148 



Failure of the Van Yean Reservoir. 



compared with Mount Disappointment, and these are covered 

 with snow during the winter months. But it has yet to he 

 proved that the Yarra undergoes so remarkahle an increase 

 in vohmie for a period of eight months in the year. This 

 river has an average depth of thirty feet, for a distance of two 

 miles above Prince's Bridge, and 1 need scarcely say that it 

 only overflows its banks in floods, which may not occur once 

 in two years. Its level, in the beginning of December, is 

 not more than two or three feet below the average of the 

 winter months, or below the level of many portions of the 

 banks ; with any reasonable increase in the velocity, therefore, 

 how is it possible for the Yarra, with an increase of only 

 one-tenth in its depth or sectional measurement, to carry ten 

 times the volume of water for eight months? 



We have only then to compare, in the drawings furnished 

 by the Committee, the sectional area of the Plenty, at one 

 or more points, in order to see how impossible it is to beheye 

 that the river, during eight months of the year, could contain 

 ten times the volume of water that it does in December. 

 The drawings show that the stream occupies in January 

 one-half of the sectional measurement, that is, one-half of 

 the depth where the banks are perpendicular. 



Now, as above stated, it may be regarded as an axiom, 

 that when a river has defined banks these indicate its 

 ordinary limits, which it only exceeds in time of floods. 

 Thus, without actual measurements for the winter months, 

 important information as to the volume of water maybe 

 e:athered from those that are resident on the spot. Dr. 

 M^Kenna and myself put the question to Mr. Bear, who 

 has lono; resided on his own property at Yan Yean, i± a 

 measurement of the Plenty, on the 12th of December, 

 would ffive a fair average for the year. He replied, that 

 he thoul^ht it would. Now, the meteorological tables show 

 that November is our wettest month, and Mr. Blackburn s 

 measurement of 2,700 gallons per minute was taken m 

 December ; therefore, it is not unreasonable to suppose, 

 that Mr. Bear's opinion may be very nearly correct. 



But let us examine the section of the river at the entrance 

 of the aqueduct, not very far distant from Mr. Bears house 

 With a discharge in January of 2,537 gallons per mmute 

 and a velocity "of half a mile per hour the section could 

 not contain more than twice the present volume, w th the 

 same velocity, without spreading widely over its right bank, 

 Xh rneSv level. With double the volume the velocity 



