CHAPTER 3 Findings and Applications

Literature Review

After reviewing the results and recommendations of NCHRP 20-07/417, including the proposed table of contents, no major changes to the results and conclusions of that project were identified. One topic that required discussion with the Panel was how to address Chapter 7 “Surface Water Environment” in the 2014 ADM. This was a short chapter providing an overview of topics that were addressed in more detail in subsequent chapters. Rather than keeping it as a separate chapter it was decided to merge Chapter 7 material into other chapters. This reduced the total number of chapters from twenty-five, as proposed in NCHRP 20-07/417 table of contents, to twenty-four. The final table of contents used for the new highway drainage manual is provided in Table 1.

A significant update suggested by NCHRP 20-07/417 was the development of three new chapters: Climate Resiliency (title was changed to Resilience), Aquatic Organism Passage, and Stormwater Best Management Practices. Other areas with major updates included design software available, discussion on improved numerical modeling techniques, updated hydrologic methods, updated fluvial geomorphology discussion, improved scour calculation methods, hydroplaning, roundabout design, and coastal engineering.

A complication for the project was the number of AASHTO, FHWA, NCHRP projects, and other reference documents that were also being updated and/or completed during the same time as this project. Relevant documents being revised included the AASHTO LRFD Bridge Design Specifications, HEC 22 (Urban Drainage Design, published in March 2024), HEC 18 (Evaluating Scour at Bridges), HEC 20 (Stream Stability at Highway Structures), HEC 23 (Bridge Scour and Stream Instability Countermeasures), HDS 6 (Highways in the River Environment, published as HEC-16 in January, 2023), HDS 7 (Hydraulic Design of Safe Bridges, NCHRP 24-49 (Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models, published in March 2024), NCHRP 15-61a (Hydrologic and Hydraulic Design Guide to Incorporate Climate Science in Transportation Infrastructure), NCHRP Project 15-80 (Design Guide and Standards for Infrastructure Resilience), NCHRP Project 20-44(44) (Implementing the Design Practices Guide for Applying Climate Change Information to Hydrologic and Coastal Design of Transportation Infrastructure), and NOAA Atlas 15 (Updated Precipitation Frequency Standards). There were also several FHWA Tech Briefs being considered on assorted topics. Technical content from the existing documents was used in the new highway drainage manual recognizing that the concurrent timelines for updates and new documents under development will require the new manual to be revised again soon. The strategy of developing the new highway drainage manual in six technical parts plus a glossary will allow future updates without having to republish the entire document. Ultimately, the best available information was used in this project to develop the new manual, with the information expected to need update or revision based on these concurrent document efforts identified in Chapter 4 below.

Another issue was the varying design practices used on many important highway drainage topics throughout the country. Historically, the document was published as a model drainage manual (MDM) where blanks were provided in the document so that a state could substitute local policy, standards or design practice as needed when using the MDM as a template to develop a state drainage design manual. When the format of document shifted to more of a resource document as the ADM in 2014, 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 issue was addressed as needed by providing an overview of the design methods available without making recommendations.

Survey of State Transportation agencies

The person-to-person survey completed during Task 1 provided significant information for the project. The interview questions were designed to identify information that would be most valuable to the update process. The initial draft of the survey questions was developed by the Research Team and forwarded to the Panel for review and comment. The survey questions were revised based on Panel comments, and then the survey process was initiated by an email invitation. In the email, a one-hour time slot was requested for a telephone or videoconference interview conducted by a member of the Research Team. A PDF of the questions was attached to the email so respondents could preview the interview questions if they wanted. Many states responded quickly, however, those that did not were sent a follow-up email with the Word document of the questions that could be filled in and returned, like a more traditional written survey process, if they preferred that to an interview. This generated some additional responses, both written and by phone.

When completed there were responses from a total of forty states representing an 80% return rate. A Task 1 report on the Person-to Person Survey Results was prepared, including the completed questionnaires and a summary of the results in a spreadsheet for easy reference and comparisons. A summary of the interview process and results is provided in Appendix A. Results of the survey provided valuable information for many of the chapters in the new highway drainage manual and will provide useful insight for subsequent NCHRP and AASHTO projects.

Sample Chapters

The Research Plan included preparing two sample chapters for Panel Review to better define document structure, format, and the level of detail to include before starting work on the entire document. The sample chapters included one based on an update of an existing 2014 ADM chapter (Energy Dissipators) and the other one a new chapter (Aquatic Organism Passage).

