DISCUSSION—RAINFALL OF VICTORIA. 12 the upper works, serve as a partial supply to Bendigo and its neighbourhood, by interrupting the rainfall on the hills south of Crusoe Gully, and the Big Hill Reservoir, the area of this watershed being about 4000 acres. These observations do not pretend to extreme accuracy, as the data are not sufficiently exact. For instance, the rainfall is taken as the mean between.
that at Crusoe and Preston Vale ; but they- may serve to show the great difference in the quantity of water available for storage when the, rain falls in heavy or in light showers. At that time,.
discussing rainfall questions, I reckoned from April i to-Sep- tember 30 as the winter or wet season, and from October I to March 31 as the summer or drier season.
Per Cent.
Rain in Inches. Available.
October, 1874, to March, 1875 6.45 33 April, 1875, to September, 1875 19 67 180 October, 1875, to March, 1876 9.96 50 April, 1876, to September, 1876 11.70 110 October. 1876, to March, 1877 10 70 5.0 April, 1877, to September, 1877 13.21 10.9
" The above are the results of computations from the dis- trict monthly reports. The following are from special reports on particular.-falls of rain
On February 20, 1875, 0.70 in., at Crusoe, yielded i i per cent. of its volume.
" On April 21-23, 1875—when ground was wet, but water- courses not running—o.64 in. gave into storage 13.7 per cent.
" On December 12, 1875, 0.75 in. fell in thirty minutes, then light showers ; total, 0.84 in. This gave 6.25 per cent.
" On March 25, 1876, 0.97 in. fell, giving 4.76 per cent.
" On the three days of May II, 12, and 13, 1876, 1.73 in.
fell, and produced 3.70 per cent.
" On May 29, 3o, and 31, 1876, 1.54 in. fell, and gave 6.66 per cent.
" On November 15, 1876, 1.35 in. fell, and gave 9.09 per cent.
" On February 12, 1877, 1.12 in. fell, and gave 12.5 per cent.
" On February 28, 1877, 1.69 in. gave 8.33 per cent.,
" In September, 1877, 0.92 in. rain, in four days, collected from a small area of 5oo acres, gave only .i per ceilt. of rainfall.
"January 1877, 1.2 in., in three days ,(ground wet),- gave 4.9 per cent."
ì»r
I22 VICTORIAN INSTITUTE OF ENGINEERS:
Mr. W. W. Culcheth's contribution to the discussion was- read by Professor W. C. Kernot, as follows :
Mr. Baracchi has done a public service in so conveniently summarising the rainfall observations of the State, that the main results may be seen almost at a glance. Engineers have rea- son to be grateful to him, not only for this paper, but also for sa readily, from time to time, placing at their service the informa-
tion collected at the Observatory. They usually want to know what has been the rainfall for some years past under various conditions, according to the scheme they may happen'to have in hand.
"The rainfall, month by month, is always obtainable, if that is of any special use to one. But, usually, it is far more impor- tant to know what heavy falls have occurred in a day or two ; also in a few hours, or even minutes. Reliable particulars re- garding heavy falls in short periods are probably only obtain- able from a properly equipped observatory, seldom from in-
dividual onservers in the country.
" It would be of great use to engineers if some such particu- lars as the following were published regularly with the other results :
" I. Total, during each month, of rainfall registered in a day::
(a) Under half an inch ; (b) from half an inch up to one inch ; (c) above one inch.
" Generally speaking, the first (a) scarcely ever flows off the surface ; the second (b) gives probably the bulk of the water obtainable .from the rainfall, :more by the large number of such falls than by the proportion flowing off, which is small; the third (c) leads to floods. Ordinary conditions prevailing over large areas in the country are here referred to, not paved and other prepared surfaces as in towns.
" II. Heavy rain, irrespective of the dates, as usually shown ; that is to say, eliminating the arbitrary division at 9 a.m., or other hour: Some such falls are lost sight of through being shown on two dates.
