Handbook for Bridge Inspections - TSP2

9.1 ELEMENTS IN THE GROUND
103. Scouring/Erosion of River Course
J~~~~ ~~:'" '" ABOVE WATER
,- 1... ______ _
OR IGINAL LEVEL
........ 1
1... ________ -'
CAUSED BY SURFACE
WATER
Description
Damage in th is category includes erosion oflhe earth both above and below the water line. An
exampl e of the latter could be erosion of the natural river bed or of protection against erosion. If the
erosion is not stopped in time the fou ndations will become under-scoured. On the other hand damage
above the water line can be exemplified by erosion of embankments and slopes in the immediate
area of abutments and columns.
Cause of Damage
Some of the possible causes are as fo llows:
• Faulty design: During the plann ing period inadequate development or lack of attention paid to
the draining ofT of surface wate r. inadequate design of a bridge can result in a narrowing of
the waterway and a consequent increase in water speed.
• Faulty construction: not carried out as prescribed
• Unsatisfactory day-to-day run ning/maintenance; Damming up of the water course due to the
accumulat ion of branches and other debris.
• Accident impact; Erosion/scouring caused by severe water discharge from fl ooding.
• In-service damage; For example the results of changes to the water course fo ll owing work carri
ed out on the ground in the vicinity.
Degree of Damage & its Consequences
Erosion below water level should be all ocated a high degree of damage because it can develop very
quickly. Even small movements of earth around foundati ons can affect the carrying capacity.
Erosion can qui ckly turn into scouring. The degree of damage fo r situations above the water level
should be assessed based on the local conditions and probable developments.
Un derwater erosion can affect both carry ing capacity and maintenance cost.
Carrying Capacity
• Erosion around a bridge opening can reduce the carrying capacity of the foundations when the
earth around them is removed. Foundations without pi les are especiall y vul nerable, and the risk
of under-scouring is great - please refer to th e drawing above. Base slab foundations will always
lose carrying capacity when subjected to underscouring with resultant subsidence.Subsidence in
the foundations produces deformation of the superstructure and can lead to the partial or ful l col
lapse of the bridge in question.