WATER AND THE ENGINEER. 657 



The maximum flood levels of watercourses cannot alway be found, 

 more especially in the case of large areas of flat country so often met 

 with in Western Queensland, as the light debris left after each 

 flood is soon destroyed by fire, leaving no permanent mark behind. 

 I have observed, however, that the large red, meat-eating ants 

 seldom, if ever, build below flood level, so that their beds may 

 generally be regarded as above the influence of floods. 



Where heavy timber is brought down the evidence of verj^ high 

 floods often remains for many years in the burnt stems of trees 

 where piles of heavy timber and debris have been left on the upper 

 sides of trees growing on the banks or on adjacent flats covered by 

 high floods. 



In the case of large streams where floods may rise 40 to 100 ft. 

 it is always advisable to closely examine the upper sides of the large 

 white gums that generally grow in such channels. It will often be 

 found that the upstream sides of such trees exhibit marks or rough- 

 nesses which, to the experienced eye, indicate bruises caused by large 

 floating logs or trees coming down on the top of high water, and 

 striking the growing trees with sufficient force to knock off or to 

 badly bruise the bark. These marks remain visible for many years, 

 mute evidence of former floods. I could cite cases in my own practice 

 where the only reliable information regarding the height of floods 

 was obtained in this way. 



When determining the height and sectional area of a large high- 

 level bridge over a stream that has not previously been bridged at 

 high level in the vicinity of the proposed structure, it is always 

 advisable, in addition to . such natural evidence as I have referred 

 to, or that may be ascertained by inquiries from local residents of 

 long standing, to investigate the discharge capacity of the proposed 

 structure in relation to rainfall on the whole basin of the stream. 

 Generally the area of the basin can be pretty accurately ascertained 

 by reference to the official maps of the district, near enough, at all 

 events, to enable a veiy close approximation to be made of the dis- 

 charge capacity of a bridge of known sectional area. 



The data required are as follow : — 



Let 'V/" = the available sectional area of the proposed bridge 

 in square feet, a.s measiu'ed at right angles to the direc- 

 tion of the cvu'rent. 



Let " h " = the estimated or computed mean velocity of the 

 stream when in high flood in miles per hour. 



Let "c'' = the area of the basin above site of proposed bridge in 

 square miles. 



Let ■' (/" = the maximum rainfall that could be discharged by 

 the bridge off the basin in inches per twenty-foiu' hom's 

 at the estimated or computed velocity. In this no 

 account is taken of absorption or evaporation, because if 

 a heavy rain storm were to occur when the ground is 

 thoroughly soaked by previous rains practically the 

 whole of the w^ater of the last storm may come down to 

 the bridge. 

 2b 



