Bridge Scour

Bridge scour is of significant concern in the United States, causing approximately 60 percent of all U.S. highway bridge failures (Lagasse & Richardson, 2001).  In 1993 alone, more than 2500 bridges were destroyed or severely damaged due to scour caused by severe flooding (Mueller & Wagner, 2002).  However, scour due to severe flooding is not the only concern.  The high profile catastrophic collapse of the Schoharie Creek Bridge in New York in 1997 in which 10 people died was caused more by the cumulative effect of pier scour of glacial till than the severe flood which ultimately caused its collapse (NTSB, 1988; Lagasse & Richardson, 2001).  In another high profile case that caused seven deaths, I-5 over Los Gatos Creek in California collapsed due to local pier scour during a flood event, but the underlying cause was channel degradation from the previous 28 years of service (Lagasse & Richardson, 2001).  In addition to the unacceptable human loss of life and the direct costs associated with bridge repair, the Federal Highway Administration (FHWA) estimates that indirect costs suffered by the public and local business because of long detours and lost production are five times greater than the direct costs of bridge repair (Lagasse & Richardson, 2001). 

In response to these issues, the FHWA established a national scour evaluation program as a component of the National Bridge Inspection Program, resulting in the development of the National Bridge Inspection Standards (NBIS).  The NBIS requires more than 588,000 U.S. bridges to be inspected every two years for scour and structural stability, and with divers every five years if underwater members are not visible (USDOT, 1991; Lagasse & Richardson, 2001).  In addition, the FHWA has published (and updated in 2001) three reports that define bridge scour technology and provide guidance to state DOTs -  “Hydraulic Engineering Circular No. 18 (HEC-18) Evaluating Scour at Bridges” (Richardson & Davis, 2001), “HEC-20 Stream Stability at Highway Structures” (Lagasse et al, 2001a), and “HEC-23 Bridge Scour and Stream Instability Countermeasures” (Lagasse et al., 2001b).   

The methods documented in HEC-18 are utilized in Michigan by engineers both in the private sector and within the Michigan Department of Transportation (MDOT) to predict bridge scour for the design of new bridges as well as for the evaluation of existing bridges.  However, there is documented evidence that suggests HEC-18 does not accurately predict scour for many regions of the country (Richardson & Davis, 1995; Mueller & Wagner, 2002).  Therefore, it is of specific interest to MDOT (as well as other state DOTs) for the equations to be modified and/or calibrated to yield more accurate calculations of scour for Michigan specific conditions.