Hydraulics & hydrology

Find technical support, training and policy in the fields of hydraulics, fish passage, hydrology and stormwater to ensure our roadways provide a safe, effective and efficient movement of people and goods while maintaining the natural movement and treatment of water throughout our watersheds and water bodies.

Hydraulics Manual M23-03

Design Manual M22-01

Chapter 800 Hydraulics (PDF 3.2MB)

Roadside Manual M25-30

Chapter 630 Parking Area Design (PDF 2.9MB)
Chapter 700 Soil and Soil Amendments (PDF 566KB)
Chapter 710 Erosion Prevention and Sediment Control (PDF 309KB)

Highway Runoff Manual (HRM)

Highway Runoff Manual M31-16

An integral part of the WSDOT NPDES Municipal Stormwater permit (Permit). The Department of Ecology reissued the Permit, on April 5, 2019. WSDOT revised the HRM to maintain equivalency with the Department of Ecology’s Stormwater Management Manuals. 

Upon issuance of the WSDOT Municipal Stormwater Permit in April 2019, WSDOT and the Department of Ecology adopted an implementing agreement calling for WSDOT to apply the April 2019 version of the Highway Runoff Manual throughout Washington State using the conditions in the agreement.

Projects going to AD after March 6, 2020 shall comply with the 2019 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 2019 HRM will not result in compliance with State Water Quality Standards.
  • Projects receiving Design Approval before July 1, 2019 may use the 2014 HRM on the condition that the projects go to AD by June 30, 2022.

The Summary of Major Changes from the 2014 to 2019 HRM (PDF 262KB) document summarizes the major changes to the 2019 HRM.

The HRM 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.

Questions, comments or suggestions on the HRM

As a general rule, direct questions about the content of the HRM to region environmental, hydraulic, water quality, and/or maintenance staff before contacting the Headquarters Hydraulics Section.

Questions about stormwater facilities inventory, stormwater retrofit priorities, NPDES Municipal Stormwater Permit, water quality monitoring/sampling, (use of experimental best management practices (BMPs), and BMP research and development) should be directed to Headquarters Design Office Highway Runoff Program staff.

The HRM will be periodically updated to clarify its content and 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 Alex.Nguyen@wsdot.wa.gov or mailed to:

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

Who maintains the Highway Runoff Manual (HRM)?

The Headquarters Environmental Services Office Stormwater and Watersheds Program and Headquarters Hydraulics Section are jointly responsible for revision and implementation oversight of the manual.

The manual upkeep uses a technical team made up of water quality, stormwater and erosion control specialists, designers, hydrologists, geotechnical and hydraulic 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 staff and contributions by consultants and outside reviewers.

Getting a printed copy

While there are no plans to distribute printed copies, the manual is available.

To order printed manuals, see WSDOT Printing Services.

However, users referencing printed copies and CD ROM versions should continually consult the online version and the Highway Runoff Manual web page for post publication updates to ensure they are using the most current design guidance. 

HRM Electronic Mailing List

To receive e-mails announcing updates, training opportunities, and improvements in design tools, please subscribe to the Stormwater Issues listserv.  Sign up to receive Stormwater Issues e-mail updates

To receive announcements for updates to the HRM, please subscribe to the HRM Update listserv.
Sign up to receive Highway Runoff Manual, M 31-16 email updates.

Additional HRM guidance

What happens when compliance with the HRM is not economically feasible, physically possible, or environmentally responsible?

Section 3-5 of the 2019 HRM includes two pathways for WSDOT address Adjustments and Deviations to the Minimum Requirements. Both pathways go through the WSDOT/Ecology Demonstrative Approach Team.

The 2019 HRM contains BMPs that WSDOT finds acceptable for widespread highway application. However, in recognition that site and project constraints may compel a designer to consider alternatives, the 2019 HRM outlines the process for seeking authorization for alternative BMP options. BMPs approved by the Department of Ecology, but not recommended for highway application are referred to as "Category 1 BMPs" (PDF 1.98MB). Additional information on emerging technology BMPs is also available on the Department of Ecology's website.

What if I want to use a BMP not listed in the HRM?

The HRM only contains Department of Ecology-approved BMPs that WSDOT finds acceptable for highway applications. Section 5-3.6 of the 2019 HRM describes the process for seeking authorization to use BMPs not contained in the HRM.

Where can I find guidance for the design of stormwater management facilities? Is training on the manual available?

WSDOT's stormwater management-related guidance, procedures, and tools can be found on the Hydraulics training page.  Information on HRM training is available online.

Gravel Areas Clarification Scenarios

Gravel placed along the highway is considered impervious surface and PGIS in certain scenarios. See the Gravel Areas Impervious and PGIS Scenarios (PDF 251KB) and review the impervious surface and PGIS definitions in the HRM.

How do I determine the qualifying areas of elevated roads and parking structures for Minimum Requirement 5 (Runoff Treatment)? 

When a brand new fly-over ramp is constructed over the existing highway, in plan view, the flyover ramp area directly over the existing highway would be considered a replaced pollution generation impervious surface (PGIS). This is also true in situations where an existing roadway is proposed to be significantly raised (new pavement greater than 1 foot thick directly on top of old pavement) or elevated without widening. PGIS that undergoes planing or mill and fill operations are not considered replaced PGIS. A parking lot structure that is proposed to be built over an existing parking lot without expansion of the parking area is considered replaced PGIS.

When a roadway or bridge is proposed to be widened over an existing non-pollution-generating impervious surface, that area should be considered new PGIS. An example of this is when an existing bridge spanning a lake is proposed to be widened by one lane; that new lane would be considered new PGIS.

Note: these area determinations of replaced PGIS do not require excavation of existing roadway. 

How do I determine the qualifying areas of elevated roads and parking structures for Minimum Requirement 6 (Flow Control)?

When a brand new flyover ramp is constructed over the existing highway, in plan view, the fly-over ramp area directly over the existing highway would be considered a replaced impervious surface. This is also true in situations where an existing roadway is proposed to be significantly raised (new pavement greater than 1 foot thick directly on top of old pavement) or elevated without widening. Impervious surfaces that undergo planing or mill and fill operations are not considered a replaced impervious surface. A parking lot structure that is proposed to be built over an existing parking lot without expansion of the parking area is considered a replaced impervious surface. When an existing bridge that spans a lake is proposed to be widened by one lane, that new one lane would be considered a replaced impervious surface.

Note that these area determinations of replaced impervious surface do not require excavation of the existing roadway.

See all manuals and standards

14 fish passage projects were completed in 2020

improving access to 54.2 miles of upstream habitat.

11,959 incidents responded to

by WSDOT’s incident Response teams during second quarter of 2021, 15% more than same quarter in 2020.

41 Pre-existing Funds Projects Advertised

during the eighth quarter of the 2019-2021 biennium.