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Highway Runoff Manual (HRM)

The Highway Runoff Manual represents years of extensive research, collaboration, and negotiation by an interdisciplinary technical team of water quality, stormwater, and erosion control specialists; designers; hydrologists; geotechnical and hydraulics engineers; landscape architects; and maintenance staff. The technical team also includes several county representatives and benefits from a close working relationship with Washington State Department of Ecology (Ecology) staff, with work also contributed by consultants and outside reviewers. The technical team recognized that it is inefficient, and in some instances ineffective, to try to emulate how local jurisdictions manage runoff from residential, commercial, and industrial land uses. Consequently, their approach to developing the HRM takes into consideration that WSDOT:

  • Needs a statewide approach for managing stormwater that recognizes the differences in climate, soils, and land uses.
  • Has limited control of pollutants entering its right of way (including pollutants generated from atmospheric deposition, vehicle operation, litter, organic debris, and surrounding land uses).
  • Highway projects are linear in nature and, as such, are faced with practical limitations in terms of locating and maintaining stormwater treatment facilities within state-owned right of way.
  • Lacks funding mechanisms (such as stormwater utility fees) and land use controls (zoning and land use ordinances) available to local governments.
  • Must be accountable to taxpayers to provide cost-effective stormwater facilities.

The HRM receives periodic updates to enhance content clarity as well as reflect changes in the regulatory landscape, advancements in stormwater management, and improvements in design tools.
Comments and suggestions for improving the HRM should be e-mailed to: or mailed to:

Highway Runoff Manual
Attention: HRM Program Manager
PO Box 47329
Olympia, WA 98504-7329

2014 Highway Runoff Manual
The Highway Runoff Manual (HRM) is an integral part of the WSDOT NPDES municipal stormwater permit (Permit). The Department of Ecology reissued the Permit, on March 6, 2014. WSDOT revised the HRM to maintain equivalency with the Department of Ecology’s Stormwater Management Manuals.

Projects going to AD after March 6, 2015 shall comply with the 2014 HRM except as follows: 

  • Projects requiring an individual Section 401 Water Quality Certification may be subject to additional stormwater requirements if, based on site specific information, the use of the 2014 HRM will not result in compliance with State Water Quality Standards. 
  • Projects receiving Design Approval before July 1, 2014 may use the 2011 HRM on the condition that the projects go to AD by June 30, 2017.

The major changes from the 2011 HRM in the 2014 HRM include:

  • Adding language and flow charts to meet the reissued permit’s low impact development (LID) requirements
  • Editing to enhance the manual’s use as a design manual
  • Revising some best management practices (BMP) to reflect changes approved by Ecology since the last HRM revision
  • Replacing Chapter 6 with a new manual called Temporary Erosion and Sediment Control Manual (TESCM). Chapter 6 now introduces the TESCM and provides a web link to this new standalone manual.

The Summary of Changes to the 2014 HRM summarizes the changes to the 2014 manual in table format. Also available is a table (xlsx, 14kb) that lists the comments Ecology received during the public comment period that resulted in changes to the HRM.

Temporary Erosion and Sediment Control Manual (pdf, 1.2mb)

In addition to the revised HRM, we have developed new tools and spreadsheets to aid in highway stormwater design.

New tools
LID feasibility checklist (xls, 45kb)
CAFVS LID Calculator (xlsx, 23kb)

MGSFlood modeling input (xlsx, 74kb)
Basic and wet biofiltration swale design (xls, 89kb)
Continuous inflow biofiltration swale design (xls, 141kb)
Media filter drain (MFD) underdrain design (xlsx, 12kb)
Slotted pipe flow spreader design (xlsx, 1mb)
Stormwater Design Documentation Spreadsheet for the 2014 HRM (xlsx, 136kb)
Stormwater Design Documentation Spreadsheet for the 2008-2011 HRM (xls 299kb)

Related Links
HRM Implementing Agreement
Stormwater Design NEPA/SEPA Documentation Checklist
ESA Stormwater Design Checklist
Erosion Control
NPDES Permits
Stormwater Research
Water Quality

Implementing Agreement
Upon issuance of the WSDOT Municipal Stormwater Permit in March 2014, WSDOT and the Department of Ecology adopted an implementing agreement (pdf 1.59mb) calling for WSDOT to apply the March 2014 version of the Highway Runoff Manual throughout Washington State.

WSDOT Stormwater Retrofit Guidance

Training on the analysis methods used to design stormwater runoff treatment and flow control facilities has been incorporated into the HQ Hydraulics Section's curriculum. Training on guidance and procedures for temporary construction-related erosion and sediment control facilities has been incorporated into the Environmental Services' erosion and sediment control program curriculum. Training for applying other elements of the HRM is under development.  All consultants who prepare Hydraulics Reports for WSDOT must have completed the 2014 Highway Runoff Manual training course.  2014 HRM training is required for WSDOT staff who write Hydraulics Reports.

HRM Electronic Mailing List
To receive e-mails announcing updates, training opportunities, and improvements in design tools, please subscribe to the Stormwater Issues listserve.
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To receive announcements for updates to the HRM, please subscribe to the HRM Update listserve.
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Frequently Asked Questions (FAQs)
The HRM FAQs page answers a variety of questions about the Highway Runoff Manual.