Foreword: The Highway Subcommittee on Design@ as part of its charge@ prepares and keeps current publications pertaining to principles@ methods@ and procedures of roadway design@ including practices to protect and to enhance the quality of the environment. As the subject of highway drainage is important to many disciplines and many phases of highway engineering@ the Subcommittee requested and received approval from the Standing Committee on Highways to establish a task force to prepare needed publications on the subject of highway drainage. Consequently@ the Task Force on Hydrology and Hydraulics was established and first met in February 1970@ developed a statement of purpose@ and outlined a program of activity. The purpose of the Task Force is to assist the Subcommittee in developing guidelines and in formulating policy for highway disciplines@ giving due consideration to safety and the environment. Pursuant to this purpose@ the Task Force on Hydrology and Hydraulics began the preparation of guidelines covering topics on major areas of highway hydraulic design. To the extent practicable@ each Guideline is a stand-alone volume@ but reference to other guidelines in the series is utilized to avoid unnecessary repetition. Since these Guidelines are intended to provide an overview@ discussion@ and design philosophy for each of the covered topics@ technical information is kept to a minimum by making reference to appropriate publications and to the AASHTO Model Drainage Manual which contains recommended design policy@ criteria@ procedures@ aids and example problems. The following Guidelines were originally published as separate volumes: I@ II and III in 1973@ IV in 1975@ V in 1977@ and VII in 1982. Consolidated editions were published in 1973 of I?CIII@ in 1975 of I?CIV@ in 1979 of I?CVI@ in 1982 of I?CVII@ and in 1987 of I?CVIII which was also printed in the current loose-leaf format. The 1992 edition of the Guidelines contains rewritten Volumes I?CIII@ updated volumes IV?CVIII@ and new volumes IX@ X@ and a Glossary. A new@ separate Volume XI was published in 1994. The 1999 edition of the Guidelines is a metric conversion of the 1992 edition and also contains new Guidelines@ Volumes XI@ XII@ XIII@ and XIV@ which are in metric format. This 2007 Edition contains both metric (SI) and U.S. Customary units@ replaces the Volume designations with Chapter designations@ and contains a new Guideline@ Chapter 15. The Highway Subcommittee on Design gratefully acknowledges the efforts of members of the Task Force on Hydrology and Hydraulics in preparing the Guidelines and the cooperation of the States participating in this endeavor.