"This would be only an occasional notice ; but it should show what unusual quantity of rainfall in a few minutes, or in an hour or more. Mr. Thwaites, in his report, gives such a list, though many of the falls referred to by him were-scarcely heavy enough to deserve special notice. This list is not generally known to engineers, perhaps; but, if known, it gives only Melbourne re- sults. No doubt, the Observatory could give similar data for other places in Victoria. Heavy continuous rain extending over longer periods, such as two inches in a day, three inches in two days, etc., are of special interest to engineers who have to pro- vide against floods ; or for water storage, from large catchment areas.
DISCUSSION-RAIN FALL OF VICTORIA. I 23
" Returning now to what Mr. Baracchi has so kindly done for us, his idea re ` normal values ' appears to be particularly good. As I understand it, he- proposes"taking a -fixed-value, near the average, by which to compare future results. This is illustrated by the diagram on page 107 of his paper, where the rainfall of each year from the start is compared with the latest average—taken there as the normal value. This will be better than the varying average by which the rainfall of the past few years has been judged ; and now that there are fifty years of re- liable results to work with, such normal value will be fairly cor- rect. During some years past, the average has varied from 26.06 in. in 1892 to 25.42 in. in 1899; since 1899 it has been steadily rising again. Probably, something near the mean of these figures, say 25.70 in., wOuld be a fair normal value for Melbourne. This is slightly higher than Mr. Baracchi has fixed for the whole State-25.5o in.
Now, since the object of publishing the results of the rain- fall observations is for the information of those concerned, I would like to express the opinion that these results lose a lot of their value through rigidly following the calendar. The rain- fall of the first two or three months of the calendar year is of little value to the country, while the record of it distorts the later results: Take .1904, for instance, There was unusually heavy rain in January and February, which caused the total for
• the' year to appear above the average;-whereas the other months taken together were far below it. The heavy falls at first did nothing to atone for the shortage later.
If -the rainfall -were counted from April 1, any abnormal fall at the start, as in 1901, would be reflected throughout the season ; for a good fall in April or May benefits the country generally.
" Perhaps it would be well if less notice were taken of the total for the year ; for, look at that for 1891, which was only a little over the average, and yet that was the year of the last great flood in the river Yarra.
" In my opinion, it would be preferable to take note of half- yearly results, commencing on October i and April i, respec- tively. The former would represent the results due to the sun being south of the equator; the latter would do the same when the .sun was -north of -the. equator.. , _The.normaitotals for these two seasons would probably be about 13.3o ins. for the former, and r 2.4o ins. for the latter.
" By way of illustrating the results of such arrangement—
from October I, 1904, the rainfall for the season was short of the first normal, above stated, by 4.92 in. Counting afresh from April 1, 1905, the total for the season was 1.98 ins. in ex- cess of the second normal. , I think these results reflect the
general opinion regarding the two seasons. The past winter
-I24 • VICTORIAN INSTITUTE OE ENGINEERS :
has been considered a wet one ; the rainfall stated in this way also showing this. But, as usually stated, this year appears to be a normal one, the average from January i being 18.51 in.;
while the actual was 18.66 in.—to end of last month (Septem- ber)."
Mr. 1V[. Elliot read the following notes :
" The rainfall: records in Victoria are valuable to the en- gineer, in so far as they give him some indication of what may
be expected • from a watershed. Experience will provide him with some data upon which to calculate the maximum and mini- mum discharge. The actual practice requires a considerable margin to be made in both cases. Two or three cases may be quoted from experience, to show the relationship existing be- tween the rainfall and the flow-oft.
" Some years ago I built a reservoir at Mount Teddington, near Stuart Mill, at an elevation of moo ft. above sea-level. The catchment consisted of 8000 acres mountainous country, rising up to 2000 ft. above sea-level ; steep slopes with ,valleys. only.. a few chains wide. The creek-bed consisted _of loose shingle .stones, worn smooth, and three to four feetin depth. During the years 1897-1902, with a rainfall of twelve. to fifteen inches per annum, the flow-off was below one per cent.
