Scour can be deepest near the peak of a flood, but hardly visible as floodwaters recede and scour holes refill with sediment.

Scour can be of two types: (1) clear-water, and (2) live-bed.

Clear-water scours occurs when bed material is not being transported upstream of the crossing.

Live-bed scour occurs when bed material is being transported upstream of the crossing.

Total scour consists of:

1. Long-term degradation of the river bed;

2. Contraction scour at the bridge;

3. Local scour at the pier or abutments; and

4. Possible lateral migration of the riverbed.

Degradation is the scouring or lowering of the streambed over relatively long reaches due to a deficit in sediment supply from upstream.

Contraction scour is a lowering of the streambed across the stream at the bridge. Contraction scour results from the constriction of the flow, which results in the removal of material from the bed across all or most of the channel width. Contraction scour is related to the passage of a flood.

Local scour involves the removal of material from around piers, abutments, spurs, and embankments. It is caused by the acceleration of flow and resulting vortices induced by obstructions to the flow.

During a flood event, bridges over streams with coarse bed material are often subjected to clear-water scour at low discharges, live-bed scour at the higher discharges and then clear-water scour at the lower discharges on the falling stages.

Clear-water scour occurs mainly in coarse bed material streams. Local clear-water scour may not reach a maximum until several floods.

Contraction scour occurs when the flow area of a stream at flood stage is reduced, either by a natural constriction of the stream channel or by a bridge support.

The decrease in flow area results in increase in velocity and sediment transport.

Live-bed scour is typically cyclic; for example, the bed scours during the rising stage of a flood and fills during the falling stage.

Local scour is caused by vortices at the base of obstructions (piers). The action of the vortex removes bed material from around the base of the pier, resulting in a scour hole.

Factors affecting the magnitude of local scour depth at piers and abutments are:

1. Velocity of the approach flow

2. Depth of flow;

3. Width of pier;

4. Discharge intercepted by the abutment and returned to the main channel;

5. Length of the pier, if skewed to the flow;

6. Size and gradation of bed material;

7. Angle of attack of the approach flow to pier or abutment;

8. Shape of pier or abutment;

9. Bed configuration; and

10. Ice formation or jams, and debris flows.

The greater the velocity, the deeper the scour.

An increase in flow depth can increase scour depth by as much as a factor or 2 or greater for piers.

Pier width has a direct influence on depth of scour. As pier width increases, there is an increase in scour depth.

Fine bed material (silts and clays) will have scour depths as deep as sand-bed streams. The effect of cohesion is to influence the time it takes to reach maximum scour, not the depth.

The maximum depth of scour at an embankment inclined 45° downstream is reduced by 20%; whereas the maximum scour at an embankment inclined 45° upstream is increased about 10%.