30 
BULLETIN I486, U. S. DEPARTMENT OF AGRICULTURE 
expensive portion of the bridge) will for equal navigable capacity, 
need to be of a length given by the formula 
L' = 
cos <p 
+ W tan cf> 
as shown in Figure 26. It will also be observed that line A places the 
movable span near the north bank and if the grade is fixed by certain 
r 
/ 
Fig. 26.— The effect of current direction on location and length of movable span 
bank points A and A', as is very apt to be the case, it will not be 
possible with the same ruling grades to place the movable span as 
high on line A. This means a more active bridge at line A and a 
greater inconvenience to traffic. 
The above is but one of many instances illustrating the need for 
special care in preliminary location study for navigable crossings. 
At best the navigation of a large boat through the comparatively 
restricted horizontal opening afforded by a movable span is a task 
requiring skill and judgment. Complicate this by a location where 