"At periods, a fall of half an inch would occur, but very little would be given off. These periods were at long intervals, with dry weather between, varied with a few light showers. This shows that with a very suitable catchment area, the quantity available for storage, •when the rainfall does not exceed twelve inches per annum, well distributed, is very small, and quite un- reliable.
" Maximum discharges from watersheds are even more cliffi-
•cult to estimate, and in all cases some risk must be taken, as there is no ,reason why.,previous records- should, not be-ex- ceeded very considerably, as took place, some months ago, at Durban, in South Africa. It can only be expected that the en- gineer is to inform- himself of pretiioüs• falls and floods, and pro- vide accordingly.
" Mr. Baracchi has quoted the Dandenong watershed, which embraces about 120. sq. miles. The flood of 1891 from this watershed reached a maximum flow of about 400,000 cub. ft.
per min. or 3751 cub. ft. per min., from each sq. mile of catchment, giving a flow-off equal to a depth of two inches over whole catchment area in the twenty-four hours. In 1899 the volume discharge over the Malmsbury reservoir weirs amounted to 450,000 cub. ft.' per -mien: from • ri2 sq. miles, being at' the rate of 4017 cub; ft. per min. per sq. mile of catchment.
" The'Chamier formula: may be quoted as an approximation for fixing flood openings. It is thus expressed: • -
DISCUSSION—RA INFAI,I, OF V I CTORIA. I25
Q = 64oXRXCXì~'l
R = average rate of rainfall in inches per hour.
C = co-efficient of surface discharge, determined •by in-
spection on the ground.. -
M = catchment area 'in: quare miles. -.
"Mr. Cardew makes an interesting statement as to calculation foe maximum discharge from watersheds. These will be found in Volume clii., page 242, ` Minutes of Proceedings,' Inst. C.E.
He states that the maximum rainfall for one day in Bathurst dis- trict is very nearly one-eighth of the mean annual rainfall. The maximum rainfall for one hour is nearly one-fourth of the great- est daily rainfall. In a period of •eight hours the maximum rainfall is double for one hour; in a. period of sixteen hours is treble, and in twenty-four hours four times as great as the rain- fall for one hour.
" In this case the catchment is 8.3 sq. miles. The waste weir is capable of carrying three and a half inches of rainfall in twenty-four hours, from a full reservoir, with three-fifths flow- ing off with a depth of one foot three inches. The length of waste weir is 123 ft., and was calculated by the formula :
L =
m x d x 1/2 gd, in which m= 0.80.
" The figures and formula are quoted as corresponding nearly to what has been found to be safe practice: They are a little below requirements of the flood, in 1899, over the Malmsbury weirs."
Mr. J. A. Smith said Mr. Baracchi had not told them, but the speaker knew that the paper was the epitome of a very great amount of work ; not merely work that had been recorded in the past, but work which Mr. Baracchi had specially undertaken for the purposes of the paper. He thought that as it stood thé paper was probably the most reliable precis of the rainfall statistics of Victoria that had beenpublished.
Adverting to the paper : on page 98 a formula was given and he would like to ask Mr. Baracchi whether that was a general formula intended to apply to any altitude—for instance, the variation of height between Melbourne, and the summit of Mount Macedon ; or whether it was only to apply where the variation was slight, as, for instance, between the bottom of a tower and the top. It would be noted that when the square root of H in that formula became the reciprocal of the numerical coefficient- o.o161—then the reduced rainfall would be infinite, however small the actual might be; while at a height of, say, 90o ft. the apparent rainfall would be doubled. •
Obviously, unless some other factor were introduced the
126 VICTORIAN INSTITUTE OF ENGINEERS:
formula would not apply to considerable elevations, but perhaps it was not intended to be so used.
Mr. Baracchi said the formula simply applied to elevations above the surrounding surface.
