HIGHWAY BRIDGE LOCATION 
19 
cedure is, of course, not the best practice and should in general be 
avoided. If this is impossible, the impact stresses due to drift and ice 
must be calculated and provided for in the design. Where this is to 
be done, a study should be made of the direction and strength of 
flood currents and advantage taken of any slack water or eddy which 
will operate to maintain comparatively still water at the point where 
arch ribs or other structural members are submerged. 
Whether this dipping of chords or ribs below extreme flood level, 
as above described, should receive serious consideration, will of 
course depend in a large degree upon the frequency of high water 
and probable recurrence of extreme floods. In this connection, a 
study of the character of watershed may prove of value. As more 
and more of the watershed is put under cultivation each year the 
SECTION AT LOCATION B 
SECTION AT LOCATION A 
Tig. 17. — Location where a street fixes the elevation of one end of bridge and drift clearance must be 
secured. Crossing A gives much greater clearance than crossing B 
greater will be the proportion of rainfall absorbed through percolation, 
thus lessening the immediate run-off and decreasing the height of 
flood water. Forest denudation, on the other hand, has the opposite 
effect and may cause a more frequent recurrence of floods than 
formerly. As between cut logs and natural drift, the former will 
generally prove much more destructive, owing to the absence of 
any limbs, branches, or root mats which act to cushion the blow. For 
this reason a study of the possibility of future logging operations 
may yield data which will influence the above decision and thus 
modify the general location scheme. For some streams in forest 
areas, the greater portion of the cutting has been done and the 
danger from log rafts or breakage of booms will be a constantly 
decreasing one. For other locations, this contingency must be con- 
sidered and provided for. 
