Construction methods · Site investigation and testing

Scour investigation and hydraulic study

Before a pier goes into a river, the river is measured: discharge, velocity, bed material, historic bed changes and the scour that the design flood will cause at each pier and at the contracted waterway. More bridges fail by scour than by any structural cause.

How bridges get built · Site investigation and testing · Scour investigation and hydraulic study

Also called: hydraulic analysis, bed material sampling, flood modelling

How it is done

Gather discharge records and derive the design flood (typically 100-year for design, 500-year for check).
Survey the channel and sample the bed material; look for armouring and for the depth of erodible sediment above rock.
Model the flood profile and velocities; compute local pier scour, contraction scour and long-term degradation (HEC-18 or the local method).
Set pile embedment below the total scour depth with a margin, or design scour protection.

Where it fits

Every river, estuary and tidal crossing; also canals with fast flows.

Where it does not

Using a single average velocity for a braided or meandering river; the thalweg can move to any pier.

Plant, pace and money

PlantSurvey boat with echo sounder, ADCP for velocities, bed samplers.

ProductivityA few days of fieldwork and a few weeks of modelling.

CostA small fraction of the project; the game's scour check is HEC-18 and it is the most common reason a design is marked poor.

Risks and controls

What goes wrong

Bed changes during construction (cofferdams narrow the channel).
Debris rafts on piers increasing effective pier width.

Quality assurance

Post-construction bed survey and a monitoring plan with a trigger level.

Examples

Schoharie Creek 1987 (US) and many monsoon-region collapses are scour failures.