CHAPTER 4 Conclusions and Suggested Research

Major Changes from the 2014 ADM

The Research Plan was successful in efficiently producing a new highway drainage manual within the required 3-year project schedule. The technical content of the new manual was significantly updated from the 2014 ADM reflecting recent research results, changes in design practice, and more widespread use of engineering software. The three new chapters developed will make a significant contribution to engineering practice and provide needed design guidance to transportation agencies on emerging technologies.

As discussed in Chapter 3, Findings and Recommendations, a major change from the 2014 ADM was merging the document back into one volume, which will be published in six technical parts plus the glossary. Selected information was added from the HDG, which will continue to be available as an archived document but is no longer supported or updated by AASHTO. This included the HDG glossary that was referenced, but not provided, in the 2014 ADM. The glossary was updated and is now included with the new highway drainage manual as the seventh part of the document (six technical parts plus the glossary).

Global changes made throughout the document included creating a consistent format, particularly in the beginning of each chapter which now includes:

The role of the hydraulic engineer was created to address the issue that for some topics in the ADM the hydraulic engineer might not be in charge of the work being done but rather is a supporting team member. For example, the hydraulic engineer typically has the leading role in culvert design but will coordinate development of design plans with structural and geotechnical engineers, a fluvial geomorphologist when unstable channel conditions exist, biologists and environmental scientists when AOP concerns exist, and for tidally influenced waterways a coastal engineer. But on other projects, such as a wetland project, the hydraulic engineer is typically not leading the work, but is an important team member providing the hydrologic and hydraulic information necessary for a wetland specialist to develop the design.

Regarding policy statements, the 2014 ADM was not always consistent in the structure and language used to define policy. Throughout the new highway drainage manual active voice, rather than passive voice, has been used whenever possible, particularly for policy statements. For example, in passive voice the statement that “The exact location of the sampling will be determined by the Engineer” in active voice would be changed to “the Engineer will determine the exact location of the sampling,” making the engineer the subject of the sentence. Furthermore, imperative mood has generally been used when it was necessary to describe responsibilities of a contractor during construction. For example, the statement that “a mechanical broom or sweeper shall be provided” would be changed to “provide a mechanical broom or sweeper” when using imperative mood. While this publication has been developed to provide hydraulic engineers with a working knowledge of hydrology and hydraulics, AASHTO recognizes that where state standards, policy, and/or technical procedures have been developed in accordance with both local laws and engineering conditions that the state standards prevail.

A summary of major changes in each chapter follows. While many changes were made throughout the entire document in terms of editing, formatting and minor technical updates, only significant changes are discussed below. Technical changes not described often represented supplemental knowledge and experience factors provided during detailed review of draft chapters by Panel members, particularly as it related to practical matters in design and construction. If a chapter is not listed the changes were not considered major.

Chapter 2 – Legal and Permitting

• Addition of National Marine Fishery Service under federal agencies that implement federal regulations affecting highways.
• Addition of Endangered Species Act.
• Addition of Safe Drinking Water Act.

Chapter 5 - Software

• Software names, features and sources were updated to reflect what is currently being used by practitioners.
• Software version numbers are not included as software is continuously updated.
• An additional section for software used in coastal zones was added.
• A decision was made to remove all proprietary software and a statement was added that all software referenced is public domain software or available to transportation agencies through licensing agreements provided by FHWA.

Chapter 7 – Wetlands

• Reorganized and updated chapter based on current regulatory standards.
• Removed Sections 7.3.3.2 “Seasonal Patterns” and 7.3.8.9 “Island Wetlands.”
• Renamed Subsections titled “Chemical Regime” to “Salinity,” and “Pollutants” to “Chemical Pollutants.”
• Added references to Chapter 16 “Stormwater Best Management Practices.”

Chapter 8 - Hydrology

• Addition of hydrologic model taxonomy and procedure selection to aid in model choice.
• Added section 8.3.2 on “Extreme Events, Climate Change and Resilience Considerations.”
• Expanded on linear interpolation versus linear regression and when to use each method.
• Expanded on rational method variations and time of concentration calculations.
• Discussion added on Natural Resource Conservation Service (NRCS) Updated Rainfall Distributions based on NOAA Atlas 14.
• New Section 8.9, “Design Frequency for Temporary Structures,” based on content previously included in Chapter 19 “Bridges,” Appendix 19a.

