HIGHWAY BEIDGE LOCATION 
17 
points, and preparing estimates of cost for the necessary wing dams, 
revetments, dikes, retards, and other control and protection work 
necessary at the various possible sites, balancing the cost of all such 
protection work against the cost of spanning the entire flood plain 
and letting the stream run where it will. If a comparatively narrow 
point in the flood plain can be utilized, this last method may prove 
the cheapest solution. The effect of unchecked channel movement 
is clearly illustrated in Plate 3, A. Plate 3, B illustrates the usual 
zigzag current effect produced by wing dam installations. The cross- 
current effect induced by the formation of sand or gravel bars is 
indicated in Plate 4, A. Plate 4, B illustrates a further stage of 
development wherein the bars have been converted into permanent 
islands and channel currents are deflected thereby. 
Figure 16 is a map of the Willamette River at Harrisburg, Oreg., 
showing the position of this stream in 1894, 1909, and 1923. This 
stream may well be taken as exemplifying the characteristics of 
shifting stream channels above described. The stream itself flows 
PROGRESSIVE MOVEMENT DOWNSTREAM 
Fig. 16.— The Willamette River at Harrisburg, Oreg., has a shifting channel which presents a 
difficult problem in bridge location 
in a broad flood plain and is fed by mountain tributaries which 
deliver to it during flood periods more load than it can carry without 
erosion. The formation of bars at points of low velocity and the 
constant shifting and changing of the channel is, therefore, greatly 
accentuated, especially along the upper reaches of the stream. 
A drawbridge crossing the river somewhere near point B is now 
under consideration and the writer has made a rather extended study 
of the stream in this connection. The high bank on the east shore is 
subjected to the eroding action of cross currents at B, but at high 
water these currents shift in direction as indicated, impinging at B'. 
It thus seems advisable to place a bridge structure above B' in order 
to place the bank erosion at flood periods, below rather than above 
the structure. As against this argument, it will be observed that the 
tangent D D' has for the last 29 years been progressing steadily north- 
ward, so that a location at B" would be more apt to remain intact for 
a longer time. The curved bank C B is undoubtedly going to progress 
downstream or northward, endangering the roadway and also the 
