle was the only vegetation. Furthermore, this new cultivation is permanent and free from danger from all but the highest of the Irrawaddy floods, while the margin of absolute safety is annually being moved farther downstream.

The primary object of the Forest Department River Training Works is to obtain a channel through which to float teak logs as cheaply as possible, and the channel, once formed, is very easy to maintain. The Civil Authorities now propose to demarcate the entire area which has been affected by the training works since 1914, and over which no land-holders rights have accrued, and this will constitute a Government estate. This estate will include all the land which may be required in the future for further stream training and, if the necessity arises, to move the course of any stream, either for cheaper floating of teak logs or for additional land reclamation, costly acquisition will not have to be faced. If this estate is constituted as now proposed it is contended that many more thousand acres of valuable paddy land will be reclaimed in a few years and the process of frequent changes in the course of each channel or of opening large silt-carrying irrigation cuts to fill up depressions will in course of time render the whole of the Myitmaka Valley culturable."

« 上一頁繼續 »