Chapter 9 – Resilience (new chapter)

• A 5-step process for incorporating resilience into hydraulic related transportation infrastructure assets was included.
• A summary of research projects that are completed, ongoing, or planned that focus on transportation infrastructure resilience was included.

Chapter 10 – Channels

• Updated to emphasize the use of 2D hydraulic models.
• Additional section on importance of terrain for use in hydraulic model development.
• Additional guidance for Manning’s n value selection.
• Added FHWA Hydraulic Toolbox as an available tool to assist with various calculations.
• New sections on two-dimensional analysis provide guidance for domain study limits, mesh development, and boundary conditions.
• Additional guidance for when three-dimensional analysis or physical modeling may be appropriate.

Chapter 11 - Fluvial Geomorphology

• Significant rewrite with a change of focus from binary stream stability diagnosis to wholistic stream system response to categorize stream sensitivity, meta-stability, and threshold concepts.
• Incorporated elements from the recently updated FHWA HEC-16.
• General guidance for highly dynamic geomorphic and hydraulic factors are listed from HEC-20 to aid in identifying potentially vulnerable infrastructure.
• Several stream environments that offer unique challenges to infrastructure are also listed and discussed.
• Several applied examples to illustrate the uses and limitations of stream response models.

• Section on stream reconnaissance divided into its two major components: desktop and field reconnaissance.

Chapter 12 – Aquatic Organism Passage (new chapter)

• The Research Team had significant discussions with the Panel about what to include and how to format this chapter, given the wide range of methods and significance assigned to AOP nationwide, as revealed during the person-to-person interviews. Ultimately, the chapter was structured to provide an overview of common practices used nationwide.
• These practices were grouped into three categories: stream simulation surrogate methods (both geomorphic and hydraulic surrogate methods), geomorphic design methods, and hydraulic design methods.
• Each method was outlined with references to commonly used design guidelines.
• In addition to new water crossings, a section on retrofits was included.
• A section on construction of streambed and habitat features best practices was included.
• A section on the importance of monitoring was included.

Chapter 13 – Culverts

• Cross reference as needed to the new Chapter 12 “Aquatic Organism Passage.”
• New information on use of filter diaphragms to mitigate seepage along culvert barrels included to supplement use of seepage collars.
• Deleted long-hand (nomograph) culvert design examples, providing a step-by-step overview and a reference to FHWA HDS 5 for examples.

Chapter 14 – Energy Dissipators

• Addition of upstream drop structures.

Chapter 15 – Storm Drainage Systems

• Expanded discussion on hydroplaning that included:
  1. Hydroplaning study by ARGONNE National Laboratory.
  2. NCHRP Project 15-55, Guidance to Predict and Mitigate Dynamic Hydroplaning on Roadways, 2021, which developed a software tool for assessing hydroplaning potential.
  3. Hydroplaning study by Florida DOT, Hydroplaning on Multi-Lane Facilities, 2012.
  4. Hydroplaning assessment tool developed by North Carolina DOT.
• New procedure for determining the capacity of curb opening inlets on grade provided by FHWA Publication FHWA-HRT-22-061.
• Added discussion of conveyance and storage in deep tunnel systems and concerns for entrained air mixed with plunging flows and needed ventilation to prevent geysers.
• New section on drainage of roundabouts.

Chapter 16 – Stormwater Best Management Practices (new chapter)

• The Research Team had significant discussions with the Panel about what to include and how to format this chapter, given the wide range of methods, regulatory requirements, and design approaches being implemented across the country.
• Included a section on State and Federal Stormwater Policy.
• Provided stormwater management planning guidelines.
• Design recommendations are provided including managing stormwater runoff project by project, using watershed approaches/regional facilities, and mitigation banking.
• Covered common analysis approaches for runoff peak rate, volume, and water quality management.
• Included a discussion of pollutants of concern and effective treatment processes for each.
• Reviewed design challenges for linear transportation systems and stormwater management in karst regions.
• Summarized soil analysis and infiltration testing protocols.
• Introduced and provided design guidance for dry detention basins, wet ponds, constructed wetlands, detention galleries, infiltration basins, trenches, and galleries, bioretention facilities, vegetated conveyance systems, landscape restoration, filter strips, media filters, open graded friction courses, permeable pavement, and regenerative stormwater conveyance.