Mr. Smith said a point had been raised as to the abnormal year 1849, when there was a rainfall of 45 in. In connection with that, he would like to ask Mr. Baracchi whether there was any certitude that `such'ä phenomenal rainfall would-not occnr again. It was a difficult question to answer, but Mr. Baracchi might guess at it with more chance of his guess'becoming pro- phetic than most could. There had been a rainfall of 20 inches in a day in Sydney. Was there any reason why the monsoonal results of planetary (in contradistinction to merely local) weather should not come yet further south, and result in a heavy fall in the Yarra valley? In the long extended history of various European countries phenomenal floods were infrequent, they did not come every century even, but when they did arrive they altogether outclassed the ordinary maximum conditions. With the Melbourne records and traditions of three-quarters of a cen- tury only, could it be affirmed that the maximum conditions had been experienced?
He exhibited photographs taken during the 1863 flood, show- ing the condition of certain Melbourne streets and areas now built upgn. It would be seen that there was reason for the question he had raised.
Mr. P. Baracchi, in replying to the discussion, said the first point was in connection with Mr. Culcheth's remarks. It was a very good criticism of the paper, indeed. The question was as to heavy rainfalls during short periods, such as the frequency of falls of half an inch, one inch, and more than one inch. This was a most important matter. Unfortunately, he was not in a position (nor was anyone else in Victoria, at present) to show that. The reason was that the returns that came from up- country were daily returns. The observers were instructed to read the gauge once a day; at 9 a.m. This was necessary, be- cause if the observers were told to measure each fall, or each shower as it fell, they would have no reliable record of rainfall at all. He took the opportunity to say that great help could be given in that respect by the many engineers who were scat- tered up-country. Every shire had its engineer. There were engineers all over the place. To show how difficult it was to obtain reliable assistance, he mentioned that a country observer had deprived the Observatory of two years' records, because he wanted two guineas for his services. As far as the Melbourne Observatory was concerned, he had an instrument which gave a continuous record of every fall that occurred. From that they had made a superficial review of all the heavy falls which had taken place, and slime of the results will be found in one of
DISCUSSION—RAINFALL OF VICTORIA. ' I27 the Reports of the Chief Engineer of the Metropolitan Board of Works. Mr. Culcheth was quite justified in asking for the in-
formation, but unfortunately it could ndt be supplied at pre- sent, excepting for Melbourne itself. The records were open
-to anyone who wished to personally investigate the heavy falls.
Someone had mentioned the flood of 1891. At the time of that flood there were fourteen stations on the catchment area of the Parra and-Dandenong Creek; whieh' recorded; it the "space of six days, from five to ten and a quarter inches of rain. It was raining all over the basin, and for clays there were continu- ous gales from the south-west, which were recognised as co- operating to cause a flood. In 1863, when five and a quarter inches were recorded, there were also heavy westerly gales.
The flood of 1863 did more damage than the flood of 1891, be- cause, in the former year, the conditions of discharge were not as good.
Mr. Culcheth had also said the division of the year was not a fair division. In this he was quite right. He (the speaker)
•did not give the annual, only, but the monthly rainfall also. In whatever order the returns were arranged, the month was the unit, and not the year. ' They could ascertain the rainfall for the first quarter, or any other quarter which was required. The month was the smallest division they could make. Even that was rather rough, but they must strike a limit.
He had many times laid stress upon the fact that the annual value of the rainfall was simply meaningless. There had been .a year in the Mallee country when only seven inches of rain had fallen, which proved to be sufficient to ensure good seasons, so that the farmers were enabled to fill the railway sheds with _grain. This was because the rain came at the proper time. It fell mostly in the months of March, April, September, and Oc- tober, and hardly any fell during the other eight months of the _year ; so that the farmers were making money, and were at the :same time complaining about the severity of the drought. But during another year they had over twenty inches, which fell
-mostly during the summer and the winter months. They could not complain of drought, and they had no crops at all. This showed, of course, that the distribution of rain in the various seasons was even more important than the annual amount regis- tered in the year.
They could not very well put the full returns on the maps; be-
,cause it would make them too crowded. To those who were interested the tables supplied all the seasonal information wanted. There they could obtain the rainfall value of each
-month.
The normal value for Melbourne (which was considered to be the most probable value they could at present work out, could :not be legitimately deduced within a probable error of 5 per