The contraction of the flow by the bridge can be caused by a decrease in flow area of the
stream channel by the abutments projecting into the channel and/or the piers taking up a large
portion of the flow area. Also, the contraction can be caused by the approaches to the bridge
cutting off the overland flow that normally goes across the floodplain during high flow. This
latter case causes clear-water scour at the bridge section because the overland flow normally
does not transport any bed material sediments. This clear-water flow picks up additional
sediment from the bed when it returns to the bridge crossing. In addition, if it returns to the
stream channel at an abutment can increase the local scour there. A guide bank at that
abutment decreases the risk from scour from this returning overbank flow. Also, relief bridges
in the approaches decrease the scour problem at the bridge cross section by decreasing the
amount of flow returning to the natural channel.
Other Factors that can cause contraction scour are:
(1)
a natural stream constriction
(2)
long approaches over the floodplain to the bridge
(3)
ice formation or jams
(4)
berm forming along the banks by sediment deposits
(5)
island or bar formations upstream or downstream of the bridge opening
(6)
debris
(7)
growth of vegetation in the channel or floodplain
To determine the magnitude of general scour from a variable backwater requires a study of the
stream system to (1) determine if this condition exists and (2) determine the magnitude of
general scour for this condition. Of particular value in determining if backwater effects exist
and the magnitude of the effects on the velocity and depth is the WSPRO computer model.
The difference in depth between the highest expected bed elevation and the lowest expected
bed elevation for the design discharge is the value of the general scour.
General scour of the bridge opening may be concentrated in one area. If the bridge is located
on or close to a bend the scour will be concentrated on the outer part of the bend. In fact,
there may be deposition on the inner portion of the bend, further concentrating the flow, which
increases the scour at the outer part of the bend. Also at bends, the thalweg (the part of the
stream where the flow or velocity is largest) will shift toward the center of the stream as the flow
increases. This can increase scour and the non-uniform distribution of the scour in the bridge
opening.
Often the magnitude of general scour cannot be predicted and inspection is the solution for
general scour problems. Also, a physical model study can be used to determine general scour.
7.6.1 Contraction Scour
Contraction Scour Conditions. Contraction scour equations are based on the principle of
conservation of sediment transport (continuity). In the case of live-bed scour, the fully
developed scour in the bridge cross section reaches equilibrium when sediment transported
into the contracted section equals sediment transported out. As scour develops, the shear
stress in the contracted section decreases as a result of a larger flow area and decreasing
average velocity. For live-bed scour, maximum scour occurs when the shear stress reaches
the point that bed-material transported in equals the bed-material transported out and the
conditions for sediment continuity are in balance. For clear-water scour, the bed-material
7.8
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