Chapter 17 – Storage Facilities

• Information was added from the Minnesota Department of Transportation on clear zone guidance for detention ponds in interchange borrow pits.
• Refocused chapter discussions around the three items of information needed for hydrologic routing using the Modified Puls procedure: inflow hydrograph, stage-storage curve, and stage-discharge curve.
• Introduced the concept of dynamic routing.
• Information on water quality related ponds (Sections 17.7-17.11) was moved to the new Chapter 16 “Stormwater Best Management Practices” so that Chapter 17 focused primarily on hydrologic routing for detention ponds using the Modified Puls method.

Chapter 18 – Pump Stations

• Introduction of variable speed pumps,
• Significant update in references of revised National American Standards of the Hydraulic Institute.

Chapter 19 – Bridges

• Reorganized and updated chapter to be more in alignment with AASHTO LRFD.
• Added pertinent definitions including scour design flood and scour check flood to be more in alignment with AASHTO LRFD.

• Removed sections that were a copy from HEC-23 and provided references.
• Updates to bridge deck drainage and bridge end drainage sections
• Decision made to move Appendix 19-A (Design Frequency for Temporary Structures) to Chapter 8 “Hydrology” with a cross reference to Chapter 23 “Construction.”

Chapter 20 – Channel and Bank Protection

• Reorganized and updated chapter.
• Removed sections that were a copy from HEC-23 and provided references.
• Included importance of first evaluating stream stability and cross-referenced Chapter 11 “Fluvial Geomorphology.”
• Provided section for traditional hard stabilization techniques and a section for nature-based, bioengineered, and biotechnical techniques.
• Added new section, Protection Extents, which provides guidance for longitudinal and vertical extents of protection.

Chapter 21 – Coastal Zone

• Reorganized and updated the chapter.
• Added new concepts and ideas from FHWA HEC-25 Third Edition (FHWA, 2020).
• Added content on Tsunamis including reference to new AASHTO document Guide Specifications for Bridges Subject to Tsunami Effects (AAHSTO, 2022).
• Expanded the discussion of tidal datums and added reference to NOAA online VDatum conversion tool.
• Added discussion of the Coastal Emergency Risks Assessment (CERA) website and of the USACE ADCIRC software.
• Expanded discussion of Sea Level Rise.
• Added section on application of climate change information, with reference to NCHRP Project 15-61.
• Expanded discussion of road overwashing, incorporating information from HEC-25 Third Edition.
• Added subsection on nature-based solutions with heavy reference to FHWA document Nature-Based Solutions for Coastal Highway Resilience: An Implementation Guide (FHWA, 2019).
• Added multiple references to FEMA coastal zone guidance documents.

Chapter 22 – Temporary Erosion and Sediment Control

• Renamed, reorganized, and updated chapter.
• Defined categories of control measures: site access, soil stabilization, sediment trapping, outlet protection, water control, and construction practices.
• Added 10 control measures commonly used in highway construction, e.g., compost filter sock, rock construction entrance, concrete washout, drainage system protection.
• Replaced all photos; added photos and figures.

Chapter 23 – Construction

• Information added on maintaining positive drainage on the roadway during construction.
• New section on temporary facilities during construction
• New section on Temporary Access Fills (TAF)
• Expanded information on construction timing and use of low-flow information to define discharge and water quality concerns.
• Updated discussion on hydrologic risk for temporary facilities, including reference to the method that was an appendix to the bridge chapter (Appendix 19A) which was moved to Chapter 8 “Hydrology.”

Chapter 24 – Maintenance

• New section on asset management of drainage facilities
• New section on BMP maintenance to parallel new chapter on BMP’s

Future Updates Needed

Given concurrent ongoing research and updated technical documents (AASHTO, FHWA, NOAA, etc.) during this project, the new highway drainage manual developed by this project will need to be updated soon. Ultimately, the best available information was used in this project to develop the new manual. The strategy of developing the manual in six technical parts plus a glossary will allow future updates without having to republish the entire 1,000+ page document. A summary of possible future updates required includes:

• Jurisdictional Wetlands and Waterways and the methods of compensatory mitigation for impacts to these resources have changed often in recent years and depending on future updates the information in Chapter 2 “Legal and Permitting” and Chapter 7 “Wetlands” may need revision.
• Any other changes in federal law, regulations, rules published in the Code of Federal Regulations (CFR), or Executive Orders may require updating information in Chapter 2 “Legal and Permitting Aspects,” and/or in policy statements in various chapters.
• AEP standards in various chapters should be reviewed by TCHH for concurrence and/or updating based on other agency updates (including FHWA, FEMA)
• AASHTO LRFD Bridge Design Specifications – Updates will be needed in the ADM, particularly in Chapter 19 “Bridges.”
• AASHTO Roadside Design Guide - Updates possible throughout the ADM based on safety considerations for various hydraulic structures.
• HEC-26 (Culvert Design for Aquatic Organism Passage) - Determine if FHWA will continue supporting HEC 26 (Stream simulation, hydraulic surrogate method) and whether it should be included in the ADM in Chapter 12 “Aquatic Organism Passage.”

