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Local Pier Scour A worst case estimate of local pier scour can be made using a method developed by Chang from a study of all relevant data and equations. It is an envelope equation that is valid for clear water and live bed scour and produces values of maximum scour depths: y. = [~'J 0.33 1.6 K~ Kz (12) - a Y. = scour depth (ft) a = pier width K~ = pier shape coefficient (Chapter 4) Kz = correction for flow angle of attack (Chapter 4) y~ = depth of flow just upstream of pier Chang found that the maximum scour depths are 2.3 times the pier width. Thus Y. (max) = 2.3 Kz a Where Where debris or ice buildup on piers represents a significant problem, it can be assessed by increasing the width of .a. in the above formula. Local Abutment Scour Determination of local abutment scour using the equations in Chapter 4 requires an understanding of flow depths and velocities, and the flow distribution on the floodplain upstream of the bridge. However, some preliminary judgments may be developed as to the expected scour potential through an assessment of the abutment location, the amount of flow in the floodplain beyond the abutment and the extent of protection provided (riprap, guidebanks, etc.) D. OOCIMENT1NG BRIDGE SCOUR ASSESSMENTS A record should be made of the results of field and office reviews of bridge scour assessments, and item 113 .Scour Critical Bridges. of the revised FHWA Recording and Coding Guide for the Structural Inventory and Appraisal of the Nation's Bridges should be marked for inclusion in the national bridge inventory. The Minnesota OOT is presently developing a comprehensive approach to conducting field and office bridge scour assessments. Their preliminary guidance and forms for accomplishing the evaluations are included in Appendix B. 50 I I r> ( I I 1 I I I I / 'I 1 I I 1 I I I I I