96 Notes on Hydrology. 



consequently lack the art of making figures pleasant, I will 

 as rapidly as possible draw to a close. 



The value of a knowledge of the rainfall in all its varying 

 phases is of special use to the engineer. By knowing the 

 heaviest monthly, weekly, daily, and hourly rain, and 

 also the maximum fall for still shorter periods, he is better 

 enabled to calculate the necessary sizes of bridges, culverts, 

 and water conduits. Of course other data are also necessary, 

 such as the nature of the surface and subsoil, the general 

 inclination of the ground, and the state of the surface. 



The method adopted generally for fixing the dimensions 

 of bridges over large rivers, viz., gauging the velocity and 

 obtaining numerous cross-sections of the largest known 

 floods will not apply to artificial watercourses, and is, in my 

 opinion, unreliable when applied to the partially dry creeks 

 of Australia, for the reason that the channels generally vary 

 very greatly, the sectional area being in some cases very 

 much larger at one spot than probably a short distance 

 lower down. Furthermore, information of this kind is 

 generally unreliable. Excessively high floods may be caused 

 by obstructions which were not noted by the observer sub- 

 sequently removed. Information of this kind is necessary, 

 but it is equally necessary that something of the local 

 hydrology should be also known and applied. 



In my opinion, to estimate the requisite waterway at a 

 certain point, it, is necessary to know the area and form of 

 the watershed ; next the levels to find the time it will take 

 the first drop of water to travel from the greatest distance 

 to the culvert or bridge ; then to know what proportion of 

 the water soaks into the ground, and what portion is held 

 back, and the rainfall. 



As an illustration, suppose an area of five acres, and the 

 greatest distance the water has to travel ten chains, and 

 assume the nature and inclination of the surface to be such 

 that nine-tenths of the water flows off at the maximum 

 period, and the water travels the ten chains in six minutes, 

 we must then know the heaviest rain that falls in six 

 minutes; for it is evident that if the storm only lasts 

 four minutes the rain will have ceased for two 

 minutes before the extreme particle of water will have 

 reached the culvert — hence the flood will not be a maximum. 

 Should the rain be uniformly heavy for eight minutes, it is 

 also evident that the flood will arrive at its maximum in 

 six minutes, stay so for two minutes, and then subside. In 



