Current guidance in the Federal Highway Administration’s Hydraulic Engineering Circular No. 18 (HEC-18), “Evaluating Scour at Bridges,” recommends that estimates of total scour at new or existing bridges consist of computing the individual components of scour separately and combining them as a simple summation. Individual scour components at a particular bridge may include any or all of the following: contraction (general) scour, local scour at piers, local scour at abutments, and pressure scour (sometimes referred to as “vertical contraction scour”). These scour processes are considered on an event-based time scale, and during a single flood event all four scour processes, if present, are generally recognized to occur simultaneously. While long-term degradation is not considered to be a local bridge scour phenomenon, its effects are recognized as a system-wide response driven by an imbalance between water and sediment transport and/or a decrease in elevation of a base-level control. Typically, long-term degradation occurring over the life of the structure is estimated and is added to the local scour components as well. The assumption that the calculated scour depths for each individual scour component can simply be superposed to provide an estimate of total scour has been challenged for many years by bridge engineers as being excessively conservative. Specifically, the interaction between scour types remains an unanswered question.
The objective of this research was to determine the relationship between combined, independent estimates of the individual scour components and total scour actually observed for the same event.
The research agency’s final report that documents the entire research effort is available as NCHRP Web-Only Document 249: Combining Individual Scour Components to Determine Total Scour.
A detailed PowerPoint presentation with embedded photos, videos (appear in Slides 19, 20, 21, and 23), and text explanations is available at: http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP24-37_CombiningIndividualScourComponents.pptx