Development of a New Highway Drainage Manual (2025)
Chapter: 1 Background
CHAPTER 1 Background
Problem Statement
The AASHTO Drainage Manual (2014) provides a template and guidelines that facilitates state transportation agencies in the development of a highway drainage design manual. The 2014 publication superseded the AASTHO publication, Model Drainage Manual (AASHTO, 2005). However, the Model Drainage Manual has a long history having been first published in 1991. That document set a high standard of excellence and practical value for all subsequent versions of the document.
The AASHTO Highway Drainage Guidelines (AASHTO, 2007) also have a long and valuable history. Shortly after formation of the AASHTO Technical Committee for Hydrology and Hydraulics (TCHH) in 1970, preparation began on guidelines covering major topics in highway drainage design. The original format was to develop a guideline on a specific topic (e.g., hydrology, culverts, storm drains, etc.) that was a stand-alone volume. The first three volumes were published in 1973, and then over the next 20 years additional volumes were written until a total of fourteen volumes were available covering a wide range of highway drainage topics.
In many ways the Highway Drainage Guidelines (HDG) parallel the AASHTO Drainage Manual (ADM), but the use and purpose of the two documents is distinctly different. The purpose of the guidelines was to provide guidance on hydraulic design considerations that should be made during planning and design of highway drainage facilities. The ADM was intended as a base document to allow development of a state specific drainage manual with policy, criteria, and design procedures (including example problems). However, both documents cover many of the same topics, and both documents are quite large.
The physical size of these two documents, and the wide range of topics included, often meant their content lagged technology changes and advancements due to the effort required to update the complete documents. Given these and other emerging technology and changes in the industry and acknowledging that the rate of technological change will only increase in the coming years, it became critical to provide faster implementation of such changes to transportation engineers and designers. To address these concerns, NCHRP Project 20-07/417 (NCHRP, 2019) evaluated options for updating both the ADM and the HDG. In consultation with the TCHH, that research concluded the current two-volume ADM (Volume 1 provided policy, criteria and standards for each chapter; Volume 2 provided design procedures by chapter) should be combined back into one volume and recommended the HDG be archived with relevant information in that document merged into the new highway drainage manual. Results from NCHRP 20-07/417 also recommended the new manual be published as a six-part document allowing a single part of the document to be updated without having to reissue the entire manual, providing for faster implementation of new technology.
Research Objective
The objective of this research was to produce a new, up-to-date highway drainage manual informed by the 2014 ADM and the results of NCHRP Project 20-07/417 that provides state and local transportation agencies with design policy and guidance and the technical background to support them. The new manual will be developed in modules that enable faster implementation and future updates to incorporate research findings, emerging issues and technologies, and new design responsibilities.
The new highway drainage manual developed by this research project will be provided to AASHTO for possible consideration as a replacement for the 2014 ADM. If adopted, AASHTO will determine the title for the new drainage manual, but for purposes of this report the document developed has been referred to as the “new highway drainage manual.”