• HEC-18 (Evaluating Scour at Bridges) –may result in new guidance for assessing scour that will impact Chapter 19 “Bridges.”
• HEC-20 (Stream Stability at Highway Structures) – New information may impact material presented in Chapter 11 “Fluvial Geomorphology.”
• HEC-23 (Bridge Scour and Stream Instability Countermeasures) – FHWA is currently updating HEC-23 which may eliminate the need for references to the related FHWA Tech Briefs in Chapter 20 “Channel and Bank Stabilization.” FHWA Tech Briefs also may be outdated due to updates in AASHTO LRFD Bridge Design Specifications.
• HDS-7 (Hydraulic Design of Safe Bridges) – may impact information in Chapter 19 “Bridges.”
• More detailed information for Chapter 9 on climate change and creating resilience design of hydraulic structures based on the following projects:
  • 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”).
  • NOAA Atlas 15 –an update to Atlas 14 that will provide new national precipitation frequency standard that accounts for climate change.

Suggested Research Topics

Developing a new highway drainage manual during this project identified areas where the available technology and engineering practice is currently limited or inadequate. Suggested future research projects include:

• Updated guidance on design frequency for temporary facilities given in Chapter 8 “Hydrology”.
• Nationwide study of coincidental probability of concurrent storms occurring on a main channel and adjacent tributaries to supplement information in Chapter 8 “Hydrology.”
• Design standard for filter diaphragms (commonly used for conduits through dam embankments) for large culvert barrels that are relatively short (creating different site conditions from a typically small conduit through a large dam) for Chapter 13 “Culverts.”
• Rock placement design standards for culvert inlets and outlet locations to include in Chapter 13 “Culverts and/or Chapter 20 “Channel and Bank Stabilization.”
• Additional research to definitively define curb open pavement inlet interception on grade and identify a recommended design procedure in Chapter 15 ‘Storm Drainage Systems.”

• Additional research on interception rates for ADA pavement grates and a recommended procedure to be included in Chapter 15 “Storm Drainage Systems.”
• Stormwater BMP pollutant removal research for the various practices introduced in this update as presented in Chapter 16 “Stormwater Best Management Practices.”
• Funding to support the International BMP Database discussed in Chapter 16 “Stormwater Best Management Practices.”
• Risk Assessment for Encroachment Design Form (Chapter 19 “Bridges”) – This form is outdated.

Implementation Plan

This implementation plan identifies ideas and methods to help move the research results into practice. The audience or "market" for this research is primarily state and local transportation agencies looking for drainage design manuals and standards that reflect the latest technology and advancements in the industry. The ownership and endorsement of the results of this research by AASHTO and particularly the TCHH, is a critical first step in implementation. This should occur seamlessly given the ongoing coordination and involvement of the TCHH throughout this project.

As with the results of any research, there may be segments of the ultimate target audience reluctant to adopt or implement new approaches or methods identified in the new highway drainage manual. The Research Team is committed to promoting the new manual at professional conferences, both at the NHEC (Task 7 in the Research Plan), but also at other conferences and professional meetings in the future. TCHH members will have similar opportunities. Such forums can help generate interest and excitement about the new document and ways it could improve the state-of-the-practice for design professionals.

A unique opportunity is the involvement of the Research Team in training courses that are provided nationwide through the FHWA National Highway Institute (NHI). This work is on-going and will provide multiple opportunities to highlight the new highway drainage manual in the immediate future to end users throughout the country. In a smaller setting, the Research Team consists of practicing consulting engineers working with various state transportation agencies across the country. Through our interactions with transportation hydraulic engineers on specific project design issues we will have opportunities to implement, and/or encourage implementation, of the new manual.

The best criteria for judging the success of the implementation plan will be how quickly state transportation agencies begin to adopt the new highway drainage manual as their primary design reference or to use content and information in the manual in their own drainage design manuals and project work. The desirable consequences of this project will be better engineering design completed in a cost-effective manner, and a potential reduction in the number of failures and/or damage to highway facilities throughout the country.