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86 SECTION 1 ADMINISTRATIVE ISSUES
Figure 3.5 Sudden collapse of Shershah curved girder bridge in Karachi, Pakistan.
by the author, requires precautionary scour protection at all times based on a scour analysis.
Countermeasures are needed based on HEC-23.
Other examples of bridge failure include a bridge in Ellis County, Texas in 2004 and an-
other major failure in New York (Schoharie Creek bridge located on the NY state thruway)
in 1987.
3. Scour from tidal flow: Tidal scour analysis using procedures outlined in HEC-18 and HEC-25
is required.
• Tidal scour is more common than the fl uvial fl ood flows of shorter durations. The destruc-
tive nature of scour evolves in cumulative smaller steps rather than occurring in a single
fl ood event.
Figure 3.6 Abutment settlement, backwall collapse, and bearing failure.
