PAPERS READ IN SECTION H. 



1.— THE AREA OF WATERWAYS. 



By JAMES VICARS, M.C.E. 



The question of area of waterway to be provided for carrying off 

 stormwater from a catchment is one bristling with difficulties of no 

 ordinary character. It must, however, be apparent at the outset 

 that the area of waterway to be provided cannot be determined 

 until the flood discharge is known. This latter depends upon so 

 many unknown quantities — quantities which in general practice 

 cannot be approximately determined or even guessed — that it is 

 well to concede at once the futility of assigning definite coefficients 

 or factors for them in estimating flood discharges. 



The elements referred to are the effective average values for 

 the whole catchment area to be assigned to rainfall, absorbing 

 power of the soil, evaporation, slope of surface, and nature of sur- 

 face, such as forest growth or grass or plain country. 



Monsoonal rains may be of less intensity than that of the 

 recorded maximum fall at individual stations, yet may be more 

 generally distributed, last longer, and produce the maximum flood 

 flow. In another case it may be a cyclonic or anticyclonic storm 

 which produces this result. Or it may be a storm of high maximum 

 and great variation which causes maximum discharge. The 

 steeper the slope and the harder and barer the surface, the less the 

 evaporation and absorption ; the more the vegetation and forest 

 growth or sandy or open character of soil, the greater the absorption, 

 and the less the velocity of flow on even steep slopes, while high 

 winds increase evaporation. 



Thus the question seems at first impossible of rational treat- 

 ment in a manner to be at all useful for practical purposes. Under 

 such circumstances, as in so many other problems, the average 

 values of the effects of all these elements may be ascertained as a 

 whole, and only in this way is the problem rationally capable of 

 practical solution. 



The history of failure is one of the best means, as it frequently dis- 

 closes inaccuracy of assigned values or fundamental errors in pre- 

 viously accepted formulae, or makes known exceptional circumstances. 

 Then, the gaugings and records of flood discharges from well-known 

 catchments for water supplies should be of equal, if not greater 

 importance, being more numerous, and the volume of flow being 

 more accurately determined. 



Working from such data various formulae have been devised 

 by various authors according to their conception of the elements to 

 be taken directly into consideration, and of the values to be as- 



