Geoscience Reference
In-Depth Information
The flow pattern around a bridge abutment (or spur-dike) also involves downward
flow, localized pressure gradient, and vortices, which induce local scour around the
base of the abutment. The obstruction of the flow forms a horizontal vortex starting
at the upstream end and running along the toe of the abutment and a vertical trailing
wake vortex at the downstream end. The horizontal and wake vortices at the abutment
are similar to the horseshoe and wake vortices at the pier.
Fig. 7.10 shows the flow features often existing downstream of weirs, spillways,
sluice gates, and headcuts. Overflow appears in form of a jet and plunges into the
downstream pool, while underflow appears as a submerged or free horizontal jet.
Flow may separate laterally due to the sudden expansion of channel width. Consider-
able scour occurs in the downstream pool. Vortices exist and affect the magnitude of
scour. The sediment eroded from the scour hole may deposit as a mound downstream,
forming an “S” shaped bed profile.
Figure 7.10
Flow over/under a structure.
The total scour around in-stream structures includes three components: (a) general
degradation or aggradation due to natural or man-induced causes in upstream channels
and watershed; (b) contraction scour due to reduction of the flow area by the struc-
tures and approach embankments; and (c) local scour caused by flow acceleration, jet
impingement, and vortices induced by the structures. The contraction scour and local
scour may be clear-water or live-bed (see Fig. 7.11). The clear-water scour indicates
that no sediment comes from the approach reach, or the sediments transported are
so fine that they wash through the reach. In the clear-water scour process, the scour
depth increases gradually and approaches an asymptotic value when the capacity of
transport out of the scour hole is zero. In the live-bed scour, the scour depth increases
rapidly and attains an equilibrium value when the capacity of sediment transport out
of the scour hole is equal to the one into the scour hole. The live-bed scour can be
cyclic in nature, typically scouring during the rising stage of a flood event and refilling
during the falling stage.
In addition, the channel migration in floodplains may affect the scour around bridge
piers and erode abutments, spurs, and embankments by changing the course of main
flow and the flow angle of attack. Debris accumulation and vegetation growth around
structures may also affect the scour significantly.
