ENGINEERING CONSTRUCTION AND RAINFALL. LI- 



Though much more might be said concerning this part of the 

 paper, the second part will now be entered upon. 



The question of providing for rainfall in connection with City 

 or Municipal Engineering, is much more simple than the branch 

 dealt with. The areas, as a rule, are of limited extent, the value 

 of the coefficient of run off is much better denned, and no un- 

 certainty should exist as to the rainfall. 



The question of area presents no difficulty, and can be cheaply 

 and easily arrived at by means of maps. The method of treat- 

 ment, however, differs somewhat from the case of a railway, for 

 whilst in that instance the configuration of a catchment as 

 regards the existence of " feeder " creeks, running into the main 

 one, is only considered, as regards the volume of run-off at some 

 point on the main creek, in this instance the extent of the catch- 

 ments of these several " feeder " creeks is of importance. 



Take, for example, the case of a creek running through land 

 in the neighbourhood of a city ; whilst in this condition, rain 

 water finds its way into the creek, at probably innumerable point, 

 along its course. As population increases, the area within the 

 catchment of this creek becomes subdivided and built on. The 

 original condition, regulating the flow of rainfall into the creek 

 is altered — streets are constructed, the water channels of which 

 discharge the rainfall into the creek in a concentrated form, in 

 fact, the original catchment, which formerly could be treated as 

 a whole, must now be considered as consisting of a number of 

 catchments, discharging at definite points into the creek. The 

 extent of these sub-catchments must be ascertained, and if a 

 storm- water channel be constructed to supersede the original 

 creek, its size must be proportioned to the several volumes from 

 these sub- catchments discharging into it at definite points along 

 its course. 



Opinions differ as to the value to be given to the coefficient of 

 run-off". As regards the City of Sydney and the western suburbs, 

 the writer is of opinion its value should be a maximum, or in 

 other words unity. There can be no question in regard to this 

 in reference to a city — parks and open spaces being specially 



