18 BULLETIN I486, U. S. DEPARTMENT OF AGRICULTURE 
approach on the west end. A heavy revetment or a series of wing 
dams between C and B will be needed for the location at B. On the 
other hand, if the structure is placed at B" ', a revetment is needed 
from B" to E, which is much more costly than for one from B' to E. 
Cutting a channel through at point A looks like a possible plan, but 
there is always the possibility that this would so increase the gradient 
of the stream that further erosion along the east bank would result. 
This would damage the village of Harrisburg to a great extent. There 
are many arguments to abvance for different locations and different 
methods of procedure. The point to be emphasized, however, is the 
necessity for a great deal of preliminary field work and preliminary 
study before locations are made across meandering or shifting streams. 
EFFECT OF ICE AND DRIFT 
Ice jams or gorges are apt to be destructive to bridges not only 
because of the tendency to erode at banks and around footings, but also 
because such gorges form a great dam holding back the water until 
it reaches a height sufficient to break through the ice, whereupon 
it rushes upon the structure with destructive force. Ice and drift 
dams of this kind are to be feared at bends or kinks in the stream or at 
points of constriction of stream banks, and this feature should be 
thoroughly looked into. The following points should also be taken 
into consideration: 
Water stage at which ice jams are to be feared. 
Maximum probable vertical reach of ice jams. 
Will the formation of jams above the site operate in any way to deflect the cur- 
rents from their natural course or to increase the tendency to bank erosion at 
any point. 
Frequency of jams. 
Possible protection against effect of ice. 
In the matter of drift the following points should be considered: 
Character and size of drift. 
Position of drift in channel at flood stage. 
It has been observed that in the majority of cases the greater 
volume of drift passes through the thread of the channel. At certain 
points this may be the approximate center of the stream, whereas 
at other points drift may hug one of the banks. Back-water cur- 
rents, eddies, and secondary currents may be such as to pile up drift 
at some particular point or against a pier location. Where the struc- 
ture is to be on a continuous grade, from end to end, it may be possible 
to locate it in such manner that the drift will be carried under the 
spans on the high side, thus giving a little more clearance. For 
example, in Figure 17 are shown two possible crossings of a river 
paralleling a city street which fixes the bridge grade on the right. 
At A the drift hugs the left bank, making it possible to secure a 
greater clearance between the lower chord and the projecting roots 
and limbs of logs and trees than is possible at B, since at this point 
drift tends to hug the right-hand bank. If the main river crossing- 
is 1,000 feet wide and if the street grade is the same at C and D, a 
2 per cent grade will give at least 10 feet additional drift clearance 
for line A. 
Velocity of drift. — There are certain locations where it becomes 
nearly impossible to avoid placing the lower portions of arch ribs 
or even truss chords slightly below extreme flood water. Such a pro- 
