Mechanism: A bridge pier in a body of water has a width perpendicular to water flow. As water hits the pier it becomes turbulent, and any downward force can remove sediment at the pier base. Remove enough support and the pier can become unstable.
Risk factors for a scour hole:
(1) high volume water flow (during flood, heavy rains, etc)
(2) fast-moving water current
(3) soft sediment adjacent to the bridge pier
(4) misalignment of the pier relative to water flow
Prevention of scour hole collapse:
(1) firmly anchor the pier into rock
(2) place a breakwater upstream to the pier such as large stones ("riprap"), concrete, etc
(3) inspect the pier especially after a flood
A scour hole can become filled with sediment, masking the hole. This can be detected by using sonar mapping around the pier.
