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Wetland Delineation and Assessment

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Hydric soil is one factor of wetlands.



Wetland Data Needs

Please consult the WSDOT regional environmental office to determine whether a Wetland Inventory or Wetland Delineation is appropriate for project needs. Spring and early summer are the ideal time of year for wetland field work. Early coordination with your environmental staff is important to get your work appropriately scheduled.

During Transportation Planning (pdf 312 kb), it may be prudent to request a Wetland Inventory to provide field evidence to confirm the presence or absence of wetlands in the potential project alternative alignments. Relying on the generalized information contained in the GIS Workbench may be insufficient to plan advance mitigation and the early stages of some projects. 

During the Project Scoping (pdf 426 kb) phase, a Wetland Inventory is recommended to provide field evidence to confirm the presence or absence of wetlands in the potential project alternative alignments. Relying on the generalized information contained in the GIS Workbench may result in mis-classifying the project or poorly estimating mitigation needs. 

During Environmental Review (pdf 265 kb), a survey of delineated wetlands is required unless a recent wetland inventory proved wetlands were absent in the project corridor.  Accurate wetland boundaries are needed to evaluate the effects of project alternatives. 

After a permittee-responsible mitigation site has been approved for use, a Wetland Delineation is required to inform the mitigation design.  The digital delineation should be provided to the Wetland Monitoring Manager.

During the Construction phase (pdf 251 kb), at an early to mid-term point in the monitoring period, wetland mitigation sites are delineated to verify that the correct amount of compensation will be provided. A final wetland delineation is conducted at the end of the monitoring period to confirm the correct amount of mitigation has been provided.


Wetland Inventory

Procedure 300-a and Task 300-a (pdf 41 kb) describe how WSDOT wetland biologists or qualified consultants perform a Wetland Inventory.  The Wetland Inventory memo provides general information to assist planners to determine or compare project alternatives. This work can take from 20 hours for a small area with few wetlands to more than 60 hours for a large area. This estimate does not include survey work.

Wetland Assessment

A Wetland Assessment report includes:

  1. A detailed field evaluation and survey of wetland boundaries (delineation).
  2. How easy the wetland would be to replace (rating).
  3. What functions the wetland performs
  4. Information on other aquatic habitats, when present.

A Wetland Assessment report does not need updating unless the project area changes, or the field work was conducted more than 5 years ago. If the project area expands into an area not evaluated by the wetland delineation field work, wetlands and buffers should be re-evaluated by a wetland expert to avoid unpermitted wetland activities or disturbance to the buffer of a nearby wetland.

Procedure and Task 431-a (pdf 47 kb) describes how WSDOT wetland biologists or qualified consultants perform Wetland Assessments. Consultant contracts may be based on a Generic Scope of Work for Delineation (doc 131 kb) template.  

1. Wetland Delineation

A wetland delineation shows the boundaries between a wetland and the adjacent upland. The wetland map has survey-level accuracy.  Ecology provides additional information on their Wetland Delineation web page. This work can take from 80 hours for a small area with few wetlands to several months work by several delineators for a corridor.  

A wetland delineator working for WSDOT must be trained and have appropriate experience. WSDOT delineators follow all WSDOT guidance on specific delineation issues unique to roadways, and are encouraged to use other resources in the Delineation Toolbox.  

Ecology's Isolated Wetlands web page provides information about regulation and permitting for isolated wetlands. WSDOT provides guidance on permitting isolated wetlands (pdf 130 kb). 

2. Wetland Ratings

WSDOT uses the Department of Ecology’s Washington State Wetland Rating Systems to categorize wetlands based on their sensitivity to disturbance, significance, our ability to replace them, and the functions they provide. When determining which system to use, refer to the Boundary Between Eastern and Western Washington for the Wetland Rating System (pdf 55 kb).

Note: In Oct. 2008, the Department of Ecology updated the above Rating System manuals with new WDFW priority habitat definitions. Use Version 2 of the ratings forms supplied by Ecology or the format-modified versions below. Annotated versions are also available from Ecology, but they do not include the WDFW priority habitat definitions.

WSDOT also applies any applicable county and city wetland rating systems defined in their Critical Areas Ordinances. Wetland Biologists must check local requirements. The WSDOT Delineation Toolbox contains additional ratings resources.

3. Wetland Functions

Executive Order EO 89-10 (pdf 19 kb), Protection of Wetlands, includes a no net loss goal that includes both area and function. A delineation provides the area of a wetland.  There are several options available to evaluate the functions a wetland provides.

  • The Department of Ecology’s Washington State Wetland Rating System web page describes the systems used to evaluate how difficult it would be to replace a wetland, and is required to determine the ratio to for replacement wetland area by many Critical Areas Ordinances.
  • Credit-Debit DISCUSSION HERE
  • There are a number of full scale wetland functions assessment methods, but there are none that are both completely accepted in Washington, and practical for use in linear projects.

If a functions assessment is completed for the impacted wetland, it can be used to design replacement wetlands so that the functions are replaced. In order to evaluate if the mitigation site has providing the desired functions, the method used should be documented in the mitigation report so it can be used again at the end of the monitoring period.

4. Other Aquatic Habitats

Aquatic habitats or resources other than wetlands may be encountered on project sites and may also need to be assessed documented and permitted if impacted. Examples of other regulated aquatic habitats include:

Special training is required to delineate these habitats. For a list of staff available to regions to delineate other aquatic habitats, contact your Regional Environmental Coordinator, Regional Permit Coordinator, or Regional Biologist.

Wetland Buffers

Buffers are defined by the Department of Ecology, but are regulated by local Critical Areas Ordinances. Ecology defines buffers in Wetlands in Washington State Volume 2: Guidance for Protecting and Managing Wetlands, (Appendix 8-C and Appendix 8-E). Ecology also provides guidance on buffer widths. Generally, buffers are:

  • the uplands [or wetlands] adjacent to an aquatic resource,
  • protect and maintain the functions and values of wetlands,
  • provide the terrestrial [or wetland] habitats for many species of wildlife,
  • can reduce impacts to the wetland from adjacent land uses.

How wide are wetland buffers?

WSDOT applies the corresponding buffer requirements from federal, state, and local governments to all wetlands identified within a project area based on each wetland's rating. Many local Critical Areas Ordinances have adopted the buffer widths suggested by Department of Ecology's guidance in Chapter 6.6 of Wetlands in Washington State.

How are buffers incorporated into the project?

Wetland biologists and consultants must follow WSDOT guidance on Delineating Wetlands and Buffers adjacent to road prisms (pdf 72 kb) and Buffers Across Roadways (pdf 19 kb). Wetland biologists inform the Project Design Team of:

  • The required buffer width of each delineated wetland within the project area.
  • Any wetland buffers that extend into the project area, even though the wetland itself is located outside the project area and may not have been delineated.

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