Memorandum
U.S. Department of Transportation
Federal Highway Administration

Attached is a package containing the new editions of the FHWA's primary hydraulics publications pertaining to scour, stream stability and countermeasures: Hydraulic Engineering Circular (HEC) 18, "Evaluating Scour at Bridges," fourth edition; HEC 20, "Stream Stability at Highway Structures," third edition; and HEC 23, "Bridge Scour and Stream Instability Countermeasures," second edition. We are sending two sets of these HECs to each State department of transportation under separate cover. The new editions of these publications have been made possible thanks to the joint efforts of the FHWA's Office Bridge Technology, Office of Infrastructure Research and Development, Resource Centers, and National Highway Institute; the United States Geological Survey; consultants; and academia.

Advances in technology obtained through research and two scanning tours have been implemented in the new HECs. These HECs have been converted to dual units and example problems have been added to the major technical topics of each publication. Highlights of these editions are:

• HEC 18 - The fourth edition has been reorganized by topic. New procedures are added for estimating: pier scour in coarse streambed material; scour on complex pier foundations; and pressure flow scour. Guidance is included for developing a plan of action for a scour critical bridge and for evaluating scour in cohesive soils and in rock formations. The riprap countermeasure section for piers and abutments has been moved to HEC 23 and converted to a design guideline so that all scour countermeasures are consolidated and are in consistent format.
• HEC 20 - The third edition includes: new guidance on stream reconnaissance, classification and response; a rapid stream stability assessment method; quantitative analyses techniques for estimating lateral and vertical stability; channel restoration concepts; and geotechnical considerations for bank stability. The countermeasure sections on stream stability presented in former Chapters 5 and 6 were moved to Chapters 3 through 6 of HEC 23 to expand selection and design guidance on stream stability countermeasures.
• HEC 23 - The second edition is now the primary reference for countermeasure selection and design. New material includes: strategies for developing a plan of action for a scour critical bridge; techniques for estimating scour at transverse and longitudinal structures; biotechnical engineering countermeasures; and guidance for scour instrumentation and monitoring (from the former FHWA Demonstration Project 97). Detailed design guidelines for 12 countermeasures are presented in the second edition.

The new editions of HECs 18, 20 and 23 will be provided to participants of the National Highway Institute (NHI) Course 135046, "Stream Stability and Scour at Highway Bridges." The new material added to HECs 18 and 20 will be integrated into the revised NHI Course 135046, which is scheduled for completion in Spring 2002.

HEC 23 will be the resource manual for a new NHI Course, 135048, "Countermeasure Design for Bridge Scour and Stream Instability," which should also be available in Spring 2002. These HECs are posted in the Office of Bridge Technology's website and are available for downloading on the Hydraulics Engineering Publications page. If you have any questions, please contact Mr. Jorge E. Pagán-Ortiz of my staff at (202) 366-4604.

Attachment