12 
BULLETIN 1486, U. S. DEPAKTMENT OF AGRICULTTJEE 
.Conditions as shown in Figure 10 are apt to be encountered: 
Where a river gorge has become sedimented due to land subsidence or. other 
geologic action converting streams into lakes or bays. 
Where lava flows have filled up river gorges of a former period. 
Where drift deposits in glaciated country have obliterated prior channels. 
Where the river gorge has had its origin in a volcanic fissure. 
Where the river gorge is cutting through an intrusion of soft lava or a deposit 
of volcanic ash. 
PERMANENCY OF CHANNEL 
In general, any movement or shifting of a stream channel is a factor 
which operates to increase maintenance costs rather than first cost of 
construction and this factor will be discussed more fully under main- 
tenance considerations. In the matter of first cost, however, shift- 
ing or meandering streams do many times necessitate a longer crossing 
than would be necessary to carry maximum flood discharge. Cross- 
ings over such streams are also apt to be more expensive, owing to 
( 
DESIGN B 
Fig. 11.— Two designs for the saine location. Design A is based on the assumption that the main 
channel will remain fixed, whereas design B is based on the assumption that the channel may shift 
the necessity for long span and deep pier construction from side to 
side of the flood plain a larger portion of which construction could, 
were the stream stable, be replaced by much less expensive viaduct 
construction. In Figure 11, design A is a typical crossing of a main 
channel having a stable right-hand bank and its entire flood plain on 
the other side. If the main channel is stable and permanent, two 
river spans will suffice and the balance of the flood plain may be 
bridged with viaduct construction at a considerable less cost per 
linear foot. If, however, the stream channel is likely to shift from 
side to side of the flood plain during the estimated life of the bridge, 
construction as shown in design B will be necessitated. In the latter 
case, six stream piers are needed as against three and five steel spans 
as against two. 
In view of the foregoing, a careful study should be made for bridge 
crossings over meandering or shifting streams and a site chosen where 
conditions are to the greatest possible extent favorable to channel 
permanency. 
