6.58 PROCEEDINGS OF SECTION H. 



To obtain '" d '" by the ordinary rules of arithmetic would prove 

 a long and tedious calculation, but " d " can readily be obtained by 

 inv formula — 



55c ^ 



Having obtained " </,"' the engineer should consider whether the 

 result is such as to afford a reasonable margin of safety. 



It should be borne in mind that in the case of very extensive 

 basins, such as those of the Fitzroy and Burdekin Rivers in Central 

 and Northern Queensland, " d " might give satisfactory results if 

 it only amounted to 1 in. or less of rain per twenty-four hours, whilst 

 in the case of comparatively small areas comprised within the same 

 general basin, an}i;hing less ttian 15 or 20 in. might not be sufficient 

 to ensure good results. 



The following examples will illustrate my argument : — 



The quantity of water that passed down the Brisbane River 

 during the maximum height of the great flood of 1893 was equivalent 

 to a discharge of about 3 in. of rain off the entire basin per twenty- 

 four hours. It would not do, however, to assume, in the case of 

 a small tributary of the Brisbane River, draining, say, fifty square 

 miles, that a bridge having a sectional area equal to a discharge 

 of 3 in. per twenty-four hours would give satisfactory results. 



With a mean velocity of five miles an hour, such a bridge would 

 have a sectional area of only 550 square feet, whereas experience 

 has taught me that the sectional area should be fully five times as 

 great in the case of a high-level structure. 



Many formulas have been given to approximately compute the 

 maximum discharge that may be expected from rivers of known area. 



One of the best known of these and the one most commonly used 

 in Indian engineering practice is that of Colonel Ryves, namely, 

 D = C (Mf), where "D" equals the maximum probable discharge in 

 cubic feet per second, " M" equals the area of the basin in square 

 miles, and "C" is a co-efficient according to experience. It has been 

 usual to limit " C" to 800, but from the experience gained in con- 

 nection with the great floods in the Brisbane River of Februaiy, 

 1893, and from other records, I am of opinion that for the coastal 

 rivers of Southern Queensland the value of '' C" may be taken as 

 follows : — 



Value ot " C." 



3,000 



2,750 



2,500 



2,250 



2,000 • 



1,750 



In the flat country of Western Queensland, where the fall often 

 does not exceed one foot per mile, the value of " C" is very much 

 less than on the coast, and may be taken at about 600 for a basin 

 of 1,000 square miles situated in the rolling downs formation of 

 Central Queensland. I would like to give more information on this 

 subject, but I am writing this pa]ier in the country away from the 

 data nA-ailable in mv office. 



