The President said Mr. Baracchi had given a great deal of valuable inforrnatian, which it would be well for the members to study at leisure.
It gave them some idea of the vast amount of work that was undertaken at the Observatory, in order that the demands which the public made upon the knowledge of weather conditions, etc., might be complied with He did not know whether the rainfall in other countries would be as varied as in Melbourne. His own impression was that the rainfall of Great Britain would be more regular. An engineer, looking at the diagram of annual rainfall, would probably think there was some error, as the variety was so abnormal, but very likely there was a natural cause for it.
Mr. J. M. Coane said he would like to ask Mr. Baracchi a question regarding a matter which had often puzzled him. Ìf they took, for example, a steep hill of loo feet, and put a raingauge io feet above the ground at a height of 15 feet from the base, and another 2 feet above the ground at the summit, the respective elevations would be 25;
feet and, 102 feet. Would the condensation above the raingauge be- the same in each case?
Mr. Baracchi said ,at the level of 102 feet the rainfall registered would be less than at 25 feet, because on the higher altitude the wind was striking against the surface and driving upwards. The rain was, formed 'in much higher strata of the atmosphere; thus the difference in level mentioned was, in this respect, negligible.
Mr. Hill said that one point on which Mr. Baracchi had not touched was the maximum rainfall during a short period of time. In Mr.
Thwaites' report to the Melbourne Board of Works there was an item upon which he had always looked with considerable doubt. It was a record of a rainfall at rate of 6 inches per hour. It did not last more than two minutes.
Mr. Baracchi said the highest rainfall of which he had a note was.
on 24th January, 1883, and it was at the rate of 3 inches per hour.
Twenty-five points of rain fell in five minutes. That occurred on the 27th of October, 1887. But they had a formal record of the deluge at Heathcote, when 7 inches of rain fell in 4 hours. The person who took the observation was a most careful observer.
Mr. Hill asked if the surrounding country showed anything like a similar result, or was it a waterspout.
Mr. J. M. Coane said it had covered at least four square miles.
Mr. J. A. Smith inquired whether Mr. Coane was aware what effect had ensued as regarded culverts, etc.
Mr. Coane said he had visited the locality, and had found that ex- tensive damage had been done to culverts, embankments, and so on.
Professor Kernot said he had listened with great interest to Mr.
Baracchi's paper. It was a subject of very great importance to civil;
engineers from a water supply point of view. He was interested in
DISCUSSION-RAINFALL OF VICTORIA. I17 the minimum rainfall. He had found that the Melbourne water sup- ply had met the demand without any difficulty whatever during the year in which they had the minimum of rainfall. He thought this should inspire confidence in the water supply, especially as there were dry years preceding, which gave the reservoirs no chance of becoming specially charged. Melbourne was not likely to have a water famine such as had occurred in Sydney.
The other point of view from which the civil engineer had to look at the rainfall was that of floods, and flood provision. They knew what had happened in 1849-the wet year—and if they had a similar rain- fall again he would not like to say exactly what would occur. In connection with the subject of the maximum rainfall, the question arose as to its distribution. There might be a tremendous rainfall without a flood if it were distributed over a lengthy period. The years 1863 and 1891 were years of very great floods on the .Parra. They expe- rienced a very high rainfall for a week. In 1863 the heaviest flood occurred. In the old days, when Elizabeth-street used to be flooded, the worst floods took place from a rainfall lasting only half-an-hour or
so.
It was .a heavy rainfall in a, short pepod over a small area ofa square mile or so which caused the trouble. That would not affect the Yarra. The Yarra could take a week's heavy rainfall, anal the bigger the river the longer the time the water took to come down. The Yarra had been much improved since the dates of those floods.
The duty of the civil engineer in dealing with the rainfall was to know how far he should go in guarding against floods, and to what extent he should risk them. It would be possible to ruin a municipality by putting huge bridges over little creeks. It would be better to run the risk of an occasional flood. But the question was—Where should they draw the line? He thought if their works were such as would carry off an inch of rainfall in an hour they were safe. If a heavier flood carne they must be prepared to submit to their streets being flooded for a little while. It was a difficulty of the engineer's position. If he provided for extra rainfalls of this kind he ruined a place with the expense of the works he put up, and if he did not make_ sufficient pro-
vision for rainfalls that come every few years he became distinctly un- popular with the people.
He had studied the subject of culverts for railways and roads, and found a great many inconsistencies. There were to be found large works for draining small areas, and small works for draining large areas. At Cootamundra, where the great railway accident took place, an 8-ft. culvert drained an area of zo miles, whilst a bridge at Haw- thorn several times larger drained two square miles.
In connection with the drainage of the Koo-wee-rup and Moe Swamps, the drains were constructed too small for maximum conditions, with the knowledge, that they would be enlarged by natural erosion, and they were gradually being made larger; but it so happened that a heavy rainfall occurred;' and the channels overflowed to .sow extent.
If there had been two or three dry years, the natural erosion. would have sufficed. As engineers, they had to run some risks in these mat- ters. Reference to the Heathcote incident recalled stories that were
I18 VICTORIAN INSTITUTE OF ENGINEERS:
frequently heard (especially from America) of cloud-bursts, in which the water seemed to come down like a Niagara over a small area. It was useless to attempt to provide against such an experience as 17 inches of rain in an hour. There had been heavy records quoted from Sydney of 20 inches in a day, which were somewhat serious.
- In Mr. Baracchi's maps of the Yarra Valley he noticed the distribu- tion of the raingauges to be very irregular. The area around Wood's Point, which had a record rainfall, had few gauges, whilst the dry part of the area around Melbourne was full of gauges:
He would move a hearty vote of thanks to Mr. Baracchi for his very interesting, paper. It was kind of Mr. Baracchi to attend and furnish such information relating to water supply, which was a matter of the greatest importance to civil engineers. If only they could get a little more accurate data than they had, it would relieve them of a good deal of anxiety in getting water when they wanted it, or getting rid of it when they did not want it.
Mr. J. M. Coane had much pleasure in seconding the vote of thanks. Referring to heavy rains, he had had an experience of 17 inches of rain in twelve hours in Queensland. That was a serious matter. On the 3rd of February, 1893, also, there was a record of 35.91. inches in 24 hours, and the total at that particular station for four days was 82 inches. As to the Heathcote record, he had passed over the area within a month after the occurrence, and was sure the rainfall had covered an area of at least four square miles. It carried culverts, bridges, and almost everything before it.
Mr. J. A. Smith said that in reference to the. doubt that had been expressed as to the authenticity of the abnormal fall in the year 1849, bethought that the record might be fully accepted. He had had the opportunity in youth of hearing the version_ s of many of the old pioneers, who had personally seen the floods of that year, and the old flood marks had been pointed out to him; and if the same condition recurred, many modern works and structures would be in considerable danger.
The vote of thanks was carried by acclamation.
Mr. Baracchi, in reply, said he hoped the paper would prove useful.
It contained all the results of the records at the Observatory. He thanked the members for listening so carefully to the paper, and hoped the information would prove of value to them.