Civil Engineering Reference
In-Depth Information
High water level
Base-of-rail
Local scour
D
S
d
g
d
1
General scour
Elevation of bridge
d
g
=
d
d
+
d
c
d
1
Local scour
General scour
Detail at pier footing
FIGURE 3.4
Scour at bridge crossings due to constrictions and obstructions.
3.2.3.2.2 Scour at Bridge Crossings
Once the required bridge opening and general geometry of the crossing is established,
scour conditions at constrictions and obstructions must be investigated in order to
sustain overall stability of the bridge foundations and substructure (Figure 3.4). Scour
can occur when the streambed is composed of cohesive or cohesionless materials.
However, scour generally occurs at a much higher rate for cohesionless materials,
which will be the focus of the present discussion.
General scour occurs due to streambed degradation at the constriction or obstruc-
tionofthewaterwayopeningcausedbythebridgesubstructures(abutmentsandpiers).
Contraction scour (due to opening constriction and/or obstruction) and local scour
(due to obstruction) are also components of total scour that may occur under both
live-bed and clear-water (the upstream streambed is at rest and there is no sediment in
water) conditions. General scour may also occur due to degradation, or adjustment of
the river bed elevation, due to overall hydraulic changes not specifically related to the
bridge crossing. This component of the general scour may occur with live-bed scour
(the streambed material upstream of the bridge is moving) conditions. For prelimi-
nary design scour, degradation can be estimated by assuming that the cross-sectional
area of the degradation scour is equal to the loss of cross-sectional discharge area due