Based on NCHRP 20-07/417 three formats were suggested for the updated document based on varying degrees of referencing to existing publications and standards:

• Option 1 was based on the current ADM blended format, where some design information is presented in detail and other information is provided by reference to a source document.
• Option 2 was to minimize the use of referencing to make the ADM a stand-alone document as much as possible, which would increase page length over the existing ADM.
• Option 3 was to increase referencing, particularly when the source document is stable and relatively unchanging, which would decrease the page count from the current ADM.

During NCHRP Project 20-07/417 the TCHH recommended the Option 1 blended approach, given the concern that TCHH should be vetting and recommending any policy and design procedures included in an AASHTO manual, with possible increased use of referencing when appropriate given established design policy and procedures well documented in readily available publications (Option 3). To evaluate that conclusion the Energy Dissipator chapter was updated using Option 1 and the AOP chapter based more on Option 3. A Panel meeting was held to discuss the sample chapters which resulted in the following conclusions:

Development of the New Highway Drainage Manual

After completion of Task 1 and 2, and approval of the Amplified Work Plan, the Research Team initiated Phase II work on the new highway drainage manual. Based on the conclusions from Phase I, the work in Phase II focused on updating the current ADM to meet core scope-of-work requirements defined as:

• Merge back onto one volume published in six technical parts.
• Integrate HDG content, Panel comments, and recent research results.
• Write three new chapters.
• Create a document that is easier to update and keep current in the future.

The Task 3 draft document was developed directly from the marked-up 2014 ADM Word files for each chapter showing the consolidated Panel/TCHH review comments. This allowed the Research Team to make suggested changes and respond directly to comments provided by a wide range of reviewers. The chapter delivery for the first draft (Task 3) was grouped into three intermediate milestones to avoid overwhelming the Panel with the entire document at one time at the required Task 3 delivery date. Chapters submitted under Task 3 focused on technical content and changes, rather than formatting and presentation. Document format issues and creating a final product consistent the AASHTO Publications Guide was completed during Tasks 4 (second draft) and 5 (Final Draft).

Some chapters were impacted by AASHTO and FHWA documents and/or NCHRP research still in progress. Additionally, while milestone delivery made the review process more manageable during Task 3, as work progressed during later milestone dates some information was found that was duplicated, or inconsistent with an earlier chapter, or needed to be moved to a chapter previously submitted for review. Consequently, the Task 4 Revised Draft document was delivered as a single Word file to facilitate review of consistency issues and cross referencing between chapters. Task 5 was delivered in the six separate Word files that will be the basis for publication and distribution. A key objective in Task 5 was to provide a document that minimized work the TCHH might need to do once AASHTO Publications starts the process to finalize and publish the new highway drainage manual, which will continue after the completion date for project NCHRP 24-50.

As noted by the Panel, one issue with the 2014 edition was widespread use of passive voice sentence structure, and how some of that should be changed to active voice and/or imperative voice mood. This was particularly problematic in policy statements and so work in both Tasks 4 and 5 included tightening up the language used whenever appropriate. Given that active voice/imperative mood is often used to demand or require an action to be taken, such language is appropriate in policy statements but is less so in design procedure or methods used, unless there is widespread acceptance and use throughout the country.

Panel Workshops were held after Tasks 3, 4 and 5 to discuss recommended changes. The Task 3 Workshop included discussion on the use of hyperlinks in the new highway drainage manual. The recommendation was to eliminate hyperlinks as they often become broken and would require significant work in future updates to find and correct. Additionally, it was suggested that people routinely use a web search engine to find references online given the problematic nature of hyperlinks. The hyperlink conclusion was presented to AASHTO Publications staff, and they disagreed believing that users know hyperlinks can become broken, but they are still helpful, particularly for their editors when they are getting the document ready for final publication. Publications did note that hyperlinks should not be used anywhere in the body of the document, only in reference lists. A compromise solution was reached where hyperlinks were included only for “key” references, not every citation used.

At the Task 3 Workshop the Panel requested an update to the TCHH at the 2023 Annual Meeting in Atlanta, Georgia. The TCHH meeting identified three major items of interest: MS4 requirements; roundabout drainage design standards; and risk considerations for temporary structures during construction. TCHH formed subgroups to address the first two issues and provided written guidance to include in the Task 4 document. On the temporary structures issue, it was decided to move the existing Appendix 19A, Design Frequency for Temporary Facilities, to the Chapter 8 “Hydrology” and leave Appendix 19B, Risk Assessment Form, in Chapter 19 “Bridges” as the method described was primarily applicable to large, permanent structures, with the caveat that this is “one example” and other research and information is needed.

Most of the major technical and organizational issues were addressed in the Task 3 Workshop. Panel Comments on both the Task 4 and 5 documents were limited and easily addressed at the corresponding Workshops.