ENGINEERING CONSTRUCTION AND RAINFALL. XXXIX. 



It is noticeable how closely, for practical purposes, these 

 formulae agree, within such limits as they would probably be 

 used. 



Thus for a catchment of 100 acres, the run off is given as 

 follows : — 



Adams ... 62 cubic feet per second. 



Biirkli Ziegler ... 67 ,, ,, 



McMath ... 72 



Hawksley ... 80 ,, ,, 



The consideration of the factors, used in these formulae, will 

 now be entered on. They are divided into four heads, viz : — 



1. The rainfall. 



2. The kind and condition of soil. 



3. The area of the catchment. 



4. The general character and condition of same. 



The rainfall. — The maximum rainfall during the most 

 severe rain storms is what is to be considered. 



It wi]l be generally conceded that rainfall of this description 

 is limited in extent and of short duration, so that the size of the 

 area must be considered. In the Southern portion of this colony 

 a catchment of 10 acres might be subject to a rainfall at the rate 

 of six inches per hour, but an adjoining catchment of, say, 500 

 acres would only be partly affected by this local intensity of 

 rainfall. 



There is considerable difficulty in dealing with this part of the 

 subject in a general manner, since it hardly lends itself to such 

 treatment ; each particular case must be dealt with on its merits. 

 The only definite thing which may be predicated about each 

 district is that it has its own peculiarities ; these must be 

 thoroughly known and understood to enable the question to be 

 treated intelligently. 



The run off from a catchment, due to melting snow, is of the 

 greatest importance in some districts. Its volume is always in 

 excess of that caused by the maximum rainfall, but as its effects, 

 in the shape of flood-marks, are invariably observable, the ques- 

 tion of suitable formula need not be discussed here. 



