Figure 7.7. Topwidth of scour hole (HEC-18).
7.7.8 Physical Model Studies
For unusual or complex pier foundation configurations a physical model study should be
made. The scale between model and prototype is based on the Froude criteria, that is, the
Froude number for the model should be the same as for the prototype. In general, it is not
possible to scale the bed material size. Also, at flood flows in sand bed streams the
sediment transport conditions will be live-bed and the bed configuration will be plane bed.
Whereas, in the model live-bed transport conditions may be ripples or dunes. These are
incomparable pier scour conditions. Therefore, it is recommended that a bed material be
used that has a critical velocity just below the model velocity (i.e., clear-water scour
conditions). This will usually give the maximum scour depth; but a careful study of the
results need to be made by persons with field and model scour experience. For additional
discussion of the use of physical modeling in hydraulic design, see Chapter 5 and HEC-23
(Lagasse et al. 2001).
The two causes of scour when the flow is obstructed by the abutment and approach highway
embankment are: (1) A horizontal vortex starting at the upstream end of the abutment and
running along the toe of the abutment, and (2) a vertical wake vortex at the downstream end
of the abutment (Figure 7.8).