Synthesis of Information Related to Highway Practices. Topic 57-02. Application and Selection of the Rational Method for Hydrologic Analysis of Watersheds

State departments of transportation (DOTs) must accurately estimate stormwater drainage to design infrastructure that minimizes risks such as roadway flooding, 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. There is no uniform agreement on the maximum drainage area for which the Rational Method 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, and many state DOTs do not specify when more advanced hydrologic methods should be applied. Complexities may include the presence of tributaries, nonuniform and longer duration rainfall, soil and land use variability, and storage within the watershed. Objective: The objective of this synthesis is to document state DOT policies, practices, and guidance in the application of the Rational Method for hydrologic analysis of watersheds. Information to be gathered includes (but is not limited to): (1) State DOT written policies and guidance on the applicability of the Rational Method, including: thresholds or limits for drainage area; and when the Rational Method is explicitly not used or is not the preferred approach, the alternative methods used, and related guidance; (2) Common practices for determining key input parameters such as basin area, runoff coefficient, time of concentration, and rainfall intensity – duration - frequency data; (3) The use of comparative analyses to determine the appropriate hydrologic method, and criteria to determine the selected method; (4) How the presence of tributaries, nonuniform and longer duration rainfall, soil and land use variability, storage within the watershed, or other factors influence the selection of the Rational Method; (5) Resource considerations, including relative effort, staff time, data, tools, and costs when selecting and designing with the Rational Method compared with other hydrologic methods; (6) Variations in practice across state DOTs and the implications of these differences for project design and cost; (7) Typical applications of the Rational Method, including project types, infrastructure categories, and required outputs (e.g., peak flow estimates vs. full hydrographs); (8) Use of the Modified Rational Method, including how it is used. 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. Information sources (partial): (1) Butler, David, et al. Urban Drainage. 4th edition. CRC Press: Boca Raton, 2018. (2) Crobeddu, E., et al. “Improved Rational Hydrograph Method.” Journal of Hydrology, Vol. 338, pgs. 63–72, 2007. (3) Guo, James. Urban Flood Mitigation and Stormwater Management. CRC Press: Boca Raton, 2019. (4) Jain, Sharad and Vijay Singh. Engineering Hydrology: An Introduction to Processes, Analysis, and Modeling. McGraw-Hill Education: New York, 2019. (5) Karamouz, M., et al. Hydrology and Hydroclimatology: Principles and Applications. Boca Raton: CRC Press, 2013. (6) Mile High Flood District. Urban Storm Drainage Criteria Manual. Vol. 1 – Management, Hydrology, and Hydraulics. Denver, 2018. (7) State DOT drainage design manuals. (8) U.S. FHWA. Highway Hydrology. Hydraulic Design Series No. 2 (HDS-2). Washington, DC, 2002. (9) United States Geological Survey. Comparison of Peak-Flow Estimation Methods for Small Drainage Basins in Maine. USGS Scientific Investigations Report 2007-5170. Prepared in cooperation with the Maine Department of Transportation. Reston, VA, 2007. (10) The Development of Hydrological Concepts in Britain and Ireland Between 1674 and 1874. (11) https://ocw.camins.upc.edu/materials_guia/250144/2015/MetRacio1.pdf (12) “Sizing of Highway Culverts and Bridges: A Historical Review of Methods and Criteria”, Bruce M. McEnroe, The University of Kansas, Report No. K-TRAN: KU-05-4.

• Record URL:
  • http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=5979

Language

• English

Project

• Status: Active
• 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:

  Larson, Sandra

• Performing Organizations:

  MAPLE Consulting, LLC

  Auburn, AL  United States 
• Principal Investigators:

  Perez, Michael

• Start Date: 20260701
• Expected Completion Date: 20270630
• 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