ENGINEERING CONSTRUCTION AND RAINFALL. LITI. 



rainfall meant a rainfall of an extraordinary nature, out of the 

 common, and such as was not justified by past experience." 



The practice of the Works Department is to consider a rainfall 

 of 2 inches per hour, with only a percentage, rising sometimes as 

 high as 50, flowing off. Of course, where large sums are proposed 

 to be expended in storm- water channels, it may be debatable 

 whether, from a pecuniary point of view, it is not advisable to 

 provide for less than the maximum rainfall, so as to reduce the 

 cost. 



When the maximum quantity to be provided for has been 

 decided, the engineer can easily provide facilities for its getting 

 away in a safe manner, that is, so as not to injure property ; and 

 the question resolves itself into the calculation of the necessary 

 size of pipe-drain, culvert, or open channel to carry off a given 

 volume of water in a given time on a fixed or determined grade. 

 Many formulae have been devised for this purpose, but none is so 

 generally applicable as that of Ganguillet and Kutter. 



The question of rainfall, when considered from the point of 

 view of water conservation, has to be dealt with in an entirely 

 different manner. 



With the exception of the bye-pass from a storage reservoir, it 

 is not the question of maximum rainfall in a limited period that 

 has to be considered, but the available volume from the minimum, 

 mean, and annual rainfall. 



The question of the length of time the water from a catchment 

 will take to flow into a reservoir or channel need not generally 

 be considered, so that slope or length of fetch has but little 

 weight. It is the amount which may be expected to run off a 

 catchment, and be available for conservation for future distri- 

 bution, such as town supply or irrigation, which has to be 

 considered. The question of evaporation does not under either 

 of the other heads enter into practical consideration, but is here 

 of vital importance. Percolation must also be taken account of. 

 Both phenomena have been classed by recent writers under the 

 head of absorption. This varies with the soil and climate. In 

 many cases, however, water which is apparently lost by percola- 



