HIGHWAY BRIDGE LOCATION 
13 
MAINTENANCE CONSIDERATIONS AFFECTING BRIDGE LOCATION 
SCOUR AROUND PIERS 
Probably one of the largest single items of maintenence cost for 
bridges is "the protection of footings against scour, or the repairing 
of damage already done by erosion. It is, therefore, desirable to 
so select the site for each crossing as to eliminate such tendencies, as 
far as possible. 
THE TENDENCY HERE IS TO ERODE 
UNLESS BOTTOM IS VERY SOLID 
AND RESISTANT. 
IF STREAM BED IS SOFT OR IF STREAM 
CARRIES HEAVY AND SHARP SEDIMENT 
LOAD A POT HOLE IS LIKELY TO BE FORMED 
AT THIS POINT. 
Fig. 12.— Stream profile showing points at which erosion is likely to occur 
In general, scour or erosion may be expected: 
(1) Wherever there are vertices in the stream bed profile in connec- 
tion with a stream bed of erodable material. Figure 12 illustrates a 
stream profile of this land. Generally a rapid will be observed just 
below the grade vertex. This rapid will generally possess a tendency 
to move upstream as the vertex behind it is eroded back. Just below 
the rapid the erosion of a pot hole is quite possible. The cause of 
pot-hole formation generally lies in the occurance of eddies or whorls 
carrying sharp cutting sediment. 
Fig. 13. — Action of drift accumulation in causing scour 
(2) When any artificial obstacle like drift or ice deflects flood 
currents downward against the footings. (Fig. 13 and pi. 2, A.) 
(3) When the bottom, for a considerable distance below the sur- 
face is very soft, or, in case of harder bottoms, when the stream car- 
ries a large quantity of sediment which acts as an abrasive. In this 
latter case, there must of course be sufficient current velocity to carry 
the sediment load else bar formation will occur. 
