SUMMARY

The American Association of State Highway and Transportation Officials (AASHTO) Drainage Manual (2014) provides a template and guidelines that facilitates state transportation agencies in the development of a highway drainage design manual. Since 1991 this document has set a high standard of technical excellence and practical value for many state and local agencies completing highway drainage design.

The AASHTO Highway Drainage Guidelines (2007) also has a long and valuable history. Shortly after the formation of the AASHTO Technical Committee for Hydrology and Hydraulics (TCHH) in 1970, preparation began on guidelines covering major topics in highway drainage design. Over a period of 20 years a total of fourteen volumes covering specific highway drainage guidelines were produced by TCHH.

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 written 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. To address these concerns, National Cooperative Highway Research Program (NCHRP) Project 20-07/417 (NCHRP, 2019) evaluated options for updating both the ADM and the HDG to provide the technical content updates needed, but also ways to provide faster implementation of new technology in the future. 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 a new highway drainage manual. Results from NCHRP 20-07/417 also recommended a new highway drainage manual be published as a 6-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.

Given this background, 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 was to be developed in a format that would enable faster implementation and updates to more efficiently incorporate future research findings, emerging issues and technologies, and new design responsibilities.

The result of this research was a new highway drainage manual based largely on the results and recommendations of NCHRP 20-07/417 and the guidance and information provided by the NCHRP Panel. The research results reflected in the 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.”

Technical content in the 2014 ADM was updated throughout the new highway drainage manual, including more information on software options based on the design tools available in the public domain. Document organization and chapter titles were based largely on the outline suggested in NCHRP Project 20-07/417 with the exception that Chapter 7 “Surface Water Environment” was deleted, and its content moved into other chapters. Additionally, the glossary published with the HDG, and referenced in the 2014 ADM, was updated, and will now be included as Part 7 of the new highway drainage manual (six technical parts plus the glossary). A major addition to the new manual was the development of three new chapters: Resilience, Aquatic Organism Passage (AOP), and Stormwater Best Management Practices (BMPs).

Another valuable contribution of the project to the transportation industry was the person-to-person survey completed. Survey questions were developed to guide the discussions and provide consistency in the topics covered. The interviews were conducted by key Research Team members by telephone or videoconference, with the interviewer making notes on the information provided for each question. A total of forty states participated in the survey representing an 80% return rate. A summary of the results and the completed questionnaires was compiled for easy reference and comparisons. The results of the survey not only provided valuable information for updating chapters in the 2014 ADM but will also provide useful insight for subsequent NCHRP and AASHTO projects.

A complication for the project was the number of Federal Highway Administration (FHWA), AASHTO, NCHRP projects, and other reference documents that were also being updated and/or completed during the same time as this project. Most notably those documents included:

• AASHTO publications:
  • LRFD Bridge Design Specifications (AASHTO, 2020)
  • Roadside Design Guide (AASHTO, 2011)
• FHWA publications:
  • HEC 22 Urban Drainage Design (FHWA, 2024)
  • HEC 18 Evaluating Scour at Bridges (FHWA, 2012)
  • HEC 20 Stream Stability at Highway Structures (FHWA, 2012)
  • HEC 23 Bridge Scour and Stream Instability Countermeasures (FHWA, 2009)
  • HEC 16 Highways in the River Environment (FHWA, 2023)
  • HDS 7 Hydraulic Design of Safe Bridge (FHWA, 2012)

• NCHRP projects:
  • NCHRP 24-49: Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models (NCHRP, 2024)
  • NCHRP 15-61a: Hydrologic and Hydraulic Design Guide to Incorporate Climate Science in Transportation Infrastructure (expected completion 2026/2027)
  • NCHRP Project 15-80: Design Guide and Standards for Infrastructure Resilience (expected completion 2024)
  • NCHRP Project 20-44(44) Implementing the Design Practices Guide for Applying Climate Change Information to Hydrologic and Coastal Design of Transportation Infrastructure (expected completion 2026/2027)
• National Oceanic and Atmospheric Administration (NOAA) publications
  • Atlas 15 Updated Precipitation Frequency Standards (expected completion 2027)

Ultimately, the best available information was used in this project to develop the new highway drainage manual, recognizing that the document developed will need to be updated soon. The strategy of developing the new manual in six technical parts plus a glossary will allow future updates without having to republish the entire manual.

Another issue was the varying design practices used throughout the country on many important topics. Historically, the document began as a model drainage manual (referred to as the MDM) where blanks were provided in the document so that a state could substitute local standards or design practice as needed when using the MDM as the basis for a state drainage design manual. When the format of document shifted in 2014 to more of a resource document as the ADM, it began to read more as an AASHTO recommended design practice manual. For some topics (e.g., scour analysis, resilience, BMP, or AOP design) this somewhat subtle change in the document format made it inherently more difficult to reach a consensus on what should be included in the new highway drainage manual. This document addressed that issue as needed by providing an overview of the design methods available without making recommendations.

The project successfully completed the research objective by providing a new highway drainage manual based on the latest technology and design practices. Just as importantly, the modular document structure used will facilitate future revisions, which will be required more often as the speed of technology improvements continues to increase.