Synthesis of Information Related to Highway Practices. Topic 5-02. Rational Method for Hydrology Upper Area Thresholds and Hierarchy of Use

State departments of transportation (DOTs) must accurately estimate stormwater drainage to design infrastructure that minimizes risks such as roadway overtopping and embankment failures. A range of hydrologic methods are available, and method selection influences infrastructure performance and safety. For small watersheds draining to highway rights-of-way, the Rational Method remains widely used due to its simplicity and long-standing application. However, there is no uniform agreement on the maximum drainage area for which it is appropriate. Federal and state guidance, academic literature, and engineering texts present differing limits, leading to inconsistent application. The method’s underlying assumptions also diminish in validity as watershed size and complexity increase, yet many state DOTs do not specify when more advanced hydrologic methods should be applied. The objective of this synthesis is to document state DOT practices in the application of the Rational Method for hydrologic analysis of small watersheds. Information to be gathered includes (but is not limited to): State DOT policies, guidance, or criteria on the applicability of the Rational Method, including thresholds or limits for drainage area; Factors influencing engineers’ selection of the Rational Method versus alternative hydrologic approaches; Typical applications of the Rational Method, including project types, infrastructure categories, and required outputs (e.g., peak flow estimates vs. full hydrographs); Common practices for determining key input parameters such as basin area, time of concentration, and rainfall intensity–duration–frequency data; Resource considerations, including relative effort, staff time, and costs compared with other hydrologic methods; and Variations in standards of practice across state DOTs and the implications of these differences for project design and cost. Information will be gathered through a literature review, a survey of state DOTs, and follow-up interviews with selected DOTs for the development of case examples. Information gaps and suggestions for research to address those gaps will be identified.

• Record URL:
  • http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=5979
• Supplemental Notes:
  • Contract to a Performing Organization has not yet been awarded.

Language

• English

Project

• Funding: $65,000.00
• Contract Numbers:

  Project 20-05, Topic 57-02

• Sponsor Organizations:

  National Cooperative Highway Research Program (NCHRP) Synthesis

  Transportation Research Board
  500 Fifth Street, NW
  Washington, DC    20001

  American Association of State Highway and Transportation Officials (AASHTO)

  444 North Capitol Street, NW
  Washington, DC  United States  20001

  Federal Highway Administration

  1200 New Jersey Avenue, SE
  Washington, DC  United States  20590
• Project Managers:

  Nasri, Arefeh

• Start Date: 20241001
• Expected Completion Date: 0
• Actual Completion Date: 0

Subject/Index Terms

• TRT Terms: Drainage; Hydrologic phenomena; Runoff; State departments of transportation; Watersheds
• Subject Areas: Highways; Hydraulics and Hydrology; Maintenance and Preservation; Planning and Forecasting;

Filing Info

• Accession Number: 01972945
• Record Type: Research project
• Source Agency: Transportation Research Board
• Contract Numbers: Project 20-05, Topic 57-02
• Files: TRB, RIP
• Created Date: Nov 26 2025 5:54PM