7.2 TOTAL SCOUR
Total scour at a highway crossing is composed of three components:
(1) Long-Term Aggradation or Degradation
The change in river bed elevation (aggradation or degradation) over long lengths and time due
to changes in controls, such as dams, changes in sediment discharge, head cuts, daily tidal
flow, and changes in river geomorphology, such as changing from a meandering to a braided
stream. These processes may be natural or human induced.
(2) General Scour
The scour that results from the acceleration of the flow due to either a natural or bridge
contraction or both (contraction scour). General scour may also result from the location of the
bridge on the stream. For example, its location with respect to a stream bend or its location
upstream from the confluence with another stream. In this latter case, the elevation of the
downstream water surface will affect the backwater on the bridge, hence, the velocity and
scour. General scour may occur during the passage of a flood and the stream may fill in on the
falling stage. This type of scour involves the removal of material from the bed and banks
across all or most of the width of a channel.
(3) Local Scour
flow. These obstructions accelerate the flow and create vortices that remove the material
around them.
Generally, scour depths from local scour are much larger than long-term degradation or
general scour, often by a factor of ten. But, if there are major changes in the stream
conditions, such as a large dam built upstream or downstream of the bridge or severe
straightening of the stream, long-term bed elevation changes can be the larger element in the
total scour. Also, scour depths from severe contraction of the flow, (often causing ponding
upstream of the bridge) can be larger than local scour.
(4) Lateral Shifting of the Stream
In addition to the above, lateral shifting of the stream may also erode the approach roadway
to the bridge and, by changing the angle of the flow in the waterway at the bridge crossing,
change the total scour.
7.3 CLEAR-WATER AND LIVE-BED SCOUR
The two conditions of general and local scour are (1) clear-water scour and (2) live-bed scour.
Clear-water scour occurs when there is no movement of the bed material in the main channel
of the stream upstream of the crossing, or the sediment transport in the upstream reach or
floodplain is transported through the bridge opening or local scour holes in suspension. The
increase in velocity by contraction of the flow by the bridge or the acceleration of the flow and
vortices created by the piers or abutments causes the bed material in the bridge opening or at
their base to move.
7.4
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